1 The LTTng Documentation
2 =======================
3 Philippe Proulx <pproulx@efficios.com>
7 include::../common/copyright.txt[]
10 include::../common/welcome.txt[]
13 include::../common/audience.txt[]
17 === What's in this documentation?
19 The LTTng Documentation is divided into the following sections:
21 * **<<nuts-and-bolts,Nuts and bolts>>** explains the
22 rudiments of software tracing and the rationale behind the
25 You can skip this section if you’re familiar with software tracing and
26 with the LTTng project.
28 * **<<installing-lttng,Installation>>** describes the steps to
29 install the LTTng packages on common Linux distributions and from
32 You can skip this section if you already properly installed LTTng on
35 * **<<getting-started,Quick start>>** is a concise guide to
36 getting started quickly with LTTng kernel and user space tracing.
38 We recommend this section if you're new to LTTng or to software tracing
41 You can skip this section if you're not new to LTTng.
43 * **<<core-concepts,Core concepts>>** explains the concepts at
46 It's a good idea to become familiar with the core concepts
47 before attempting to use the toolkit.
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.
55 Instrumenting source code is essential to provide a meaningful
58 You can skip this section if you do not have a programming background.
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.
68 include::../common/convention.txt[]
71 include::../common/acknowledgements.txt[]
75 == What's new in LTTng {revision}?
77 * **Tracing control**:
78 ** You can attach <<java-application-context,Java application-specific
79 context fields>> to a <<channel,channel>> with the
80 man:lttng-add-context(1) command:
85 lttng add-context --jul --type='$app.retriever:cur_msg_id'
89 Here, `$app` is the prefix of all application-specific context fields,
90 `retriever` names a _context information retriever_ defined at the
91 application level, and `cur_msg_id` names a context field read from this
94 Both the `java.util.logging` and Apache log4j <<domain,tracing domains>>
97 ** You can use Java application-specific <<adding-context,context>>
98 fields in the <<enabling-disabling-events,filter expression>> of an
104 lttng enable-event --log4j my_logger \
105 --filter='$app.retriever:cur_msg_id == 23'
109 ** New `lttng status` command which is the equivalent of +lttng list
110 __CUR__+, where +__CUR__+ is the name of the current
111 <<tracing-session,tracing session>>.
113 See man:lttng-status(1).
115 ** New `lttng metadata regenerate` command to
116 <<metadata-regenerate,regenerate the metadata file of an LTTng
117 trace>> at any moment. This command is meant to be used to resample
118 the wall time following a major
119 https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction
120 so that a system which boots with an incorrect wall time can be
121 traced before its wall time is NTP-corrected.
123 See man:lttng-metadata(1).
125 ** New command-line interface warnings when <<event,event records>> or
126 whole sub-buffers are
127 <<channel-overwrite-mode-vs-discard-mode,lost>>. The warning messages
128 are printed when a <<tracing-session,tracing session>> is
129 <<basic-tracing-session-control,stopped>> (man:lttng-stop(1)
132 * **User space tracing**:
133 ** Shared object base address dump in order to map <<event,event
134 records>> to original source location (file and line number).
136 If you attach the `ip` and `vpid` <<adding-context,context fields>> to a
137 user space <<channel,channel>> and if you use the
138 <<liblttng-ust-dl,path:{liblttng-ust-dl.so} helper>>, you can retrieve
139 the source location where a given event record was generated.
141 The http://diamon.org/babeltrace/[Babeltrace] trace viewer supports this
142 state dump and those context fields since version 1.4 to print the
143 source location of a given event record. http://tracecompass.org/[Trace
144 Compass] also supports this since version 2.0.
146 ** A <<java-application,Java application>> which uses
147 `java.util.logging` now adds an LTTng-UST log handler to the desired
150 The previous workflow was to initialize the LTTng-UST Java agent
151 by calling `LTTngAgent.getLTTngAgent()`. This had the effect of adding
152 an LTTng-UST log handler to the root loggers.
154 ** A <<java-application,Java application>> which uses Apache log4j now
155 adds an LTTng-UST log appender to the desired log4j loggers.
157 The previous workflow was to initialize the LTTng-UST Java agent
158 by calling `LTTngAgent.getLTTngAgent()`. This had the effect of adding
159 an LTTng-UST appender to the root loggers.
161 ** Any <<java-application,Java application>> can provide
162 <<java-application-context,dynamic context fields>> while running
163 thanks to a new API provided by the <<lttng-ust-agents,LTTng-UST Java
164 agent>>. You can require LTTng to record specific context fields in
165 event records, and you can use them in the filter expression of
166 <<event,event rules>>.
168 * **Linux kernel tracing**:
169 ** The LTTng kernel modules can now be built into a Linux kernel image,
170 that is, not as loadable modules.
173 https://github.com/lttng/lttng-modules/blob/stable-{revision}/README.md#kernel-built-in-support[`README.md`]
176 ** New instrumentation:
177 *** ARM64 architecture support.
179 *** x86 `irq_vectors`.
180 ** New <<adding-context,context fields>>:
183 *** `need_reschedule`
184 *** `migratable` (specific to RT-Preempt)
185 ** Clock source plugin support for advanced cases where a custom source
186 of time is needed to timestamp LTTng event records.
188 See https://github.com/lttng/lttng-modules/blob/stable-{revision}/lttng-clock.h[`lttng-clock.h`]
189 for an overview of the small API.
192 ** The link:/man[man pages] of the man:lttng(1) command-line tool are
193 split into one man page per command (à la Git), for example:
198 man lttng-enable-event
202 You can also use the `--help` option of any man:lttng(1) command to
205 The content and formatting of all the LTTng man pages has improved
212 What is LTTng? As its name suggests, the _Linux Trace Toolkit: next
213 generation_ is a modern toolkit for tracing Linux systems and
214 applications. So your first question might be:
221 As the history of software engineering progressed and led to what
222 we now take for granted--complex, numerous and
223 interdependent software applications running in parallel on
224 sophisticated operating systems like Linux--the authors of such
225 components, software developers, began feeling a natural
226 urge to have tools that would ensure the robustness and good performance
227 of their masterpieces.
229 One major achievement in this field is, inarguably, the
230 https://www.gnu.org/software/gdb/[GNU debugger (GDB)],
231 an essential tool for developers to find and fix bugs. But even the best
232 debugger won't help make your software run faster, and nowadays, faster
233 software means either more work done by the same hardware, or cheaper
234 hardware for the same work.
236 A _profiler_ is often the tool of choice to identify performance
237 bottlenecks. Profiling is suitable to identify _where_ performance is
238 lost in a given software. The profiler outputs a profile, a statistical
239 summary of observed events, which you may use to discover which
240 functions took the most time to execute. However, a profiler won't
241 report _why_ some identified functions are the bottleneck. Bottlenecks
242 might only occur when specific conditions are met, conditions that are
243 sometimes impossible to capture by a statistical profiler, or impossible
244 to reproduce with an application altered by the overhead of an
245 event-based profiler. For a thorough investigation of software
246 performance issues, a history of execution is essential, with the
247 recorded values of variables and context fields you choose, and
248 with as little influence as possible on the instrumented software. This
249 is where tracing comes in handy.
251 _Tracing_ is a technique used to understand what goes on in a running
252 software system. The software used for tracing is called a _tracer_,
253 which is conceptually similar to a tape recorder. When recording,
254 specific instrumentation points placed in the software source code
255 generate events that are saved on a giant tape: a _trace_ file. You
256 can trace user applications and the operating system at the same time,
257 opening