2.12/lttng-docs-2.12.txt

   1 The LTTng Documentation
   2 =======================
   3 Philippe Proulx <pproulx@efficios.com>
   4 v2.12, 25 February 2021
   5
   6
   7 include::../common/copyright.txt[]
   8
   9
  10 include::../common/welcome.txt[]
  11
  12
  13 include::../common/audience.txt[]
  14
  15
  16 [[chapters]]
  17 === What's in this documentation?
  18
  19 The LTTng Documentation is divided into the following sections:
  20
  21 * **<<nuts-and-bolts,Nuts and bolts>>** explains the
  22   rudiments of software tracing and the rationale behind the
  23   LTTng project.
  24 +
  25 Skip this section if you’re familiar with software tracing and with the
  26 LTTng project.
  27
  28 * **<<installing-lttng,Installation>>** describes the steps to
  29   install the LTTng packages on common Linux distributions and from
  30   their sources.
  31 +
  32 Skip this section if you already properly installed LTTng on your target
  33 system.
  34
  35 * **<<getting-started,Quick start>>** is a concise guide to
  36   getting started quickly with LTTng kernel and user space tracing.
  37 +
  38 We recommend this section if you're new to LTTng or to software tracing
  39 in general.
  40 +
  41 Skip this section if you're not new to LTTng.
  42
  43 * **<<core-concepts,Core concepts>>** explains the concepts at
  44   the heart of LTTng.
  45 +
  46 It's a good idea to become familiar with the core concepts
  47 before attempting to use the toolkit.
  48
  49 * **<<plumbing,Components of LTTng>>** describes the various components
  50   of the LTTng machinery, like the daemons, the libraries, and the
  51   command-line interface.
  52 * **<<instrumenting,Instrumentation>>** shows different ways to
  53   instrument user applications and the Linux kernel.
  54 +
  55 Instrumenting source code is essential to provide a meaningful
  56 source of events.
  57 +
  58 Skip this section if you don't have a programming background.
  59
  60 * **<<controlling-tracing,Tracing control>>** is divided into topics
  61   which demonstrate how to use the vast array of features that
  62   LTTng{nbsp}{revision} offers.
  63 * **<<reference,Reference>>** contains reference tables.
  64 * **<<glossary,Glossary>>** is a specialized dictionary of terms related
  65   to LTTng or to the field of software tracing.
  66
  67
  68 include::../common/convention.txt[]
  69
  70
  71 include::../common/acknowledgements.txt[]
  72
  73
  74 [[whats-new]]
  75 == What's new in LTTng{nbsp}{revision}?
  76
  77 LTTng{nbsp}{revision} bears the name _Ta Meilleure_, a Northeast IPA
  78 beer brewed by https://lagabiere.com/[Lagabière]. Translating to ``Your
  79 best one'', this beer gives out strong aromas of passion fruit, lemon,
  80 and peaches. Tastewise, expect a lot of fruit, a creamy texture, and a
  81 smooth lingering hop bitterness.
  82
  83 New features and changes in LTTng{nbsp}{revision}:
  84
  85 Tracing control::
  86 +
  87 * Clear the contents of one or more <<tracing-session,tracing sessions>>
  88   without having to destroy and reconfigure them
  89   with the new man:lttng-clear(1) command.
  90 +
  91 This is especially useful to clear the tracing data of a tracing session
  92 between attempts to reproduce a problem.
  93 +
  94 See <<clear,Clear a tracing session>>.
  95
  96 * Before LTTng{nbsp}{revision}, the man:lttng-track(1) and
  97   man:lttng-untrack(1) commands used to add and remove process IDs
  98   (PIDs) to a whitelist so that LTTng would only trace processes with
  99   specific PIDs.
 100 +
 101 LTTng{nbsp}{revision} adds Unix user IDs (UIDs) and Unix group IDs
 102 (GIDs) to the available <<pid-tracking,process attributes to track>>.
 103 You can specify numeric user/group IDs and user/group names to track,
 104 for example:
 105 +
 106 [role="term"]
 107 ----
 108 $ lttng track --userspace --vuid=http,999 --vgid=mysql,9
 109 ----
 110 +
 111 While you can also track UIDs and GIDs with the
 112 opt:lttng-enable-event(1):--filter option of the `enable-event` command,
 113 this dedicated process attribute tracking approach reduces tracing
 114 overhead and prevents the creation of <<def-sub-buffer,sub-buffers>> for
 115 the users and groups which LTTng doesn't track.
 116 +
 117 In the command manual pages, the term ``whitelist'' is renamed to
 118 ``inclusion set'' to clarify the concept.
 119
 120 * The <<lttng-relayd,relay daemon>> can now maintain many files
 121   virtually opened without using as many file descriptors (FD). It does
 122   so by closing and reopening FDs as needed.
 123 +
 124 This feature is meant as a workaround for users who can't bump the
 125 system limit because of permission restrictions.
 126 +
 127 The new opt:lttng-relayd(8):--fd-pool-size relay daemon option
 128 sets the maximum number of simultaneously opened file descriptors
 129 (using the soft `RLIMIT_NOFILE` resource limit of the process by
 130 default; see man:getrlimit(2)).
 131
 132 * By default, the relay daemon writes its traces under a predefined
 133   directory hierarchy,
 134   +$LTTNG_HOME/lttng-traces/__host__/__session__/__domain__+, with:
 135 +
 136 --
 137 +__host__+::
 138     Remote hostname.
 139
 140 +__session__+::
 141     <<tracing-session,Tracing session>> name.
 142
 143 +__domain__+::
 144     <<domain,Tracing domain>> name (`ust` or `kernel`).
 145 --
 146 +
 147 Change this hierarchy to group traces by tracing session name rather
 148 than by hostname
 149 (+$LTTNG_HOME/lttng-traces/__session__/__host__/__domain__+) with the
 150 new opt:lttng-relayd(8):--group-output-by-session option of the
 151 relay daemon.
 152 +
 153 This feature is especially useful if you're tracing two or more hosts,
 154 having different hostnames, which share the same tracing session name as
 155 part of their configuration. In this scenario, you can use a descriptive
 156 tracing session name (for example, `connection-hang`) across a fleet of
 157 machines streaming to a single relay daemon.
 158
 159 * The relay daemon has a new opt:lttng-relayd(8):--working-directory
 160   option to override its working directory.
 161
 162 Linux kernel tracing::
 163 +
 164 * New instrumentation hooks to trace the entry and exit tracepoints of
 165   the network reception code paths of the Linux kernel.
 166 +
 167 Use the resulting event records to identify the bounds of a network
 168 reception and link the events that occur in the interim (for example,
 169 wake-ups) to a specific network reception instance. You can also
 170 analyze the latency of the network stack thanks to those event records.
 171
 172 * The `thread` field of the `irqaction` structure, which specifies the
 173   process to wake up when a threaded interrupt request (IRQ) occurs, is
 174   now part of the `lttng_statedump_interrupt` event record.
 175 +
 176 Use this information to discover which processes handle the various
 177 IRQs. You can also associate the events occurring in the context of
 178 those processes with their respective IRQ.
 179
 180 * New `lttng_statedump_cpu_topology` tracepoint to record the active
 181   CPU/NUMA topology.
 182 +
 183 Use this information to discover which CPUs are SMT siblings or part of
 184 the same socket. As of LTTng{nbsp}{revision}, only the x86 architecture
 185 is supported since all architectures describe their topologies
 186 differently.
 187 +
 188 The `architecture` field of the tracepoint is statically defined and
 189 exists for all architecture implementations. Analysis tools can
 190 therefore anticipate the layout of the event record.
 191 +
 192 Event record example:
 193 +
 194 [source,yaml]
 195 ----
 196 lttng_statedump_cpu_topology:
 197   architecture: x86
 198   cpu_id: 0
 199   vendor: GenuineIntel
 200   family: 6
 201   model: 142
 202   model_name: Intel(R) Core(TM) i7-7600U CPU @ 2.80GHz
 203   physical_id: 0
 204   core_id: 0
 205   cores: 2
 206 ----
 207
 208 * New product UUID metadata environment field, `product_uuid`,
 209   which LTTng copies from the
 210   https://en.wikipedia.org/wiki/Desktop_Management_Interface[Desktop
 211   Management Interface] (DMI).
 212 +
 213 Use this environment field to uniquely identify a machine (virtual or
 214 physical) in order to correlate traces from multiple virtual machines.
 215
 216
 217 [[nuts-and-bolts]]
 218 == Nuts and bolts
 219
 220 What is LTTng? As its name suggests, the _Linux Trace Toolkit: next
 221 generation_ is a modern toolkit for tracing Linux systems and
 222 applications. So your first question might be:
 223 **what is tracing?**
 224
 225
 226 [[what-is-tracing]]
 227 === What is tracing?
 228
 229 As the history of software engineering progressed and led to what
 230 we now take for granted--complex, numerous and
 231 interdependent software applications running in parallel on
 232 sophisticated operating systems like Linux--the authors of such
 233 components, software developers, began feeling a natural
 234 urge to have tools that would ensure the robustness and good performance
 235 of their masterpieces.
 236
 237 One major achievement in this field is, inarguably, the
 238 https://www.gnu.org/software/gdb/[GNU debugger (GDB)],
 239 an essential tool for developers to find and fix bugs. But even the best
 240 debugger won't help make your software run faster, and nowadays, faster
 241 software means either more work done by the same hardware, or cheaper
 242 hardware for the same work.
 243
 244 A _profiler_ is often the tool of choice to identify performance
 245 bottlenecks. Profiling is suitable to identify _where_ performance is
 246 lost in a given software. The profiler outputs a profile, a statistical
 247 summary of observed events, which you may use to discover which
 248 functions took the most time to execute. However, a profiler won't
 249 report _why_ some identified functions are the bottleneck. Bottlenecks
 250 might only occur when specific conditions are met, conditions that are
 251 sometimes impossible to capture by a statistical profiler, or impossible
 252 to reproduce with an application altered by the overhead of an
 253 event-based profiler. For a thorough investigation of software
 254 performance issues, a history of execution is essential, with the
 255 recorded values of variables and context fields you choose, and
 256 with as little influence as possible on the instrumented software. This
 257 is where tracing comes in handy.
 258
 259 _Tracing_ is a technique used to understand what goes on in a running
 260 software system. The software used for tracing is called a _tracer_,
 261 which is conceptually similar to a tape recorder. When recording,
 262 specific instrumentation points placed in the software source code
 263 generate events that are saved on a giant tape: a _trace_ file. You
 264 can trace user applications and the operating system at the same time,
 265 opening the possibility of resolving a wide range of problems that would
 266 otherwise be extremely challenging.
 267
 268 Tracing is often compared to _logging_. However, tracers and loggers are
 269 two different tools, serving two different purposes. Tracers are
 270 designed to record much lower-level events that occur much more
 271 frequently than log messages, often in the range of thousands per
 272 second, with very little execution overhead. Logging is more appropriate
 273 for a very high-level analysis of less frequent events: user accesses,
 274 exceptional conditions (errors and warnings, for example), database
 275 transactions, instant messaging communications, and such. Simply put,
 276 logging is one of the many use cases that can be satisfied with tracing.
 277
 278 The list of recorded events inside a trace file can be read manually
 279 like a log file for the maximum level of detail, but it is generally
 280 much more interesting to perform application-specific analyses to
 281 produce reduced statistics and graphs that are useful to resolve a
 282 given problem. Trace viewers and analyzers are specialized tools
 283 designed to do this.
 284
 285 In the end, this is what LTTng is: a powerful, open source set of
 286 tools to trace the Linux kernel and user applications at the same time.
 287 LTTng is composed of several components actively maintained and
 288 developed by its link:/community/#where[community].
 289
 290
 291 [[lttng-alternatives]]
 292 === Alternatives to noch:{LTTng}
 293
 294 Excluding proprietary solutions, a few competing software tracers
 295 exist for Linux:
 296
 297 https://github.com/dtrace4linux/linux[dtrace4linux]::
 298     A port of Sun Microsystems' DTrace to Linux.
 299 +
 300 The cmd:dtrace tool interprets user scripts and is responsible for
 301 loading code into the Linux kernel for further execution and collecting
 302 the outputted data.
 303
 304 https://en.wikipedia.org/wiki/Berkeley_Packet_Filter[eBPF]::
 305     A subsystem in the Linux kernel in which a virtual machine can
 306     execute programs passed from the user space to the kernel.
 307 +
 308 You can attach such programs to tracepoints and kprobes thanks to a
 309 system call, and they can output data to the user space when executed
 310 thanks to different mechanisms (pipe, VM register values, and eBPF maps,
 311 to name a few).
 312
 313 https://www.kernel.org/doc/Documentation/trace/ftrace.txt[ftrace]::
 314     The de facto function tracer of the Linux kernel.
 315 +
 316 Its user interface is a set of special files in sysfs.
 317
 318 https://perf.wiki.kernel.org/[perf]::
 319     A performance analysis tool for Linux which supports hardware
 320     performance counters, tracepoints, as well as other counters and
 321     types of probes.
 322 +
 323 The controlling utility of perf is the cmd:perf command line/text UI
 324 tool.
 325
 326 http://linux.die.net/man/1/strace[strace]::
 327     A command-line utility which records system calls made by a
 328     user process, as well as signal deliveries and changes of process
 329     state.
 330 +
 331 strace makes use of https://en.wikipedia.org/wiki/Ptrace[ptrace] to
 332 fulfill its function.
 333
 334 http://www.sysdig.org/[sysdig]::
 335     Like SystemTap, uses scripts to analyze Linux kernel events.
 336 +
 337 You write scripts, or _chisels_ in the jargon of sysdig, in Lua and
 338 sysdig executes them while it traces the system or afterwards. The
 339 interface of sysdig is the cmd:sysdig command-line tool as well as the
 340 text UI-based cmd:csysdig tool.
 341
 342 https://sourceware.org/systemtap/[SystemTap]::
 343     A Linux kernel and user space tracer which uses custom user scripts
 344     to produce plain text traces.
 345 +
 346 SystemTap converts the scripts to the C language, and then compiles them
 347 as Linux kernel modules which are loaded to produce trace data. The
 348 primary user interface of SystemTap is the cmd:stap command-line tool.
 349
 350 The main distinctive features of LTTng is that it produces correlated
 351 kernel and user space traces, as well as doing so with the lowest
 352 overhead amongst other solutions. It produces trace files in the
 353 http://diamon.org/ctf[CTF] format, a file format optimized
 354 for the production and analyses of multi-gigabyte data.
 355
 356 LTTng is the result of more than 10{nbsp}years of active open source
 357 development by a community of passionate developers. LTTng is currently
 358 available on major desktop and server Linux distributions.
 359
 360 The main interface for tracing control is a single command-line tool
 361 named cmd:lttng. The latter can create several tracing sessions, enable
 362 and disable events on the fly, filter events efficiently with custom
 363 user expressions, start and stop tracing, and much more. LTTng can
 364 record the traces on the file system or send them over the network, and
 365 keep them totally or partially. You can view the traces once tracing
 366 becomes inactive or in real-time.
 367
 368 <<installing-lttng,Install LTTng now>> and
 369 <<getting-started,start tracing>>!
 370
 371
 372 [[installing-lttng]]
 373 == Installation
 374
 375 **LTTng** is a set of software <<plumbing,components>> which interact to
 376 <<instrumenting,instrument>> the Linux kernel and user applications, and
 377 to <<controlling-tracing,control tracing>> (start and stop
 378 tracing, enable and disable event rules, and the rest). Those
 379 components are bundled into the following packages:
 380
 381 LTTng-tools::
 382     Libraries and command-line interface to control tracing.
 383
 384 LTTng-modules::
 385     Linux kernel modules to instrument and trace the kernel.
 386
 387 LTTng-UST::
 388     Libraries and Java/Python packages to instrument and trace user
 389     applications.
 390
 391 Most distributions mark the LTTng-modules and LTTng-UST packages as
 392 optional when installing LTTng-tools (which is always required). In the
 393 following sections, we always provide the steps to install all three,
 394 but note that:
 395
 396 * You only need to install LTTng-modules if you intend to trace the
 397   Linux kernel.
 398 * You only need to install LTTng-UST if you intend to trace user
 399   applications.
 400
 401 [role="growable"]
 402 .Availability of LTTng{nbsp}{revision} for major Linux distributions as of 25{nbsp}February{nbsp}2021.
 403 |====
 404 |Distribution |Available in releases
 405
 406 |https://www.ubuntu.com/[Ubuntu]
 407 |<<ubuntu,Ubuntu 20.10 _Groovy Gorilla_>>.
 408
 409 Ubuntu{nbsp}16.04 _Xenial Xerus_, Ubuntu{nbsp}18.04 _Bionic Beaver_,
 410 and Ubuntu{nbsp}20.04 _Focal Fossa_:
 411 <<ubuntu-ppa,use the LTTng Stable{nbsp}{revision} PPA>>.
 412
 413 |https://www.debian.org/[Debian]
 414 |<<debian,Debian "bullseye" (testing)>>.
 415
 416 |https://getfedora.org/[Fedora]
 417 |xref:fedora[Fedora{nbsp}33, Fedora{nbsp}34, and Fedora{nbsp}35].
 418
 419 |https://www.archlinux.org/[Arch Linux]
 420 |<<arch-linux,_Community_ repository and AUR>>.
 421
 422 |https://www.redhat.com/[RHEL] and https://www.suse.com/[SLES]
 423 |See http://packages.efficios.com/[EfficiOS Enterprise Packages].
 424
 425 |https://alpinelinux.org/[Alpine Linux]
 426 |<<alpine-linux,Alpine Linux{nbsp}3.12 and Alpine Linux{nbsp}3.13>>.
 427
 428 |https://buildroot.org/[Buildroot]
 429 |xref:buildroot[Buildroot{nbsp}2020.08 and Buildroot{nbsp}2020.11].
 430
 431 |https://www.openembedded.org/wiki/Main_Page[OpenEmbedded] and
 432 https://www.yoctoproject.org/[Yocto]
 433 |<<oe-yocto,Yocto Project{nbsp}3.2 _Gatesgarth_>>
 434 (`openembedded-core` layer).
 435 |====
 436
 437
 438 [[ubuntu]]
 439 === [[ubuntu-official-repositories]]Ubuntu
 440
 441 LTTng{nbsp}{revision} is available on Ubuntu{nbsp}20.10 _Groovy
 442 Gorilla_. For previous supported releases of Ubuntu, <<ubuntu-ppa,use
 443 the LTTng Stable{nbsp}{revision} PPA>>.
 444
 445 To install LTTng{nbsp}{revision} on Ubuntu{nbsp}20.10 _Groovy Gorilla_:
 446
 447 . Install the main LTTng{nbsp}{revision} packages:
 448 +
 449 --
 450 [role="term"]
 451 ----
 452 # apt-get install lttng-tools
 453 # apt-get install lttng-modules-dkms
 454 # apt-get install liblttng-ust-dev
 455 ----
 456 --
 457
 458 . **If you need to instrument and trace
 459   <<java-application,Java applications>>**, install the LTTng-UST
 460   Java agent:
 461 +
 462 --
 463 [role="term"]
 464 ----
 465 # apt-get install liblttng-ust-agent-java
 466 ----
 467 --
 468
 469 . **If you need to instrument and trace
 470   <<python-application,Python{nbsp}3 applications>>**, install the
 471   LTTng-UST Python agent:
 472 +
 473 --
 474 [role="term"]
 475 ----
 476 # apt-get install python3-lttngust
 477 ----
 478 --
 479
 480
 481 [[ubuntu-ppa]]
 482 === Ubuntu: noch:{LTTng} Stable {revision} PPA
 483
 484 The https://launchpad.net/~lttng/+archive/ubuntu/stable-{revision}[LTTng
 485 Stable{nbsp}{revision} PPA] offers the latest stable
 486 LTTng{nbsp}{revision} packages for Ubuntu{nbsp}16.04 _Xenial Xerus_,
 487 Ubuntu{nbsp}18.04 _Bionic Beaver_, and Ubuntu{nbsp}20.04 _Focal Fossa_.
 488
 489 To install LTTng{nbsp}{revision} from the LTTng Stable{nbsp}{revision}
 490 PPA:
 491
 492 . Add the LTTng Stable{nbsp}{revision} PPA repository and update the
 493   list of packages:
 494 +
 495 --
 496 [role="term"]
 497 ----
 498 # apt-add-repository ppa:lttng/stable-2.12
 499 # apt-get update
 500 ----
 501 --
 502
 503 . Install the main LTTng{nbsp}{revision} packages:
 504 +
 505 --
 506 [role="term"]
 507 ----
 508 # apt-get install lttng-tools
 509 # apt-get install lttng-modules-dkms
 510 # apt-get install liblttng-ust-dev
 511 ----
 512 --
 513
 514 . **If you need to instrument and trace
 515   <<java-application,Java applications>>**, install the LTTng-UST
 516   Java agent:
 517 +
 518 --
 519 [role="term"]
 520 ----
 521 # apt-get install liblttng-ust-agent-java
 522 ----
 523 --
 524
 525 . **If you need to instrument and trace
 526   <<python-application,Python{nbsp}3 applications>>**, install the
 527   LTTng-UST Python agent:
 528 +
 529 --
 530 [role="term"]
 531 ----
 532 # apt-get install python3-lttngust
 533 ----
 534 --
 535
 536
 537 [[debian]]
 538 === Debian
 539
 540 To install LTTng{nbsp}{revision} on Debian "bullseye" (testing):
 541
 542 . Install the main LTTng{nbsp}{revision} packages:
 543 +
 544 --
 545 [role="term"]
 546 ----
 547 # apt-get install lttng-modules-dkms
 548 # apt-get install liblttng-ust-dev
 549 # apt-get install lttng-tools
 550 ----
 551 --
 552
 553 . **If you need to instrument and trace <<java-application,Java
 554   applications>>**, install the LTTng-UST Java agent:
 555 +
 556 --
 557 [role="term"]
 558 ----
 559 # apt-get install liblttng-ust-agent-java
 560 ----
 561 --
 562
 563 . **If you need to instrument and trace <<python-application,Python
 564   applications>>**, install the LTTng-UST Python agent:
 565 +
 566 --
 567 [role="term"]
 568 ----
 569 # apt-get install python3-lttngust
 570 ----
 571 --
 572
 573
 574 [[fedora]]
 575 === Fedora
 576
 577 To install LTTng{nbsp}{revision} on Fedora{nbsp}33, Fedora{nbsp}34, or
 578 Fedora{nbsp}35:
 579
 580 . Install the LTTng-tools{nbsp}{revision} and LTTng-UST{nbsp}{revision}
 581   packages:
 582 +
 583 --
 584 [role="term"]
 585 ----
 586 # yum install lttng-tools
 587 # yum install lttng-ust
 588 ----
 589 --
 590
 591 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
 592 +
 593 --
 594 [role="term"]
 595 ----
 596 $ cd $(mktemp -d) &&
 597 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.12.tar.bz2 &&
 598 tar -xf lttng-modules-latest-2.12.tar.bz2 &&
 599 cd lttng-modules-2.12.* &&
 600 make &&
 601 sudo make modules_install &&
 602 sudo depmod -a
 603 ----
 604 --
 605
 606 [IMPORTANT]
 607 .Java and Python application instrumentation and tracing
 608 ====
 609 If you need to instrument and trace <<java-application,Java
 610 applications>> on Fedora, you need to build and install
 611 LTTng-UST{nbsp}{revision} <<building-from-source,from source>> and pass
 612 the `--enable-java-agent-jul`, `--enable-java-agent-log4j`, or
 613 `--enable-java-agent-all` options to the `configure` script, depending
 614 on which Java logging framework you use.
 615
 616 If you need to instrument and trace <<python-application,Python
 617 applications>> on Fedora, you need to build and install
 618 LTTng-UST{nbsp}{revision} from source and pass the
 619 `--enable-python-agent` option to the `configure` script.
 620 ====
 621
 622
 623 [[arch-linux]]
 624 === Arch Linux
 625
 626 LTTng-UST{nbsp}{revision} is available in the _community_
 627 repository of Arch Linux, while LTTng-tools{nbsp}{revision} and
 628 LTTng-modules{nbsp}{revision} are available in the
 629 https://aur.archlinux.org/[AUR].
 630
 631 To install LTTng{nbsp}{revision} on Arch Linux, using
 632 https://github.com/Jguer/yay[yay] for the AUR packages:
 633
 634 . Install the main LTTng{nbsp}{revision} packages:
 635 +
 636 --
 637 [role="term"]
 638 ----
 639 # pacman -Sy lttng-ust
 640 $ yay -Sy lttng-tools
 641 $ yay -Sy lttng-modules
 642 ----
 643 --
 644
 645 . **If you need to instrument and trace <<python-application,Python
 646   applications>>**, install the LTTng-UST Python agent:
 647 +
 648 --
 649 [role="term"]
 650 ----
 651 # pacman -Sy python-lttngust
 652 # pacman -Sy python2-lttngust
 653 ----
 654 --
 655
 656
 657 [[alpine-linux]]
 658 === Alpine Linux
 659
 660 To install LTTng-tools{nbsp}{revision} and LTTng-UST{nbsp}{revision} on
 661 Alpine Linux{nbsp}3.12 or Alpine Linux{nbsp}3.13:
 662
 663 . Add the LTTng packages:
 664 +
 665 --
 666 [role="term"]
 667 ----
 668 # apk add lttng-tools
 669 # apk add lttng-ust-dev
 670 ----
 671 --
 672
 673 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
 674 +
 675 --
 676 [role="term"]
 677 ----
 678 $ cd $(mktemp -d) &&
 679 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.12.tar.bz2 &&
 680 tar -xf lttng-modules-latest-2.12.tar.bz2 &&
 681 cd lttng-modules-2.12.* &&
 682 make &&
 683 sudo make modules_install &&
 684 sudo depmod -a
 685 ----
 686 --
 687
 688
 689 [[buildroot]]
 690 === Buildroot
 691
 692 To install LTTng{nbsp}{revision} on Buildroot{nbsp}2020.08 or
 693 Buildroot{nbsp}2020.11:
 694
 695 . Launch the Buildroot configuration tool:
 696 +
 697 --
 698 [role="term"]
 699 ----
 700 $ make menuconfig
 701 ----
 702 --
 703
 704 . In **Kernel**, check **Linux kernel**.
 705 . In **Toolchain**, check **Enable WCHAR support**.
 706 . In **Target packages**{nbsp}&#8594; **Debugging, profiling and benchmark**,
 707   check **lttng-modules** and **lttng-tools**.
 708 . In **Target packages**{nbsp}&#8594; **Libraries**{nbsp}&#8594;
 709   **Other**, check **lttng-libust**.
 710
 711
 712 [[oe-yocto]]
 713 === OpenEmbedded and Yocto
 714
 715 LTTng{nbsp}{revision} recipes are available in the
 716 https://layers.openembedded.org/layerindex/branch/master/layer/openembedded-core/[`openembedded-core`]
 717 layer for Yocto Project{nbsp}3.2 _Gatesgarth_ under the following names:
 718
 719 * `lttng-tools`
 720 * `lttng-modules`
 721 * `lttng-ust`
 722
 723 With BitBake, the simplest way to include LTTng recipes in your target
 724 image is to add them to `IMAGE_INSTALL_append` in path:{conf/local.conf}:
 725
 726 ----
 727 IMAGE_INSTALL_append = " lttng-tools lttng-modules lttng-ust"
 728 ----
 729
 730 If you use Hob:
 731
 732 . Select a machine and an image recipe.
 733 . Click **Edit image recipe**.
 734 . Under the **All recipes** tab, search for **lttng**.
 735 . Check the desired LTTng recipes.
 736
 737
 738 [[building-from-source]]
 739 === Build from source
 740
 741 To build and install LTTng{nbsp}{revision} from source:
 742
 743 . Using the package manager of your distribution, or from source,
 744   install the following dependencies of LTTng-tools and LTTng-UST:
 745 +
 746 --
 747 * https://sourceforge.net/projects/libuuid/[libuuid]
 748 * http://directory.fsf.org/wiki/Popt[popt]
 749 * http://liburcu.org/[Userspace RCU]
 750 * http://www.xmlsoft.org/[libxml2]
 751 * **Optional**: https://github.com/numactl/numactl[numactl]
 752 --
 753
 754 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
 755 +
 756 --
 757 [role="term"]
 758 ----
 759 $ cd $(mktemp -d) &&
 760 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.12.tar.bz2 &&
 761 tar -xf lttng-modules-latest-2.12.tar.bz2 &&
 762 cd lttng-modules-2.12.* &&
 763 make &&
 764 sudo make modules_install &&
 765 sudo depmod -a
 766 ----
 767 --
 768
 769 . Download, build, and install the latest LTTng-UST{nbsp}{revision}:
 770 +
 771 --
 772 [role="term"]
 773 ----
 774 $ cd $(mktemp -d) &&
 775 wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.12.tar.bz2 &&
 776 tar -xf lttng-ust-latest-2.12.tar.bz2 &&
 777 cd lttng-ust-2.12.* &&
 778 ./configure &&
 779 make &&
 780 sudo make install &&
 781 sudo ldconfig
 782 ----
 783 --
 784 +
 785 Add `--disable-numa` to `./configure` if you don't have
 786 https://github.com/numactl/numactl[numactl].
 787 +
 788 --
 789 [IMPORTANT]
 790 .Java and Python application tracing
 791 ====
 792 If you need to instrument and trace <<java-application,Java
 793 applications>>, pass the `--enable-java-agent-jul`,
 794 `--enable-java-agent-log4j`, or `--enable-java-agent-all` options to the
 795 `configure` script, depending on which Java logging framework you use.
 796
 797 If you need to instrument and trace <<python-application,Python
 798 applications>>, pass the `--enable-python-agent` option to the
 799 `configure` script. You can set the `PYTHON` environment variable to the
 800 path to the Python interpreter for which to install the LTTng-UST Python
 801 agent package.
 802 ====
 803 --
 804 +
 805 --
 806 [NOTE]
 807 ====
 808 By default, LTTng-UST libraries are installed to
 809 dir:{/usr/local/lib}, which is the de facto directory in which to
 810 keep self-compiled and third-party libraries.
 811
 812 When <<building-tracepoint-providers-and-user-application,linking an
 813 instrumented user application with `liblttng-ust`>>:
 814
 815 * Append `/usr/local/lib` to the env:LD_LIBRARY_PATH environment
 816   variable.
 817 * Pass the `-L/usr/local/lib` and `-Wl,-rpath,/usr/local/lib` options to
 818   man:gcc(1), man:g++(1), or man:clang(1).
 819 ====
 820 --
 821
 822 . Download, build, and install the latest LTTng-tools{nbsp}{revision}:
 823 +
 824 --
 825 [role="term"]
 826 ----
 827 $ cd $(mktemp -d) &&
 828 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.12.tar.bz2 &&
 829 tar -xf lttng-tools-latest-2.12.tar.bz2 &&
 830 cd lttng-tools-2.12.* &&
 831 ./configure &&
 832 make &&
 833 sudo make install &&
 834 sudo ldconfig
 835 ----
 836 --
 837
 838 TIP: The https://github.com/eepp/vlttng[vlttng tool] can do all the
 839 previous steps automatically for a given version of LTTng and confine
 840 the installed files in a specific directory. This can be useful to test
 841 LTTng without installing it on your system.
 842
 843
 844 [[getting-started]]
 845 == Quick start
 846
 847 This is a short guide to get started quickly with LTTng kernel and user
 848 space tracing.
 849
 850 Before you follow this guide, make sure to <<installing-lttng,install>>
 851 LTTng.
 852
 853 This tutorial walks you through the steps to:
 854
 855 . <<tracing-the-linux-kernel,Trace the Linux kernel>>.
 856 . <<tracing-your-own-user-application,Trace a user application>> written
 857   in C.
 858 . <<viewing-and-analyzing-your-traces,View and analyze the
 859   recorded events>>.
 860
 861
 862 [[tracing-the-linux-kernel]]
 863 === Trace the Linux kernel
 864
 865 The following command lines start with the `#` prompt because you need
 866 root privileges to trace the Linux kernel. You can also trace the kernel
 867 as a regular user if your Unix user is a member of the
 868 <<tracing-group,tracing group>>.
 869
 870 . Create a <<tracing-session,tracing session>> which writes its traces
 871   to dir:{/tmp/my-kernel-trace}:
 872 +
 873 --
 874 [role="term"]
 875 ----
 876 # lttng create my-kernel-session --output=/tmp/my-kernel-trace
 877 ----
 878 --
 879
 880 . List the available kernel tracepoints and system calls:
 881 +
 882 --
 883 [role="term"]
 884 ----
 885 # lttng list --kernel
 886 # lttng list --kernel --syscall
 887 ----
 888 --
 889
 890 . Create <<event,event rules>> which match the desired instrumentation
 891   point names, for example the `sched_switch` and `sched_process_fork`
 892   tracepoints, and the man:open(2) and man:close(2) system calls:
 893 +
 894 --
 895 [role="term"]
 896 ----
 897 # lttng enable-event --kernel sched_switch,sched_process_fork
 898 # lttng enable-event --kernel --syscall open,close
 899 ----
 900 --
 901 +
 902 Create an event rule which matches _all_ the Linux kernel
 903 tracepoints with the opt:lttng-enable-event(1):--all option
 904 (this will generate a lot of data when tracing):
 905 +
 906 --
 907 [role="term"]
 908 ----
 909 # lttng enable-event --kernel --all
 910 ----
 911 --
 912
 913 . <<basic-tracing-session-control,Start tracing>>:
 914 +
 915 --
 916 [role="term"]
 917 ----
 918 # lttng start
 919 ----
 920 --
 921
 922 . Do some operation on your system for a few seconds. For example,
 923   load a website, or list the files of a directory.
 924 . <<creating-destroying-tracing-sessions,Destroy>> the current
 925   tracing session:
 926 +
 927 --
 928 [role="term"]
 929 ----
 930 # lttng destroy
 931 ----
 932 --
 933 +
 934 The man:lttng-destroy(1) command doesn't destroy the trace data; it
 935 only destroys the state of the tracing session.
 936 +
 937 The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command
 938 implicitly (see <<basic-tracing-session-control,Start and stop a tracing
 939 session>>). You need to stop tracing to make LTTng flush the remaining
 940 trace data and make the trace readable.
 941
 942 . For the sake of this example, make the recorded trace accessible to
 943   the non-root users:
 944 +
 945 --
 946 [role="term"]
 947 ----
 948 # chown -R $(whoami) /tmp/my-kernel-trace
 949 ----
 950 --
 951
 952 See <<viewing-and-analyzing-your-traces,View and analyze the
 953 recorded events>> to view the recorded events.
 954
 955
 956 [[tracing-your-own-user-application]]
 957 === Trace a user application
 958
 959 This section steps you through a simple example to trace a
 960 _Hello world_ program written in C.
 961
 962 To create the traceable user application:
 963
 964 . Create the tracepoint provider header file, which defines the
 965   tracepoints and the events they can generate:
 966 +
 967 --
 968 [source,c]
 969 .path:{hello-tp.h}
 970 ----
 971 #undef TRACEPOINT_PROVIDER
 972 #define TRACEPOINT_PROVIDER hello_world
 973
 974 #undef TRACEPOINT_INCLUDE
 975 #define TRACEPOINT_INCLUDE "./hello-tp.h"
 976
 977 #if !defined(_HELLO_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
 978 #define _HELLO_TP_H
 979
 980 #include <lttng/tracepoint.h>
 981
 982 TRACEPOINT_EVENT(
 983     hello_world,
 984     my_first_tracepoint,
 985     TP_ARGS(
 986         int, my_integer_arg,
 987         char*, my_string_arg
 988     ),
 989     TP_FIELDS(
 990         ctf_string(my_string_field, my_string_arg)
 991         ctf_integer(int, my_integer_field, my_integer_arg)
 992     )
 993 )
 994
 995 #endif /* _HELLO_TP_H */
 996
 997 #include <lttng/tracepoint-event.h>
 998 ----
 999 --
1000
1001 . Create the tracepoint provider package source file:
1002 +
1003 --
1004 [source,c]
1005 .path:{hello-tp.c}
1006 ----
1007 #define TRACEPOINT_CREATE_PROBES
1008 #define TRACEPOINT_DEFINE
1009
1010 #include "hello-tp.h"
1011 ----
1012 --
1013
1014 . Build the tracepoint provider package:
1015 +
1016 --
1017 [role="term"]
1018 ----
1019 $ gcc -c -I. hello-tp.c
1020 ----
1021 --
1022
1023 . Create the _Hello World_ application source file:
1024 +
1025 --
1026 [source,c]
1027 .path:{hello.c}
1028 ----
1029 #include <stdio.h>
1030 #include "hello-tp.h"
1031
1032 int main(int argc, char *argv[])
1033 {
1034     int x;
1035
1036     puts("Hello, World!\nPress Enter to continue...");
1037
1038     /*
1039      * The following getchar() call is only placed here for the purpose
1040      * of this demonstration, to pause the application in order for
1041      * you to have time to list its tracepoints. It's not needed
1042      * otherwise.
1043      */
1044     getchar();
1045
1046     /*
1047      * A tracepoint() call.
1048      *
1049      * Arguments, as defined in hello-tp.h:
1050      *
1051      * 1. Tracepoint provider name   (required)
1052      * 2. Tracepoint name            (required)
1053      * 3. my_integer_arg             (first user-defined argument)
1054      * 4. my_string_arg              (second user-defined argument)
1055      *
1056      * Notice the tracepoint provider and tracepoint names are
1057      * NOT strings: they are in fact parts of variables that the
1058      * macros in hello-tp.h create.
1059      */
1060     tracepoint(hello_world, my_first_tracepoint, 23, "hi there!");
1061
1062     for (x = 0; x < argc; ++x) {
1063         tracepoint(hello_world, my_first_tracepoint, x, argv[x]);
1064     }
1065
1066     puts("Quitting now!");
1067     tracepoint(hello_world, my_first_tracepoint, x * x, "x^2");
1068
1069     return 0;
1070 }
1071 ----
1072 --
1073
1074 . Build the application:
1075 +
1076 --
1077 [role="term"]
1078 ----
1079 $ gcc -c hello.c
1080 ----
1081 --
1082
1083 . Link the application with the tracepoint provider package,
1084   `liblttng-ust`, and `libdl`:
1085 +
1086 --
1087 [role="term"]
1088 ----
1089 $ gcc -o hello hello.o hello-tp.o -llttng-ust -ldl
1090 ----
1091 --
1092
1093 Here's the whole build process:
1094
1095 [role="img-100"]
1096 .Build steps of the user space tracing tutorial.
1097 image::ust-flow.png[]
1098
1099 To trace the user application:
1100
1101 . Run the application with a few arguments:
1102 +
1103 --
1104 [role="term"]
1105 ----
1106 $ ./hello world and beyond
1107 ----
1108 --
1109 +
1110 You see:
1111 +
1112 --
1113 ----
1114 Hello, World!
1115 Press Enter to continue...
1116 ----
1117 --
1118
1119 . Start an LTTng <<lttng-sessiond,session daemon>>:
1120 +
1121 --
1122 [role="term"]
1123 ----
1124 $ lttng-sessiond --daemonize
1125 ----
1126 --
1127 +
1128 Note that a session daemon might already be running, for example as a
1129 service that the service manager of the distribution started.
1130
1131 . List the available user space tracepoints:
1132 +
1133 --
1134 [role="term"]
1135 ----
1136 $ lttng list --userspace
1137 ----
1138 --
1139 +
1140 You see the `hello_world:my_first_tracepoint` tracepoint listed
1141 under the `./hello` process.
1142
1143 . Create a <<tracing-session,tracing session>>:
1144 +
1145 --
1146 [role="term"]
1147 ----
1148 $ lttng create my-user-space-session
1149 ----
1150 --
1151
1152 . Create an <<event,event rule>> which matches the
1153   `hello_world:my_first_tracepoint` event name:
1154 +
1155 --
1156 [role="term"]
1157 ----
1158 $ lttng enable-event --userspace hello_world:my_first_tracepoint
1159 ----
1160 --
1161
1162 . <<basic-tracing-session-control,Start tracing>>:
1163 +
1164 --
1165 [role="term"]
1166 ----
1167 $ lttng start
1168 ----
1169 --
1170
1171 . Go back to the running `hello` application and press Enter. The
1172   program executes all `tracepoint()` instrumentation points and exits.
1173 . <<creating-destroying-tracing-sessions,Destroy>> the current
1174   tracing session:
1175 +
1176 --
1177 [role="term"]
1178 ----
1179 $ lttng destroy
1180 ----
1181 --
1182 +
1183 The man:lttng-destroy(1) command doesn't destroy the trace data; it
1184 only destroys the state of the tracing session.
1185 +
1186 The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command
1187 implicitly (see <<basic-tracing-session-control,Start and stop a tracing
1188 session>>). You need to stop tracing to make LTTng flush the remaining
1189 trace data and make the trace readable.
1190
1191 By default, LTTng saves the traces in
1192 +$LTTNG_HOME/lttng-traces/__name__-__date__-__time__+,
1193 where +__name__+ is the tracing session name. The
1194 env:LTTNG_HOME environment variable defaults to `$HOME` if not set.
1195
1196 See <<viewing-and-analyzing-your-traces,View and analyze the
1197 recorded events>> to view the recorded events.
1198
1199
1200 [[viewing-and-analyzing-your-traces]]
1201 === View and analyze the recorded events
1202
1203 Once you have completed the <<tracing-the-linux-kernel,Trace the Linux
1204 kernel>> and <<tracing-your-own-user-application,Trace a user
1205 application>> tutorials, you can inspect the recorded events.
1206
1207 There are many tools you can use to read LTTng traces:
1208
1209 https://babeltrace.org/[Babeltrace{nbsp}2]::
1210     A rich, flexible trace manipulation toolkit which includes
1211     a versatile command-line interface
1212     (https://babeltrace.org/docs/v2.0/man1/babeltrace2.1/[cmd:babeltrace2]),
1213     a https://babeltrace.org/docs/v2.0/libbabeltrace2/[C library],
1214     and https://babeltrace.org/docs/v2.0/python/bt2/[Python{nbsp}3 bindings]
1215     so that you can easily process or convert an LTTng trace with
1216     your own script.
1217 +
1218 The Babeltrace{nbsp}2 project ships with a
1219 https://babeltrace.org/docs/v2.0/man7/babeltrace2-plugin-ctf.7/[plugin]
1220 which supports the format of the traces which LTTng produces,
1221 https://diamon.org/ctf/[CTF].
1222
1223 http://tracecompass.org/[Trace Compass]::
1224     A graphical user interface for viewing and analyzing any type of
1225     logs or traces, including those of LTTng.
1226
1227 https://github.com/lttng/lttng-analyses[**LTTng analyses**]::
1228     An experimental project which includes many high-level analyses of
1229     LTTng kernel traces, like scheduling statistics, interrupt
1230     frequency distribution, top CPU usage, and more.
1231
1232 NOTE: This section assumes that LTTng saved the traces it recorded
1233 during the previous tutorials to their default location, in the
1234 dir:{$LTTNG_HOME/lttng-traces} directory. The env:LTTNG_HOME
1235 environment variable defaults to `$HOME` if not set.
1236
1237
1238 [[viewing-and-analyzing-your-traces-bt]]
1239 ==== Use the cmd:babeltrace2 command-line tool
1240
1241 The simplest way to list all the recorded events of an LTTng trace is to
1242 pass its path to
1243 https://babeltrace.org/docs/v2.0/man1/babeltrace2.1/[cmd:babeltrace2]
1244 without options:
1245
1246 [role="term"]
1247 ----
1248 $ babeltrace2 ~/lttng-traces/my-user-space-session*
1249 ----
1250
1251 cmd:babeltrace2 finds all traces recursively within the given path and
1252 prints all their events, sorting them chronologically.
1253
1254 Pipe the output of cmd:babeltrace2 into a tool like man:grep(1) for
1255 further filtering:
1256
1257 [role="term"]
1258 ----
1259 $ babeltrace2 /tmp/my-kernel-trace | grep _switch
1260 ----
1261
1262 Pipe the output of cmd:babeltrace2 into a tool like man:wc(1) to count
1263 the recorded events:
1264
1265 [role="term"]
1266 ----
1267 $ babeltrace2 /tmp/my-kernel-trace | grep _open | wc --lines
1268 ----
1269
1270
1271 [[viewing-and-analyzing-your-traces-bt-python]]
1272 ==== Use the Babeltrace{nbsp}2 Python bindings
1273
1274 The <<viewing-and-analyzing-your-traces-bt,text output of
1275 cmd:babeltrace2>> is useful to isolate events by simple matching using
1276 man:grep(1) and similar utilities. However, more elaborate filters, such
1277 as keeping only event records with a field value falling within a
1278 specific range, are not trivial to write using a shell. Moreover,
1279 reductions and even the most basic computations involving multiple event
1280 records are virtually impossible to implement.
1281
1282 Fortunately, Babeltrace{nbsp}2 ships with
1283 https://babeltrace.org/docs/v2.0/python/bt2/[Python{nbsp}3 bindings]
1284 which make it easy to read the event records of an LTTng trace
1285 sequentially and compute the desired information.
1286
1287 The following script accepts an LTTng Linux kernel trace path as its
1288 first argument and prints the short names of the top five running
1289 processes on CPU{nbsp}0 during the whole trace:
1290
1291 [source,python]
1292 .path:{top5proc.py}
1293 ----
1294 import bt2
1295 import sys
1296 import collections
1297
1298 def top5proc():
1299     # Get the trace path from the first command-line argument.
1300     it = bt2.TraceCollectionMessageIterator(sys.argv[1])
1301
1302     # This counter dictionary will hold execution times:
1303     #
1304     #     Task command name -> Total execution time (ns)
1305     exec_times = collections.Counter()
1306
1307     # This holds the last `sched_switch` timestamp.
1308     last_ts = None
1309
1310     for msg in it:
1311         # We only care about event messages.
1312         if type(msg) is not bt2._EventMessageConst:
1313             continue
1314
1315         # Event of the event message.
1316         event = msg.event
1317
1318         # Keep only `sched_switch` events.
1319         if event.cls.name != 'sched_switch':
1320             continue
1321
1322         # Keep only events which occurred on CPU 0.
1323         if event.packet.context_field['cpu_id'] != 0:
1324             continue
1325
1326         # Event timestamp (ns).
1327         cur_ts = msg.default_clock_snapshot.ns_from_origin
1328
1329         if last_ts is None:
1330             # We start here.
1331             last_ts = cur_ts
1332
1333         # (Short) name of the previous task command.
1334         prev_comm = str(event.payload_field['prev_comm'])
1335
1336         # Initialize an entry in our dictionary if not yet done.
1337         if prev_comm not in exec_times:
1338             exec_times[prev_comm] = 0
1339
1340         # Compute previous command execution time.
1341         diff = cur_ts - last_ts
1342
1343         # Update execution time of this command.
1344         exec_times[prev_comm] += diff
1345
1346         # Update last timestamp.
1347         last_ts = cur_ts
1348
1349     # Print top 5.
1350     for name, ns in exec_times.most_common(5):
1351         print('{:20}{} s'.format(name, ns / 1e9))
1352
1353
1354 if __name__ == '__main__':
1355     top5proc()
1356 ----
1357
1358 Run this script:
1359
1360 [role="term"]
1361 ----
1362 $ python3 top5proc.py /tmp/my-kernel-trace/kernel
1363 ----
1364
1365 Output example:
1366
1367 ----
1368 swapper/0           48.607245889 s
1369 chromium            7.192738188 s
1370 pavucontrol         0.709894415 s
1371 Compositor          0.660867933 s
1372 Xorg.bin            0.616753786 s
1373 ----
1374
1375 Note that `swapper/0` is the ``idle'' process of CPU{nbsp}0 on Linux;
1376 since we weren't using the CPU that much when tracing, its first
1377 position in the list makes sense.
1378
1379
1380 [[core-concepts]]
1381 == [[understanding-lttng]]Core concepts
1382
1383 From a user's perspective, the LTTng system is built on a few concepts,
1384 or objects, on which the <<lttng-cli,cmd:lttng command-line tool>>
1385 operates by sending commands to the <<lttng-sessiond,session daemon>>.
1386 Understanding how those objects relate to eachother is key in mastering
1387 the toolkit.
1388
1389 The core concepts are:
1390
1391 * <<tracing-session,Tracing session>>
1392 * <<domain,Tracing domain>>
1393 * <<channel,Channel and ring buffer>>
1394 * <<"event","Instrumentation point, event rule, event, and event record">>
1395
1396
1397 [[tracing-session]]
1398 === Tracing session
1399
1400 A _tracing session_ is a stateful dialogue between you and
1401 a <<lttng-sessiond,session daemon>>. You can
1402 <<creating-destroying-tracing-sessions,create a new tracing
1403 session>> with the `lttng create` command.
1404
1405 Most of what you do when you control LTTng tracers happens within a
1406 tracing session. In particular, a tracing session:
1407
1408 * Has its own name.
1409 * Has its own set of trace files.
1410 * Has its own state of activity (started or stopped).
1411 * Has its own <<tracing-session-mode,mode>> (local, network streaming,
1412   snapshot, or live).
1413 * Has its own <<channel,channels>> to which are associated their own
1414   <<event,event rules>>.
1415 * Has its own <<pid-tracking,process attribute tracking>> inclusion
1416   sets.
1417
1418 [role="img-100"]
1419 .A _tracing session_ contains <<channel,channels>> that are members of <<domain,tracing domains>> and contain <<event,event rules>>.
1420 image::concepts.png[]
1421
1422 Those attributes and objects are completely isolated between different
1423 tracing sessions.
1424
1425 A tracing session is analogous to a cash machine session:
1426 the operations you do on the banking system through the cash machine do
1427 not alter the data of other users of the same system. In the case of
1428 the cash machine, a session lasts as long as your bank card is inside.
1429 In the case of LTTng, a tracing session lasts from the `lttng create`
1430 command to the `lttng destroy` command.
1431
1432 [role="img-100"]
1433 .Each Unix user has its own set of tracing sessions.
1434 image::many-sessions.png[]
1435
1436
1437 [[tracing-session-mode]]
1438 ==== Tracing session mode
1439
1440 LTTng can send the generated trace data to different locations. The
1441 _tracing session mode_ dictates where to send it. The following modes
1442 are available in LTTng{nbsp}{revision}:
1443
1444 [[local-mode]]Local mode::
1445   LTTng writes the traces to the file system of the machine it traces
1446   (target system).
1447
1448 [[net-streaming-mode]]Network streaming mode::
1449   LTTng sends the traces over the network to a
1450   <<lttng-relayd,relay daemon>> running on a remote system.
1451
1452 Snapshot mode::
1453   LTTng doesn't write the traces by default.
1454 +
1455 Instead, you can request LTTng to <<taking-a-snapshot,take a snapshot>>,
1456 that is, a copy of the current sub-buffers of the tracing session, and
1457 to write it to the file system of the target or to send it over the
1458 network to a <<lttng-relayd,relay daemon>> running on a remote system.
1459
1460 [[live-mode]]Live mode::
1461   This mode is similar to the network streaming mode, but a live
1462   trace viewer can connect to the distant relay daemon to
1463   <<lttng-live,view event records as LTTng generates them>>.
1464
1465
1466 [[domain]]
1467 === Tracing domain
1468
1469 A _tracing domain_ is a namespace for event sources. A tracing domain
1470 has its own properties and features.
1471
1472 There are currently five available tracing domains:
1473
1474 * Linux kernel
1475 * User space
1476 * `java.util.logging` (JUL)
1477 * log4j
1478 * Python
1479
1480 You must specify a tracing domain when using some commands to avoid
1481 ambiguity. For example, since all the domains support named tracepoints
1482 as event sources (instrumentation points that you manually insert in the
1483 source code), you need to specify a tracing domain when
1484 <<enabling-disabling-events,creating an event rule>> because all the
1485 tracing domains could have tracepoints with the same names.
1486
1487 You can create <<channel,channels>> in the Linux kernel and user space
1488 tracing domains. The other tracing domains have a single default
1489 channel.
1490
1491
1492 [[channel]]
1493 === Channel and ring buffer
1494
1495 A _channel_ is an object which is responsible for a set of ring buffers.
1496 Each ring buffer is divided into multiple sub-buffers. When an LTTng
1497 tracer emits an event, it can record it to one or more
1498 sub-buffers. The attributes of a channel determine what to do when
1499 there's no space left for a new event record because all sub-buffers
1500 are full, where to send a full sub-buffer, and other behaviours.
1501
1502 A channel is always associated to a <<domain,tracing domain>>. The
1503 `java.util.logging` (JUL), log4j, and Python tracing domains each have
1504 a default channel which you can't configure.
1505
1506 A channel also owns <<event,event rules>>. When an LTTng tracer emits
1507 an event, it records it to the sub-buffers of all
1508 the enabled channels with a satisfied event rule, as long as those
1509 channels are part of active <<tracing-session,tracing sessions>>.
1510
1511
1512 [[channel-buffering-schemes]]
1513 ==== Per-user vs. per-process buffering schemes
1514
1515 A channel has at least one ring buffer _per CPU_. LTTng always
1516 records an event to the ring buffer associated to the CPU on which it
1517 occurs.
1518
1519 Two _buffering schemes_ are available when you
1520 <<enabling-disabling-channels,create a channel>> in the
1521 user space <<domain,tracing domain>>:
1522
1523 Per-user buffering::
1524   Allocate one set of ring buffers--one per CPU--shared by all the
1525   instrumented processes of each Unix user.
1526 +
1527 --
1528 [role="img-100"]
1529 .Per-user buffering scheme.
1530 image::per-user-buffering.png[]
1531 --
1532
1533 Per-process buffering::
1534   Allocate one set of ring buffers--one per CPU--for each
1535   instrumented process.
1536 +
1537 --
1538 [role="img-100"]
1539 .Per-process buffering scheme.
1540 image::per-process-buffering.png[]
1541 --
1542 +
1543 The per-process buffering scheme tends to consume more memory than the
1544 per-user option because systems generally have more instrumented
1545 processes than Unix users running instrumented processes. However, the
1546 per-process buffering scheme ensures that one process having a high
1547 event throughput won't fill all the shared sub-buffers of the same
1548 user, only its own.
1549
1550 The Linux kernel tracing domain has only one available buffering scheme
1551 which is to allocate a single set of ring buffers for the whole system.
1552 This scheme is similar to the per-user option, but with a single, global
1553 user ``running'' the kernel.
1554
1555
1556 [[channel-overwrite-mode-vs-discard-mode]]
1557 ==== Overwrite vs. discard event record loss modes
1558
1559 When an event occurs, LTTng records it to a specific sub-buffer (yellow
1560 arc in the following animations) of the ring buffer of a specific
1561 channel. When there's no space left in a sub-buffer, the tracer marks it
1562 as consumable (red) and another, empty sub-buffer starts receiving the
1563 following event records. A <<lttng-consumerd,consumer daemon>>
1564 eventually consumes the marked sub-buffer (returns to white).
1565
1566 [NOTE]
1567 [role="docsvg-channel-subbuf-anim"]
1568 ====
1569 {note-no-anim}
1570 ====
1571
1572 In an ideal world, sub-buffers are consumed faster than they are filled,
1573 as it is the case in the previous animation. In the real world,
1574 however, all sub-buffers can be full at some point, leaving no space to
1575 record the following events.
1576
1577 By default, LTTng-modules and LTTng-UST are _non-blocking_ tracers: when
1578 no empty sub-buffer is available, it is acceptable to lose event records
1579 when the alternative would be to cause substantial delays in the
1580 execution of the instrumented application. LTTng privileges performance
1581 over integrity; it aims at perturbing the target system as little as
1582 possible in order to make tracing of subtle race conditions and rare
1583 interrupt cascades possible.
1584
1585 Since LTTng{nbsp}2.10, the LTTng user space tracer, LTTng-UST, supports
1586 a _blocking mode_. See the <<blocking-timeout-example,blocking timeout
1587 example>> to learn how to use the blocking mode.
1588
1589 When it comes to losing event records because no empty sub-buffer is
1590 available, or because the <<opt-blocking-timeout,blocking timeout>> is
1591 reached, the _event record loss mode_ of the channel determines what to
1592 do. The available event record loss modes are:
1593
1594 Discard mode::
1595   Drop the newest event records until the tracer releases a sub-buffer.
1596 +
1597 This is the only available mode when you specify a
1598 <<opt-blocking-timeout,blocking timeout>>.
1599
1600 Overwrite mode::
1601   Clear the sub-buffer containing the oldest event records and start
1602   writing the newest event records there.
1603 +
1604 This mode is sometimes called _flight recorder mode_ because it's
1605 similar to a
1606 https://en.wikipedia.org/wiki/Flight_recorder[flight recorder]:
1607 always keep a fixed amount of the latest data.
1608
1609 Which mechanism you should choose depends on your context: prioritize
1610 the newest or the oldest event records in the ring buffer?
1611
1612 Beware that, in overwrite mode, the tracer abandons a _whole sub-buffer_
1613 as soon as a there's no space left for a new event record, whereas in
1614 discard mode, the tracer only discards the event record that doesn't
1615 fit.
1616
1617 In discard mode, LTTng increments a count of lost event records when an
1618 event record is lost and saves this count to the trace. Since
1619 LTTng{nbsp}2.8, in overwrite mode, LTTng writes to a given sub-buffer
1620 its sequence number within its data stream. With a <<local-mode,local>>,
1621 <<net-streaming-mode,network streaming>>, or <<live-mode,live>>
1622 <<tracing-session,tracing session>>, a trace reader can use such
1623 sequence numbers to report lost packets. In overwrite mode, LTTng
1624 doesn't write to the trace the exact number of lost event records in
1625 those lost sub-buffers.
1626
1627 Trace analyses can use saved discarded event record and sub-buffer
1628 (packet) counts of the trace to decide whether or not to perform the
1629 analyses even if trace data is known to be missing.
1630
1631 There are a few ways to decrease your probability of losing event
1632 records.
1633 <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>> shows
1634 how to fine-tune the sub-buffer count and size of a channel to virtually
1635 stop losing event records, though at the cost of greater memory usage.
1636
1637
1638 [[channel-subbuf-size-vs-subbuf-count]]
1639 ==== Sub-buffer count and size
1640
1641 When you <<enabling-disabling-channels,create a channel>>, you can
1642 set its number of sub-buffers and their size.
1643
1644 Note that there is noticeable CPU overhead introduced when
1645 switching sub-buffers (marking a full one as consumable and switching
1646 to an empty one for the following events to be recorded). Knowing this,
1647 the following list presents a few practical situations along with how
1648 to configure the sub-buffer count and size for them:
1649
1650 * **High event throughput**: In general, prefer bigger sub-buffers to
1651   lower the risk of losing event records.
1652 +
1653 Having bigger sub-buffers also ensures a lower
1654 <<channel-switch-timer,sub-buffer switching frequency>>.
1655 +
1656 The number of sub-buffers is only meaningful if you create the channel
1657 in overwrite mode: in this case, if a sub-buffer overwrite happens, the
1658 other sub-buffers are left unaltered.
1659
1660 * **Low event throughput**: In general, prefer smaller sub-buffers
1661   since the risk of losing event records is low.
1662 +
1663 Because events occur less frequently, the sub-buffer switching frequency
1664 should remain low and thus the overhead of the tracer shouldn't be a
1665 problem.
1666
1667 * **Low memory system**: If your target system has a low memory
1668   limit, prefer fewer first, then smaller sub-buffers.
1669 +
1670 Even if the system is limited in memory, you want to keep the
1671 sub-buffers as big as possible to avoid a high sub-buffer switching
1672 frequency.
1673
1674 Note that LTTng uses http://diamon.org/ctf/[CTF] as its trace format,
1675 which means event data is very compact. For example, the average
1676 LTTng kernel event record weights about 32{nbsp}bytes. Thus, a
1677 sub-buffer size of 1{nbsp}MiB is considered big.
1678
1679 The previous situations highlight the major trade-off between a few big
1680 sub-buffers and more, smaller sub-buffers: sub-buffer switching
1681 frequency vs. how much data is lost in overwrite mode. Assuming a
1682 constant event throughput and using the overwrite mode, the two
1683 following configurations have the same ring buffer total size:
1684
1685 [NOTE]
1686 [role="docsvg-channel-subbuf-size-vs-count-anim"]
1687 ====
1688 {note-no-anim}
1689 ====
1690
1691 * **Two sub-buffers of 4{nbsp}MiB each**: Expect a very low sub-buffer
1692   switching frequency, but if a sub-buffer overwrite happens, half of
1693   the event records so far (4{nbsp}MiB) are definitely lost.
1694 * **Eight sub-buffers of 1{nbsp}MiB each**: Expect four times the
1695   overhead of the tracer as the previous configuration, but if a
1696   sub-buffer overwrite happens, only the eighth of event records so far
1697   are definitely lost.
1698
1699 In discard mode, the sub-buffers count parameter is pointless: use two
1700 sub-buffers and set their size according to the requirements of your
1701 situation.
1702
1703
1704 [[channel-switch-timer]]
1705 ==== Switch timer period
1706
1707 The _switch timer period_ is an important configurable attribute of
1708 a channel to ensure periodic sub-buffer flushing.
1709
1710 When the _switch timer_ expires, a sub-buffer switch happens. Set
1711 the switch timer period attribute when you
1712 <<enabling-disabling-channels,create a channel>> to ensure that LTTng
1713 consumes and commits trace data to trace files or to a distant relay
1714 daemon periodically in case of a low event throughput.
1715
1716 [NOTE]
1717 [role="docsvg-channel-switch-timer"]
1718 ====
1719 {note-no-anim}
1720 ====
1721
1722 This attribute is also convenient when you use big sub-buffers to cope
1723 with a sporadic high event throughput, even if the throughput is
1724 normally low.
1725
1726
1727 [[channel-read-timer]]
1728 ==== Read timer period
1729
1730 By default, the LTTng tracers use a notification mechanism to signal a
1731 full sub-buffer so that a consumer daemon can consume it. When such
1732 notifications must be avoided, for example in real-time applications,
1733 use the _read timer_ of the channel instead. When the read timer fires,
1734 the <<lttng-consumerd,consumer daemon>> checks for full, consumable
1735 sub-buffers.
1736
1737
1738 [[tracefile-rotation]]
1739 ==== Trace file count and size
1740
1741 By default, trace files can grow as large as needed. Set the maximum
1742 size of each trace file that a channel writes when you
1743 <<enabling-disabling-channels,create a channel>>. When the size of a
1744 trace file reaches the fixed maximum size of the channel, LTTng creates
1745 another file to contain the next event records. LTTng appends a file
1746 count to each trace file name in this case.
1747
1748 If you set the trace file size attribute when you create a channel, the
1749 maximum number of trace files that LTTng creates is _unlimited_ by
1750 default. To limit them, set a maximum number of trace files. When the
1751 number of trace files reaches the fixed maximum count of the channel,
1752 the oldest trace file is overwritten. This mechanism is called _trace
1753 file rotation_.
1754
1755 [IMPORTANT]
1756 ====
1757 Even if you don't limit the trace file count, you can't assume that
1758 LTTng doesn't manage any trace file.
1759
1760 In other words, there is no safe way to know if LTTng still holds a
1761 given trace file open with the trace file rotation feature.
1762
1763 The only way to obtain an unmanaged, self-contained LTTng trace before
1764 you <<creating-destroying-tracing-sessions,destroy>> the tracing session
1765 is with the <<session-rotation,tracing session rotation>> feature
1766 (available since LTTng{nbsp}2.11).
1767 ====
1768
1769
1770 [[event]]
1771 === Instrumentation point, event rule, event, and event record
1772
1773 An _event rule_ is a set of conditions which must be **all** satisfied
1774 for LTTng to record an occuring event.
1775
1776 You set the conditions when you <<enabling-disabling-events,create
1777 an event rule>>.
1778
1779 You always attach an event rule to a <<channel,channel>> when you create
1780 it.
1781
1782 When an event passes the conditions of an event rule, LTTng records it
1783 in one of the sub-buffers of the attached channel.
1784
1785 The available conditions, as of LTTng{nbsp}{revision}, are:
1786
1787 * The event rule _is enabled_.
1788 * The type of the instrumentation point _is{nbsp}T_.
1789 * The name of the instrumentation point (sometimes called _event name_)
1790   _matches{nbsp}N_, but _isn't{nbsp}E_.
1791 * The log level of the instrumentation point _is as severe as{nbsp}L_, or
1792   _is exactly{nbsp}L_.
1793 * The fields of the payload of the event _satisfy_ a filter
1794   expression{nbsp}__F__.
1795
1796 As you can see, all the conditions but the dynamic filter are related to
1797 the status of the event rule or to the instrumentation point, not to the
1798 occurring events. This is why, without a filter, checking if an event
1799 passes an event rule isn't a dynamic task: when you create or modify an
1800 event rule, all the tracers of its tracing domain enable or disable the
1801 instrumentation points themselves once. This is possible because the
1802 attributes of an instrumentation point (type, name, and log level) are
1803 defined statically. In other words, without a dynamic filter, the tracer
1804 _doesn't evaluate_ the arguments of an instrumentation point unless it
1805 matches an enabled event rule.
1806
1807 Note that, for LTTng to record an event, the <<channel,channel>> to
1808 which a matching event rule is attached must also be enabled, and the
1809 <<tracing-session,tracing session>> owning this channel must be active
1810 (started).
1811
1812 [role="img-100"]
1813 .Logical path from an instrumentation point to an event record.
1814 image::event-rule.png[]
1815
1816 .Event, event record, or event rule?
1817 ****
1818 With so many similar terms, it's easy to get confused.
1819
1820 An **event** is the consequence of the execution of an _instrumentation
1821 point_, like a tracepoint that you manually place in some source code,
1822 or a Linux kernel kprobe. An event is said to _occur_ at a specific
1823 time. Different actions can be taken upon the occurrence of an event,
1824 like record the payload of the event to a buffer.
1825
1826 An **event record** is the representation of an event in a sub-buffer. A
1827 tracer is responsible for capturing the payload of an event, current
1828 context variables, the ID of the event, and its timestamp. LTTng
1829 can append this sub-buffer to a trace file.
1830
1831 An **event rule** is a set of conditions which must _all_ be satisfied
1832 for LTTng to record an occuring event. Events still occur without
1833 satisfying event rules, but LTTng doesn't record them.
1834 ****
1835
1836
1837 [[plumbing]]
1838 == Components of noch:{LTTng}
1839
1840 The second _T_ in _LTTng_ stands for _toolkit_: it would be wrong
1841 to call LTTng a simple _tool_ since it is composed of multiple
1842 interacting components. This section describes those components,
1843 explains their respective roles, and shows how they connect together to
1844 form the LTTng ecosystem.
1845
1846 The following diagram shows how the most important components of LTTng
1847 interact with user applications, the Linux kernel, and you:
1848
1849 [role="img-100"]
1850 .Control and trace data paths between LTTng components.
1851 image::plumbing.png[]
1852
1853 The LTTng project incorporates:
1854
1855 * **LTTng-tools**: Libraries and command-line interface to
1856   control tracing sessions.
1857 ** <<lttng-sessiond,Session daemon>> (man:lttng-sessiond(8)).
1858 ** <<lttng-consumerd,Consumer daemon>> (cmd:lttng-consumerd).
1859 ** <<lttng-relayd,Relay daemon>> (man:lttng-relayd(8)).
1860 ** <<liblttng-ctl-lttng,Tracing control library>> (`liblttng-ctl`).
1861 ** <<lttng-cli,Tracing control command-line tool>> (man:lttng(1)).
1862 * **LTTng-UST**: Libraries and Java/Python packages to trace user
1863   applications.
1864 ** <<lttng-ust,User space tracing library>> (`liblttng-ust`) and its
1865    headers to instrument and trace any native user application.
1866 ** <<prebuilt-ust-helpers,Preloadable user space tracing helpers>>:
1867 *** `liblttng-ust-libc-wrapper`
1868 *** `liblttng-ust-pthread-wrapper`
1869 *** `liblttng-ust-cyg-profile`
1870 *** `liblttng-ust-cyg-profile-fast`
1871 *** `liblttng-ust-dl`
1872 ** User space tracepoint provider source files generator command-line
1873    tool (man:lttng-gen-tp(1)).
1874 ** <<lttng-ust-agents,LTTng-UST Java agent>> to instrument and trace
1875    Java applications using `java.util.logging` or
1876    Apache log4j{nbsp}1.2 logging.
1877 ** <<lttng-ust-agents,LTTng-UST Python agent>> to instrument
1878    Python applications using the standard `logging` package.
1879 * **LTTng-modules**: <<lttng-modules,Linux kernel modules>> to trace
1880   the kernel.
1881 ** LTTng kernel tracer module.
1882 ** Tracing ring buffer kernel modules.
1883 ** Probe kernel modules.
1884 ** LTTng logger kernel module.
1885
1886
1887 [[lttng-cli]]
1888 === Tracing control command-line interface
1889
1890 [role="img-100"]
1891 .The tracing control command-line interface.
1892 image::plumbing-lttng-cli.png[]
1893
1894 The _man:lttng(1) command-line tool_ is the standard user interface to
1895 control LTTng <<tracing-session,tracing sessions>>. The cmd:lttng tool
1896 is part of LTTng-tools.
1897
1898 The cmd:lttng tool is linked with
1899 <<liblttng-ctl-lttng,`liblttng-ctl`>> to communicate with
1900 one or more <<lttng-sessiond,session daemons>> behind the scenes.
1901
1902 The cmd:lttng tool has a Git-like interface:
1903
1904 [role="term"]
1905 ----
1906 $ lttng <GENERAL OPTIONS> <COMMAND> <COMMAND OPTIONS>
1907 ----
1908
1909 The <<controlling-tracing,Tracing control>> section explores the
1910 available features of LTTng using the cmd:lttng tool.
1911
1912
1913 [[liblttng-ctl-lttng]]
1914 === Tracing control library
1915
1916 [role="img-100"]
1917 .The tracing control library.
1918 image::plumbing-liblttng-ctl.png[]
1919
1920 The _LTTng control library_, `liblttng-ctl`, is used to communicate
1921 with a <<lttng-sessiond,session daemon>> using a C API that hides the
1922 underlying details of the protocol. `liblttng-ctl` is part of LTTng-tools.
1923
1924 The <<lttng-cli,cmd:lttng command-line tool>>
1925 is linked with `liblttng-ctl`.
1926
1927 Use `liblttng-ctl` in C or $$C++$$ source code by including its
1928 ``master'' header:
1929
1930 [source,c]
1931 ----
1932 #include <lttng/lttng.h>
1933 ----
1934
1935 Some objects are referenced by name (C string), such as tracing
1936 sessions, but most of them require to create a handle first using
1937 `lttng_create_handle()`.
1938
1939 As of LTTng{nbsp}{revision}, the best available developer documentation for
1940 `liblttng-ctl` is its installed header files. Every function and structure is
1941 thoroughly documented.
1942
1943
1944 [[lttng-ust]]
1945 === User space tracing library
1946
1947 [role="img-100"]
1948 .The user space tracing library.
1949 image::plumbing-liblttng-ust.png[]
1950
1951 The _user space tracing library_, `liblttng-ust` (see man:lttng-ust(3)),
1952 is the LTTng user space tracer. It receives commands from a
1953 <<lttng-sessiond,session daemon>>, for example to
1954 enable and disable specific instrumentation points, and writes event
1955 records to ring buffers shared with a
1956 <<lttng-consumerd,consumer daemon>>.
1957 `liblttng-ust` is part of LTTng-UST.
1958
1959 Public C header files are installed beside `liblttng-ust` to
1960 instrument any <<c-application,C or $$C++$$ application>>.
1961
1962 <<lttng-ust-agents,LTTng-UST agents>>, which are regular Java and Python
1963 packages, use their own library providing tracepoints which is
1964 linked with `liblttng-ust`.
1965
1966 An application or library doesn't have to initialize `liblttng-ust`
1967 manually: its constructor does the necessary tasks to properly register
1968 to a session daemon. The initialization phase also enables the
1969 instrumentation points matching the <<event,event rules>> that you
1970 already created.
1971
1972
1973 [[lttng-ust-agents]]
1974 === User space tracing agents
1975
1976 [role="img-100"]
1977 .The user space tracing agents.
1978 image::plumbing-lttng-ust-agents.png[]
1979
1980 The _LTTng-UST Java and Python agents_ are regular Java and Python
1981 packages which add LTTng tracing capabilities to the
1982 native logging frameworks. The LTTng-UST agents are part of LTTng-UST.
1983
1984 In the case of Java, the
1985 https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[`java.util.logging`
1986 core logging facilities] and
1987 https://logging.apache.org/log4j/1.2/[Apache log4j{nbsp}1.2] are supported.
1988 Note that Apache Log4{nbsp}2 isn't supported.
1989
1990 In the case of Python, the standard
1991 https://docs.python.org/3/library/logging.html[`logging`] package
1992 is supported. Both Python{nbsp}2 and Python{nbsp}3 modules can import the
1993 LTTng-UST Python agent package.
1994
1995 The applications using the LTTng-UST agents are in the
1996 `java.util.logging` (JUL),
1997 log4j, and Python <<domain,tracing domains>>.
1998
1999 Both agents use the same mechanism to trace the log statements. When an
2000 agent initializes, it creates a log handler that attaches to the root
2001 logger. The agent also registers to a <<lttng-sessiond,session daemon>>.
2002 When the application executes a log statement, the root logger passes it
2003 to the log handler of the agent. The log handler of the agent calls a
2004 native function in a tracepoint provider package shared library linked
2005 with <<lttng-ust,`liblttng-ust`>>, passing the formatted log message and
2006 other fields, like its logger name and its log level. This native
2007 function contains a user space instrumentation point, hence tracing the
2008 log statement.
2009
2010 The log level condition of an
2011 <<event,event rule>> is considered when tracing
2012 a Java or a Python application, and it's compatible with the standard
2013 JUL, log4j, and Python log levels.
2014
2015
2016 [[lttng-modules]]
2017 === LTTng kernel modules
2018
2019 [role="img-100"]
2020 .The LTTng kernel modules.
2021 image::plumbing-lttng-modules.png[]
2022
2023 The _LTTng kernel modules_ are a set of Linux kernel modules
2024 which implement the kernel tracer of the LTTng project. The LTTng
2025 kernel modules are part of LTTng-modules.
2026
2027 The LTTng kernel modules include:
2028
2029 * A set of _probe_ modules.
2030 +
2031 Each module attaches to a specific subsystem
2032 of the Linux kernel using its tracepoint instrument points. There are
2033 also modules to attach to the entry and return points of the Linux
2034 system call functions.
2035
2036 * _Ring buffer_ modules.
2037 +
2038 A ring buffer implementation is provided as kernel modules. The LTTng
2039 kernel tracer writes to the ring buffer; a
2040 <<lttng-consumerd,consumer daemon>> reads from the ring buffer.
2041
2042 * The _LTTng kernel tracer_ module.
2043 * The _LTTng logger_ module.
2044 +
2045 The LTTng logger module implements the special path:{/proc/lttng-logger}
2046 (and path:{/dev/lttng-logger} since LTTng{nbsp}2.11) files so that any
2047 executable can generate LTTng events by opening and writing to those
2048 files.
2049 +
2050 See <<proc-lttng-logger-abi,LTTng logger>>.
2051
2052 Generally, you don't have to load the LTTng kernel modules manually
2053 (using man:modprobe(8), for example): a root <<lttng-sessiond,session
2054 daemon>> loads the necessary modules when starting. If you have extra
2055 probe modules, you can specify to load them to the session daemon on
2056 the command line.
2057
2058 The LTTng kernel modules are installed in
2059 +/usr/lib/modules/__release__/extra+ by default, where +__release__+ is
2060 the kernel release (see `uname --kernel-release`).
2061
2062
2063 [[lttng-sessiond]]
2064 === Session daemon
2065
2066 [role="img-100"]
2067 .The session daemon.
2068 image::plumbing-sessiond.png[]
2069
2070 The _session daemon_, man:lttng-sessiond(8), is a daemon responsible for
2071 managing tracing sessions and for controlling the various components of
2072 LTTng. The session daemon is part of LTTng-tools.
2073
2074 The session daemon sends control requests to and receives control
2075 responses from:
2076
2077 * The <<lttng-ust,user space tracing library>>.
2078 +
2079 Any instance of the user space tracing library first registers to
2080 a session daemon. Then, the session daemon can send requests to
2081 this instance, such as:
2082 +
2083 --
2084 ** Get the list of tracepoints.
2085 ** Share an <<event,event rule>> so that the user space tracing library
2086    can enable or disable tracepoints. Amongst the possible conditions
2087    of an event rule is a filter expression which `liblttng-ust` evalutes
2088    when an event occurs.
2089 ** Share <<channel,channel>> attributes and ring buffer locations.
2090 --
2091 +
2092 The session daemon and the user space tracing library use a Unix
2093 domain socket for their communication.
2094
2095 * The <<lttng-ust-agents,user space tracing agents>>.
2096 +
2097 Any instance of a user space tracing agent first registers to
2098 a session daemon. Then, the session daemon can send requests to
2099 this instance, such as:
2100 +
2101 --
2102 ** Get the list of loggers.
2103 ** Enable or disable a specific logger.
2104 --
2105 +
2106 The session daemon and the user space tracing agent use a TCP connection
2107 for their communication.
2108
2109 * The <<lttng-modules,LTTng kernel tracer>>.
2110 * The <<lttng-consumerd,consumer daemon>>.
2111 +
2112 The session daemon sends requests to the consumer daemon to instruct
2113 it where to send the trace data streams, amongst other information.
2114
2115 * The <<lttng-relayd,relay daemon>>.
2116
2117 The session daemon receives commands from the
2118 <<liblttng-ctl-lttng,tracing control library>>.
2119
2120 The root session daemon loads the appropriate
2121 <<lttng-modules,LTTng kernel modules>> on startup. It also spawns
2122 a <<lttng-consumerd,consumer daemon>> as soon as you create
2123 an <<event,event rule>>.
2124
2125 The session daemon doesn't send and receive trace data: this is the
2126 role of the <<lttng-consumerd,consumer daemon>> and
2127 <<lttng-relayd,relay daemon>>. It does, however, generate the
2128 http://diamon.org/ctf/[CTF] metadata stream.
2129
2130 Each Unix user can have its own session daemon instance. The
2131 tracing sessions which different session daemons manage are completely
2132 independent.
2133
2134 The root user's session daemon is the only one which is
2135 allowed to control the LTTng kernel tracer, and its spawned consumer
2136 daemon is the only one which is allowed to consume trace data from the
2137 LTTng kernel tracer. Note, however, that any Unix user which is a member
2138 of the <<tracing-group,tracing group>> is allowed
2139 to create <<channel,channels>> in the
2140 Linux kernel <<domain,tracing domain>>, and thus to trace the Linux
2141 kernel.
2142
2143 The <<lttng-cli,cmd:lttng command-line tool>> automatically starts a
2144 session daemon when using its `create` command if none is currently
2145 running. You can also start the session daemon manually.
2146
2147
2148 [[lttng-consumerd]]
2149 === Consumer daemon
2150
2151 [role="img-100"]
2152 .The consumer daemon.
2153 image::plumbing-consumerd.png[]
2154
2155 The _consumer daemon_, cmd:lttng-consumerd, is a daemon which shares
2156 ring buffers with user applications or with the LTTng kernel modules to
2157 collect trace data and send it to some location (on disk or to a
2158 <<lttng-relayd,relay daemon>> over the network). The consumer daemon
2159 is part of LTTng-tools.
2160
2161 You don't start a consumer daemon manually: a consumer daemon is always
2162 spawned by a <<lttng-sessiond,session daemon>> as soon as you create an
2163 <<event,event rule>>, that is, before you start tracing. When you kill
2164 its owner session daemon, the consumer daemon also exits because it is
2165 the child process of the session daemon. Command-line options of
2166 man:lttng-sessiond(8) target the consumer daemon process.
2167
2168 There are up to two running consumer daemons per Unix user, whereas only
2169 one session daemon can run per user. This is because each process can be
2170 either 32-bit or 64-bit: if the target system runs a mixture of 32-bit
2171 and 64-bit processes, it is more efficient to have separate
2172 corresponding 32-bit and 64-bit consumer daemons. The root user is an
2173 exception: it can have up to _three_ running consumer daemons: 32-bit
2174 and 64-bit instances for its user applications, and one more
2175 reserved for collecting kernel trace data.
2176
2177
2178 [[lttng-relayd]]
2179 === Relay daemon
2180
2181 [role="img-100"]
2182 .The relay daemon.
2183 image::plumbing-relayd.png[]
2184
2185 The _relay daemon_, man:lttng-relayd(8), is a daemon acting as a bridge
2186 between remote session and consumer daemons, local trace files, and a
2187 remote live trace viewer. The relay daemon is part of LTTng-tools.
2188
2189 The main purpose of the relay daemon is to implement a receiver of
2190 <<sending-trace-data-over-the-network,trace data over the network>>.
2191 This is useful when the target system doesn't have much file system
2192 space to record trace files locally.
2193
2194 The relay daemon is also a server to which a
2195 <<lttng-live,live trace viewer>> can
2196 connect. The live trace viewer sends requests to the relay daemon to
2197 receive trace data as the target system emits events. The
2198 communication protocol is named _LTTng live_; it is used over TCP
2199 connections.
2200
2201 Note that you can start the relay daemon on the target system directly.
2202 This is the setup of choice when the use case is to view events as
2203 the target system emits them without the need of a remote system.
2204
2205
2206 [[instrumenting]]
2207 == [[using-lttng]]Instrumentation
2208
2209 There are many examples of tracing and monitoring in our everyday life:
2210
2211 * You have access to real-time and historical weather reports and
2212   forecasts thanks to weather stations installed around the country.
2213 * You know your heart is safe thanks to an electrocardiogram.
2214 * You make sure not to drive your car too fast and to have enough fuel
2215   to reach your destination thanks to gauges visible on your dashboard.
2216
2217 All the previous examples have something in common: they rely on
2218 **instruments**. Without the electrodes attached to the surface of your
2219 body skin, cardiac monitoring is futile.
2220
2221 LTTng, as a tracer, is no different from those real life examples. If
2222 you're about to trace a software system or, in other words, record its
2223 history of execution, you better have **instrumentation points** in the
2224 subject you're tracing, that is, the actual software.
2225
2226 Various ways were developed to instrument a piece of software for LTTng
2227 tracing. The most straightforward one is to manually place
2228 instrumentation points, called _tracepoints_, in the source code of the
2229 software. It is also possible to add instrumentation points dynamically
2230 in the Linux kernel <<domain,tracing domain>>.
2231
2232 If you're only interested in tracing the Linux kernel, your
2233 instrumentation needs are probably already covered by the built-in
2234 <<lttng-modules,Linux kernel tracepoints>> of LTTng. You may also wish
2235 to trace a user application which is already instrumented for LTTng
2236 tracing. In such cases, skip this whole section and read the topics of
2237 the <<controlling-tracing,Tracing control>> section.
2238
2239 Many methods are available to instrument a piece of software for LTTng
2240 tracing. They are:
2241
2242 * <<c-application,User space instrumentation for C and $$C++$$
2243   applications>>.
2244 * <<prebuilt-ust-helpers,Prebuilt user space tracing helpers>>.
2245 * <<java-application,User space Java agent>>.
2246 * <<python-application,User space Python agent>>.
2247 * <<proc-lttng-logger-abi,LTTng logger>>.
2248 * <<instrumenting-linux-kernel,LTTng kernel tracepoints>>.
2249
2250
2251 [[c-application]]
2252 === [[cxx-application]]User space instrumentation for C and $$C++$$ applications
2253
2254 The procedure to instrument a C or $$C++$$ user application with
2255 the <<lttng-ust,LTTng user space tracing library>>, `liblttng-ust`, is:
2256
2257 . <<tracepoint-provider,Create the source files of a tracepoint provider
2258   package>>.
2259 . <<probing-the-application-source-code,Add tracepoints to
2260   the source code of the application>>.
2261 . <<building-tracepoint-providers-and-user-application,Build and link
2262   a tracepoint provider package and the user application>>.
2263
2264 If you need quick, man:printf(3)-like instrumentation, skip
2265 those steps and use <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>>
2266 instead.
2267
2268 IMPORTANT: You need to <<installing-lttng,install>> LTTng-UST to
2269 instrument a user application with `liblttng-ust`.
2270
2271
2272 [[tracepoint-provider]]
2273 ==== Create the source files of a tracepoint provider package
2274
2275 A _tracepoint provider_ is a set of compiled functions which provide
2276 **tracepoints** to an application, the type of instrumentation point
2277 supported by LTTng-UST. Those functions can emit events with
2278 user-defined fields and serialize those events as event records to one
2279 or more LTTng-UST <<channel,channel>> sub-buffers. The `tracepoint()`
2280 macro, which you <<probing-the-application-source-code,insert in the
2281 source code of a user application>>, calls those functions.
2282
2283 A _tracepoint provider package_ is an object file (`.o`) or a shared
2284 library (`.so`) which contains one or more tracepoint providers.
2285 Its source files are:
2286
2287 * One or more <<tpp-header,tracepoint provider header>> (`.h`).
2288 * A <<tpp-source,tracepoint provider package source>> (`.c`).
2289
2290 A tracepoint provider package is dynamically linked with `liblttng-ust`,
2291 the LTTng user space tracer, at run time.
2292
2293 [role="img-100"]
2294 .User application linked with `liblttng-ust` and containing a tracepoint provider.
2295 image::ust-app.png[]
2296
2297 NOTE: If you need quick, man:printf(3)-like instrumentation,
2298 skip creating and using a tracepoint provider and use
2299 <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>> instead.
2300
2301
2302 [[tpp-header]]
2303 ===== Create a tracepoint provider header file template
2304
2305 A _tracepoint provider header file_ contains the tracepoint
2306 definitions of a tracepoint provider.
2307
2308 To create a tracepoint provider header file:
2309
2310 . Start from this template:
2311 +
2312 --
2313 [source,c]
2314 .Tracepoint provider header file template (`.h` file extension).
2315 ----
2316 #undef TRACEPOINT_PROVIDER
2317 #define TRACEPOINT_PROVIDER provider_name
2318
2319 #undef TRACEPOINT_INCLUDE
2320 #define TRACEPOINT_INCLUDE "./tp.h"
2321
2322 #if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
2323 #define _TP_H
2324
2325 #include <lttng/tracepoint.h>
2326
2327 /*
2328  * Use TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(),
2329  * TRACEPOINT_EVENT_INSTANCE(), and TRACEPOINT_LOGLEVEL() here.
2330  */
2331
2332 #endif /* _TP_H */
2333
2334 #include <lttng/tracepoint-event.h>
2335 ----
2336 --
2337
2338 . Replace:
2339 +
2340 * `provider_name` with the name of your tracepoint provider.
2341 * `"tp.h"` with the name of your tracepoint provider header file.
2342
2343 . Below the `#include <lttng/tracepoint.h>` line, put your
2344   <<defining-tracepoints,tracepoint definitions>>.
2345
2346 Your tracepoint provider name must be unique amongst all the possible
2347 tracepoint provider names used on the same target system. We
2348 suggest to include the name of your project or company in the name,
2349 for example, `org_lttng_my_project_tpp`.
2350
2351 TIP: [[lttng-gen-tp]]Use the man:lttng-gen-tp(1) tool to create
2352 this boilerplate for you. When using cmd:lttng-gen-tp, all you need to
2353 write are the <<defining-tracepoints,tracepoint definitions>>.
2354
2355
2356 [[defining-tracepoints]]
2357 ===== Create a tracepoint definition
2358
2359 A _tracepoint definition_ defines, for a given tracepoint:
2360
2361 * Its **input arguments**. They are the macro parameters that the
2362   `tracepoint()` macro accepts for this particular tracepoint
2363   in the source code of the user application.
2364 * Its **output event fields**. They are the sources of event fields
2365   that form the payload of any event that the execution of the
2366   `tracepoint()` macro emits for this particular tracepoint.
2367
2368 Create a tracepoint definition by using the
2369 `TRACEPOINT_EVENT()` macro below the `#include <lttng/tracepoint.h>`
2370 line in the
2371 <<tpp-header,tracepoint provider header file template>>.
2372
2373 The syntax of the `TRACEPOINT_EVENT()` macro is:
2374
2375 [source,c]
2376 .`TRACEPOINT_EVENT()` macro syntax.
2377 ----
2378 TRACEPOINT_EVENT(
2379     /* Tracepoint provider name */
2380     provider_name,
2381
2382     /* Tracepoint name */
2383     tracepoint_name,
2384
2385     /* Input arguments */
2386     TP_ARGS(
2387         arguments
2388     ),
2389
2390     /* Output event fields */
2391     TP_FIELDS(
2392         fields
2393     )
2394 )
2395 ----
2396
2397 Replace:
2398
2399 * `provider_name` with your tracepoint provider name.
2400 * `tracepoint_name` with your tracepoint name.
2401 * `arguments` with the <<tpp-def-input-args,input arguments>>.
2402 * `fields` with the <<tpp-def-output-fields,output event field>>
2403   definitions.
2404
2405 This tracepoint emits events named `provider_name:tracepoint_name`.
2406
2407 [IMPORTANT]
2408 .Event name length limitation
2409 ====
2410 The concatenation of the tracepoint provider name and the
2411 tracepoint name must not exceed **254{nbsp}characters**. If it does, the
2412 instrumented application compiles and runs, but LTTng throws multiple
2413 warnings and you could experience serious issues.
2414 ====
2415
2416 [[tpp-def-input-args]]The syntax of the `TP_ARGS()` macro is:
2417
2418 [source,c]
2419 .`TP_ARGS()` macro syntax.
2420 ----
2421 TP_ARGS(
2422     type, arg_name
2423 )
2424 ----
2425
2426 Replace:
2427
2428 * `type` with the C type of the argument.
2429 * `arg_name` with the argument name.
2430
2431 You can repeat `type` and `arg_name` up to 10{nbsp}times to have more
2432 than one argument.
2433
2434 .`TP_ARGS()` usage with three arguments.
2435 ====
2436 [source,c]
2437 ----
2438 TP_ARGS(
2439     int, count,
2440     float, ratio,
2441     const char*, query
2442 )
2443 ----
2444 ====
2445
2446 The `TP_ARGS()` and `TP_ARGS(void)` forms are valid to create a
2447 tracepoint definition with no input arguments.
2448
2449 [[tpp-def-output-fields]]The `TP_FIELDS()` macro contains a list of
2450 `ctf_*()` macros. Each `ctf_*()` macro defines one event field. See
2451 man:lttng-ust(3) for a complete description of the available `ctf_*()`
2452 macros. A `ctf_*()` macro specifies the type, size, and byte order of
2453 one event field.
2454
2455 Each `ctf_*()` macro takes an _argument expression_ parameter. This is a
2456 C expression that the tracer evalutes at the `tracepoint()` macro site
2457 in the source code of the application. This expression provides the
2458 source of data of a field. The argument expression can include input
2459 argument names listed in the `TP_ARGS()` macro.
2460
2461 Each `ctf_*()` macro also takes a _field name_ parameter. Field names
2462 must be unique within a given tracepoint definition.
2463
2464 Here's a complete tracepoint definition example:
2465
2466 .Tracepoint definition.
2467 ====
2468 The following tracepoint definition defines a tracepoint which takes
2469 three input arguments and has four output event fields.
2470
2471 [source,c]
2472 ----
2473 #include "my-custom-structure.h"
2474
2475 TRACEPOINT_EVENT(
2476     my_provider,
2477     my_tracepoint,
2478     TP_ARGS(
2479         const struct my_custom_structure*, my_custom_structure,
2480         float, ratio,
2481         const char*, query
2482     ),
2483     TP_FIELDS(
2484         ctf_string(query_field, query)
2485         ctf_float(double, ratio_field, ratio)
2486         ctf_integer(int, recv_size, my_custom_structure->recv_size)
2487         ctf_integer(int, send_size, my_custom_structure->send_size)
2488     )
2489 )
2490 ----
2491
2492 Refer to this tracepoint definition with the `tracepoint()` macro in
2493 the source code of your application like this:
2494
2495 [source,c]
2496 ----
2497 tracepoint(my_provider, my_tracepoint,
2498            my_structure, some_ratio, the_query);
2499 ----
2500 ====
2501
2502 NOTE: The LTTng tracer only evaluates tracepoint arguments at run time
2503 if they satisfy an enabled <<event,event rule>>.
2504
2505
2506 [[using-tracepoint-classes]]
2507 ===== Use a tracepoint class
2508
2509 A _tracepoint class_ is a class of tracepoints which share the same
2510 output event field definitions. A _tracepoint instance_ is one
2511 instance of such a defined tracepoint class, with its own tracepoint
2512 name.
2513
2514 The <<defining-tracepoints,`TRACEPOINT_EVENT()` macro>> is actually a
2515 shorthand which defines both a tracepoint class and a tracepoint
2516 instance at the same time.
2517
2518 When you build a tracepoint provider package, the C or $$C++$$ compiler
2519 creates one serialization function for each **tracepoint class**. A
2520 serialization function is responsible for serializing the event fields
2521 of a tracepoint to a sub-buffer when tracing.
2522
2523 For various performance reasons, when your situation requires multiple
2524 tracepoint definitions with different names, but with the same event
2525 fields, we recommend that you manually create a tracepoint class
2526 and instantiate as many tracepoint instances as needed. One positive
2527 effect of such a design, amongst other advantages, is that all
2528 tracepoint instances of the same tracepoint class reuse the same
2529 serialization function, thus reducing
2530 https://en.wikipedia.org/wiki/Cache_pollution[cache pollution].
2531
2532 .Use a tracepoint class and tracepoint instances.
2533 ====
2534 Consider the following three tracepoint definitions:
2535
2536 [source,c]
2537 ----
2538 TRACEPOINT_EVENT(
2539     my_app,
2540     get_account,
2541     TP_ARGS(
2542         int, userid,
2543         size_t, len
2544     ),
2545     TP_FIELDS(
2546         ctf_integer(int, userid, userid)
2547         ctf_integer(size_t, len, len)
2548     )
2549 )
2550
2551 TRACEPOINT_EVENT(
2552     my_app,
2553     get_settings,
2554     TP_ARGS(
2555         int, userid,
2556         size_t, len
2557     ),
2558     TP_FIELDS(
2559         ctf_integer(int, userid, userid)
2560         ctf_integer(size_t, len, len)
2561     )
2562 )
2563
2564 TRACEPOINT_EVENT(
2565     my_app,
2566     get_transaction,
2567     TP_ARGS(
2568         int, userid,
2569         size_t, len
2570     ),
2571     TP_FIELDS(
2572         ctf_integer(int, userid, userid)
2573         ctf_integer(size_t, len, len)
2574     )
2575 )
2576 ----
2577
2578 In this case, we create three tracepoint classes, with one implicit
2579 tracepoint instance for each of them: `get_account`, `get_settings`, and
2580 `get_transaction`. However, they all share the same event field names
2581 and types. Hence three identical, yet independent serialization
2582 functions are created when you build the tracepoint provider package.
2583
2584 A better design choice is to define a single tracepoint class and three
2585 tracepoint instances:
2586
2587 [source,c]
2588 ----
2589 /* The tracepoint class */
2590 TRACEPOINT_EVENT_CLASS(
2591     /* Tracepoint provider name */
2592     my_app,
2593
2594     /* Tracepoint class name */
2595     my_class,
2596
2597     /* Input arguments */
2598     TP_ARGS(
2599         int, userid,
2600         size_t, len
2601     ),
2602
2603     /* Output event fields */
2604     TP_FIELDS(
2605         ctf_integer(int, userid, userid)
2606         ctf_integer(size_t, len, len)
2607     )
2608 )
2609
2610 /* The tracepoint instances */
2611 TRACEPOINT_EVENT_INSTANCE(
2612     /* Tracepoint provider name */
2613     my_app,
2614
2615     /* Tracepoint class name */
2616     my_class,
2617
2618     /* Tracepoint name */
2619     get_account,
2620
2621     /* Input arguments */
2622     TP_ARGS(
2623         int, userid,
2624         size_t, len
2625     )
2626 )
2627 TRACEPOINT_EVENT_INSTANCE(
2628     my_app,
2629     my_class,
2630     get_settings,
2631     TP_ARGS(
2632         int, userid,
2633         size_t, len
2634     )
2635 )
2636 TRACEPOINT_EVENT_INSTANCE(
2637     my_app,
2638     my_class,
2639     get_transaction,
2640     TP_ARGS(
2641         int, userid,
2642         size_t, len
2643     )
2644 )
2645 ----
2646 ====
2647
2648
2649 [[assigning-log-levels]]
2650 ===== Assign a log level to a tracepoint definition
2651
2652 Assign a _log level_ to a <<defining-tracepoints,tracepoint definition>>
2653 with the `TRACEPOINT_LOGLEVEL()` macro.
2654
2655 Assigning different levels of severity to tracepoint definitions can
2656 be useful: when you <<enabling-disabling-events,create an event rule>>,
2657 you can target tracepoints having a log level as severe as a specific
2658 value.
2659
2660 The concept of LTTng-UST log levels is similar to the levels found
2661 in typical logging frameworks:
2662
2663 * In a logging framework, the log level is given by the function
2664   or method name you use at the log statement site: `debug()`,
2665   `info()`, `warn()`, `error()`, and so on.
2666 * In LTTng-UST, you statically assign the log level to a tracepoint
2667   definition; any `tracepoint()` macro invocation which refers to
2668   this definition has this log level.
2669
2670 You must use `TRACEPOINT_LOGLEVEL()` _after_ the
2671 <<defining-tracepoints,`TRACEPOINT_EVENT()`>> or
2672 <<using-tracepoint-classes,`TRACEPOINT_INSTANCE()`>> macro for a given
2673 tracepoint.
2674
2675 The syntax of the `TRACEPOINT_LOGLEVEL()` macro is:
2676
2677 [source,c]
2678 .`TRACEPOINT_LOGLEVEL()` macro syntax.
2679 ----
2680 TRACEPOINT_LOGLEVEL(provider_name, tracepoint_name, log_level)
2681 ----
2682
2683 Replace:
2684
2685 * `provider_name` with the tracepoint provider name.
2686 * `tracepoint_name` with the tracepoint name.
2687 * `log_level` with the log level to assign to the tracepoint
2688   definition named `tracepoint_name` in the `provider_name`
2689   tracepoint provider.
2690 +
2691 See man:lttng-ust(3) for a list of available log level names.
2692
2693 .Assign the `TRACE_DEBUG_UNIT` log level to a tracepoint definition.
2694 ====
2695 [source,c]
2696 ----
2697 /* Tracepoint definition */
2698 TRACEPOINT_EVENT(
2699     my_app,
2700     get_transaction,
2701     TP_ARGS(
2702         int, userid,
2703         size_t, len
2704     ),
2705     TP_FIELDS(
2706         ctf_integer(int, userid, userid)
2707         ctf_integer(size_t, len, len)
2708     )
2709 )
2710
2711 /* Log level assignment */
2712 TRACEPOINT_LOGLEVEL(my_app, get_transaction, TRACE_DEBUG_UNIT)
2713 ----
2714 ====
2715
2716
2717 [[tpp-source]]
2718 ===== Create a tracepoint provider package source file
2719
2720 A _tracepoint provider package source file_ is a C source file which
2721 includes a <<tpp-header,tracepoint provider header file>> to expand its
2722 macros into event serialization and other functions.
2723
2724 Use the following tracepoint provider package source file template:
2725
2726 [source,c]
2727 .Tracepoint provider package source file template.
2728 ----
2729 #define TRACEPOINT_CREATE_PROBES
2730
2731 #include "tp.h"
2732 ----
2733
2734 Replace `tp.h` with the name of your <<tpp-header,tracepoint provider
2735 header file>> name. You may also include more than one tracepoint
2736 provider header file here to create a tracepoint provider package
2737 holding more than one tracepoint providers.
2738
2739
2740 [[probing-the-application-source-code]]
2741 ==== Add tracepoints to the source code of an application
2742
2743 Once you <<tpp-header,create a tracepoint provider header file>>, use
2744 the `tracepoint()` macro in the source code of your application to
2745 insert the tracepoints that this header
2746 <<defining-tracepoints,defines>>.
2747
2748 The `tracepoint()` macro takes at least two parameters: the tracepoint
2749 provider name and the tracepoint name. The corresponding tracepoint
2750 definition defines the other parameters.
2751
2752 .`tracepoint()` usage.
2753 ====
2754 The following <<defining-tracepoints,tracepoint definition>> defines a
2755 tracepoint which takes two input arguments and has two output event
2756 fields.
2757
2758 [source,c]
2759 .Tracepoint provider header file.
2760 ----
2761 #include "my-custom-structure.h"
2762
2763 TRACEPOINT_EVENT(
2764     my_provider,
2765     my_tracepoint,
2766     TP_ARGS(
2767         int, argc,
2768         const char*, cmd_name
2769     ),
2770     TP_FIELDS(
2771         ctf_string(cmd_name, cmd_name)
2772         ctf_integer(int, number_of_args, argc)
2773     )
2774 )
2775 ----
2776
2777 Refer to this tracepoint definition with the `tracepoint()` macro in
2778 the source code of your application like this:
2779
2780 [source,c]
2781 .Application source file.
2782 ----
2783 #include "tp.h"
2784
2785 int main(int argc, char* argv[])
2786 {
2787     tracepoint(my_provider, my_tracepoint, argc, argv[0]);
2788
2789     return 0;
2790 }
2791 ----
2792
2793 Note how the source code of the application includes
2794 the tracepoint provider header file containing the tracepoint
2795 definitions to use, path:{tp.h}.
2796 ====
2797
2798 .`tracepoint()` usage with a complex tracepoint definition.
2799 ====
2800 Consider this complex tracepoint definition, where multiple event
2801 fields refer to the same input arguments in their argument expression
2802 parameter:
2803
2804 [source,c]
2805 .Tracepoint provider header file.
2806 ----
2807 /* For `struct stat` */
2808 #include <sys/types.h>
2809 #include <sys/stat.h>
2810 #include <unistd.h>
2811
2812 TRACEPOINT_EVENT(
2813     my_provider,
2814     my_tracepoint,
2815     TP_ARGS(
2816         int, my_int_arg,
2817         char*, my_str_arg,
2818         struct stat*, st
2819     ),
2820     TP_FIELDS(
2821         ctf_integer(int, my_constant_field, 23 + 17)
2822         ctf_integer(int, my_int_arg_field, my_int_arg)
2823         ctf_integer(int, my_int_arg_field2, my_int_arg * my_int_arg)
2824         ctf_integer(int, sum4_field, my_str_arg[0] + my_str_arg[1] +
2825                                      my_str_arg[2] + my_str_arg[3])
2826         ctf_string(my_str_arg_field, my_str_arg)
2827         ctf_integer_hex(off_t, size_field, st->st_size)
2828         ctf_float(double, size_dbl_field, (double) st->st_size)
2829         ctf_sequence_text(char, half_my_str_arg_field, my_str_arg,
2830                           size_t, strlen(my_str_arg) / 2)
2831     )
2832 )
2833 ----
2834
2835 Refer to this tracepoint definition with the `tracepoint()` macro in
2836 the source code of your application like this:
2837
2838 [source,c]
2839 .Application source file.
2840 ----
2841 #define TRACEPOINT_DEFINE
2842 #include "tp.h"
2843
2844 int main(void)
2845 {
2846     struct stat s;
2847
2848     stat("/etc/fstab", &s);
2849     tracepoint(my_provider, my_tracepoint, 23, "Hello, World!", &s);
2850
2851     return 0;
2852 }
2853 ----
2854
2855 If you look at the event record that LTTng writes when tracing this
2856 program, assuming the file size of path:{/etc/fstab} is 301{nbsp}bytes,
2857 it should look like this:
2858
2859 .Event record fields
2860 |====
2861 |Field name              |Field value
2862 |`my_constant_field`     |40
2863 |`my_int_arg_field`      |23
2864 |`my_int_arg_field2`     |529
2865 |`sum4_field`            |389
2866 |`my_str_arg_field`      |`Hello, World!`
2867 |`size_field`            |0x12d
2868 |`size_dbl_field`        |301.0
2869 |`half_my_str_arg_field` |`Hello,`
2870 |====
2871 ====
2872
2873 Sometimes, the arguments you pass to `tracepoint()` are expensive to
2874 compute--they use the call stack, for example. To avoid this computation
2875 when the tracepoint is disabled, use the `tracepoint_enabled()` and
2876 `do_tracepoint()` macros.
2877
2878 The syntax of the `tracepoint_enabled()` and `do_tracepoint()` macros
2879 is:
2880
2881 [source,c]
2882 .`tracepoint_enabled()` and `do_tracepoint()` macros syntax.
2883 ----
2884 tracepoint_enabled(provider_name, tracepoint_name)
2885 do_tracepoint(provider_name, tracepoint_name, ...)
2886 ----
2887
2888 Replace:
2889
2890 * `provider_name` with the tracepoint provider name.
2891 * `tracepoint_name` with the tracepoint name.
2892
2893 `tracepoint_enabled()` returns a non-zero value if the tracepoint named
2894 `tracepoint_name` from the provider named `provider_name` is enabled
2895 **at run time**.
2896
2897 `do_tracepoint()` is like `tracepoint()`, except that it doesn't check
2898 if the tracepoint is enabled. Using `tracepoint()` with
2899 `tracepoint_enabled()` is dangerous since `tracepoint()` also contains
2900 the `tracepoint_enabled()` check, thus a race condition is
2901 possible in this situation:
2902
2903 [source,c]
2904 .Possible race condition when using `tracepoint_enabled()` with `tracepoint()`.
2905 ----
2906 if (tracepoint_enabled(my_provider, my_tracepoint)) {
2907     stuff = prepare_stuff();
2908 }
2909
2910 tracepoint(my_provider, my_tracepoint, stuff);
2911 ----
2912
2913 If the tracepoint is enabled after the condition, then `stuff` isn't
2914 prepared: the emitted event will either contain wrong data, or the whole
2915 application could crash (segmentation fault, for example).
2916
2917 NOTE: Neither `tracepoint_enabled()` nor `do_tracepoint()` have an
2918 `STAP_PROBEV()` call. If you need it, you must emit
2919 this call yourself.
2920
2921
2922 [[building-tracepoint-providers-and-user-application]]
2923 ==== Build and link a tracepoint provider package and an application
2924
2925 Once you have one or more <<tpp-header,tracepoint provider header
2926 files>> and a <<tpp-source,tracepoint provider package source file>>,
2927 create the tracepoint provider package by compiling its source
2928 file. From here, multiple build and run scenarios are possible. The
2929 following table shows common application and library configurations
2930 along with the required command lines to achieve them.
2931
2932 In the following diagrams, we use the following file names:
2933
2934 `app`::
2935   Executable application.
2936
2937 `app.o`::
2938   Application object file.
2939
2940 `tpp.o`::
2941   Tracepoint provider package object file.
2942
2943 `tpp.a`::
2944   Tracepoint provider package archive file.
2945
2946 `libtpp.so`::
2947   Tracepoint provider package shared object file.
2948
2949 `emon.o`::
2950   User library object file.
2951
2952 `libemon.so`::
2953   User library shared object file.
2954
2955 We use the following symbols in the diagrams of table below:
2956
2957 [role="img-100"]
2958 .Symbols used in the build scenario diagrams.
2959 image::ust-sit-symbols.png[]
2960
2961 We assume that path:{.} is part of the env:LD_LIBRARY_PATH environment
2962 variable in the following instructions.
2963
2964 [role="growable ust-scenarios",cols="asciidoc,asciidoc"]
2965 .Common tracepoint provider package scenarios.
2966 |====
2967 |Scenario |Instructions
2968
2969 |
2970 The instrumented application is statically linked with
2971 the tracepoint provider package object.
2972
2973 image::ust-sit+app-linked-with-tp-o+app-instrumented.png[]
2974
2975 |
2976 include::../common/ust-sit-step-tp-o.txt[]
2977
2978 To build the instrumented application:
2979
2980 . In path:{app.c}, before including path:{tpp.h}, add the following line:
2981 +
2982 --
2983 [source,c]
2984 ----
2985 #define TRACEPOINT_DEFINE
2986 ----
2987 --
2988
2989 . Compile the application source file:
2990 +
2991 --
2992 [role="term"]
2993 ----
2994 $ gcc -c app.c
2995 ----
2996 --
2997
2998 . Build the application:
2999 +
3000 --
3001 [role="term"]
3002 ----
3003 $ gcc -o app app.o tpp.o -llttng-ust -ldl
3004 ----
3005 --
3006
3007 To run the instrumented application:
3008
3009 * Start the application:
3010 +
3011 --
3012 [role="term"]
3013 ----
3014 $ ./app
3015 ----
3016 --
3017
3018 |
3019 The instrumented application is statically linked with the
3020 tracepoint provider package archive file.
3021
3022 image::ust-sit+app-linked-with-tp-a+app-instrumented.png[]
3023
3024 |
3025 To create the tracepoint provider package archive file:
3026
3027 . Compile the <<tpp-source,tracepoint provider package source file>>:
3028 +
3029 --
3030 [role="term"]
3031 ----
3032 $ gcc -I. -c tpp.c
3033 ----
3034 --
3035
3036 . Create the tracepoint provider package archive file:
3037 +
3038 --
3039 [role="term"]
3040 ----
3041 $ ar rcs tpp.a tpp.o
3042 ----
3043 --
3044
3045 To build the instrumented application:
3046
3047 . In path:{app.c}, before including path:{tpp.h}, add the following line:
3048 +
3049 --
3050 [source,c]
3051 ----
3052 #define TRACEPOINT_DEFINE
3053 ----
3054 --
3055
3056 . Compile the application source file:
3057 +
3058 --
3059 [role="term"]
3060 ----
3061 $ gcc -c app.c
3062 ----
3063 --
3064
3065 . Build the application:
3066 +
3067 --
3068 [role="term"]
3069 ----
3070 $ gcc -o app app.o tpp.a -llttng-ust -ldl
3071 ----
3072 --
3073
3074 To run the instrumented application:
3075
3076 * Start the application:
3077 +
3078 --
3079 [role="term"]
3080 ----
3081 $ ./app
3082 ----
3083 --
3084
3085 |
3086 The instrumented application is linked with the tracepoint provider
3087 package shared object.
3088
3089 image::ust-sit+app-linked-with-tp-so+app-instrumented.png[]
3090
3091 |
3092 include::../common/ust-sit-step-tp-so.txt[]
3093
3094 To build the instrumented application:
3095
3096 . In path:{app.c}, before including path:{tpp.h}, add the following line:
3097 +
3098 --
3099 [source,c]
3100 ----
3101 #define TRACEPOINT_DEFINE
3102 ----
3103 --
3104
3105 . Compile the application source file:
3106 +
3107 --
3108 [role="term"]
3109 ----
3110 $ gcc -c app.c
3111 ----
3112 --
3113
3114 . Build the application:
3115 +
3116 --
3117 [role="term"]
3118 ----
3119 $ gcc -o app app.o -ldl -L. -ltpp
3120 ----
3121 --
3122
3123 To run the instrumented application:
3124
3125 * Start the application:
3126 +
3127 --
3128 [role="term"]
3129 ----
3130 $ ./app
3131 ----
3132 --
3133
3134 |
3135 The tracepoint provider package shared object is preloaded before the
3136 instrumented application starts.
3137
3138 image::ust-sit+tp-so-preloaded+app-instrumented.png[]
3139
3140 |
3141 include::../common/ust-sit-step-tp-so.txt[]
3142
3143 To build the instrumented application:
3144
3145 . In path:{app.c}, before including path:{tpp.h}, add the
3146   following lines:
3147 +
3148 --
3149 [source,c]
3150 ----
3151 #define TRACEPOINT_DEFINE
3152 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3153 ----
3154 --
3155
3156 . Compile the application source file:
3157 +
3158 --
3159 [role="term"]
3160 ----
3161 $ gcc -c app.c
3162 ----
3163 --
3164
3165 . Build the application:
3166 +
3167 --
3168 [role="term"]
3169 ----
3170 $ gcc -o app app.o -ldl
3171 ----
3172 --
3173
3174 To run the instrumented application with tracing support:
3175
3176 * Preload the tracepoint provider package shared object and
3177   start the application:
3178 +
3179 --
3180 [role="term"]
3181 ----
3182 $ LD_PRELOAD=./libtpp.so ./app
3183 ----
3184 --
3185
3186 To run the instrumented application without tracing support:
3187
3188 * Start the application:
3189 +
3190 --
3191 [role="term"]
3192 ----
3193 $ ./app
3194 ----
3195 --
3196
3197 |
3198 The instrumented application dynamically loads the tracepoint provider
3199 package shared object.
3200
3201 image::ust-sit+app-dlopens-tp-so+app-instrumented.png[]
3202
3203 |
3204 include::../common/ust-sit-step-tp-so.txt[]
3205
3206 To build the instrumented application:
3207
3208 . In path:{app.c}, before including path:{tpp.h}, add the
3209   following lines:
3210 +
3211 --
3212 [source,c]
3213 ----
3214 #define TRACEPOINT_DEFINE
3215 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3216 ----
3217 --
3218
3219 . Compile the application source file:
3220 +
3221 --
3222 [role="term"]
3223 ----
3224 $ gcc -c app.c
3225 ----
3226 --
3227
3228 . Build the application:
3229 +
3230 --
3231 [role="term"]
3232 ----
3233 $ gcc -o app app.o -ldl
3234 ----
3235 --
3236
3237 To run the instrumented application:
3238
3239 * Start the application:
3240 +
3241 --
3242 [role="term"]
3243 ----
3244 $ ./app
3245 ----
3246 --
3247
3248 |
3249 The application is linked with the instrumented user library.
3250
3251 The instrumented user library is statically linked with the tracepoint
3252 provider package object file.
3253
3254 image::ust-sit+app-linked-with-lib+lib-linked-with-tp-o+lib-instrumented.png[]
3255
3256 |
3257 include::../common/ust-sit-step-tp-o-fpic.txt[]
3258
3259 To build the instrumented user library:
3260
3261 . In path:{emon.c}, before including path:{tpp.h}, add the
3262   following line:
3263 +
3264 --
3265 [source,c]
3266 ----
3267 #define TRACEPOINT_DEFINE
3268 ----
3269 --
3270
3271 . Compile the user library source file:
3272 +
3273 --
3274 [role="term"]
3275 ----
3276 $ gcc -I. -fpic -c emon.c
3277 ----
3278 --
3279
3280 . Build the user library shared object:
3281 +
3282 --
3283 [role="term"]
3284 ----
3285 $ gcc -shared -o libemon.so emon.o tpp.o -llttng-ust -ldl
3286 ----
3287 --
3288
3289 To build the application:
3290
3291 . Compile the application source file:
3292 +
3293 --
3294 [role="term"]
3295 ----
3296 $ gcc -c app.c
3297 ----
3298 --
3299
3300 . Build the application:
3301 +
3302 --
3303 [role="term"]
3304 ----
3305 $ gcc -o app app.o -L. -lemon
3306 ----
3307 --
3308
3309 To run the application:
3310
3311 * Start the application:
3312 +
3313 --
3314 [role="term"]
3315 ----
3316 $ ./app
3317 ----
3318 --
3319
3320 |
3321 The application is linked with the instrumented user library.
3322
3323 The instrumented user library is linked with the tracepoint provider
3324 package shared object.
3325
3326 image::ust-sit+app-linked-with-lib+lib-linked-with-tp-so+lib-instrumented.png[]
3327
3328 |
3329 include::../common/ust-sit-step-tp-so.txt[]
3330
3331 To build the instrumented user library:
3332
3333 . In path:{emon.c}, before including path:{tpp.h}, add the
3334   following line:
3335 +
3336 --
3337 [source,c]
3338 ----
3339 #define TRACEPOINT_DEFINE
3340 ----
3341 --
3342
3343 . Compile the user library source file:
3344 +
3345 --
3346 [role="term"]
3347 ----
3348 $ gcc -I. -fpic -c emon.c
3349 ----
3350 --
3351
3352 . Build the user library shared object:
3353 +
3354 --
3355 [role="term"]
3356 ----
3357 $ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
3358 ----
3359 --
3360
3361 To build the application:
3362
3363 . Compile the application source file:
3364 +
3365 --
3366 [role="term"]
3367 ----
3368 $ gcc -c app.c
3369 ----
3370 --
3371
3372 . Build the application:
3373 +
3374 --
3375 [role="term"]
3376 ----
3377 $ gcc -o app app.o -L. -lemon
3378 ----
3379 --
3380
3381 To run the application:
3382
3383 * Start the application:
3384 +
3385 --
3386 [role="term"]
3387 ----
3388 $ ./app
3389 ----
3390 --
3391
3392 |
3393 The tracepoint provider package shared object is preloaded before the
3394 application starts.
3395
3396 The application is linked with the instrumented user library.
3397
3398 image::ust-sit+tp-so-preloaded+app-linked-with-lib+lib-instrumented.png[]
3399
3400 |
3401 include::../common/ust-sit-step-tp-so.txt[]
3402
3403 To build the instrumented user library:
3404
3405 . In path:{emon.c}, before including path:{tpp.h}, add the
3406   following lines:
3407 +
3408 --
3409 [source,c]
3410 ----
3411 #define TRACEPOINT_DEFINE
3412 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3413 ----
3414 --
3415
3416 . Compile the user library source file:
3417 +
3418 --
3419 [role="term"]
3420 ----
3421 $ gcc -I. -fpic -c emon.c
3422 ----
3423 --
3424
3425 . Build the user library shared object:
3426 +
3427 --
3428 [role="term"]
3429 ----
3430 $ gcc -shared -o libemon.so emon.o -ldl
3431 ----
3432 --
3433
3434 To build the application:
3435
3436 . Compile the application source file:
3437 +
3438 --
3439 [role="term"]
3440 ----
3441 $ gcc -c app.c
3442 ----
3443 --
3444
3445 . Build the application:
3446 +
3447 --
3448 [role="term"]
3449 ----
3450 $ gcc -o app app.o -L. -lemon
3451 ----
3452 --
3453
3454 To run the application with tracing support:
3455
3456 * Preload the tracepoint provider package shared object and
3457   start the application:
3458 +
3459 --
3460 [role="term"]
3461 ----
3462 $ LD_PRELOAD=./libtpp.so ./app
3463 ----
3464 --
3465
3466 To run the application without tracing support:
3467
3468 * Start the application:
3469 +
3470 --
3471 [role="term"]
3472 ----
3473 $ ./app
3474 ----
3475 --
3476
3477 |
3478 The application is linked with the instrumented user library.
3479
3480 The instrumented user library dynamically loads the tracepoint provider
3481 package shared object.
3482
3483 image::ust-sit+app-linked-with-lib+lib-dlopens-tp-so+lib-instrumented.png[]
3484
3485 |
3486 include::../common/ust-sit-step-tp-so.txt[]
3487
3488 To build the instrumented user library:
3489
3490 . In path:{emon.c}, before including path:{tpp.h}, add the
3491   following lines:
3492 +
3493 --
3494 [source,c]
3495 ----
3496 #define TRACEPOINT_DEFINE
3497 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3498 ----
3499 --
3500
3501 . Compile the user library source file:
3502 +
3503 --
3504 [role="term"]
3505 ----
3506 $ gcc -I. -fpic -c emon.c
3507 ----
3508 --
3509
3510 . Build the user library shared object:
3511 +
3512 --
3513 [role="term"]
3514 ----
3515 $ gcc -shared -o libemon.so emon.o -ldl
3516 ----
3517 --
3518
3519 To build the application:
3520
3521 . Compile the application source file:
3522 +
3523 --
3524 [role="term"]
3525 ----
3526 $ gcc -c app.c
3527 ----
3528 --
3529
3530 . Build the application:
3531 +
3532 --
3533 [role="term"]
3534 ----
3535 $ gcc -o app app.o -L. -lemon
3536 ----
3537 --
3538
3539 To run the application:
3540
3541 * Start the application:
3542 +
3543 --
3544 [role="term"]
3545 ----
3546 $ ./app
3547 ----
3548 --
3549
3550 |
3551 The application dynamically loads the instrumented user library.
3552
3553 The instrumented user library is linked with the tracepoint provider
3554 package shared object.
3555
3556 image::ust-sit+app-dlopens-lib+lib-linked-with-tp-so+lib-instrumented.png[]
3557
3558 |
3559 include::../common/ust-sit-step-tp-so.txt[]
3560
3561 To build the instrumented user library:
3562
3563 . In path:{emon.c}, before including path:{tpp.h}, add the
3564   following line:
3565 +
3566 --
3567 [source,c]
3568 ----
3569 #define TRACEPOINT_DEFINE
3570 ----
3571 --
3572
3573 . Compile the user library source file:
3574 +
3575 --
3576 [role="term"]
3577 ----
3578 $ gcc -I. -fpic -c emon.c
3579 ----
3580 --
3581
3582 . Build the user library shared object:
3583 +
3584 --
3585 [role="term"]
3586 ----
3587 $ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
3588 ----
3589 --
3590
3591 To build the application:
3592
3593 . Compile the application source file:
3594 +
3595 --
3596 [role="term"]
3597 ----
3598 $ gcc -c app.c
3599 ----
3600 --
3601
3602 . Build the application:
3603 +
3604 --
3605 [role="term"]
3606 ----
3607 $ gcc -o app app.o -ldl -L. -lemon
3608 ----
3609 --
3610
3611 To run the application:
3612
3613 * Start the application:
3614 +
3615 --
3616 [role="term"]
3617 ----
3618 $ ./app
3619 ----
3620 --
3621
3622 |
3623 The application dynamically loads the instrumented user library.
3624
3625 The instrumented user library dynamically loads the tracepoint provider
3626 package shared object.
3627
3628 image::ust-sit+app-dlopens-lib+lib-dlopens-tp-so+lib-instrumented.png[]
3629
3630 |
3631 include::../common/ust-sit-step-tp-so.txt[]
3632
3633 To build the instrumented user library:
3634
3635 . In path:{emon.c}, before including path:{tpp.h}, add the
3636   following lines:
3637 +
3638 --
3639 [source,c]
3640 ----
3641 #define TRACEPOINT_DEFINE
3642 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3643 ----
3644 --
3645
3646 . Compile the user library source file:
3647 +
3648 --
3649 [role="term"]
3650 ----
3651 $ gcc -I. -fpic -c emon.c
3652 ----
3653 --
3654
3655 . Build the user library shared object:
3656 +
3657 --
3658 [role="term"]
3659 ----
3660 $ gcc -shared -o libemon.so emon.o -ldl
3661 ----
3662 --
3663
3664 To build the application:
3665
3666 . Compile the application source file:
3667 +
3668 --
3669 [role="term"]
3670 ----
3671 $ gcc -c app.c
3672 ----
3673 --
3674
3675 . Build the application:
3676 +
3677 --
3678 [role="term"]
3679 ----
3680 $ gcc -o app app.o -ldl -L. -lemon
3681 ----
3682 --
3683
3684 To run the application:
3685
3686 * Start the application:
3687 +
3688 --
3689 [role="term"]
3690 ----
3691 $ ./app
3692 ----
3693 --
3694
3695 |
3696 The tracepoint provider package shared object is preloaded before the
3697 application starts.
3698
3699 The application dynamically loads the instrumented user library.
3700
3701 image::ust-sit+tp-so-preloaded+app-dlopens-lib+lib-instrumented.png[]
3702
3703 |
3704 include::../common/ust-sit-step-tp-so.txt[]
3705
3706 To build the instrumented user library:
3707
3708 . In path:{emon.c}, before including path:{tpp.h}, add the
3709   following lines:
3710 +
3711 --
3712 [source,c]
3713 ----
3714 #define TRACEPOINT_DEFINE
3715 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3716 ----
3717 --
3718
3719 . Compile the user library source file:
3720 +
3721 --
3722 [role="term"]
3723 ----
3724 $ gcc -I. -fpic -c emon.c
3725 ----
3726 --
3727
3728 . Build the user library shared object:
3729 +
3730 --
3731 [role="term"]
3732 ----
3733 $ gcc -shared -o libemon.so emon.o -ldl
3734 ----
3735 --
3736
3737 To build the application:
3738
3739 . Compile the application source file:
3740 +
3741 --
3742 [role="term"]
3743 ----
3744 $ gcc -c app.c
3745 ----
3746 --
3747
3748 . Build the application:
3749 +
3750 --
3751 [role="term"]
3752 ----
3753 $ gcc -o app app.o -L. -lemon
3754 ----
3755 --
3756
3757 To run the application with tracing support:
3758
3759 * Preload the tracepoint provider package shared object and
3760   start the application:
3761 +
3762 --
3763 [role="term"]
3764 ----
3765 $ LD_PRELOAD=./libtpp.so ./app
3766 ----
3767 --
3768
3769 To run the application without tracing support:
3770
3771 * Start the application:
3772 +
3773 --
3774 [role="term"]
3775 ----
3776 $ ./app
3777 ----
3778 --
3779
3780 |
3781 The application is statically linked with the tracepoint provider
3782 package object file.
3783
3784 The application is linked with the instrumented user library.
3785
3786 image::ust-sit+app-linked-with-tp-o+app-linked-with-lib+lib-instrumented.png[]
3787
3788 |
3789 include::../common/ust-sit-step-tp-o.txt[]
3790
3791 To build the instrumented user library:
3792
3793 . In path:{emon.c}, before including path:{tpp.h}, add the
3794   following line:
3795 +
3796 --
3797 [source,c]
3798 ----
3799 #define TRACEPOINT_DEFINE
3800 ----
3801 --
3802
3803 . Compile the user library source file:
3804 +
3805 --
3806 [role="term"]
3807 ----
3808 $ gcc -I. -fpic -c emon.c
3809 ----
3810 --
3811
3812 . Build the user library shared object:
3813 +
3814 --
3815 [role="term"]
3816 ----
3817 $ gcc -shared -o libemon.so emon.o
3818 ----
3819 --
3820
3821 To build the application:
3822
3823 . Compile the application source file:
3824 +
3825 --
3826 [role="term"]
3827 ----
3828 $ gcc -c app.c
3829 ----
3830 --
3831
3832 . Build the application:
3833 +
3834 --
3835 [role="term"]
3836 ----
3837 $ gcc -o app app.o tpp.o -llttng-ust -ldl -L. -lemon
3838 ----
3839 --
3840
3841 To run the instrumented application:
3842
3843 * Start the application:
3844 +
3845 --
3846 [role="term"]
3847 ----
3848 $ ./app
3849 ----
3850 --
3851
3852 |
3853 The application is statically linked with the tracepoint provider
3854 package object file.
3855
3856 The application dynamically loads the instrumented user library.
3857
3858 image::ust-sit+app-linked-with-tp-o+app-dlopens-lib+lib-instrumented.png[]
3859
3860 |
3861 include::../common/ust-sit-step-tp-o.txt[]
3862
3863 To build the application:
3864
3865 . In path:{app.c}, before including path:{tpp.h}, add the following line:
3866 +
3867 --
3868 [source,c]
3869 ----
3870 #define TRACEPOINT_DEFINE
3871 ----
3872 --
3873
3874 . Compile the application source file:
3875 +
3876 --
3877 [role="term"]
3878 ----
3879 $ gcc -c app.c
3880 ----
3881 --
3882
3883 . Build the application:
3884 +
3885 --
3886 [role="term"]
3887 ----
3888 $ gcc -Wl,--export-dynamic -o app app.o tpp.o \
3889     -llttng-ust -ldl
3890 ----
3891 --
3892 +
3893 The `--export-dynamic` option passed to the linker is necessary for the
3894 dynamically loaded library to ``see'' the tracepoint symbols defined in
3895 the application.
3896
3897 To build the instrumented user library:
3898
3899 . Compile the user library source file:
3900 +
3901 --
3902 [role="term"]
3903 ----
3904 $ gcc -I. -fpic -c emon.c
3905 ----
3906 --
3907
3908 . Build the user library shared object:
3909 +
3910 --
3911 [role="term"]
3912 ----
3913 $ gcc -shared -o libemon.so emon.o
3914 ----
3915 --
3916
3917 To run the application:
3918
3919 * Start the application:
3920 +
3921 --
3922 [role="term"]
3923 ----
3924 $ ./app
3925 ----
3926 --
3927 |====
3928
3929
3930 [[using-lttng-ust-with-daemons]]
3931 ===== Use noch:{LTTng-UST} with daemons
3932
3933 If your instrumented application calls man:fork(2), man:clone(2),
3934 or BSD's man:rfork(2), without a following man:exec(3)-family
3935 system call, you must preload the path:{liblttng-ust-fork.so} shared
3936 object when you start the application.
3937
3938 [role="term"]
3939 ----
3940 $ LD_PRELOAD=liblttng-ust-fork.so ./my-app
3941 ----
3942
3943 If your tracepoint provider package is
3944 a shared library which you also preload, you must put both
3945 shared objects in env:LD_PRELOAD:
3946
3947 [role="term"]
3948 ----
3949 $ LD_PRELOAD=liblttng-ust-fork.so:/path/to/tp.so ./my-app
3950 ----
3951
3952
3953 [role="since-2.9"]
3954 [[liblttng-ust-fd]]
3955 ===== Use noch:{LTTng-UST} with applications which close file descriptors that don't belong to them
3956
3957 If your instrumented application closes one or more file descriptors
3958 which it did not open itself, you must preload the
3959 path:{liblttng-ust-fd.so} shared object when you start the application:
3960
3961 [role="term"]
3962 ----
3963 $ LD_PRELOAD=liblttng-ust-fd.so ./my-app
3964 ----
3965
3966 Typical use cases include closing all the file descriptors after
3967 man:fork(2) or man:rfork(2) and buggy applications doing
3968 ``double closes''.
3969
3970
3971 [[lttng-ust-pkg-config]]
3972 ===== Use noch:{pkg-config}
3973
3974 On some distributions, LTTng-UST ships with a
3975 https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config]
3976 metadata file. If this is your case, then use cmd:pkg-config to
3977 build an application on the command line:
3978
3979 [role="term"]
3980 ----
3981 $ gcc -o my-app my-app.o tp.o $(pkg-config --cflags --libs lttng-ust)
3982 ----
3983
3984
3985 [[instrumenting-32-bit-app-on-64-bit-system]]
3986 ===== [[advanced-instrumenting-techniques]]Build a 32-bit instrumented application for a 64-bit target system
3987
3988 In order to trace a 32-bit application running on a 64-bit system,
3989 LTTng must use a dedicated 32-bit
3990 <<lttng-consumerd,consumer daemon>>.
3991
3992 The following steps show how to build and install a 32-bit consumer
3993 daemon, which is _not_ part of the default 64-bit LTTng build, how to
3994 build and install the 32-bit LTTng-UST libraries, and how to build and
3995 link an instrumented 32-bit application in that context.
3996
3997 To build a 32-bit instrumented application for a 64-bit target system,
3998 assuming you have a fresh target system with no installed Userspace RCU
3999 or LTTng packages:
4000
4001 . Download, build, and install a 32-bit version of Userspace RCU:
4002 +
4003 --
4004 [role="term"]
4005 ----
4006 $ cd $(mktemp -d) &&
4007 wget http://lttng.org/files/urcu/userspace-rcu-latest-0.9.tar.bz2 &&
4008 tar -xf userspace-rcu-latest-0.9.tar.bz2 &&
4009 cd userspace-rcu-0.9.* &&
4010 ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 &&
4011 make &&
4012 sudo make install &&
4013 sudo ldconfig
4014 ----
4015 --
4016
4017 . Using the package manager of your distribution, or from source,
4018   install the following 32-bit versions of the following dependencies of
4019   LTTng-tools and LTTng-UST:
4020 +
4021 --
4022 * https://sourceforge.net/projects/libuuid/[libuuid]
4023 * http://directory.fsf.org/wiki/Popt[popt]
4024 * http://www.xmlsoft.org/[libxml2]
4025 --
4026
4027 . Download, build, and install a 32-bit version of the latest
4028   LTTng-UST{nbsp}{revision}:
4029 +
4030 --
4031 [role="term"]
4032 ----
4033 $ cd $(mktemp -d) &&
4034 wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.12.tar.bz2 &&
4035 tar -xf lttng-ust-latest-2.12.tar.bz2 &&
4036 cd lttng-ust-2.12.* &&
4037 ./configure --libdir=/usr/local/lib32 \
4038             CFLAGS=-m32 CXXFLAGS=-m32 \
4039             LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' &&
4040 make &&
4041 sudo make install &&
4042 sudo ldconfig
4043 ----
4044 --
4045 +
4046 [NOTE]
4047 ====
4048 Depending on your distribution,
4049 32-bit libraries could be installed at a different location than
4050 `/usr/lib32`. For example, Debian is known to install
4051 some 32-bit libraries in `/usr/lib/i386-linux-gnu`.
4052
4053 In this case, make sure to set `LDFLAGS` to all the
4054 relevant 32-bit library paths, for example:
4055
4056 [role="term"]
4057 ----
4058 $ LDFLAGS='-L/usr/lib/i386-linux-gnu -L/usr/lib32'
4059 ----
4060 ====
4061
4062 . Download the latest LTTng-tools{nbsp}{revision}, build, and install
4063   the 32-bit consumer daemon:
4064 +
4065 --
4066 [role="term"]
4067 ----
4068 $ cd $(mktemp -d) &&
4069 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.12.tar.bz2 &&
4070 tar -xf lttng-tools-latest-2.12.tar.bz2 &&
4071 cd lttng-tools-2.12.* &&
4072 ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \
4073             LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' \
4074             --disable-bin-lttng --disable-bin-lttng-crash \
4075             --disable-bin-lttng-relayd --disable-bin-lttng-sessiond &&
4076 make &&
4077 cd src/bin/lttng-consumerd &&
4078 sudo make install &&
4079 sudo ldconfig
4080 ----
4081 --
4082
4083 . From your distribution or from source,
4084   <<installing-lttng,install>> the 64-bit versions of
4085   LTTng-UST and Userspace RCU.
4086 . Download, build, and install the 64-bit version of the
4087   latest LTTng-tools{nbsp}{revision}:
4088 +
4089 --
4090 [role="term"]
4091 ----
4092 $ cd $(mktemp -d) &&
4093 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.12.tar.bz2 &&
4094 tar -xf lttng-tools-latest-2.12.tar.bz2 &&
4095 cd lttng-tools-2.12.* &&
4096 ./configure --with-consumerd32-libdir=/usr/local/lib32 \
4097             --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd &&
4098 make &&
4099 sudo make install &&
4100 sudo ldconfig
4101 ----
4102 --
4103
4104 . Pass the following options to man:gcc(1), man:g++(1), or man:clang(1)
4105   when linking your 32-bit application:
4106 +
4107 ----
4108 -m32 -L/usr/lib32 -L/usr/local/lib32 \
4109 -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32
4110 ----
4111 +
4112 For example, let's rebuild the quick start example in
4113 <<tracing-your-own-user-application,Trace a user application>> as an
4114 instrumented 32-bit application:
4115 +
4116 --
4117 [role="term"]
4118 ----
4119 $ gcc -m32 -c -I. hello-tp.c
4120 $ gcc -m32 -c hello.c
4121 $ gcc -m32 -o hello hello.o hello-tp.o \
4122     -L/usr/lib32 -L/usr/local/lib32 \
4123     -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 \
4124     -llttng-ust -ldl
4125 ----
4126 --
4127
4128 No special action is required to execute the 32-bit application and
4129 to trace it: use the command-line man:lttng(1) tool as usual.
4130
4131
4132 [role="since-2.5"]
4133 [[tracef]]
4134 ==== Use `tracef()`
4135
4136 man:tracef(3) is a small LTTng-UST API designed for quick,
4137 man:printf(3)-like instrumentation without the burden of
4138 <<tracepoint-provider,creating>> and
4139 <<building-tracepoint-providers-and-user-application,building>>
4140 a tracepoint provider package.
4141
4142 To use `tracef()` in your application:
4143
4144 . In the C or C++ source files where you need to use `tracef()`,
4145   include `<lttng/tracef.h>`:
4146 +
4147 --
4148 [source,c]
4149 ----
4150 #include <lttng/tracef.h>
4151 ----
4152 --
4153
4154 . In the source code of the application, use `tracef()` like you would
4155   use man:printf(3):
4156 +
4157 --
4158 [source,c]
4159 ----
4160     /* ... */
4161
4162     tracef("my message: %d (%s)", my_integer, my_string);
4163
4164     /* ... */
4165 ----
4166 --
4167
4168 . Link your application with `liblttng-ust`:
4169 +
4170 --
4171 [role="term"]
4172 ----
4173 $ gcc -o app app.c -llttng-ust
4174 ----
4175 --
4176
4177 To trace the events that `tracef()` calls emit:
4178
4179 * <<enabling-disabling-events,Create an event rule>> which matches the
4180   `lttng_ust_tracef:*` event name:
4181 +
4182 --
4183 [role="term"]
4184 ----
4185 $ lttng enable-event --userspace 'lttng_ust_tracef:*'
4186 ----
4187 --
4188
4189 [IMPORTANT]
4190 .Limitations of `tracef()`
4191 ====
4192 The `tracef()` utility function was developed to make user space tracing
4193 super simple, albeit with notable disadvantages compared to
4194 <<defining-tracepoints,user-defined tracepoints>>:
4195
4196 * All the emitted events have the same tracepoint provider and
4197   tracepoint names, respectively `lttng_ust_tracef` and `event`.
4198 * There is no static type checking.
4199 * The only event record field you actually get, named `msg`, is a string
4200   potentially containing the values you passed to `tracef()`
4201   using your own format string. This also means that you can't filter
4202   events with a custom expression at run time because there are no
4203   isolated fields.
4204 * Since `tracef()` uses the man:vasprintf(3) function of the
4205   C{nbsp}standard library behind the scenes to format the strings at run
4206   time, its expected performance is lower than with user-defined
4207   tracepoints, which don't require a conversion to a string.
4208
4209 Taking this into consideration, `tracef()` is useful for some quick
4210 prototyping and debugging, but you shouldn't consider it for any
4211 permanent and serious applicative instrumentation.
4212 ====
4213
4214
4215 [role="since-2.7"]
4216 [[tracelog]]
4217 ==== Use `tracelog()`
4218
4219 The man:tracelog(3) API is very similar to <<tracef,`tracef()`>>, with
4220 the difference that it accepts an additional log level parameter.
4221
4222 The goal of `tracelog()` is to ease the migration from logging to
4223 tracing.
4224
4225 To use `tracelog()` in your application:
4226
4227 . In the C or C++ source files where you need to use `tracelog()`,
4228   include `<lttng/tracelog.h>`:
4229 +
4230 --
4231 [source,c]
4232 ----
4233 #include <lttng/tracelog.h>
4234 ----
4235 --
4236
4237 . In the source code of the application, use `tracelog()` like you would
4238   use man:printf(3), except for the first parameter which is the log
4239   level:
4240 +
4241 --
4242 [source,c]
4243 ----
4244     /* ... */
4245
4246     tracelog(TRACE_WARNING, "my message: %d (%s)",
4247              my_integer, my_string);
4248
4249     /* ... */
4250 ----
4251 --
4252 +
4253 See man:lttng-ust(3) for a list of available log level names.
4254
4255 . Link your application with `liblttng-ust`:
4256 +
4257 --
4258 [role="term"]
4259 ----
4260 $ gcc -o app app.c -llttng-ust
4261 ----
4262 --
4263
4264 To trace the events that `tracelog()` calls emit with a log level
4265 _as severe as_ a specific log level:
4266
4267 * <<enabling-disabling-events,Create an event rule>> which matches the
4268   `lttng_ust_tracelog:*` event name and a minimum level
4269   of severity:
4270 +
4271 --
4272 [role="term"]
4273 ----
4274 $ lttng enable-event --userspace 'lttng_ust_tracelog:*'
4275                    --loglevel=TRACE_WARNING
4276 ----
4277 --
4278
4279 To trace the events that `tracelog()` calls emit with a
4280 _specific log level_:
4281
4282 * Create an event rule which matches the `lttng_ust_tracelog:*`
4283   event name and a specific log level:
4284 +
4285 --
4286 [role="term"]
4287 ----
4288 $ lttng enable-event --userspace 'lttng_ust_tracelog:*'
4289                    --loglevel-only=TRACE_INFO
4290 ----
4291 --
4292
4293
4294 [[prebuilt-ust-helpers]]
4295 === Prebuilt user space tracing helpers
4296
4297 The LTTng-UST package provides a few helpers in the form of preloadable
4298 shared objects which automatically instrument system functions and
4299 calls.
4300
4301 The helper shared objects are normally found in dir:{/usr/lib}. If you
4302 built LTTng-UST <<building-from-source,from source>>, they are probably
4303 located in dir:{/usr/local/lib}.
4304
4305 The installed user space tracing helpers in LTTng-UST{nbsp}{revision}
4306 are:
4307
4308 path:{liblttng-ust-libc-wrapper.so}::
4309 path:{liblttng-ust-pthread-wrapper.so}::
4310   <<liblttng-ust-libc-pthread-wrapper,C{nbsp}standard library
4311   memory and POSIX threads function tracing>>.
4312
4313 path:{liblttng-ust-cyg-profile.so}::
4314 path:{liblttng-ust-cyg-profile-fast.so}::
4315   <<liblttng-ust-cyg-profile,Function entry and exit tracing>>.
4316
4317 path:{liblttng-ust-dl.so}::
4318   <<liblttng-ust-dl,Dynamic linker tracing>>.
4319
4320 To use a user space tracing helper with any user application:
4321
4322 * Preload the helper shared object when you start the application:
4323 +
4324 --
4325 [role="term"]
4326 ----
4327 $ LD_PRELOAD=liblttng-ust-libc-wrapper.so my-app
4328 ----
4329 --
4330 +
4331 You can preload more than one helper:
4332 +
4333 --
4334 [role="term"]
4335 ----
4336 $ LD_PRELOAD=liblttng-ust-libc-wrapper.so:liblttng-ust-dl.so my-app
4337 ----
4338 --
4339
4340
4341 [role="since-2.3"]
4342 [[liblttng-ust-libc-pthread-wrapper]]
4343 ==== Instrument C standard library memory and POSIX threads functions
4344
4345 The path:{liblttng-ust-libc-wrapper.so} and
4346 path:{liblttng-ust-pthread-wrapper.so} helpers
4347 add instrumentation to some C standard library and POSIX
4348 threads functions.
4349
4350 [role="growable"]
4351 .Functions instrumented by preloading path:{liblttng-ust-libc-wrapper.so}.
4352 |====
4353 |TP provider name |TP name |Instrumented function
4354
4355 .6+|`lttng_ust_libc` |`malloc`         |man:malloc(3)
4356                      |`calloc`         |man:calloc(3)
4357                      |`realloc`        |man:realloc(3)
4358                      |`free`           |man:free(3)
4359                      |`memalign`       |man:memalign(3)
4360                      |`posix_memalign` |man:posix_memalign(3)
4361 |====
4362
4363 [role="growable"]
4364 .Functions instrumented by preloading path:{liblttng-ust-pthread-wrapper.so}.
4365 |====
4366 |TP provider name |TP name |Instrumented function
4367
4368 .4+|`lttng_ust_pthread` |`pthread_mutex_lock_req` |man:pthread_mutex_lock(3p) (request time)
4369                         |`pthread_mutex_lock_acq` |man:pthread_mutex_lock(3p) (acquire time)
4370                         |`pthread_mutex_trylock`  |man:pthread_mutex_trylock(3p)
4371                         |`pthread_mutex_unlock`   |man:pthread_mutex_unlock(3p)
4372 |====
4373
4374 When you preload the shared object, it replaces the functions listed
4375 in the previous tables by wrappers which contain tracepoints and call
4376 the replaced functions.
4377
4378
4379 [[liblttng-ust-cyg-profile]]
4380 ==== Instrument function entry and exit
4381
4382 The path:{liblttng-ust-cyg-profile*.so} helpers can add instrumentation
4383 to the entry and exit points of functions.
4384
4385 man:gcc(1) and man:clang(1) have an option named
4386 https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html[`-finstrument-functions`]
4387 which generates instrumentation calls for entry and exit to functions.
4388 The LTTng-UST function tracing helpers,
4389 path:{liblttng-ust-cyg-profile.so} and
4390 path:{liblttng-ust-cyg-profile-fast.so}, take advantage of this feature
4391 to add tracepoints to the two generated functions (which contain
4392 `cyg_profile` in their names, hence the name of the helper).
4393
4394 To use the LTTng-UST function tracing helper, the source files to
4395 instrument must be built using the `-finstrument-functions` compiler
4396 flag.
4397
4398 There are two versions of the LTTng-UST function tracing helper:
4399
4400 * **path:{liblttng-ust-cyg-profile-fast.so}** is a lightweight variant
4401   that you should only use when it can be _guaranteed_ that the
4402   complete event stream is recorded without any lost event record.
4403   Any kind of duplicate information is left out.
4404 +
4405 Assuming no event record is lost, having only the function addresses on
4406 entry is enough to create a call graph, since an event record always
4407 contains the ID of the CPU that generated it.
4408 +
4409 Use a tool like man:addr2line(1) to convert function addresses back to
4410 source file names and line numbers.
4411
4412 * **path:{liblttng-ust-cyg-profile.so}** is a more robust variant
4413 which also works in use cases where event records might get discarded or
4414 not recorded from application startup.
4415 In these cases, the trace analyzer needs more information to be
4416 able to reconstruct the program flow.
4417
4418 See man:lttng-ust-cyg-profile(3) to learn more about the instrumentation
4419 points of this helper.
4420
4421 All the tracepoints that this helper provides have the
4422 log level `TRACE_DEBUG_FUNCTION` (see man:lttng-ust(3)).
4423
4424 TIP: It's sometimes a good idea to limit the number of source files that
4425 you compile with the `-finstrument-functions` option to prevent LTTng
4426 from writing an excessive amount of trace data at run time. When using
4427 man:gcc(1), use the
4428 `-finstrument-functions-exclude-function-list` option to avoid
4429 instrument entries and exits of specific function names.
4430
4431
4432 [role="since-2.4"]
4433 [[liblttng-ust-dl]]
4434 ==== Instrument the dynamic linker
4435
4436 The path:{liblttng-ust-dl.so} helper adds instrumentation to the
4437 man:dlopen(3) and man:dlclose(3) function calls.
4438
4439 See man:lttng-ust-dl(3) to learn more about the instrumentation points
4440 of this helper.
4441
4442
4443 [role="since-2.4"]
4444 [[java-application]]
4445 === User space Java agent
4446
4447 You can instrument any Java application which uses one of the following
4448 logging frameworks:
4449
4450 * The https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[**`java.util.logging`**]
4451   (JUL) core logging facilities.
4452 * http://logging.apache.org/log4j/1.2/[**Apache log4j{nbsp}1.2**], since
4453   LTTng{nbsp}2.6. Note that Apache Log4j{nbsp}2 isn't supported.
4454
4455 [role="img-100"]
4456 .LTTng-UST Java agent imported by a Java application.
4457 image::java-app.png[]
4458
4459 Note that the methods described below are new in LTTng{nbsp}2.8.
4460 Previous LTTng versions use another technique.
4461
4462 NOTE: We use http://openjdk.java.net/[OpenJDK]{nbsp}8 for development
4463 and https://ci.lttng.org/[continuous integration], thus this version is
4464 directly supported. However, the LTTng-UST Java agent is also tested
4465 with OpenJDK{nbsp}7.
4466
4467
4468 [role="since-2.8"]
4469 [[jul]]
4470 ==== Use the LTTng-UST Java agent for `java.util.logging`
4471
4472 To use the LTTng-UST Java agent in a Java application which uses
4473 `java.util.logging` (JUL):
4474
4475 . In the source code of the Java application, import the LTTng-UST log
4476   handler package for `java.util.logging`:
4477 +
4478 --
4479 [source,java]
4480 ----
4481 import org.lttng.ust.agent.jul.LttngLogHandler;
4482 ----
4483 --
4484
4485 . Create an LTTng-UST JUL log handler:
4486 +
4487 --
4488 [source,java]
4489 ----
4490 Handler lttngUstLogHandler = new LttngLogHandler();
4491 ----
4492 --
4493
4494 . Add this handler to the JUL loggers which should emit LTTng events:
4495 +
4496 --
4497 [source,java]
4498 ----
4499 Logger myLogger = Logger.getLogger("some-logger");
4500
4501 myLogger.addHandler(lttngUstLogHandler);
4502 ----
4503 --
4504
4505 . Use `java.util.logging` log statements and configuration as usual.
4506   The loggers with an attached LTTng-UST log handler can emit
4507   LTTng events.
4508
4509 . Before exiting the application, remove the LTTng-UST log handler from
4510   the loggers attached to it and call its `close()` method:
4511 +
4512 --
4513 [source,java]
4514 ----
4515 myLogger.removeHandler(lttngUstLogHandler);
4516 lttngUstLogHandler.close();
4517 ----
4518 --
4519 +
4520 This isn't strictly necessary, but it is recommended for a clean
4521 disposal of the resources of the handler.
4522
4523 . Include the common and JUL-specific JAR files of the LTTng-UST Java agent,
4524   path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-jul.jar},
4525   in the
4526   https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
4527   path] when you build the Java application.
4528 +
4529 The JAR files are typically located in dir:{/usr/share/java}.
4530 +
4531 IMPORTANT: The LTTng-UST Java agent must be
4532 <<installing-lttng,installed>> for the logging framework your
4533 application uses.
4534
4535 .Use the LTTng-UST Java agent for `java.util.logging`.
4536 ====
4537 [source,java]
4538 .path:{Test.java}
4539 ----
4540 import java.io.IOException;
4541 import java.util.logging.Handler;
4542 import java.util.logging.Logger;
4543 import org.lttng.ust.agent.jul.LttngLogHandler;
4544
4545 public class Test
4546 {
4547     private static final int answer = 42;
4548
4549     public static void main(String[] argv) throws Exception
4550     {
4551         // Create a logger
4552         Logger logger = Logger.getLogger("jello");
4553
4554         // Create an LTTng-UST log handler
4555         Handler lttngUstLogHandler = new LttngLogHandler();
4556
4557         // Add the LTTng-UST log handler to our logger
4558         logger.addHandler(lttngUstLogHandler);
4559
4560         // Log at will!
4561         logger.info("some info");
4562         logger.warning("some warning");
4563         Thread.sleep(500);
4564         logger.finer("finer information; the answer is " + answer);
4565         Thread.sleep(123);
4566         logger.severe("error!");
4567
4568         // Not mandatory, but cleaner
4569         logger.removeHandler(lttngUstLogHandler);
4570         lttngUstLogHandler.close();
4571     }
4572 }
4573 ----
4574
4575 Build this example:
4576
4577 [role="term"]
4578 ----
4579 $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
4580 ----
4581
4582 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4583 <<enabling-disabling-events,create an event rule>> matching the
4584 `jello` JUL logger, and <<basic-tracing-session-control,start tracing>>:
4585
4586 [role="term"]
4587 ----
4588 $ lttng create
4589 $ lttng enable-event --jul jello
4590 $ lttng start
4591 ----
4592
4593 Run the compiled class:
4594
4595 [role="term"]
4596 ----
4597 $ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
4598 ----
4599
4600 <<basic-tracing-session-control,Stop tracing>> and inspect the
4601 recorded events:
4602
4603 [role="term"]
4604 ----
4605 $ lttng stop
4606 $ lttng view
4607 ----
4608 ====
4609
4610 In the resulting trace, an <<event,event record>> generated by a Java
4611 application using `java.util.logging` is named `lttng_jul:event` and
4612 has the following fields:
4613
4614 `msg`::
4615   Log record message.
4616
4617 `logger_name`::
4618   Logger name.
4619
4620 `class_name`::
4621   Name of the class in which the log statement was executed.
4622
4623 `method_name`::
4624   Name of the method in which the log statement was executed.
4625
4626 `long_millis`::
4627   Logging time (timestamp in milliseconds).
4628
4629 `int_loglevel`::
4630   Log level integer value.
4631
4632 `int_threadid`::
4633   ID of the thread in which the log statement was executed.
4634
4635 Use the opt:lttng-enable-event(1):--loglevel or
4636 opt:lttng-enable-event(1):--loglevel-only option of the
4637 man:lttng-enable-event(1) command to target a range of JUL log levels
4638 or a specific JUL log level.
4639
4640
4641 [role="since-2.8"]
4642 [[log4j]]
4643 ==== Use the LTTng-UST Java agent for Apache log4j
4644
4645 To use the LTTng-UST Java agent in a Java application which uses
4646 Apache log4j{nbsp}1.2:
4647
4648 . In the source code of the Java application, import the LTTng-UST log
4649   appender package for Apache log4j:
4650 +
4651 --
4652 [source,java]
4653 ----
4654 import org.lttng.ust.agent.log4j.LttngLogAppender;
4655 ----
4656 --
4657
4658 . Create an LTTng-UST log4j log appender:
4659 +
4660 --
4661 [source,java]
4662 ----
4663 Appender lttngUstLogAppender = new LttngLogAppender();
4664 ----
4665 --
4666
4667 . Add this appender to the log4j loggers which should emit LTTng events:
4668 +
4669 --
4670 [source,java]
4671 ----
4672 Logger myLogger = Logger.getLogger("some-logger");
4673
4674 myLogger.addAppender(lttngUstLogAppender);
4675 ----
4676 --
4677
4678 . Use Apache log4j log statements and configuration as usual. The
4679   loggers with an attached LTTng-UST log appender can emit LTTng events.
4680
4681 . Before exiting the application, remove the LTTng-UST log appender from
4682   the loggers attached to it and call its `close()` method:
4683 +
4684 --
4685 [source,java]
4686 ----
4687 myLogger.removeAppender(lttngUstLogAppender);
4688 lttngUstLogAppender.close();
4689 ----
4690 --
4691 +
4692 This isn't strictly necessary, but it is recommended for a clean
4693 disposal of the resources of the appender.
4694
4695 . Include the common and log4j-specific JAR
4696   files of the LTTng-UST Java agent, path:{lttng-ust-agent-common.jar} and
4697   path:{lttng-ust-agent-log4j.jar}, in the
4698   https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
4699   path] when you build the Java application.
4700 +
4701 The JAR files are typically located in dir:{/usr/share/java}.
4702 +
4703 IMPORTANT: The LTTng-UST Java agent must be
4704 <<installing-lttng,installed>> for the logging framework your
4705 application uses.
4706
4707 .Use the LTTng-UST Java agent for Apache log4j.
4708 ====
4709 [source,java]
4710 .path:{Test.java}
4711 ----
4712 import org.apache.log4j.Appender;
4713 import org.apache.log4j.Logger;
4714 import org.lttng.ust.agent.log4j.LttngLogAppender;
4715
4716 public class Test
4717 {
4718     private static final int answer = 42;
4719
4720     public static void main(String[] argv) throws Exception
4721     {
4722         // Create a logger
4723         Logger logger = Logger.getLogger("jello");
4724
4725         // Create an LTTng-UST log appender
4726         Appender lttngUstLogAppender = new LttngLogAppender();
4727
4728         // Add the LTTng-UST log appender to our logger
4729         logger.addAppender(lttngUstLogAppender);
4730
4731         // Log at will!
4732         logger.info("some info");
4733         logger.warn("some warning");
4734         Thread.sleep(500);
4735         logger.debug("debug information; the answer is " + answer);
4736         Thread.sleep(123);
4737         logger.fatal("error!");
4738
4739         // Not mandatory, but cleaner
4740         logger.removeAppender(lttngUstLogAppender);
4741         lttngUstLogAppender.close();
4742     }
4743 }
4744
4745 ----
4746
4747 Build this example (`$LOG4JPATH` is the path to the Apache log4j JAR
4748 file):
4749
4750 [role="term"]
4751 ----
4752 $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java
4753 ----
4754
4755 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4756 <<enabling-disabling-events,create an event rule>> matching the
4757 `jello` log4j logger, and <<basic-tracing-session-control,start tracing>>:
4758
4759 [role="term"]
4760 ----
4761 $ lttng create
4762 $ lttng enable-event --log4j jello
4763 $ lttng start
4764 ----
4765
4766 Run the compiled class:
4767
4768 [role="term"]
4769 ----
4770 $ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH:. Test
4771 ----
4772
4773 <<basic-tracing-session-control,Stop tracing>> and inspect the
4774 recorded events:
4775
4776 [role="term"]
4777 ----
4778 $ lttng stop
4779 $ lttng view
4780 ----
4781 ====
4782
4783 In the resulting trace, an <<event,event record>> generated by a Java
4784 application using log4j is named `lttng_log4j:event` and
4785 has the following fields:
4786
4787 `msg`::
4788   Log record message.
4789
4790 `logger_name`::
4791   Logger name.
4792
4793 `class_name`::
4794   Name of the class in which the log statement was executed.
4795
4796 `method_name`::
4797   Name of the method in which the log statement was executed.
4798
4799 `filename`::
4800   Name of the file in which the executed log statement is located.
4801
4802 `line_number`::
4803   Line number at which the log statement was executed.
4804
4805 `timestamp`::
4806   Logging timestamp.
4807
4808 `int_loglevel`::
4809   Log level integer value.
4810
4811 `thread_name`::
4812   Name of the Java thread in which the log statement was executed.
4813
4814 Use the opt:lttng-enable-event(1):--loglevel or
4815 opt:lttng-enable-event(1):--loglevel-only option of the
4816 man:lttng-enable-event(1) command to target a range of Apache log4j
4817 log levels or a specific log4j log level.
4818
4819
4820 [role="since-2.8"]
4821 [[java-application-context]]
4822 ==== Provide application-specific context fields in a Java application
4823
4824 A Java application-specific context field is a piece of state provided
4825 by the application which <<adding-context,you can add>>, using the
4826 man:lttng-add-context(1) command, to each <<event,event record>>
4827 produced by the log statements of this application.
4828
4829 For example, a given object might have a current request ID variable.
4830 You can create a context information retriever for this object and
4831 assign a name to this current request ID. You can then, using the
4832 man:lttng-add-context(1) command, add this context field by name to
4833 the JUL or log4j <<channel,channel>>.
4834
4835 To provide application-specific context fields in a Java application:
4836
4837 . In the source code of the Java application, import the LTTng-UST
4838   Java agent context classes and interfaces:
4839 +
4840 --
4841 [source,java]
4842 ----
4843 import org.lttng.ust.agent.context.ContextInfoManager;
4844 import org.lttng.ust.agent.context.IContextInfoRetriever;
4845 ----
4846 --
4847
4848 . Create a context information retriever class, that is, a class which
4849   implements the `IContextInfoRetriever` interface:
4850 +
4851 --
4852 [source,java]
4853 ----
4854 class MyContextInfoRetriever implements IContextInfoRetriever
4855 {
4856     @Override
4857     public Object retrieveContextInfo(String key)
4858     {
4859         if (key.equals("intCtx")) {
4860             return (short) 17;
4861         } else if (key.equals("strContext")) {
4862             return "context value!";
4863         } else {
4864             return null;
4865         }
4866     }
4867 }
4868 ----
4869 --
4870 +
4871 This `retrieveContextInfo()` method is the only member of the
4872 `IContextInfoRetriever` interface. Its role is to return the current
4873 value of a state by name to create a context field. The names of the
4874 context fields and which state variables they return depends on your
4875 specific scenario.
4876 +
4877 All primitive types and objects are supported as context fields.
4878 When `retrieveContextInfo()` returns an object, the context field
4879 serializer calls its `toString()` method to add a string field to
4880 event records. The method can also return `null`, which means that
4881 no context field is available for the required name.
4882
4883 . Register an instance of your context information retriever class to
4884   the context information manager singleton:
4885 +
4886 --
4887 [source,java]
4888 ----
4889 IContextInfoRetriever cir = new MyContextInfoRetriever();
4890 ContextInfoManager cim = ContextInfoManager.getInstance();
4891 cim.registerContextInfoRetriever("retrieverName", cir);
4892 ----
4893 --
4894
4895 . Before exiting the application, remove your context information
4896   retriever from the context information manager singleton:
4897 +
4898 --
4899 [source,java]
4900 ----
4901 ContextInfoManager cim = ContextInfoManager.getInstance();
4902 cim.unregisterContextInfoRetriever("retrieverName");
4903 ----
4904 --
4905 +
4906 This isn't strictly necessary, but it is recommended for a clean
4907 disposal of some resources of the manager.
4908
4909 . Build your Java application with LTTng-UST Java agent support as
4910   usual, following the procedure for either the <<jul,JUL>> or
4911   <<log4j,Apache log4j>> framework.
4912
4913
4914 .Provide application-specific context fields in a Java application.
4915 ====
4916 [source,java]
4917 .path:{Test.java}
4918 ----
4919 import java.util.logging.Handler;
4920 import java.util.logging.Logger;
4921 import org.lttng.ust.agent.jul.LttngLogHandler;
4922 import org.lttng.ust.agent.context.ContextInfoManager;
4923 import org.lttng.ust.agent.context.IContextInfoRetriever;
4924
4925 public class Test
4926 {
4927     // Our context information retriever class
4928     private static class MyContextInfoRetriever
4929     implements IContextInfoRetriever
4930     {
4931         @Override
4932         public Object retrieveContextInfo(String key) {
4933             if (key.equals("intCtx")) {
4934                 return (short) 17;
4935             } else if (key.equals("strContext")) {
4936                 return "context value!";
4937             } else {
4938                 return null;
4939             }
4940         }
4941     }
4942
4943     private static final int answer = 42;
4944
4945     public static void main(String args[]) throws Exception
4946     {
4947         // Get the context information manager instance
4948         ContextInfoManager cim = ContextInfoManager.getInstance();
4949
4950         // Create and register our context information retriever
4951         IContextInfoRetriever cir = new MyContextInfoRetriever();
4952         cim.registerContextInfoRetriever("myRetriever", cir);
4953
4954         // Create a logger
4955         Logger logger = Logger.getLogger("jello");
4956
4957         // Create an LTTng-UST log handler
4958         Handler lttngUstLogHandler = new LttngLogHandler();
4959
4960         // Add the LTTng-UST log handler to our logger
4961         logger.addHandler(lttngUstLogHandler);
4962
4963         // Log at will!
4964         logger.info("some info");
4965         logger.warning("some warning");
4966         Thread.sleep(500);
4967         logger.finer("finer information; the answer is " + answer);
4968         Thread.sleep(123);
4969         logger.severe("error!");
4970
4971         // Not mandatory, but cleaner
4972         logger.removeHandler(lttngUstLogHandler);
4973         lttngUstLogHandler.close();
4974         cim.unregisterContextInfoRetriever("myRetriever");
4975     }
4976 }
4977 ----
4978
4979 Build this example:
4980
4981 [role="term"]
4982 ----
4983 $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
4984 ----
4985
4986 <<creating-destroying-tracing-sessions,Create a tracing session>>
4987 and <<enabling-disabling-events,create an event rule>> matching the
4988 `jello` JUL logger:
4989
4990 [role="term"]
4991 ----
4992 $ lttng create
4993 $ lttng enable-event --jul jello
4994 ----
4995
4996 <<adding-context,Add the application-specific context fields>> to the
4997 JUL channel:
4998
4999 [role="term"]
5000 ----
5001 $ lttng add-context --jul --type='$app.myRetriever:intCtx'
5002 $ lttng add-context --jul --type='$app.myRetriever:strContext'
5003 ----
5004
5005 <<basic-tracing-session-control,Start tracing>>:
5006
5007 [role="term"]
5008 ----
5009 $ lttng start
5010 ----
5011
5012 Run the compiled class:
5013
5014 [role="term"]
5015 ----
5016 $ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
5017 ----
5018
5019 <<basic-tracing-session-control,Stop tracing>> and inspect the
5020 recorded events:
5021
5022 [role="term"]
5023 ----
5024 $ lttng stop
5025 $ lttng view
5026 ----
5027 ====
5028
5029
5030 [role="since-2.7"]
5031 [[python-application]]
5032 === User space Python agent
5033
5034 You can instrument a Python{nbsp}2 or Python{nbsp}3 application which
5035 uses the standard
5036 https://docs.python.org/3/library/logging.html[`logging`] package.
5037
5038 Each log statement emits an LTTng event once the
5039 application module imports the
5040 <<lttng-ust-agents,LTTng-UST Python agent>> package.
5041
5042 [role="img-100"]
5043 .A Python application importing the LTTng-UST Python agent.
5044 image::python-app.png[]
5045
5046 To use the LTTng-UST Python agent:
5047
5048 . In the source code of the Python application, import the LTTng-UST
5049   Python agent:
5050 +
5051 --
5052 [source,python]
5053 ----
5054 import lttngust
5055 ----
5056 --
5057 +
5058 The LTTng-UST Python agent automatically adds its logging handler to the
5059 root logger at import time.
5060 +
5061 Any log statement that the application executes before this import does
5062 not emit an LTTng event.
5063 +
5064 IMPORTANT: The LTTng-UST Python agent must be
5065 <<installing-lttng,installed>>.
5066
5067 . Use log statements and logging configuration as usual.
5068   Since the LTTng-UST Python agent adds a handler to the _root_
5069   logger, you can trace any log statement from any logger.
5070
5071 .Use the LTTng-UST Python agent.
5072 ====
5073 [source,python]
5074 .path:{test.py}
5075 ----
5076 import lttngust
5077 import logging
5078 import time
5079
5080
5081 def example():
5082     logging.basicConfig()
5083     logger = logging.getLogger('my-logger')
5084
5085     while True:
5086         logger.debug('debug message')
5087         logger.info('info message')
5088         logger.warn('warn message')
5089         logger.error('error message')
5090         logger.critical('critical message')
5091         time.sleep(1)
5092
5093
5094 if __name__ == '__main__':
5095     example()
5096 ----
5097
5098 NOTE: `logging.basicConfig()`, which adds to the root logger a basic
5099 logging handler which prints to the standard error stream, isn't
5100 strictly required for LTTng-UST tracing to work, but in versions of
5101 Python preceding{nbsp}3.2, you could see a warning message which indicates
5102 that no handler exists for the logger `my-logger`.
5103
5104 <<creating-destroying-tracing-sessions,Create a tracing session>>,
5105 <<enabling-disabling-events,create an event rule>> matching the
5106 `my-logger` Python logger, and <<basic-tracing-session-control,start
5107 tracing>>:
5108
5109 [role="term"]
5110 ----
5111 $ lttng create
5112 $ lttng enable-event --python my-logger
5113 $ lttng start
5114 ----
5115
5116 Run the Python script:
5117
5118 [role="term"]
5119 ----
5120 $ python test.py
5121 ----
5122
5123 <<basic-tracing-session-control,Stop tracing>> and inspect the recorded
5124 events:
5125
5126 [role="term"]
5127 ----
5128 $ lttng stop
5129 $ lttng view
5130 ----
5131 ====
5132
5133 In the resulting trace, an <<event,event record>> generated by a Python
5134 application is named `lttng_python:event` and has the following fields:
5135
5136 `asctime`::
5137   Logging time (string).
5138
5139 `msg`::
5140   Log record message.
5141
5142 `logger_name`::
5143   Logger name.
5144
5145 `funcName`::
5146   Name of the function in which the log statement was executed.
5147
5148 `lineno`::
5149   Line number at which the log statement was executed.
5150
5151 `int_loglevel`::
5152   Log level integer value.
5153
5154 `thread`::
5155   ID of the Python thread in which the log statement was executed.
5156
5157 `threadName`::
5158   Name of the Python thread in which the log statement was executed.
5159
5160 Use the opt:lttng-enable-event(1):--loglevel or
5161 opt:lttng-enable-event(1):--loglevel-only option of the
5162 man:lttng-enable-event(1) command to target a range of Python log levels
5163 or a specific Python log level.
5164
5165 When an application imports the LTTng-UST Python agent, the agent tries
5166 to register to a <<lttng-sessiond,session daemon>>. Note that you must
5167 <<start-sessiond,start the session daemon>> _before_ you run the Python
5168 application. If a session daemon is found, the agent tries to register
5169 to it during five seconds, after which the application continues
5170 without LTTng tracing support. Override this timeout value with
5171 the env:LTTNG_UST_PYTHON_REGISTER_TIMEOUT environment variable
5172 (milliseconds).
5173
5174 If the session daemon stops while a Python application with an imported
5175 LTTng-UST Python agent runs, the agent retries to connect and to
5176 register to a session daemon every three seconds. Override this
5177 delay with the env:LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment
5178 variable.
5179
5180
5181 [role="since-2.5"]
5182 [[proc-lttng-logger-abi]]
5183 === LTTng logger
5184
5185 The `lttng-tracer` Linux kernel module, part of
5186 <<lttng-modules,LTTng-modules>>, creates the special LTTng logger files
5187 path:{/proc/lttng-logger} and path:{/dev/lttng-logger} (since
5188 LTTng{nbsp}2.11) when it's loaded. Any application can write text data
5189 to any of those files to emit an LTTng event.
5190
5191 [role="img-100"]
5192 .An application writes to the LTTng logger file to emit an LTTng event.
5193 image::lttng-logger.png[]
5194
5195 The LTTng logger is the quickest method--not the most efficient,
5196 however--to add instrumentation to an application. It is designed
5197 mostly to instrument shell scripts:
5198
5199 [role="term"]
5200 ----
5201 $ echo "Some message, some $variable" > /dev/lttng-logger
5202 ----
5203
5204 Any event that the LTTng logger emits is named `lttng_logger` and
5205 belongs to the Linux kernel <<domain,tracing domain>>. However, unlike
5206 other instrumentation points in the kernel tracing domain, **any Unix
5207 user** can <<enabling-disabling-events,create an event rule>> which
5208 matches its event name, not only the root user or users in the
5209 <<tracing-group,tracing group>>.
5210
5211 To use the LTTng logger:
5212
5213 * From any application, write text data to the path:{/dev/lttng-logger}
5214   file.
5215
5216 The `msg` field of `lttng_logger` event records contains the
5217 recorded message.
5218
5219 NOTE: The maximum message length of an LTTng logger event is
5220 1024{nbsp}bytes. Writing more than this makes the LTTng logger emit more
5221 than one event to contain the remaining data.
5222
5223 You shouldn't use the LTTng logger to trace a user application which
5224 can be instrumented in a more efficient way, namely:
5225
5226 * <<c-application,C and $$C++$$ applications>>.
5227 * <<java-application,Java applications>>.
5228 * <<python-application,Python applications>>.
5229
5230 .Use the LTTng logger.
5231 ====
5232 [source,bash]
5233 .path:{test.bash}
5234 ----
5235 echo 'Hello, World!' > /dev/lttng-logger
5236 sleep 2
5237 df --human-readable --print-type / > /dev/lttng-logger
5238 ----
5239
5240 <<creating-destroying-tracing-sessions,Create a tracing session>>,
5241 <<enabling-disabling-events,create an event rule>> matching the
5242 `lttng_logger` Linux kernel tracepoint, and
5243 <<basic-tracing-session-control,start tracing>>:
5244
5245 [role="term"]
5246 ----
5247 $ lttng create
5248 $ lttng enable-event --kernel lttng_logger
5249 $ lttng start
5250 ----
5251
5252 Run the Bash script:
5253
5254 [role="term"]
5255 ----
5256 $ bash test.bash
5257 ----
5258
5259 <<basic-tracing-session-control,Stop tracing>> and inspect the recorded
5260 events:
5261
5262 [role="term"]
5263 ----
5264 $ lttng stop
5265 $ lttng view
5266 ----
5267 ====
5268
5269
5270 [[instrumenting-linux-kernel]]
5271 === LTTng kernel tracepoints
5272
5273 NOTE: This section shows how to _add_ instrumentation points to the
5274 Linux kernel. The subsystems of the kernel are already thoroughly
5275 instrumented at strategic places for LTTng when you
5276 <<installing-lttng,install>> the <<lttng-modules,LTTng-modules>>
5277 package.
5278
5279 ////
5280 There are two methods to instrument the Linux kernel:
5281
5282 . <<linux-add-lttng-layer,Add an LTTng layer>> over an existing ftrace
5283   tracepoint which uses the `TRACE_EVENT()` API.
5284 +
5285 Choose this if you want to instrumentation a Linux kernel tree with an
5286 instrumentation point compatible with ftrace, perf, and SystemTap.
5287
5288 . Use an <<linux-lttng-tracepoint-event,LTTng-only approach>> to
5289   instrument an out-of-tree kernel module.
5290 +
5291 Choose this if you don't need ftrace, perf, or SystemTap support.
5292 ////
5293
5294
5295 [[linux-add-lttng-layer]]
5296 ==== [[instrumenting-linux-kernel-itself]][[mainline-trace-event]][[lttng-adaptation-layer]]Add an LTTng layer to an existing ftrace tracepoint
5297
5298 This section shows how to add an LTTng layer to existing ftrace
5299 instrumentation using the `TRACE_EVENT()` API.
5300
5301 This section doesn't document the `TRACE_EVENT()` macro. Read the
5302 following articles to learn more about this API:
5303
5304 * http://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part{nbsp}1)]
5305 * http://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part{nbsp}2)]
5306 * http://lwn.net/Articles/383362/[Using the TRACE_EVENT() macro (Part{nbsp}3)]
5307
5308 The following procedure assumes that your ftrace tracepoints are
5309 correctly defined in their own header and that they are created in
5310 one source file using the `CREATE_TRACE_POINTS` definition.
5311
5312 To add an LTTng layer over an existing ftrace tracepoint:
5313
5314 . Make sure the following kernel configuration options are
5315   enabled:
5316 +
5317 --
5318 * `CONFIG_MODULES`
5319 * `CONFIG_KALLSYMS`
5320 * `CONFIG_HIGH_RES_TIMERS`
5321 * `CONFIG_TRACEPOINTS`
5322 --
5323
5324 . Build the Linux source tree with your custom ftrace tracepoints.
5325 . Boot the resulting Linux image on your target system.
5326 +
5327 Confirm that the tracepoints exist by looking for their names in the
5328 dir:{/sys/kernel/debug/tracing/events/subsys} directory, where `subsys`
5329 is your subsystem name.
5330
5331 . Get a copy of the latest LTTng-modules{nbsp}{revision}:
5332 +
5333 --
5334 [role="term"]
5335 ----
5336 $ cd $(mktemp -d) &&
5337 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.12.tar.bz2 &&
5338 tar -xf lttng-modules-latest-2.12.tar.bz2 &&
5339 cd lttng-modules-2.12.*
5340 ----
5341 --
5342
5343 . In dir:{instrumentation/events/lttng-module}, relative to the root
5344   of the LTTng-modules source tree, create a header file named
5345   +__subsys__.h+ for your custom subsystem +__subsys__+ and write your
5346   LTTng-modules tracepoint definitions using the LTTng-modules
5347   macros in it.
5348 +
5349 Start with this template:
5350 +
5351 --
5352 [source,c]
5353 .path:{instrumentation/events/lttng-module/my_subsys.h}
5354 ----
5355 #undef TRACE_SYSTEM
5356 #define TRACE_SYSTEM my_subsys
5357
5358 #if !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ)
5359 #define _LTTNG_MY_SUBSYS_H
5360
5361 #include "../../../probes/lttng-tracepoint-event.h"
5362 #include <linux/tracepoint.h>
5363
5364 LTTNG_TRACEPOINT_EVENT(
5365     /*
5366      * Format is identical to the TRACE_EVENT() version for the three
5367      * following macro parameters:
5368      */
5369     my_subsys_my_event,
5370     TP_PROTO(int my_int, const char *my_string),
5371     TP_ARGS(my_int, my_string),
5372
5373     /* LTTng-modules specific macros */
5374     TP_FIELDS(
5375         ctf_integer(int, my_int_field, my_int)
5376         ctf_string(my_bar_field, my_bar)
5377     )
5378 )
5379
5380 #endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */
5381
5382 #include "../../../probes/define_trace.h"
5383 ----
5384 --
5385 +
5386 The entries in the `TP_FIELDS()` section are the list of fields for the
5387 LTTng tracepoint. This is similar to the `TP_STRUCT__entry()` part of
5388 the `TRACE_EVENT()` ftrace macro.
5389 +
5390 See <<lttng-modules-tp-fields,Tracepoint fields macros>> for a
5391 complete description of the available `ctf_*()` macros.
5392
5393 . Create the kernel module C{nbsp}source file of the LTTng-modules
5394   probe, +probes/lttng-probe-__subsys__.c+, where +__subsys__+ is your
5395   subsystem name:
5396 +
5397 --
5398 [source,c]
5399 .path:{probes/lttng-probe-my-subsys.c}
5400 ----
5401 #include <linux/module.h>
5402 #include "../lttng-tracer.h"
5403
5404 /*
5405  * Build-time verification of mismatch between mainline
5406  * TRACE_EVENT() arguments and the LTTng-modules adaptation
5407  * layer LTTNG_TRACEPOINT_EVENT() arguments.
5408  */
5409 #include <trace/events/my_subsys.h>
5410
5411 /* Create LTTng tracepoint probes */
5412 #define LTTNG_PACKAGE_BUILD
5413 #define CREATE_TRACE_POINTS
5414 #define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module
5415
5416 #include "../instrumentation/events/lttng-module/my_subsys.h"
5417
5418 MODULE_LICENSE("GPL and additional rights");
5419 MODULE_AUTHOR("Your name <your-email>");
5420 MODULE_DESCRIPTION("LTTng my_subsys probes");
5421 MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "."
5422     __stringify(LTTNG_MODULES_MINOR_VERSION) "."
5423     __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION)
5424     LTTNG_MODULES_EXTRAVERSION);
5425 ----
5426 --
5427
5428 . Edit path:{probes/KBuild} and add your new kernel module object
5429   next to the existing ones:
5430 +
5431 --
5432 [source,make]
5433 .path:{probes/KBuild}
5434 ----
5435 # ...
5436
5437 obj-m += lttng-probe-module.o
5438 obj-m += lttng-probe-power.o
5439
5440 obj-m += lttng-probe-my-subsys.o
5441
5442 # ...
5443 ----
5444 --
5445
5446 . Build and install the LTTng kernel modules:
5447 +
5448 --
5449 [role="term"]
5450 ----
5451 $ make KERNELDIR=/path/to/linux
5452 # make modules_install && depmod -a
5453 ----
5454 --
5455 +
5456 Replace `/path/to/linux` with the path to the Linux source tree where
5457 you defined and used tracepoints with the `TRACE_EVENT()` ftrace macro.
5458
5459 Note that you can also use the
5460 <<lttng-tracepoint-event-code,`LTTNG_TRACEPOINT_EVENT_CODE()` macro>>
5461 instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and
5462 C code that need to be executed before the event fields are recorded.
5463
5464 The best way to learn how to use the previous LTTng-modules macros is to
5465 inspect the existing LTTng-modules tracepoint definitions in the
5466 dir:{instrumentation/events/lttng-module} header files. Compare them
5467 with the Linux kernel mainline versions in the
5468 dir:{include/trace/events} directory of the Linux source tree.
5469
5470
5471 [role="since-2.7"]
5472 [[lttng-tracepoint-event-code]]
5473 ===== Use custom C code to access the data for tracepoint fields
5474
5475 Although we recommended to always use the
5476 <<lttng-adaptation-layer,`LTTNG_TRACEPOINT_EVENT()`>> macro to describe
5477 the arguments and fields of an LTTng-modules tracepoint when possible,
5478 sometimes you need a more complex process to access the data that the
5479 tracer records as event record fields. In other words, you need local
5480 variables and multiple C{nbsp}statements instead of simple
5481 argument-based expressions that you pass to the
5482 <<lttng-modules-tp-fields,`ctf_*()` macros of `TP_FIELDS()`>>.
5483
5484 Use the `LTTNG_TRACEPOINT_EVENT_CODE()` macro instead of
5485 `LTTNG_TRACEPOINT_EVENT()` to declare custom local variables and define
5486 a block of C{nbsp}code to be executed before LTTng records the fields.
5487 The structure of this macro is:
5488
5489 [source,c]
5490 .`LTTNG_TRACEPOINT_EVENT_CODE()` macro syntax.
5491 ----
5492 LTTNG_TRACEPOINT_EVENT_CODE(
5493     /*
5494      * Format identical to the LTTNG_TRACEPOINT_EVENT()
5495      * version for the following three macro parameters:
5496      */
5497     my_subsys_my_event,
5498     TP_PROTO(int my_int, const char *my_string),
5499     TP_ARGS(my_int, my_string),
5500
5501     /* Declarations of custom local variables */
5502     TP_locvar(
5503         int a = 0;
5504         unsigned long b = 0;
5505         const char *name = "(undefined)";
5506         struct my_struct *my_struct;
5507     ),
5508
5509     /*
5510      * Custom code which uses both tracepoint arguments
5511      * (in TP_ARGS()) and local variables (in TP_locvar()).
5512      *
5513      * Local variables are actually members of a structure pointed
5514      * to by the special variable tp_locvar.
5515      */
5516     TP_code(
5517         if (my_int) {
5518             tp_locvar->a = my_int + 17;
5519             tp_locvar->my_struct = get_my_struct_at(tp_locvar->a);
5520             tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct);
5521             tp_locvar->name = my_struct_get_name(tp_locvar->my_struct);
5522             put_my_struct(tp_locvar->my_struct);
5523
5524             if (tp_locvar->b) {
5525                 tp_locvar->a = 1;
5526             }
5527         }
5528     ),
5529
5530     /*
5531      * Format identical to the LTTNG_TRACEPOINT_EVENT()
5532      * version for this, except that tp_locvar members can be
5533      * used in the argument expression parameters of
5534      * the ctf_*() macros.
5535      */
5536     TP_FIELDS(
5537         ctf_integer(unsigned long, my_struct_b, tp_locvar->b)
5538         ctf_integer(int, my_struct_a, tp_locvar->a)
5539         ctf_string(my_string_field, my_string)
5540         ctf_string(my_struct_name, tp_locvar->name)
5541     )
5542 )
5543 ----
5544
5545 IMPORTANT: The C code defined in `TP_code()` must not have any side
5546 effects when executed. In particular, the code must not allocate
5547 memory or get resources without deallocating this memory or putting
5548 those resources afterwards.
5549
5550
5551 [[instrumenting-linux-kernel-tracing]]
5552 ==== Load and unload a custom probe kernel module
5553
5554 You must load a <<lttng-adaptation-layer,created LTTng-modules probe
5555 kernel module>> in the kernel before it can emit LTTng events.
5556
5557 To load the default probe kernel modules and a custom probe kernel
5558 module:
5559
5560 * Use the opt:lttng-sessiond(8):--extra-kmod-probes option to give extra
5561   probe modules to load when starting a root <<lttng-sessiond,session
5562   daemon>>:
5563 +
5564 --
5565 .Load the `my_subsys`, `usb`, and the default probe modules.
5566 ====
5567 [role="term"]
5568 ----
5569 # lttng-sessiond --extra-kmod-probes=my_subsys,usb
5570 ----
5571 ====
5572 --
5573 +
5574 You only need to pass the subsystem name, not the whole kernel module
5575 name.
5576
5577 To load _only_ a given custom probe kernel module:
5578
5579 * Use the opt:lttng-sessiond(8):--kmod-probes option to give the probe
5580   modules to load when starting a root session daemon:
5581 +
5582 --
5583 .Load only the `my_subsys` and `usb` probe modules.
5584 ====
5585 [role="term"]
5586 ----
5587 # lttng-sessiond --kmod-probes=my_subsys,usb
5588 ----
5589 ====
5590 --
5591
5592 To confirm that a probe module is loaded:
5593
5594 * Use man:lsmod(8):
5595 +
5596 --
5597 [role="term"]
5598 ----
5599 $ lsmod | grep lttng_probe_usb
5600 ----
5601 --
5602
5603 To unload the loaded probe modules:
5604
5605 * Kill the session daemon with `SIGTERM`:
5606 +
5607 --
5608 [role="term"]
5609 ----
5610 # pkill lttng-sessiond
5611 ----
5612 --
5613 +
5614 You can also use the man:modprobe(8) `--remove` option if the session
5615 daemon terminates abnormally.
5616
5617
5618 [[controlling-tracing]]
5619 == Tracing control
5620
5621 Once an application or a Linux kernel is
5622 <<instrumenting,instrumented>> for LTTng tracing,
5623 you can _trace_ it.
5624
5625 This section is divided in topics on how to use the various
5626 <<plumbing,components of LTTng>>, in particular the <<lttng-cli,cmd:lttng
5627 command-line tool>>, to _control_ the LTTng daemons and tracers.
5628
5629 NOTE: In the following subsections, we refer to an man:lttng(1) command
5630 using its man page name. For example, instead of _Run the `create`
5631 command to..._, we use _Run the man:lttng-create(1) command to..._.
5632
5633
5634 [[start-sessiond]]
5635 === Start a session daemon
5636
5637 In some situations, you need to run a <<lttng-sessiond,session daemon>>
5638 (man:lttng-sessiond(8)) _before_ you can use the man:lttng(1)
5639 command-line tool.
5640
5641 You will see the following error when you run a command while no session
5642 daemon is running:
5643
5644 ----
5645 Error: No session daemon is available
5646 ----
5647
5648 The only command that automatically runs a session daemon is
5649 man:lttng-create(1), which you use to
5650 <<creating-destroying-tracing-sessions,create a tracing session>>. While
5651 this is most of the time the first operation that you do, sometimes it's
5652 not. Some examples are:
5653
5654 * <<list-instrumentation-points,List the available instrumentation points>>.
5655 * <<saving-loading-tracing-session,Load a tracing session configuration>>.
5656
5657 [[tracing-group]] Each Unix user must have its own running session
5658 daemon to trace user applications. The session daemon that the root user
5659 starts is the only one allowed to control the LTTng kernel tracer. Users
5660 that are part of the _tracing group_ can control the root session
5661 daemon. The default tracing group name is `tracing`; set it to something
5662 else with the opt:lttng-sessiond(8):--group option when you start the
5663 root session daemon.
5664
5665 To start a user session daemon:
5666
5667 * Run man:lttng-sessiond(8):
5668 +
5669 --
5670 [role="term"]
5671 ----
5672 $ lttng-sessiond --daemonize
5673 ----
5674 --
5675
5676 To start the root session daemon:
5677
5678 * Run man:lttng-sessiond(8) as the root user:
5679 +
5680 --
5681 [role="term"]
5682 ----
5683 # lttng-sessiond --daemonize
5684 ----
5685 --
5686
5687 In both cases, remove the opt:lttng-sessiond(8):--daemonize option to
5688 start the session daemon in foreground.
5689
5690 To stop a session daemon, use man:kill(1) on its process ID (standard
5691 `TERM` signal).
5692
5693 Note that some Linux distributions could manage the LTTng session daemon
5694 as a service. In this case, you should use the service manager to
5695 start, restart, and stop session daemons.
5696
5697
5698 [[creating-destroying-tracing-sessions]]
5699 === Create and destroy a tracing session
5700
5701 Almost all the LTTng control operations happen in the scope of
5702 a <<tracing-session,tracing session>>, which is the dialogue between the
5703 <<lttng-sessiond,session daemon>> and you.
5704
5705 To create a tracing session with a generated name:
5706
5707 * Use the man:lttng-create(1) command:
5708 +
5709 --
5710 [role="term"]
5711 ----
5712 $ lttng create
5713 ----
5714 --
5715
5716 The name of the created tracing session is `auto` followed by the
5717 creation date.
5718
5719 To create a tracing session with a specific name:
5720
5721 * Use the optional argument of the man:lttng-create(1) command:
5722 +
5723 --
5724 [role="term"]
5725 ----
5726 $ lttng create my-session
5727 ----
5728 --
5729 +
5730 Replace `my-session` with the specific tracing session name.
5731
5732 LTTng appends the creation date to the name of the created tracing
5733 session.
5734
5735 LTTng writes the traces of a tracing session in
5736 +$LTTNG_HOME/lttng-trace/__name__+ by default, where +__name__+ is the
5737 name of the tracing session. Note that the env:LTTNG_HOME environment
5738 variable defaults to `$HOME` if not set.
5739
5740 To output LTTng traces to a non-default location:
5741
5742 * Use the opt:lttng-create(1):--output option of the man:lttng-create(1) command:
5743 +
5744 --
5745 [role="term"]
5746 ----
5747 $ lttng create my-session --output=/tmp/some-directory
5748 ----
5749 --
5750
5751 You may create as many tracing sessions as you wish.
5752
5753 To list all the existing tracing sessions for your Unix user:
5754
5755 * Use the man:lttng-list(1) command:
5756 +
5757 --
5758 [role="term"]
5759 ----
5760 $ lttng list
5761 ----
5762 --
5763
5764 [[cur-tracing-session]]When you create a tracing session, it is set as
5765 the _current tracing session_. The following man:lttng(1) commands
5766 operate on the current tracing session when you don't specify one:
5767
5768 [role="list-3-cols"]
5769 * man:lttng-add-context(1)
5770 * man:lttng-clear(1)
5771 * man:lttng-destroy(1)
5772 * man:lttng-disable-channel(1)
5773 * man:lttng-disable-event(1)
5774 * man:lttng-disable-rotation(1)
5775 * man:lttng-enable-channel(1)
5776 * man:lttng-enable-event(1)
5777 * man:lttng-enable-rotation(1)
5778 * man:lttng-load(1)
5779 * man:lttng-regenerate(1)
5780 * man:lttng-rotate(1)
5781 * man:lttng-save(1)
5782 * man:lttng-snapshot(1)
5783 * man:lttng-start(1)
5784 * man:lttng-status(1)
5785 * man:lttng-stop(1)
5786 * man:lttng-track(1)
5787 * man:lttng-untrack(1)
5788 * man:lttng-view(1)
5789
5790 To change the current tracing session:
5791
5792 * Use the man:lttng-set-session(1) command:
5793 +
5794 --
5795 [role="term"]
5796 ----
5797 $ lttng set-session new-session
5798 ----
5799 --
5800 +
5801 Replace `new-session` by the name of the new current tracing session.
5802
5803 When you're done tracing in a given tracing session, destroy it. This
5804 operation frees the resources taken by the tracing session to destroy;
5805 it doesn't destroy the trace data that LTTng wrote for this tracing
5806 session (see <<clear,Clear a tracing session>> for one way to do this).
5807
5808 To destroy the current tracing session:
5809
5810 * Use the man:lttng-destroy(1) command:
5811 +
5812 --
5813 [role="term"]
5814 ----
5815 $ lttng destroy
5816 ----
5817 --
5818
5819 The man:lttng-destroy(1) command also runs the man:lttng-stop(1)
5820 command implicitly (see <<basic-tracing-session-control,Start and stop a
5821 tracing session>>). You need to stop tracing to make LTTng flush the
5822 remaining trace data and make the trace readable.
5823
5824
5825 [[list-instrumentation-points]]
5826 === List the available instrumentation points
5827
5828 The <<lttng-sessiond,session daemon>> can query the running instrumented
5829 user applications and the Linux kernel to get a list of available
5830 instrumentation points. For the Linux kernel <<domain,tracing domain>>,
5831 they are tracepoints and system calls. For the user space tracing
5832 domain, they are tracepoints. For the other tracing domains, they are
5833 logger names.
5834
5835 To list the available instrumentation points:
5836
5837 * Use the man:lttng-list(1) command with the option of the requested
5838   tracing domain amongst:
5839 +
5840 --
5841 opt:lttng-list(1):--kernel::
5842     Linux kernel tracepoints (your Unix user must be a root user, or it
5843     must be a member of the <<tracing-group,tracing group>>).
5844
5845 opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall::
5846     Linux kernel system calls (your Unix user must be a root user, or it
5847     must be a member of the tracing group).
5848
5849 opt:lttng-list(1):--userspace::
5850     User space tracepoints.
5851
5852 opt:lttng-list(1):--jul::
5853    `java.util.logging` loggers.
5854
5855 opt:lttng-list(1):--log4j::
5856     Apache log4j loggers.
5857
5858 opt:lttng-list(1):--python::
5859     Python loggers.
5860 --
5861
5862 .List the available user space tracepoints.
5863 ====
5864 [role="term"]
5865 ----
5866 $ lttng list --userspace
5867 ----
5868 ====
5869
5870 .List the available Linux kernel system call tracepoints.
5871 ====
5872 [role="term"]
5873 ----
5874 $ lttng list --kernel --syscall
5875 ----
5876 ====
5877
5878
5879 [[enabling-disabling-events]]
5880 === Create and enable an event rule
5881
5882 Once you <<creating-destroying-tracing-sessions,create a tracing
5883 session>>, you can create <<event,event rules>> with the
5884 man:lttng-enable-event(1) command.
5885
5886 You specify each condition with a command-line option. The available
5887 condition arguments are shown in the following table.
5888
5889 [role="growable",cols="asciidoc,asciidoc,default"]
5890 .Condition command-line arguments for the man:lttng-enable-event(1) command.
5891 |====
5892 |Argument |Description |Applicable tracing domains
5893
5894 |
5895 One of:
5896
5897 . `--syscall`
5898 . +--probe=__ADDR__+
5899 . +--function=__ADDR__+
5900 . +--userspace-probe=__PATH__:__SYMBOL__+
5901 . +--userspace-probe=sdt:__PATH__:__PROVIDER__:__NAME__+
5902
5903 |
5904 Instead of using the default _tracepoint_ instrumentation type, use:
5905
5906 . A Linux system call (entry and exit).
5907 . A Linux https://lwn.net/Articles/132196/[kprobe] (symbol or address).
5908 . The entry and return points of a Linux function (symbol or address).
5909 . The entry point of a user application or library function (path to
5910   application/library and symbol).
5911 . A https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[SystemTap
5912   Statically Defined Tracing] (USDT) probe (path to application/library,
5913   provider and probe names).
5914
5915 |Linux kernel.
5916
5917 |First positional argument.
5918
5919 |
5920 Tracepoint or system call name.
5921
5922 With the opt:lttng-enable-event(1):--probe,
5923 opt:lttng-enable-event(1):--function, and
5924 opt:lttng-enable-event(1):--userspace-probe options, this is a custom
5925 name given to the event rule. With the JUL, log4j, and Python domains,
5926 this is a logger name.
5927
5928 With a tracepoint, logger, or system call name, use the special
5929 `*` globbing character to match anything (for example, `sched_*`,
5930 `my_comp*:*msg_*`).
5931
5932 |All.
5933
5934 |
5935 One of:
5936
5937 . +--loglevel=__LEVEL__+
5938 . +--loglevel-only=__LEVEL__+
5939
5940 |
5941 . Match only tracepoints or log statements with a logging level at
5942   least as severe as +__LEVEL__+.
5943 . Match only tracepoints or log statements with a logging level
5944   equal to +__LEVEL__+.
5945
5946 See man:lttng-enable-event(1) for the list of available logging level
5947 names.
5948
5949 |User space, JUL, log4j, and Python.
5950
5951 |+--exclude=__EXCLUSIONS__+
5952
5953 |
5954 When you use a `*` character at the end of the tracepoint or logger
5955 name (first positional argument), exclude the specific names in the
5956 comma-delimited list +__EXCLUSIONS__+.
5957
5958 |
5959 User space, JUL, log4j, and Python.
5960
5961 |+--filter=__EXPR__+
5962
5963 |
5964 Match only events which satisfy the expression +__EXPR__+.
5965
5966 See man:lttng-enable-event(1) to learn more about the syntax of a
5967 filter expression.
5968
5969 |All.
5970
5971 |====
5972
5973 You attach an event rule to a <<channel,channel>> on creation. If you do
5974 not specify the channel with the opt:lttng-enable-event(1):--channel
5975 option, and if the event rule to create is the first in its
5976 <<domain,tracing domain>> for a given tracing session, then LTTng
5977 creates a _default channel_ for you. This default channel is reused in
5978 subsequent invocations of the man:lttng-enable-event(1) command for the
5979 same tracing domain.
5980
5981 An event rule is always enabled at creation time.
5982
5983 The following examples show how to combine the previous
5984 command-line options to create simple to more complex event rules.
5985
5986 .Create an event rule targetting a Linux kernel tracepoint (default channel).
5987 ====
5988 [role="term"]
5989 ----
5990 $ lttng enable-event --kernel sched_switch
5991 ----
5992 ====
5993
5994 .Create an event rule matching four Linux kernel system calls (default channel).
5995 ====
5996 [role="term"]
5997 ----
5998 $ lttng enable-event --kernel --syscall open,write,read,close
5999 ----
6000 ====
6001
6002 .Create event rules matching tracepoints with filter expressions (default channel).
6003 ====
6004 [role="term"]
6005 ----
6006 $ lttng enable-event --kernel sched_switch --filter='prev_comm == "bash"'
6007 ----
6008
6009 [role="term"]
6010 ----
6011 $ lttng enable-event --kernel --all \
6012                    --filter='$ctx.tid == 1988 || $ctx.tid == 1534'
6013 ----
6014
6015 [role="term"]
6016 ----
6017 $ lttng enable-event --jul my_logger \
6018                    --filter='$app.retriever:cur_msg_id > 3'
6019 ----
6020
6021 IMPORTANT: Make sure to always quote the filter string when you
6022 use man:lttng(1) from a shell.
6023
6024 See also <<pid-tracking,Track process attributes>> which offers another,
6025 more efficient filtering mechanism for process ID, user ID, and group
6026 ID attributes.
6027 ====
6028
6029 .Create an event rule matching any user space tracepoint of a given tracepoint provider with a log level range (default channel).
6030 ====
6031 [role="term"]
6032 ----
6033 $ lttng enable-event --userspace my_app:'*' --loglevel=TRACE_INFO
6034 ----
6035
6036 IMPORTANT: Make sure to always quote the wildcard character when you
6037 use man:lttng(1) from a shell.
6038 ====
6039
6040 .Create an event rule matching multiple Python loggers with a wildcard and with exclusions (default channel).
6041 ====
6042 [role="term"]
6043 ----
6044 $ lttng enable-event --python my-app.'*' \
6045                    --exclude='my-app.module,my-app.hello'
6046 ----
6047 ====
6048
6049 .Create an event rule matching any Apache log4j logger with a specific log level (default channel).
6050 ====
6051 [role="term"]
6052 ----
6053 $ lttng enable-event --log4j --all --loglevel-only=LOG4J_WARN
6054 ----
6055 ====
6056
6057 .Create an event rule attached to a specific channel matching a specific user space tracepoint provider and tracepoint.
6058 ====
6059 [role="term"]
6060 ----
6061 $ lttng enable-event --userspace my_app:my_tracepoint --channel=my-channel
6062 ----
6063 ====
6064
6065 .Create an event rule matching the `malloc` function entry in path:{/usr/lib/libc.so.6}:
6066 ====
6067 [role="term"]
6068 ----
6069 $ lttng enable-event --kernel --userspace-probe=/usr/lib/libc.so.6:malloc \
6070                    libc_malloc
6071 ----
6072 ====
6073
6074 .Create an event rule matching the `server`/`accept_request` https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[USDT probe] in path:{/usr/bin/serv}:
6075 ====
6076 [role="term"]
6077 ----
6078 $ lttng enable-event --kernel --userspace-probe=sdt:serv:server:accept_request \
6079                   server_accept_request
6080 ----
6081 ====
6082
6083 The event rules of a given channel form a whitelist: as soon as an
6084 emitted event passes one of them, LTTng can record the event. For
6085 example, an event named `my_app:my_tracepoint` emitted from a user space
6086 tracepoint with a `TRACE_ERROR` log level passes both of the following
6087 rules:
6088
6089 [role="term"]
6090 ----
6091 $ lttng enable-event --userspace my_app:my_tracepoint
6092 $ lttng enable-event --userspace my_app:my_tracepoint \
6093                    --loglevel=TRACE_INFO
6094 ----
6095
6096 The second event rule is redundant: the first one includes
6097 the second one.
6098
6099
6100 [[disable-event-rule]]
6101 === Disable an event rule
6102
6103 To disable an event rule that you <<enabling-disabling-events,created>>
6104 previously, use the man:lttng-disable-event(1) command. This command
6105 disables _all_ the event rules (of a given tracing domain and channel)
6106 which match an instrumentation point. The other conditions aren't
6107 supported as of LTTng{nbsp}{revision}.
6108
6109 The LTTng tracer doesn't record an emitted event which passes
6110 a _disabled_ event rule.
6111
6112 .Disable an event rule matching a Python logger (default channel).
6113 ====
6114 [role="term"]
6115 ----
6116 $ lttng disable-event --python my-logger
6117 ----
6118 ====
6119
6120 .Disable an event rule matching all `java.util.logging` loggers (default channel).
6121 ====
6122 [role="term"]
6123 ----
6124 $ lttng disable-event --jul '*'
6125 ----
6126 ====
6127
6128 .Disable _all_ the event rules of the default channel.
6129 ====
6130 The opt:lttng-disable-event(1):--all-events option isn't, like the
6131 opt:lttng-enable-event(1):--all option of man:lttng-enable-event(1), the
6132 equivalent of the event name `*` (wildcard): it disables _all_ the event
6133 rules of a given channel.
6134
6135 [role="term"]
6136 ----
6137 $ lttng disable-event --jul --all-events
6138 ----
6139 ====
6140
6141 NOTE: You can't delete an event rule once you create it.
6142
6143
6144 [[status]]
6145 === Get the status of a tracing session
6146
6147 To get the status of the <<cur-tracing-session,current tracing
6148 session>>, that is, its parameters, its channels, event rules, and their
6149 attributes:
6150
6151 * Use the man:lttng-status(1) command:
6152 +
6153 --
6154 [role="term"]
6155 ----
6156 $ lttng status
6157 ----
6158 --
6159
6160 To get the status of any tracing session:
6161
6162 * Use the man:lttng-list(1) command with the name of the tracing
6163   session:
6164 +
6165 --
6166 [role="term"]
6167 ----
6168 $ lttng list my-session
6169 ----
6170 --
6171 +
6172 Replace `my-session` with the desired tracing session name.
6173
6174
6175 [[basic-tracing-session-control]]
6176 === Start and stop a tracing session
6177
6178 Once you <<creating-destroying-tracing-sessions,create a tracing
6179 session>> and
6180 <<enabling-disabling-events,create one or more event rules>>,
6181 you can start and stop the tracers for this tracing session.
6182
6183 To start tracing in the <<cur-tracing-session,current tracing session>>:
6184
6185 * Use the man:lttng-start(1) command:
6186 +
6187 --
6188 [role="term"]
6189 ----
6190 $ lttng start
6191 ----
6192 --
6193
6194 LTTng is very flexible: you can launch user applications before
6195 or after the you start the tracers. The tracers only record the events
6196 if they pass enabled event rules and if they occur while the tracers are
6197 started.
6198
6199 To stop tracing in the current tracing session:
6200
6201 * Use the man:lttng-stop(1) command:
6202 +
6203 --
6204 [role="term"]
6205 ----
6206 $ lttng stop
6207 ----
6208 --
6209 +
6210 If there were <<channel-overwrite-mode-vs-discard-mode,lost event
6211 records>> or lost sub-buffers since the last time you ran
6212 man:lttng-start(1), warnings are printed when you run the
6213 man:lttng-stop(1) command.
6214
6215 IMPORTANT: You need to stop tracing to make LTTng flush the remaining
6216 trace data and make the trace readable. Note that the
6217 man:lttng-destroy(1) command (see
6218 <<creating-destroying-tracing-sessions,Create and destroy a tracing
6219 session>>) also runs the man:lttng-stop(1) command implicitly.
6220
6221 [role="since-2.12"]
6222 [[clear]]
6223 === Clear a tracing session
6224
6225 You might need to remove all the current tracing data of one or more
6226 <<tracing-session,tracing sessions>> between multiple attempts to
6227 reproduce a problem without interrupting the LTTng tracing activity.
6228
6229 To clear the tracing data of the
6230 <<cur-tracing-session,current tracing session>>:
6231
6232 * Use the man:lttng-clear(1) command:
6233 +
6234 --
6235 [role="term"]
6236 ----
6237 $ lttng clear
6238 ----
6239 --
6240
6241 To clear the tracing data of all the tracing sessions:
6242
6243 * Use the `lttng clear` command with the opt:lttng-clear(1):--all
6244   option:
6245 +
6246 --
6247 [role="term"]
6248 ----
6249 $ lttng clear --all
6250 ----
6251 --
6252
6253
6254 [[enabling-disabling-channels]]
6255 === Create a channel
6256
6257 Once you create a tracing session, you can create a <<channel,channel>>
6258 with the man:lttng-enable-channel(1) command.
6259
6260 Note that LTTng automatically creates a default channel when, for a
6261 given <<domain,tracing domain>>, no channels exist and you
6262 <<enabling-disabling-events,create>> the first event rule. This default
6263 channel is named `channel0` and its attributes are set to reasonable
6264 values. Therefore, you only need to create a channel when you need
6265 non-default attributes.
6266
6267 You specify each non-default channel attribute with a command-line
6268 option when you use the man:lttng-enable-channel(1) command. The
6269 available command-line options are:
6270
6271 [role="growable",cols="asciidoc,asciidoc"]
6272 .Command-line options for the man:lttng-enable-channel(1) command.
6273 |====
6274 |Option |Description
6275
6276 |`--overwrite`
6277
6278 |
6279 Use the _overwrite_
6280 <<channel-overwrite-mode-vs-discard-mode,event record loss mode>> instead
6281 of the default _discard_ mode.
6282
6283 |`--buffers-pid` (user space tracing domain only)
6284
6285 |
6286 Use the per-process <<channel-buffering-schemes,buffering scheme>>
6287 instead of the default per-user buffering scheme.
6288
6289 |+--subbuf-size=__SIZE__+
6290
6291 |
6292 Allocate sub-buffers of +__SIZE__+ bytes (power of two), for each CPU,
6293 either for each Unix user (default), or for each instrumented process.
6294
6295 See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
6296
6297 |+--num-subbuf=__COUNT__+
6298
6299 |
6300 Allocate +__COUNT__+ sub-buffers (power of two), for each CPU, either
6301 for each Unix user (default), or for each instrumented process.
6302
6303 See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
6304
6305 |+--tracefile-size=__SIZE__+
6306
6307 |
6308 Set the maximum size of each trace file that this channel writes within
6309 a stream to +__SIZE__+ bytes instead of no maximum.
6310
6311 See <<tracefile-rotation,Trace file count and size>>.
6312
6313 |+--tracefile-count=__COUNT__+
6314
6315 |
6316 Limit the number of trace files that this channel creates to
6317 +__COUNT__+ channels instead of no limit.
6318
6319 See <<tracefile-rotation,Trace file count and size>>.
6320
6321 |+--switch-timer=__PERIODUS__+
6322
6323 |
6324 Set the <<channel-switch-timer,switch timer period>>
6325 to +__PERIODUS__+{nbsp}µs.
6326
6327 |+--read-timer=__PERIODUS__+
6328
6329 |
6330 Set the <<channel-read-timer,read timer period>>
6331 to +__PERIODUS__+{nbsp}µs.
6332
6333 |[[opt-blocking-timeout]]+--blocking-timeout=__TIMEOUTUS__+
6334
6335 |
6336 Set the timeout of user space applications which load LTTng-UST
6337 in blocking mode to +__TIMEOUTUS__+:
6338
6339 0 (default)::
6340     Never block (non-blocking mode).
6341
6342 `inf`::
6343     Block forever until space is available in a sub-buffer to record
6344     the event.
6345
6346 __n__, a positive value::
6347     Wait for at most __n__ µs when trying to write into a sub-buffer.
6348
6349 Note that, for this option to have any effect on an instrumented
6350 user space application, you need to run the application with a set
6351 env:LTTNG_UST_ALLOW_BLOCKING environment variable.
6352
6353 |+--output=__TYPE__+ (Linux kernel tracing domain only)
6354
6355 |
6356 Set the output type of the channel to +__TYPE__+, either `mmap` or
6357 `splice`.
6358
6359 |====
6360
6361 You can only create a channel in the Linux kernel and user space
6362 <<domain,tracing domains>>: other tracing domains have their own channel
6363 created on the fly when <<enabling-disabling-events,creating event
6364 rules>>.
6365
6366 [IMPORTANT]
6367 ====
6368 Because of a current LTTng limitation, you must create all channels
6369 _before_ you <<basic-tracing-session-control,start tracing>> in a given
6370 tracing session, that is, before the first time you run
6371 man:lttng-start(1).
6372
6373 Since LTTng automatically creates a default channel when you use the
6374 man:lttng-enable-event(1) command with a specific tracing domain, you
6375 can't, for example, create a Linux kernel event rule, start tracing,
6376 and then create a user space event rule, because no user space channel
6377 exists yet and it's too late to create one.
6378
6379 For this reason, make sure to configure your channels properly
6380 before starting the tracers for the first time!
6381 ====
6382
6383 The following examples show how to combine the previous
6384 command-line options to create simple to more complex channels.
6385
6386 .Create a Linux kernel channel with default attributes.
6387 ====
6388 [role="term"]
6389 ----
6390 $ lttng enable-channel --kernel my-channel
6391 ----
6392 ====
6393
6394 .Create a user space channel with four sub-buffers or 1{nbsp}MiB each, per CPU, per instrumented process.
6395 ====
6396 [role="term"]
6397 ----
6398 $ lttng enable-channel --userspace --num-subbuf=4 --subbuf-size=1M \
6399                      --buffers-pid my-channel
6400 ----
6401 ====
6402
6403 .[[blocking-timeout-example]]Create a default user space channel with an infinite blocking timeout.
6404 ====
6405 <<creating-destroying-tracing-sessions,Create a tracing-session>>,
6406 create the channel, <<enabling-disabling-events,create an event rule>>,
6407 and <<basic-tracing-session-control,start tracing>>:
6408
6409 [role="term"]
6410 ----
6411 $ lttng create
6412 $ lttng enable-channel --userspace --blocking-timeout=inf blocking-channel
6413 $ lttng enable-event --userspace --channel=blocking-channel --all
6414 $ lttng start
6415 ----
6416
6417 Run an application instrumented with LTTng-UST and allow it to block:
6418
6419 [role="term"]
6420 ----
6421 $ LTTNG_UST_ALLOW_BLOCKING=1 my-app
6422 ----
6423 ====
6424
6425 .Create a Linux kernel channel which rotates eight trace files of 4{nbsp}MiB each for each stream
6426 ====
6427 [role="term"]
6428 ----
6429 $ lttng enable-channel --kernel --tracefile-count=8 \
6430                      --tracefile-size=4194304 my-channel
6431 ----
6432 ====
6433
6434 .Create a user space channel in overwrite (or _flight recorder_) mode.
6435 ====
6436 [role="term"]
6437 ----
6438 $ lttng enable-channel --userspace --overwrite my-channel
6439 ----
6440 ====
6441
6442 <<enabling-disabling-events,Create>> the same event rule in
6443 two different channels:
6444
6445 [role="term"]
6446 ----
6447 $ lttng enable-event --userspace --channel=my-channel app:tp
6448 $ lttng enable-event --userspace --channel=other-channel app:tp
6449 ----
6450
6451 If both channels are enabled, when a tracepoint named `app:tp` is
6452 reached, LTTng records two events, one for each channel.
6453
6454
6455 [[disable-channel]]
6456 === Disable a channel
6457
6458 To disable a specific channel that you <<enabling-disabling-channels,created>>
6459 previously, use the man:lttng-disable-channel(1) command.
6460
6461 .Disable a specific Linux kernel channel.
6462 ====
6463 [role="term"]
6464 ----
6465 $ lttng disable-channel --kernel my-channel
6466 ----
6467 ====
6468
6469 The state of a channel precedes the individual states of event rules
6470 attached to it: event rules which belong to a disabled channel, even if
6471 they are enabled, are also considered disabled.
6472
6473
6474 [[adding-context]]
6475 === Add context fields to a channel
6476
6477 Event record fields in trace files provide important information about
6478 events that occured previously, but sometimes some external context may
6479 help you solve a problem faster.
6480
6481 Examples of context fields are:
6482
6483 * The **process ID**, **thread ID**, **process name**, and
6484   **process priority** of the thread in which the event occurs.
6485 * The **hostname** of the system on which the event occurs.
6486 * The Linux kernel and user call stacks (since
6487   LTTng{nbsp}2.11).
6488 * The current values of many possible **performance counters** using
6489   perf, for example:
6490 ** CPU cycles, stalled cycles, idle cycles, and the other cycle types.
6491 ** Cache misses.
6492 ** Branch instructions, misses, and loads.
6493 ** CPU faults.
6494 * Any context defined at the application level (supported for the
6495   JUL and log4j <<domain,tracing domains>>).
6496
6497 To get the full list of available context fields, see
6498 `lttng add-context --list`. Some context fields are reserved for a
6499 specific <<domain,tracing domain>> (Linux kernel or user space).
6500
6501 You add context fields to <<channel,channels>>. All the events
6502 that a channel with added context fields records contain those fields.
6503
6504 To add context fields to one or all the channels of a given tracing
6505 session:
6506
6507 * Use the man:lttng-add-context(1) command.
6508
6509 .Add context fields to all the channels of the current tracing session.
6510 ====
6511 The following command line adds the virtual process identifier and
6512 the per-thread CPU cycles count fields to all the user space channels
6513 of the
6514 <<cur-tracing-session,current tracing session>>.
6515
6516 [role="term"]
6517 ----
6518 $ lttng add-context --userspace --type=vpid --type=perf:thread:cpu-cycles
6519 ----
6520 ====
6521
6522 .Add performance counter context fields by raw ID
6523 ====
6524 See man:lttng-add-context(1) for the exact format of the context field
6525 type, which is partly compatible with the format used in
6526 man:perf-record(1).
6527
6528 [role="term"]
6529 ----
6530 $ lttng add-context --userspace --type=perf:thread:raw:r0110:test
6531 $ lttng add-context --kernel --type=perf:cpu:raw:r0013c:x86unhalted
6532 ----
6533 ====
6534
6535 .Add context fields to a specific channel.
6536 ====
6537 The following command line adds the thread identifier and user call
6538 stack context fields to the Linux kernel channel named `my-channel` in
6539 the current tracing session.
6540
6541 [role="term"]
6542 ----
6543 $ lttng add-context --kernel --channel=my-channel \
6544      --type=tid --type=callstack-user
6545 ----
6546 ====
6547
6548 .Add an application-specific context field to a specific channel.
6549 ====
6550 The following command line adds the `cur_msg_id` context field of the
6551 `retriever` context retriever for all the instrumented
6552 <<java-application,Java applications>> recording <<event,event records>>
6553 in the channel named `my-channel`:
6554
6555 [role="term"]
6556 ----
6557 $ lttng add-context --kernel --channel=my-channel \
6558                   --type='$app:retriever:cur_msg_id'
6559 ----
6560
6561 IMPORTANT: Make sure to always quote the `$` character when you
6562 use man:lttng-add-context(1) from a shell.
6563 ====
6564
6565 NOTE: You can't remove context fields from a channel once you add it.
6566
6567
6568 [role="since-2.7"]
6569 [[pid-tracking]]
6570 === Track process attributes
6571
6572 It's often useful to only allow processes with specific attributes to
6573 emit events. For example, you may wish to record all the system calls
6574 which a given process makes (à la
6575 http://linux.die.net/man/1/strace[strace]).
6576
6577 The man:lttng-track(1) and man:lttng-untrack(1) commands serve this
6578 purpose. Both commands operate on _inclusion sets_ of process attribute
6579 values. The available process attribute types are:
6580
6581 Linux kernel <<domain,tracing domain>> only::
6582 +
6583 * Process ID (PID).
6584
6585 * Virtual process ID (VPID).
6586 +
6587 This is the PID as seen by the application.
6588
6589 * Unix user ID (UID) (since LTTng{nbsp}2.12).
6590
6591 * Virtual Unix user ID (VUID) (since LTTng{nbsp}2.12).
6592 +
6593 This is the UID as seen by the application.
6594
6595 * Unix group ID (GID) (since LTTng{nbsp}2.12).
6596
6597 * Virtual Unix group ID (VGID) (since LTTng{nbsp}2.12).
6598 +
6599 This is the GID as seen by the application.
6600
6601
6602 User space tracing domain::
6603 +
6604 * VPID.
6605 * VUID (since LTTng{nbsp}2.12).
6606 * VGID (since LTTng{nbsp}2.12).
6607
6608 Each tracing domain has one inclusion set per process attribute type:
6609 the Linux kernel tracing domain has six while the user space tracing
6610 domain has three.
6611
6612 For a given event which passes an enabled <<event,event rule>> to be
6613 recorded, _all_ the attributes of its executing process must be part of
6614 the inclusion sets of the tracing domain of the event rule.
6615
6616 Add entries to an inclusion set with the man:lttng-track(1) command and
6617 remove entries with the man:lttng-untrack(1) command. A process
6618 attribute is _tracked_ when it's part of an inclusion set and
6619 _untracked_ otherwise.
6620
6621 [NOTE]
6622 ====
6623 The process attribute values are _numeric_.
6624
6625 Should a process with a given tracked process ID, for example, exit, and
6626 then a new process be given this ID, then the latter would also be
6627 allowed to emit events.
6628
6629 With the `lttng track` command, you can add Unix user and group _names_
6630 to the user and group inclusion sets: the <<lttng-sessiond,session
6631 daemon>> finds the corresponding UID, VUID, GID, or VGID once on
6632 _addition_ to the inclusion set. This means that if you rename the user
6633 or group after you run `lttng track`, its user/group ID remains tracked.
6634 ====
6635
6636 .Track and untrack virtual process IDs.
6637 ====
6638 For the sake of the following example, assume the target system has
6639 16{nbsp}possible VPIDs.
6640
6641 When you
6642 <<creating-destroying-tracing-sessions,create a tracing session>>,
6643 the user space VPID inclusion set contains _all_ the possible VPIDs:
6644
6645 [role="img-100"]
6646 .All VPIDs are tracked.
6647 image::track-all.png[]
6648
6649 When the inclusion set is full and you use the man:lttng-track(1)
6650 command to specify some VPIDs to track, LTTng first clears the inclusion
6651 set, and then it adds the specific VPIDs to track. After:
6652
6653 [role="term"]
6654 ----
6655 $ lttng track --userspace --vpid=3,4,7,10,13
6656 ----
6657
6658 the VPID inclusion set is:
6659
6660 [role="img-100"]
6661 .VPIDs 3, 4, 7, 10, and 13 are tracked.
6662 image::track-3-4-7-10-13.png[]
6663
6664 Add more VPIDs to the inclusion set afterwards:
6665
6666 [role="term"]
6667 ----
6668 $ lttng track --userspace --vpid=1,15,16
6669 ----
6670
6671 The result is:
6672
6673 [role="img-100"]
6674 .VPIDs 1, 15, and 16 are added to the inclusion set.
6675 image::track-1-3-4-7-10-13-15-16.png[]
6676
6677 The man:lttng-untrack(1) command removes entries from process attribute
6678 inclusion sets. Given the previous example, the following command:
6679
6680 [role="term"]
6681 ----
6682 $ lttng untrack --userspace --vpid=3,7,10,13
6683 ----
6684
6685 leads to this VPID inclusion set:
6686
6687 [role="img-100"]
6688 .VPIDs 3, 7, 10, and 13 are removed from the inclusion set.
6689 image::track-1-4-15-16.png[]
6690
6691 LTTng can track all the possible VPIDs again using the
6692 opt:lttng-track(1):--all option:
6693
6694 [role="term"]
6695 ----
6696 $ lttng track --userspace --vpid --all
6697 ----
6698
6699 The result is, again:
6700
6701 [role="img-100"]
6702 .All VPIDs are tracked.
6703 image::track-all.png[]
6704 ====
6705
6706 .Track only specific process attributes.
6707 ====
6708 A typical use case with process attribute tracking is to start with an
6709 empty inclusion set, then <<basic-tracing-session-control,start the
6710 tracers>>, and then add entries manually while the tracers are active.
6711
6712 Use the opt:lttng-untrack(1):--all option of the
6713 man:lttng-untrack(1) command to clear the inclusion set after you
6714 <<creating-destroying-tracing-sessions,create a tracing session>>, for
6715 example (with UIDs):
6716
6717 [role="term"]
6718 ----
6719 $ lttng untrack --kernel --uid --all
6720 ----
6721
6722 gives:
6723
6724 [role="img-100"]
6725 .No UIDs are tracked.
6726 image::untrack-all.png[]
6727
6728 If you trace with this inclusion set configuration, the LTTng kernel
6729 tracer records no events within the <<cur-tracing-session,current
6730 tracing session>> because it doesn't track any UID. Use the
6731 man:lttng-track(1) command as usual to track specific UIDs when you need
6732 to, for example:
6733
6734 [role="term"]
6735 ----
6736 $ lttng track --kernel --uid=http,11
6737 ----
6738
6739 Result:
6740
6741 [role="img-100"]
6742 .UIDs 6 (`http`) and 11 are tracked.
6743 image::track-6-11.png[]
6744 ====
6745
6746
6747 [role="since-2.5"]
6748 [[saving-loading-tracing-session]]
6749 === Save and load tracing session configurations
6750
6751 Configuring a <<tracing-session,tracing session>> can be long. Some of
6752 the tasks involved are:
6753
6754 * <<enabling-disabling-channels,Create channels>> with
6755   specific attributes.
6756 * <<adding-context,Add context fields>> to specific channels.
6757 * <<enabling-disabling-events,Create event rules>> with specific log
6758   level and filter conditions.
6759
6760 If you use LTTng to solve real world problems, chances are you have to
6761 record events using the same tracing session setup over and over,
6762 modifying a few variables each time in your instrumented program
6763 or environment. To avoid constant tracing session reconfiguration,
6764 the man:lttng(1) command-line tool can save and load tracing session
6765 configurations to/from XML files.
6766
6767 To save a given tracing session configuration:
6768
6769 * Use the man:lttng-save(1) command:
6770 +
6771 --
6772 [role="term"]
6773 ----
6774 $ lttng save my-session
6775 ----
6776 --
6777 +
6778 Replace `my-session` with the name of the tracing session to save.
6779
6780 LTTng saves tracing session configurations to
6781 dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the
6782 env:LTTNG_HOME environment variable defaults to `$HOME` if not set. Use
6783 the opt:lttng-save(1):--output-path option to change this destination
6784 directory.
6785
6786 LTTng saves all configuration parameters, for example:
6787
6788 * The tracing session name.
6789 * The trace data output path.
6790 * The channels with their state and all their attributes.
6791 * The context fields you added to channels.
6792 * The event rules with their state, log level and filter conditions.
6793
6794 To load a tracing session:
6795
6796 * Use the man:lttng-load(1) command:
6797 +
6798 --
6799 [role="term"]
6800 ----
6801 $ lttng load my-session
6802 ----
6803 --
6804 +
6805 Replace `my-session` with the name of the tracing session to load.
6806
6807 When LTTng loads a configuration, it restores your saved tracing session
6808 as if you just configured it manually.
6809
6810 See man:lttng-load(1) for the complete list of command-line options. You
6811 can also save and load many sessions at a time, and decide in which
6812 directory to output the XML files.
6813
6814
6815 [[sending-trace-data-over-the-network]]
6816 === Send trace data over the network
6817
6818 LTTng can send the recorded trace data to a remote system over the
6819 network instead of writing it to the local file system.
6820
6821 To send the trace data over the network:
6822
6823 . On the _remote_ system (which can also be the target system),
6824   start an LTTng <<lttng-relayd,relay daemon>> (man:lttng-relayd(8)):
6825 +
6826 --
6827 [role="term"]
6828 ----
6829 $ lttng-relayd
6830 ----
6831 --
6832
6833 . On the _target_ system, create a tracing session configured to
6834   send trace data over the network:
6835 +
6836 --
6837 [role="term"]
6838 ----
6839 $ lttng create my-session --set-url=net://remote-system
6840 ----
6841 --
6842 +
6843 Replace `remote-system` by the host name or IP address of the
6844 remote system. See man:lttng-create(1) for the exact URL format.
6845
6846 . On the target system, use the man:lttng(1) command-line tool as usual.
6847   When tracing is active, the consumer daemon of the target sends
6848   sub-buffers to the relay daemon running on the remote system instead
6849   of flushing them to the local file system. The relay daemon writes the
6850   received packets to the local file system.
6851
6852 The relay daemon writes trace files to
6853 +$LTTNG_HOME/lttng-traces/__hostname__/__session__+ by default, where
6854 +__hostname__+ is the host name of the target system and +__session__+
6855 is the tracing session name. Note that the env:LTTNG_HOME environment
6856 variable defaults to `$HOME` if not set. Use the
6857 opt:lttng-relayd(8):--output option of man:lttng-relayd(8) to write
6858 trace files to another base directory.
6859
6860
6861 [role="since-2.4"]
6862 [[lttng-live]]
6863 === View events as LTTng emits them (noch:{LTTng} live)
6864
6865 LTTng live is a network protocol implemented by the <<lttng-relayd,relay
6866 daemon>> (man:lttng-relayd(8)) to allow compatible trace viewers to
6867 display events as LTTng emits them on the target system while tracing is
6868 active.
6869
6870 The relay daemon creates a _tee_: it forwards the trace data to both
6871 the local file system and to connected live viewers:
6872
6873 [role="img-90"]
6874 .The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live viewer.
6875 image::live.png[]
6876
6877 To use LTTng live:
6878
6879 . On the _target system_, create a <<tracing-session,tracing session>>
6880   in _live mode_:
6881 +
6882 --
6883 [role="term"]
6884 ----
6885 $ lttng create my-session --live
6886 ----
6887 --
6888 +
6889 This spawns a local relay daemon.
6890
6891 . Start the live viewer and configure it to connect to the relay
6892   daemon. For example, with
6893   https://babeltrace.org/docs/v2.0/man1/babeltrace2.1/[cmd:babeltrace2]:
6894 +
6895 --
6896 [role="term"]
6897 ----
6898 $ babeltrace2 net://localhost/host/hostname/my-session
6899 ----
6900 --
6901 +
6902 Replace:
6903 +
6904 --
6905 * `hostname` with the host name of the target system.
6906 * `my-session` with the name of the tracing session to view.
6907 --
6908
6909 . Configure the tracing session as usual with the man:lttng(1)
6910   command-line tool, and <<basic-tracing-session-control,start tracing>>.
6911
6912 List the available live tracing sessions with Babeltrace{nbsp}2:
6913
6914 [role="term"]
6915 ----
6916 $ babeltrace2 net://localhost
6917 ----
6918
6919 You can start the relay daemon on another system. In this case, you need
6920 to specify the URL of the relay daemon when you create the tracing
6921 session with the opt:lttng-create(1):--set-url option. You also need to
6922 replace `localhost` in the procedure above with the host name of the
6923 system on which the relay daemon is running.
6924
6925 See man:lttng-create(1) and man:lttng-relayd(8) for the complete list of
6926 command-line options.
6927
6928
6929 [role="since-2.3"]
6930 [[taking-a-snapshot]]
6931 === Take a snapshot of the current sub-buffers of a tracing session
6932
6933 The normal behavior of LTTng is to append full sub-buffers to growing
6934 trace data files. This is ideal to keep a full history of the events
6935 that occurred on the target system, but it can
6936 represent too much data in some situations. For example, you may wish
6937 to trace your application continuously until some critical situation
6938 happens, in which case you only need the latest few recorded
6939 events to perform the desired analysis, not multi-gigabyte trace files.
6940
6941 With the man:lttng-snapshot(1) command, you can take a snapshot of the
6942 current sub-buffers of a given <<tracing-session,tracing session>>.
6943 LTTng can write the snapshot to the local file system or send it over
6944 the network.
6945
6946 [role="img-100"]
6947 .A snapshot is a copy of the current sub-buffers, which aren't cleared after the operation.
6948 image::snapshot.png[]
6949
6950 If you wish to create unmanaged, self-contained, non-overlapping
6951 trace chunk archives instead of a simple copy of the current
6952 sub-buffers, see the <<session-rotation,tracing session rotation>>
6953 feature (available since LTTng{nbsp}2.11).
6954
6955 To take a snapshot:
6956
6957 . Create a tracing session in _snapshot mode_:
6958 +
6959 --
6960 [role="term"]
6961 ----
6962 $ lttng create my-session --snapshot
6963 ----
6964 --
6965 +
6966 The <<channel-overwrite-mode-vs-discard-mode,event record loss mode>> of
6967 <<channel,channels>> created in this mode is automatically set to
6968 _overwrite_ (flight recorder mode).
6969
6970 . Configure the tracing session as usual with the man:lttng(1)
6971   command-line tool, and <<basic-tracing-session-control,start tracing>>.
6972
6973 . **Optional**: When you need to take a snapshot,
6974   <<basic-tracing-session-control,stop tracing>>.
6975 +
6976 You can take a snapshot when the tracers are active, but if you stop
6977 them first, you're sure that the data in the sub-buffers doesn't
6978 change before you actually take the snapshot.
6979
6980 . Take a snapshot:
6981 +
6982 --
6983 [role="term"]
6984 ----
6985 $ lttng snapshot record --name=my-first-snapshot
6986 ----
6987 --
6988 +
6989 LTTng writes the current sub-buffers of all the channels of the
6990 <<cur-tracing-session,current tracing session>> to
6991 trace files on the local file system. Those trace files have
6992 `my-first-snapshot` in their name.
6993
6994 There is no difference between the format of a normal trace file and the
6995 format of a snapshot: viewers of LTTng traces also support LTTng
6996 snapshots.
6997
6998 By default, LTTng writes snapshot files to the path shown by
6999 `lttng snapshot list-output`. You can change this path or decide to send
7000 snapshots over the network using either:
7001
7002 . An output path or URL that you specify when you
7003   <<creating-destroying-tracing-sessions,create the tracing session>>.
7004 . A snapshot output path or URL that you add using
7005   `lttng snapshot add-output`.
7006 . An output path or URL that you provide directly to the
7007   `lttng snapshot record` command.
7008
7009 Method{nbsp}3 overrides method{nbsp}2, which overrides method 1. When
7010 you specify a URL, a relay daemon must listen on a remote system (see
7011 <<sending-trace-data-over-the-network,Send trace data over the
7012 network>>).
7013
7014
7015 [role="since-2.11"]
7016 [[session-rotation]]
7017 === Archive the current trace chunk (rotate a tracing session)
7018
7019 The <<taking-a-snapshot,snapshot user guide>> shows how to dump the
7020 current sub-buffers of a tracing session to the file system or send them
7021 over the network. When you take a snapshot, LTTng doesn't clear the ring
7022 buffers of the tracing session: if you take another snapshot immediately
7023 after, both snapshots could contain overlapping trace data.
7024
7025 Inspired by https://en.wikipedia.org/wiki/Log_rotation[log rotation],
7026 _tracing session rotation_ is a feature which appends the content of the
7027 ring buffers to what's already on the file system or sent over the
7028 network since the creation of the tracing session or since the last
7029 rotation, and then clears those ring buffers to avoid trace data
7030 overlaps.
7031
7032 What LTTng is about to write when performing a tracing session rotation
7033 is called the _current trace chunk_. When this current trace chunk is
7034 written to the file system or sent over the network, it becomes a _trace
7035 chunk archive_. Therefore, a tracing session rotation _archives_ the
7036 current trace chunk.
7037
7038 [role="img-100"]
7039 .A tracing session rotation operation _archives_ the current trace chunk.
7040 image::rotation.png[]
7041
7042 A trace chunk archive is a self-contained LTTng trace which LTTng
7043 doesn't manage anymore: you can read it, modify it, move it, or remove
7044 it.
7045
7046 There are two methods to perform a tracing session rotation: immediately
7047 or with a rotation schedule.
7048
7049 To perform an immediate tracing session rotation:
7050
7051 . <<creating-destroying-tracing-sessions,Create a tracing session>>
7052   in _normal mode_ or _network streaming mode_
7053   (only those two creation modes support tracing session rotation):
7054 +
7055 --
7056 [role="term"]
7057 ----
7058 $ lttng create my-session
7059 ----
7060 --
7061
7062 . <<enabling-disabling-events,Create one or more event rules>>
7063   and <<basic-tracing-session-control,start tracing>>:
7064 +
7065 --
7066 [role="term"]
7067 ----
7068 $ lttng enable-event --kernel sched_'*'
7069 $ lttng start
7070 ----
7071 --
7072
7073 . When needed, immediately rotate the
7074   <<cur-tracing-session,current tracing session>>:
7075 +
7076 --
7077 [role="term"]
7078 ----
7079 $ lttng rotate
7080 ----
7081 --
7082 +
7083 The cmd:lttng-rotate command prints the path to the created trace
7084 chunk archive. See man:lttng-rotate(1) to learn about the format
7085 of trace chunk archive directory names.
7086 +
7087 Perform other immediate rotations while the tracing session is
7088 active. It is guaranteed that all the trace chunk archives don't
7089 contain overlapping trace data. You can also perform an immediate
7090 rotation once you have <<basic-tracing-session-control,stopped>> the
7091 tracing session.
7092
7093 . When you're done tracing,
7094   <<creating-destroying-tracing-sessions,destroy the current tracing
7095   session>>:
7096 +
7097 --
7098 [role="term"]
7099 ----
7100 $ lttng destroy
7101 ----
7102 --
7103 +
7104 The tracing session destruction operation creates one last trace
7105 chunk archive from the current trace chunk.
7106
7107 A tracing session rotation schedule is a planned rotation which LTTng
7108 performs automatically based on one of the following conditions:
7109
7110 * A timer with a configured period times out.
7111
7112 * The total size of the flushed part of the current trace chunk
7113   becomes greater than or equal to a configured value.
7114
7115 To schedule a tracing session rotation, set a _rotation schedule_:
7116
7117 . <<creating-destroying-tracing-sessions,Create a tracing session>>
7118   in _normal mode_ or _network streaming mode_
7119   (only those two creation modes support tracing session rotation):
7120 +
7121 --
7122 [role="term"]
7123 ----
7124 $ lttng create my-session
7125 ----
7126 --
7127
7128 . <<enabling-disabling-events,Create one or more event rules>>:
7129 +
7130 --
7131 [role="term"]
7132 ----
7133 $ lttng enable-event --kernel sched_'*'
7134 ----
7135 --
7136
7137 . Set a tracing session rotation schedule:
7138 +
7139 --
7140 [role="term"]
7141 ----
7142 $ lttng enable-rotation --timer=10s
7143 ----
7144 --
7145 +
7146 In this example, we set a rotation schedule so that LTTng performs a
7147 tracing session rotation every ten seconds.
7148 +
7149 See man:lttng-enable-rotation(1) to learn more about other ways to set a
7150 rotation schedule.
7151
7152 . <<basic-tracing-session-control,Start tracing>>:
7153 +
7154 --
7155 [role="term"]
7156 ----
7157 $ lttng start
7158 ----
7159 --
7160 +
7161 LTTng performs tracing session rotations automatically while the tracing
7162 session is active thanks to the rotation schedule.
7163
7164 . When you're done tracing,
7165   <<creating-destroying-tracing-sessions,destroy the current tracing
7166   session>>:
7167 +
7168 --
7169 [role="term"]
7170 ----
7171 $ lttng destroy
7172 ----
7173 --
7174 +
7175 The tracing session destruction operation creates one last trace chunk
7176 archive from the current trace chunk.
7177
7178 Use man:lttng-disable-rotation(1) to unset a tracing session
7179 rotation schedule.
7180
7181 NOTE: man:lttng-rotate(1) and man:lttng-enable-rotation(1) list
7182 limitations regarding those two commands.
7183
7184
7185 [role="since-2.6"]
7186 [[mi]]
7187 === Use the machine interface
7188
7189 With any command of the man:lttng(1) command-line tool, set the
7190 opt:lttng(1):--mi option to `xml` (before the command name) to get an
7191 XML machine interface output, for example:
7192
7193 [role="term"]
7194 ----
7195 $ lttng --mi=xml enable-event --kernel --syscall open
7196 ----
7197
7198 A schema definition (XSD) is
7199 https://github.com/lttng/lttng-tools/blob/stable-{revision}/src/common/src/common/mi-lttng-4.0.xsd[available]
7200 to ease the integration with external tools as much as possible.
7201
7202
7203 [role="since-2.8"]
7204 [[metadata-regenerate]]
7205 === Regenerate the metadata of an LTTng trace
7206
7207 An LTTng trace, which is a http://diamon.org/ctf[CTF] trace, has both
7208 data stream files and a metadata file. This metadata file contains,
7209 amongst other things, information about the offset of the clock sources
7210 used to timestamp <<event,event records>> when tracing.
7211
7212 If, once a <<tracing-session,tracing session>> is
7213 <<basic-tracing-session-control,started>>, a major
7214 https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction
7215 happens, the clock offset of the trace also needs to be updated. Use
7216 the `metadata` item of the man:lttng-regenerate(1) command to do so.
7217
7218 The main use case of this command is to allow a system to boot with
7219 an incorrect wall time and trace it with LTTng before its wall time
7220 is corrected. Once the system is known to be in a state where its
7221 wall time is correct, it can run `lttng regenerate metadata`.
7222
7223 To regenerate the metadata of an LTTng trace:
7224
7225 * Use the `metadata` item of the man:lttng-regenerate(1) command:
7226 +
7227 --
7228 [role="term"]
7229 ----
7230 $ lttng regenerate metadata
7231 ----
7232 --
7233
7234 [IMPORTANT]
7235 ====
7236 `lttng regenerate metadata` has the following limitations:
7237
7238 * Tracing session <<creating-destroying-tracing-sessions,created>>
7239   in non-live mode.
7240 * User space <<channel,channels>>, if any, are using
7241   <<channel-buffering-schemes,per-user buffering>>.
7242 ====
7243
7244
7245 [role="since-2.9"]
7246 [[regenerate-statedump]]
7247 === Regenerate the state dump of a tracing session
7248
7249 The LTTng kernel and user space tracers generate state dump
7250 <<event,event records>> when the application starts or when you
7251 <<basic-tracing-session-control,start a tracing session>>. An analysis
7252 can use the state dump event records to set an initial state before it
7253 builds the rest of the state from the following event records.
7254 http://tracecompass.org/[Trace Compass] is a notable example of an
7255 application which uses the state dump of an LTTng trace.
7256
7257 When you <<taking-a-snapshot,take a snapshot>>, it's possible that the
7258 state dump event records aren't included in the snapshot because they
7259 were recorded to a sub-buffer that has been consumed or overwritten
7260 already.
7261
7262 Use the `lttng regenerate statedump` command to emit the state
7263 dump event records again.
7264
7265 To regenerate the state dump of the current tracing session, provided
7266 create it in snapshot mode, before you take a snapshot:
7267
7268 . Use the `statedump` item of the man:lttng-regenerate(1) command:
7269 +
7270 --
7271 [role="term"]
7272 ----
7273 $ lttng regenerate statedump
7274 ----
7275 --
7276
7277 . <<basic-tracing-session-control,Stop the tracing session>>:
7278 +
7279 --
7280 [role="term"]
7281 ----
7282 $ lttng stop
7283 ----
7284 --
7285
7286 . <<taking-a-snapshot,Take a snapshot>>:
7287 +
7288 --
7289 [role="term"]
7290 ----
7291 $ lttng snapshot record --name=my-snapshot
7292 ----
7293 --
7294
7295 Depending on the event throughput, you should run steps 1 and 2
7296 as closely as possible.
7297
7298 NOTE: To record the state dump events, you need to
7299 <<enabling-disabling-events,create event rules>> which enable them.
7300 LTTng-UST state dump tracepoints start with `lttng_ust_statedump:`.
7301 LTTng-modules state dump tracepoints start with `lttng_statedump_`.
7302
7303
7304 [role="since-2.7"]
7305 [[persistent-memory-file-systems]]
7306 === Record trace data on persistent memory file systems
7307
7308 https://en.wikipedia.org/wiki/Non-volatile_random-access_memory[Non-volatile random-access memory]
7309 (NVRAM) is random-access memory that retains its information when power
7310 is turned off (non-volatile). Systems with such memory can store data
7311 structures in RAM and retrieve them after a reboot, without flushing
7312 to typical _storage_.
7313
7314 Linux supports NVRAM file systems thanks to either
7315 http://pramfs.sourceforge.net/[PRAMFS] or
7316 https://www.kernel.org/doc/Documentation/filesystems/dax.txt[DAX]{nbsp}+{nbsp}http://lkml.iu.edu/hypermail/linux/kernel/1504.1/03463.html[pmem]
7317 (requires Linux{nbsp}4.1+).
7318
7319 This section doesn't describe how to operate such file systems;
7320 we assume that you have a working persistent memory file system.
7321
7322 When you create a <<tracing-session,tracing session>>, you can specify
7323 the path of the shared memory holding the sub-buffers. If you specify a
7324 location on an NVRAM file system, then you can retrieve the latest
7325 recorded trace data when the system reboots after a crash.
7326
7327 To record trace data on a persistent memory file system and retrieve the
7328 trace data after a system crash:
7329
7330 . Create a tracing session with a sub-buffer shared memory path located
7331   on an NVRAM file system:
7332 +
7333 --
7334 [role="term"]
7335 ----
7336 $ lttng create my-session --shm-path=/path/to/shm
7337 ----
7338 --
7339
7340 . Configure the tracing session as usual with the man:lttng(1)
7341   command-line tool, and <<basic-tracing-session-control,start tracing>>.
7342
7343 . After a system crash, use the man:lttng-crash(1) command-line tool to
7344   view the trace data recorded on the NVRAM file system:
7345 +
7346 --
7347 [role="term"]
7348 ----
7349 $ lttng-crash /path/to/shm
7350 ----
7351 --
7352
7353 The binary layout of the ring buffer files isn't exactly the same as
7354 the trace files layout. This is why you need to use man:lttng-crash(1)
7355 instead of your preferred trace viewer directly.
7356
7357 To convert the ring buffer files to LTTng trace files:
7358
7359 * Use the opt:lttng-crash(1):--extract option of man:lttng-crash(1):
7360 +
7361 --
7362 [role="term"]
7363 ----
7364 $ lttng-crash --extract=/path/to/trace /path/to/shm
7365 ----
7366 --
7367
7368
7369 [role="since-2.10"]
7370 [[notif-trigger-api]]
7371 === Get notified when the buffer usage of a channel is too high or too low
7372
7373 With the $$C/C++$$ notification and trigger API of LTTng, your user
7374 application can get notified when the buffer usage of one or more
7375 <<channel,channels>> becomes too low or too high. Use this API
7376 and enable or disable <<event,event rules>> during tracing to avoid
7377 <<channel-overwrite-mode-vs-discard-mode,discarded event records>>.
7378
7379 .Have a user application get notified when the buffer usage of an LTTng channel is too high.
7380 ====
7381 In this example, we create and build an application which gets notified
7382 when the buffer usage of a specific LTTng channel is higher than
7383 75{nbsp}%. We only print that it is the case in the example, but we
7384 could as well use the API of <<liblttng-ctl-lttng,`liblttng-ctl`>> to
7385 disable event rules when this happens.
7386
7387 . Create the C{nbsp}source file of application:
7388 +
7389 --
7390 [source,c]
7391 .path:{notif-app.c}
7392 ----
7393 #include <stdio.h>
7394 #include <assert.h>
7395 #include <lttng/domain.h>
7396 #include <lttng/action/action.h>
7397 #include <lttng/action/notify.h>
7398 #include <lttng/condition/condition.h>
7399 #include <lttng/condition/buffer-usage.h>
7400 #include <lttng/condition/evaluation.h>
7401 #include <lttng/notification/channel.h>
7402 #include <lttng/notification/notification.h>
7403 #include <lttng/trigger/trigger.h>
7404 #include <lttng/endpoint.h>
7405
7406 int main(int argc, char *argv[])
7407 {
7408     int exit_status = 0;
7409     struct lttng_notification_channel *notification_channel;
7410     struct lttng_condition *condition;
7411     struct lttng_action *action;
7412     struct lttng_trigger *trigger;
7413     const char *tracing_session_name;
7414     const char *channel_name;
7415
7416     assert(argc >= 3);
7417     tracing_session_name = argv[1];
7418     channel_name = argv[2];
7419
7420     /*
7421      * Create a notification channel. A notification channel
7422      * connects the user application to the LTTng session daemon.
7423      * This notification channel can be used to listen to various
7424      * types of notifications.
7425      */
7426     notification_channel = lttng_notification_channel_create(
7427         lttng_session_daemon_notification_endpoint);
7428
7429     /*
7430      * Create a "high buffer usage" condition. In this case, the
7431      * condition is reached when the buffer usage is greater than or
7432      * equal to 75 %. We create the condition for a specific tracing
7433      * session name, channel name, and for the user space tracing
7434      * domain.
7435      *
7436      * The "low buffer usage" condition type also exists.
7437      */
7438     condition = lttng_condition_buffer_usage_high_create();
7439     lttng_condition_buffer_usage_set_threshold_ratio(condition, .75);
7440     lttng_condition_buffer_usage_set_session_name(
7441         condition, tracing_session_name);
7442     lttng_condition_buffer_usage_set_channel_name(condition,
7443         channel_name);
7444     lttng_condition_buffer_usage_set_domain_type(condition,
7445         LTTNG_DOMAIN_UST);
7446
7447     /*
7448      * Create an action (get a notification) to take when the
7449      * condition created above is reached.
7450      */
7451     action = lttng_action_notify_create();
7452
7453     /*
7454      * Create a trigger. A trigger associates a condition to an
7455      * action: the action is executed when the condition is reached.
7456      */
7457     trigger = lttng_trigger_create(condition, action);
7458
7459     /* Register the trigger to LTTng. */
7460     lttng_register_trigger(trigger);
7461
7462     /*
7463      * Now that we have registered a trigger, a notification will be
7464      * emitted everytime its condition is met. To receive this
7465      * notification, we must subscribe to notifications that match
7466      * the same condition.
7467      */
7468     lttng_notification_channel_subscribe(notification_channel,
7469         condition);
7470
7471     /*
7472      * Notification loop. Put this in a dedicated thread to avoid
7473      * blocking the main thread.
7474      */
7475     for (;;) {
7476         struct lttng_notification *notification;
7477         enum lttng_notification_channel_status status;
7478         const struct lttng_evaluation *notification_evaluation;
7479         const struct lttng_condition *notification_condition;
7480         double buffer_usage;
7481
7482         /* Receive the next notification. */
7483         status = lttng_notification_channel_get_next_notification(
7484             notification_channel, &notification);
7485
7486         switch (status) {
7487         case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK:
7488             break;
7489         case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED:
7490             /*
7491              * The session daemon can drop notifications if a monitoring
7492              * application isn't consuming the notifications fast
7493              * enough.
7494              */
7495             continue;
7496         case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED:
7497             /*
7498              * The notification channel has been closed by the
7499              * session daemon. This is typically caused by a session
7500              * daemon shutting down.
7501              */
7502             goto end;
7503         default:
7504             /* Unhandled conditions or errors. */
7505             exit_status = 1;
7506             goto end;
7507         }
7508
7509         /*
7510          * A notification provides, amongst other things:
7511          *
7512          * * The condition that caused this notification to be
7513          *   emitted.
7514          * * The condition evaluation, which provides more
7515          *   specific information on the evaluation of the
7516          *   condition.
7517          *
7518          * The condition evaluation provides the buffer usage
7519          * value at the moment the condition was reached.
7520          */
7521         notification_condition = lttng_notification_get_condition(
7522             notification);
7523         notification_evaluation = lttng_notification_get_evaluation(
7524             notification);
7525
7526         /* We're subscribed to only one condition. */
7527         assert(lttng_condition_get_type(notification_condition) ==
7528             LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH);
7529
7530         /*
7531          * Get the exact sampled buffer usage from the
7532          * condition evaluation.
7533          */
7534         lttng_evaluation_buffer_usage_get_usage_ratio(
7535             notification_evaluation, &buffer_usage);
7536
7537         /*
7538          * At this point, instead of printing a message, we
7539          * could do something to reduce the buffer usage of the channel,
7540          * like disable specific events.
7541          */
7542         printf("Buffer usage is %f %% in tracing session \"%s\", "
7543             "user space channel \"%s\".\n", buffer_usage * 100,
7544             tracing_session_name, channel_name);
7545         lttng_notification_destroy(notification);
7546     }
7547
7548 end:
7549     lttng_action_destroy(action);
7550     lttng_condition_destroy(condition);
7551     lttng_trigger_destroy(trigger);
7552     lttng_notification_channel_destroy(notification_channel);
7553     return exit_status;
7554 }
7555 ----
7556 --
7557
7558 . Build the `notif-app` application, linking it to `liblttng-ctl`:
7559 +
7560 --
7561 [role="term"]
7562 ----
7563 $ gcc -o notif-app notif-app.c -llttng-ctl
7564 ----
7565 --
7566
7567 . <<creating-destroying-tracing-sessions,Create a tracing session>>,
7568   <<enabling-disabling-events,create an event rule>> matching all the
7569   user space tracepoints, and
7570   <<basic-tracing-session-control,start tracing>>:
7571 +
7572 --
7573 [role="term"]
7574 ----
7575 $ lttng create my-session
7576 $ lttng enable-event --userspace --all
7577 $ lttng start
7578 ----
7579 --
7580 +
7581 If you create the channel manually with the man:lttng-enable-channel(1)
7582 command, control how frequently LTTng samples the current values of the
7583 channel properties to evaluate user conditions with the
7584 opt:lttng-enable-channel(1):--monitor-timer option.
7585
7586 . Run the `notif-app` application. This program accepts the
7587   <<tracing-session,tracing session>> name and the user space channel
7588   name as its two first arguments. The channel which LTTng automatically
7589   creates with the man:lttng-enable-event(1) command above is named
7590   `channel0`:
7591 +
7592 --
7593 [role="term"]
7594 ----
7595 $ ./notif-app my-session channel0
7596 ----
7597 --
7598
7599 . In another terminal, run an application with a very high event
7600   throughput so that the 75{nbsp}% buffer usage condition is reached.
7601 +
7602 In the first terminal, the application should print lines like this:
7603 +
7604 ----
7605 Buffer usage is 81.45197 % in tracing session "my-session", user space
7606 channel "channel0".
7607 ----
7608 +
7609 If you don't see anything, try modifying the condition in
7610 path:{notif-app.c} to a lower value (0.1, for example), rebuilding it
7611 (step{nbsp}2) and running it again (step{nbsp}4).
7612 ====
7613
7614
7615 [[reference]]
7616 == Reference
7617
7618 [[lttng-modules-ref]]
7619 === noch:{LTTng-modules}
7620
7621
7622 [role="since-2.9"]
7623 [[lttng-tracepoint-enum]]
7624 ==== `LTTNG_TRACEPOINT_ENUM()` usage
7625
7626 Use the `LTTNG_TRACEPOINT_ENUM()` macro to define an enumeration:
7627
7628 [source,c]
7629 ----
7630 LTTNG_TRACEPOINT_ENUM(name, TP_ENUM_VALUES(entries))
7631 ----
7632
7633 Replace:
7634
7635 * `name` with the name of the enumeration (C identifier, unique
7636   amongst all the defined enumerations).
7637 * `entries` with a list of enumeration entries.
7638
7639 The available enumeration entry macros are:
7640
7641 +ctf_enum_value(__name__, __value__)+::
7642   Entry named +__name__+ mapped to the integral value +__value__+.
7643
7644 +ctf_enum_range(__name__, __begin__, __end__)+::
7645   Entry named +__name__+ mapped to the range of integral values between
7646   +__begin__+ (included) and +__end__+ (included).
7647
7648 +ctf_enum_auto(__name__)+::
7649   Entry named +__name__+ mapped to the integral value following the
7650   last mapping value.
7651 +
7652 The last value of a `ctf_enum_value()` entry is its +__value__+
7653 parameter.
7654 +
7655 The last value of a `ctf_enum_range()` entry is its +__end__+ parameter.
7656 +
7657 If `ctf_enum_auto()` is the first entry in the list, its integral
7658 value is 0.
7659
7660 Use the `ctf_enum()` <<lttng-modules-tp-fields,field definition macro>>
7661 to use a defined enumeration as a tracepoint field.
7662
7663 .Define an enumeration with `LTTNG_TRACEPOINT_ENUM()`.
7664 ====
7665 [source,c]
7666 ----
7667 LTTNG_TRACEPOINT_ENUM(
7668     my_enum,
7669     TP_ENUM_VALUES(
7670         ctf_enum_auto("AUTO: EXPECT 0")
7671         ctf_enum_value("VALUE: 23", 23)
7672         ctf_enum_value("VALUE: 27", 27)
7673         ctf_enum_auto("AUTO: EXPECT 28")
7674         ctf_enum_range("RANGE: 101 TO 303", 101, 303)
7675         ctf_enum_auto("AUTO: EXPECT 304")
7676     )
7677 )
7678 ----
7679 ====
7680
7681
7682 [role="since-2.7"]
7683 [[lttng-modules-tp-fields]]
7684 ==== Tracepoint fields macros (for `TP_FIELDS()`)
7685
7686 [[tp-fast-assign]][[tp-struct-entry]]The available macros to define
7687 tracepoint fields, which must be listed within `TP_FIELDS()` in
7688 `LTTNG_TRACEPOINT_EVENT()`, are:
7689
7690 [role="func-desc growable",cols="asciidoc,asciidoc"]
7691 .Available macros to define LTTng-modules tracepoint fields
7692 |====
7693 |Macro |Description and parameters
7694
7695 |
7696 +ctf_integer(__t__, __n__, __e__)+
7697
7698 +ctf_integer_nowrite(__t__, __n__, __e__)+
7699
7700 +ctf_user_integer(__t__, __n__, __e__)+
7701
7702 +ctf_user_integer_nowrite(__t__, __n__, __e__)+
7703 |
7704 Standard integer, displayed in base{nbsp}10.
7705
7706 +__t__+::
7707   Integer C type (`int`, `long`, `size_t`, ...).
7708
7709 +__n__+::
7710   Field name.
7711
7712 +__e__+::
7713   Argument expression.
7714
7715 |
7716 +ctf_integer_hex(__t__, __n__, __e__)+
7717
7718 +ctf_user_integer_hex(__t__, __n__, __e__)+
7719 |
7720 Standard integer, displayed in base{nbsp}16.
7721
7722 +__t__+::
7723   Integer C type.
7724
7725 +__n__+::
7726   Field name.
7727
7728 +__e__+::
7729   Argument expression.
7730
7731 |+ctf_integer_oct(__t__, __n__, __e__)+
7732 |
7733 Standard integer, displayed in base{nbsp}8.
7734
7735 +__t__+::
7736   Integer C type.
7737
7738 +__n__+::
7739   Field name.
7740
7741 +__e__+::
7742   Argument expression.
7743
7744 |
7745 +ctf_integer_network(__t__, __n__, __e__)+
7746
7747 +ctf_user_integer_network(__t__, __n__, __e__)+
7748 |
7749 Integer in network byte order (big-endian), displayed in base{nbsp}10.
7750
7751 +__t__+::
7752   Integer C type.
7753
7754 +__n__+::
7755   Field name.
7756
7757 +__e__+::
7758   Argument expression.
7759
7760 |
7761 +ctf_integer_network_hex(__t__, __n__, __e__)+
7762
7763 +ctf_user_integer_network_hex(__t__, __n__, __e__)+
7764 |
7765 Integer in network byte order, displayed in base{nbsp}16.
7766
7767 +__t__+::
7768   Integer C type.
7769
7770 +__n__+::
7771   Field name.
7772
7773 +__e__+::
7774   Argument expression.
7775
7776 |
7777 +ctf_enum(__N__, __t__, __n__, __e__)+
7778
7779 +ctf_enum_nowrite(__N__, __t__, __n__, __e__)+
7780
7781 +ctf_user_enum(__N__, __t__, __n__, __e__)+
7782
7783 +ctf_user_enum_nowrite(__N__, __t__, __n__, __e__)+
7784 |
7785 Enumeration.
7786
7787 +__N__+::
7788   Name of a <<lttng-tracepoint-enum,previously defined enumeration>>.
7789
7790 +__t__+::
7791   Integer C type (`int`, `long`, `size_t`, ...).
7792
7793 +__n__+::
7794   Field name.
7795
7796 +__e__+::
7797   Argument expression.
7798
7799 |
7800 +ctf_string(__n__, __e__)+
7801
7802 +ctf_string_nowrite(__n__, __e__)+
7803
7804 +ctf_user_string(__n__, __e__)+
7805
7806 +ctf_user_string_nowrite(__n__, __e__)+
7807 |
7808 Null-terminated string; undefined behavior if +__e__+ is `NULL`.
7809
7810 +__n__+::
7811   Field name.
7812
7813 +__e__+::
7814   Argument expression.
7815
7816 |
7817 +ctf_array(__t__, __n__, __e__, __s__)+
7818
7819 +ctf_array_nowrite(__t__, __n__, __e__, __s__)+
7820
7821 +ctf_user_array(__t__, __n__, __e__, __s__)+
7822
7823 +ctf_user_array_nowrite(__t__, __n__, __e__, __s__)+
7824 |
7825 Statically-sized array of integers.
7826
7827 +__t__+::
7828   Array element C type.
7829
7830 +__n__+::
7831   Field name.
7832
7833 +__e__+::
7834   Argument expression.
7835
7836 +__s__+::
7837   Number of elements.
7838
7839 |
7840 +ctf_array_bitfield(__t__, __n__, __e__, __s__)+
7841
7842 +ctf_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
7843
7844 +ctf_user_array_bitfield(__t__, __n__, __e__, __s__)+
7845
7846 +ctf_user_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
7847 |
7848 Statically-sized array of bits.
7849
7850 The type of +__e__+ must be an integer type. +__s__+ is the number
7851 of elements of such type in +__e__+, not the number of bits.
7852
7853 +__t__+::
7854   Array element C type.
7855
7856 +__n__+::
7857   Field name.
7858
7859 +__e__+::
7860   Argument expression.
7861
7862 +__s__+::
7863   Number of elements.
7864
7865 |
7866 +ctf_array_text(__t__, __n__, __e__, __s__)+
7867
7868 +ctf_array_text_nowrite(__t__, __n__, __e__, __s__)+
7869
7870 +ctf_user_array_text(__t__, __n__, __e__, __s__)+
7871
7872 +ctf_user_array_text_nowrite(__t__, __n__, __e__, __s__)+
7873 |
7874 Statically-sized array, printed as text.
7875
7876 The string doesn't need to be null-terminated.
7877
7878 +__t__+::
7879   Array element C type (always `char`).
7880
7881 +__n__+::
7882   Field name.
7883
7884 +__e__+::
7885   Argument expression.
7886
7887 +__s__+::
7888   Number of elements.
7889
7890 |
7891 +ctf_sequence(__t__, __n__, __e__, __T__, __E__)+
7892
7893 +ctf_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
7894
7895 +ctf_user_sequence(__t__, __n__, __e__, __T__, __E__)+
7896
7897 +ctf_user_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
7898 |
7899 Dynamically-sized array of integers.
7900
7901 The type of +__E__+ must be unsigned.
7902
7903 +__t__+::
7904   Array element C type.
7905
7906 +__n__+::
7907   Field name.
7908
7909 +__e__+::
7910   Argument expression.
7911
7912 +__T__+::
7913   Length expression C type.
7914
7915 +__E__+::
7916   Length expression.
7917
7918 |
7919 +ctf_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
7920
7921 +ctf_user_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
7922 |
7923 Dynamically-sized array of integers, displayed in base{nbsp}16.
7924
7925 The type of +__E__+ must be unsigned.
7926
7927 +__t__+::
7928   Array element C type.
7929
7930 +__n__+::
7931   Field name.
7932
7933 +__e__+::
7934   Argument expression.
7935
7936 +__T__+::
7937   Length expression C type.
7938
7939 +__E__+::
7940   Length expression.
7941
7942 |+ctf_sequence_network(__t__, __n__, __e__, __T__, __E__)+
7943 |
7944 Dynamically-sized array of integers in network byte order (big-endian),
7945 displayed in base{nbsp}10.
7946
7947 The type of +__E__+ must be unsigned.
7948
7949 +__t__+::
7950   Array element C type.
7951
7952 +__n__+::
7953   Field name.
7954
7955 +__e__+::
7956   Argument expression.
7957
7958 +__T__+::
7959   Length expression C type.
7960
7961 +__E__+::
7962   Length expression.
7963
7964 |
7965 +ctf_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
7966
7967 +ctf_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
7968
7969 +ctf_user_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
7970
7971 +ctf_user_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
7972 |
7973 Dynamically-sized array of bits.
7974
7975 The type of +__e__+ must be an integer type. +__s__+ is the number
7976 of elements of such type in +__e__+, not the number of bits.
7977
7978 The type of +__E__+ must be unsigned.
7979
7980 +__t__+::
7981   Array element C type.
7982
7983 +__n__+::
7984   Field name.
7985
7986 +__e__+::
7987   Argument expression.
7988
7989 +__T__+::
7990   Length expression C type.
7991
7992 +__E__+::
7993   Length expression.
7994
7995 |
7996 +ctf_sequence_text(__t__, __n__, __e__, __T__, __E__)+
7997
7998 +ctf_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
7999
8000 +ctf_user_sequence_text(__t__, __n__, __e__, __T__, __E__)+
8001
8002 +ctf_user_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
8003 |
8004 Dynamically-sized array, displayed as text.
8005
8006 The string doesn't need to be null-terminated.
8007
8008 The type of +__E__+ must be unsigned.
8009
8010 The behaviour is undefined if +__e__+ is `NULL`.
8011
8012 +__t__+::
8013   Sequence element C type (always `char`).
8014
8015 +__n__+::
8016   Field name.
8017
8018 +__e__+::
8019   Argument expression.
8020
8021 +__T__+::
8022   Length expression C type.
8023
8024 +__E__+::
8025   Length expression.
8026 |====
8027
8028 Use the `_user` versions when the argument expression, `e`, is
8029 a user space address. In the cases of `ctf_user_integer*()` and
8030 `ctf_user_float*()`, `&e` must be a user space address, thus `e` must
8031 be addressable.
8032
8033 The `_nowrite` versions omit themselves from the session trace, but are
8034 otherwise identical. This means the `_nowrite` fields won't be written
8035 in the recorded trace. Their primary purpose is to make some
8036 of the event context available to the
8037 <<enabling-disabling-events,event filters>> without having to
8038 commit the data to sub-buffers.
8039
8040
8041 [[glossary]]
8042 == Glossary
8043
8044 Terms related to LTTng and to tracing in general:
8045
8046 Babeltrace::
8047   The http://diamon.org/babeltrace[Babeltrace] project, which includes:
8048 +
8049 * The
8050   https://babeltrace.org/docs/v2.0/man1/babeltrace2.1/[cmd:babeltrace2]
8051   command-line interface.
8052 * The libbabeltrace2 library which offers a
8053   https://babeltrace.org/docs/v2.0/libbabeltrace2/[C API].
8054 * https://babeltrace.org/docs/v2.0/python/bt2/[Python{nbsp}3 bindings].
8055 * Plugins.
8056
8057 [[def-buffering-scheme]]<<channel-buffering-schemes,buffering scheme>>::
8058   A layout of <<def-sub-buffer,sub-buffers>> applied to a given channel.
8059
8060 [[def-channel]]<<channel,channel>>::
8061   An entity which is responsible for a set of
8062   <<def-ring-buffer,ring buffers>>.
8063 +
8064 <<def-event-rule,Event rules>> are always attached to a specific
8065 channel.
8066
8067 clock::
8068   A source of time for a <<def-tracer,tracer>>.
8069
8070 [[def-consumer-daemon]]<<lttng-consumerd,consumer daemon>>::
8071   A process which is responsible for consuming the full
8072   <<def-sub-buffer,sub-buffers>> and write them to a file system or
8073   send them over the network.
8074
8075 [[def-current-trace-chunk]]current trace chunk::
8076   A <<def-trace-chunk,trace chunk>> which includes the current content
8077   of all the <<def-sub-buffer,sub-buffers>> of the
8078   <<def-tracing-session-rotation,tracing session>> and the stream files
8079   produced since the latest event amongst:
8080 +
8081 * The creation of the <<def-tracing-session,tracing session>>.
8082 * The last tracing session rotation, if any.
8083
8084 <<channel-overwrite-mode-vs-discard-mode,discard mode>>::
8085   The <<def-event-record-loss-mode,event record loss mode>> in which
8086   the <<def-tracer,tracer>> _discards_ new event records when there's no
8087   <<def-sub-buffer,sub-buffer>> space left to store them.
8088
8089 [[def-event]]event::
8090   The consequence of the execution of an
8091   <<def-instrumentation-point,instrumentation point>>, like a
8092   <<def-tracepoint,tracepoint>> that you manually place in some source
8093   code, or a Linux kernel kprobe.
8094 +
8095 An event is said to _occur_ at a specific time. <<def-lttng,LTTng>> can
8096 take various actions upon the occurrence of an event, like record its
8097 payload to a <<def-sub-buffer,sub-buffer>>.
8098
8099 [[def-event-name]]event name::
8100   The name of an <<def-event,event>>, which is also the name of the
8101   <<def-event-record,event record>>.
8102 +
8103 This is also called the _instrumentation point name_.
8104
8105 [[def-event-record]]event record::
8106   A record, in a <<def-trace,trace>>, of the payload of an
8107   <<def-event,event>> which occured.
8108
8109 [[def-event-record-loss-mode]]<<channel-overwrite-mode-vs-discard-mode,event record loss mode>>::
8110   The mechanism by which event records of a given
8111   <<def-channel,channel>> are lost (not recorded) when there is no
8112   <<def-sub-buffer,sub-buffer>> space left to store them.
8113
8114 [[def-event-rule]]<<event,event rule>>::
8115   Set of conditions which must be satisfied for one or more occuring
8116   <<def-event,events>> to be recorded.
8117
8118 [[def-incl-set]]inclusion set::
8119   In the <<pid-tracking,process attribute tracking>> context: a
8120   set of <<def-proc-attr,process attributes>> of a given type.
8121
8122 <<instrumenting,instrumentation>>::
8123   The use of <<def-lttng,LTTng>> probes to make a piece of software
8124   traceable.
8125
8126 [[def-instrumentation-point]]instrumentation point::
8127   A point in the execution path of a piece of software that, when
8128   reached by this execution, can emit an <<def-event,event>>.
8129
8130 instrumentation point name::
8131   See _<<def-event-name,event name>>_.
8132
8133 `java.util.logging`::
8134   The
8135   https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities]
8136   of the Java platform.
8137
8138 log4j::
8139   A http://logging.apache.org/log4j/1.2/[logging library] for Java
8140   developed by the Apache Software Foundation.
8141
8142 log level::
8143   Level of severity of a log statement or user space
8144   <<def-instrumentation-point,instrumentation point>>.
8145
8146 [[def-lttng]]LTTng::
8147   The _Linux Trace Toolkit: next generation_ project.
8148
8149 <<lttng-cli,cmd:lttng>>::
8150   A command-line tool provided by the <<def-lttng-tools,LTTng-tools>>
8151   project which you can use to send and receive control messages to and
8152   from a <<def-session-daemon,session daemon>>.
8153
8154 LTTng analyses::
8155   The https://github.com/lttng/lttng-analyses[LTTng analyses] project,
8156   which is a set of analyzing programs that you can use to obtain a
8157   higher level view of an <<def-lttng,LTTng>> <<def-trace,trace>>.
8158
8159 cmd:lttng-consumerd::
8160   The name of the <<def-consumer-daemon,consumer daemon>> program.
8161
8162 cmd:lttng-crash::
8163   A utility provided by the <<def-lttng-tools,LTTng-tools>> project
8164   which can convert <<def-ring-buffer,ring buffer>> files (usually
8165   <<persistent-memory-file-systems,saved on a persistent memory file
8166   system>>) to <<def-trace,trace>> files.
8167 +
8168 See man:lttng-crash(1).
8169
8170 LTTng Documentation::
8171   This document.
8172
8173 <<lttng-live,LTTng live>>::
8174   A communication protocol between the <<lttng-relayd,relay daemon>> and
8175   live viewers which makes it possible to see <<def-event-record,event
8176   records>> ``live'', as they are received by the
8177   <<def-relay-daemon,relay daemon>>.
8178
8179 <<lttng-modules,LTTng-modules>>::
8180   The https://github.com/lttng/lttng-modules[LTTng-modules] project,
8181   which contains the Linux kernel modules to make the Linux kernel
8182   <<def-instrumentation-point,instrumentation points>> available for
8183   <<def-lttng,LTTng>> tracing.
8184
8185 cmd:lttng-relayd::
8186   The name of the <<def-relay-daemon,relay daemon>> program.
8187
8188 cmd:lttng-sessiond::
8189   The name of the <<def-session-daemon,session daemon>> program.
8190
8191 [[def-lttng-tools]]LTTng-tools::
8192   The https://github.com/lttng/lttng-tools[LTTng-tools] project, which
8193   contains the various programs and libraries used to
8194   <<controlling-tracing,control tracing>>.
8195
8196 [[def-lttng-ust]]<<lttng-ust,LTTng-UST>>::
8197   The https://github.com/lttng/lttng-ust[LTTng-UST] project, which
8198   contains libraries to instrument
8199   <<def-user-application,user applications>>.
8200
8201 <<lttng-ust-agents,LTTng-UST Java agent>>::
8202   A Java package provided by the <<def-lttng-ust,LTTng-UST>> project to
8203   allow the LTTng instrumentation of `java.util.logging` and Apache
8204   log4j{nbsp}1.2 logging statements.
8205
8206 <<lttng-ust-agents,LTTng-UST Python agent>>::
8207   A Python package provided by the <<def-lttng-ust,LTTng-UST>> project
8208   to allow the <<def-lttng,LTTng>> instrumentation of Python logging
8209   statements.
8210
8211 <<channel-overwrite-mode-vs-discard-mode,overwrite mode>>::
8212   The <<def-event-record-loss-mode,event record loss mode>> in which new
8213   <<def-event-record,event records>> _overwrite_ older event records
8214   when there's no <<def-sub-buffer,sub-buffer>> space left to store
8215   them.
8216
8217 <<channel-buffering-schemes,per-process buffering>>::
8218   A <<def-buffering-scheme,buffering scheme>> in which each instrumented
8219   process has its own <<def-sub-buffer,sub-buffers>> for a given user
8220   space <<def-channel,channel>>.
8221
8222 <<channel-buffering-schemes,per-user buffering>>::
8223   A <<def-buffering-scheme,buffering scheme>> in which all the processes
8224   of a Unix user share the same <<def-sub-buffer,sub-buffers>> for a
8225   given user space <<def-channel,channel>>.
8226
8227 [[def-proc-attr]]process attribute::
8228   In the <<pid-tracking,process attribute tracking>> context:
8229 +
8230 * A process ID.
8231 * A virtual process ID.
8232 * A Unix user ID.
8233 * A virtual Unix user ID.
8234 * A Unix group ID.
8235 * A virtual Unix group ID.
8236
8237 [[def-relay-daemon]]<<lttng-relayd,relay daemon>>::
8238   A process which is responsible for receiving the <<def-trace,trace>>
8239   data which a distant <<def-consumer-daemon,consumer daemon>> sends.
8240
8241 [[def-ring-buffer]]ring buffer::
8242   A set of <<def-sub-buffer,sub-buffers>>.
8243
8244 rotation::
8245   See _<<def-tracing-session-rotation,tracing session rotation>>_.
8246
8247 [[def-session-daemon]]<<lttng-sessiond,session daemon>>::
8248   A process which receives control commands from you and orchestrates
8249   the <<def-tracer,tracers>> and various <<def-lttng,LTTng>> daemons.
8250
8251 <<taking-a-snapshot,snapshot>>::
8252   A copy of the current data of all the <<def-sub-buffer,sub-buffers>>
8253   of a given <<def-tracing-session,tracing session>>, saved as
8254   <<def-trace,trace>> files.
8255
8256 [[def-sub-buffer]]sub-buffer::
8257   One part of an <<def-lttng,LTTng>> <<def-ring-buffer,ring buffer>>
8258   which contains <<def-event-record,event records>>.
8259
8260 timestamp::
8261   The time information attached to an <<def-event,event>> when it is
8262   emitted.
8263
8264 [[def-trace]]trace (_noun_)::
8265   A set of:
8266 +
8267 * One http://diamon.org/ctf/[CTF] metadata stream file.
8268 * One or more CTF data stream files which are the concatenations of one
8269   or more flushed <<def-sub-buffer,sub-buffers>>.
8270
8271 [[def-trace-verb]]trace (_verb_)::
8272   The action of recording the <<def-event,events>> emitted by an
8273   application or by a system, or to initiate such recording by
8274   controlling a <<def-tracer,tracer>>.
8275
8276 [[def-trace-chunk]]trace chunk::
8277   A self-contained <<def-trace,trace>> which is part of a
8278   <<def-tracing-session,tracing session>>. Each
8279   <<def-tracing-session-rotation, tracing session rotation>> produces a
8280   <<def-trace-chunk-archive,trace chunk archive>>.
8281
8282 [[def-trace-chunk-archive]]trace chunk archive::
8283   The result of a <<def-tracing-session-rotation, tracing session rotation>>.
8284 +
8285 <<def-lttng,LTTng>> doesn't manage any trace chunk archive, even if its
8286 containing <<def-tracing-session,tracing session>> is still active: you
8287 are free to read it, modify it, move it, or remove it.
8288
8289 Trace Compass::
8290   The http://tracecompass.org[Trace Compass] project and application.
8291
8292 [[def-tracepoint]]tracepoint::
8293   An instrumentation point using the tracepoint mechanism of the Linux
8294   kernel or of <<def-lttng-ust,LTTng-UST>>.
8295
8296 tracepoint definition::
8297   The definition of a single <<def-tracepoint,tracepoint>>.
8298
8299 tracepoint name::
8300   The name of a <<def-tracepoint,tracepoint>>.
8301
8302 [[def-tracepoint-provider]]tracepoint provider::
8303   A set of functions providing <<def-tracepoint,tracepoints>> to an
8304   instrumented <<def-user-application,user application>>.
8305 +
8306 Not to be confused with a <<def-tracepoint-provider-package,tracepoint
8307 provider package>>: many tracepoint providers can exist within a
8308 tracepoint provider package.
8309
8310 [[def-tracepoint-provider-package]]tracepoint provider package::
8311   One or more <<def-tracepoint-provider,tracepoint providers>> compiled
8312   as an https://en.wikipedia.org/wiki/Object_file[object file] or as a
8313   link:https://en.wikipedia.org/wiki/Library_(computing)#Shared_libraries[shared
8314   library].
8315
8316 [[def-tracer]]tracer::
8317   A software which records emitted <<def-event,events>>.
8318
8319 <<domain,tracing domain>>::
8320   A namespace for <<def-event,event>> sources.
8321
8322 <<tracing-group,tracing group>>::
8323   The Unix group in which a Unix user can be to be allowed to
8324   <<def-trace-verb,trace>> the Linux kernel.
8325
8326 [[def-tracing-session]]<<tracing-session,tracing session>>::
8327   A stateful dialogue between you and a <<lttng-sessiond,session daemon>>.
8328
8329 [[def-tracing-session-rotation]]<<session-rotation,tracing session rotation>>::
8330   The action of archiving the
8331   <<def-current-trace-chunk,current trace chunk>> of a
8332   <<def-tracing-session,tracing session>>.
8333
8334 tracked <<def-proc-attr,process attribute>>::
8335   A process attribute which is part of an <<def-incl-set,inclusion
8336   set>>.
8337
8338 untracked process attribute::
8339   A process attribute which isn't part of an <<def-incl-set,inclusion
8340   set>>.
8341
8342 [[def-user-application]]user application::
8343   An application running in user space, as opposed to a Linux kernel
8344   module, for example.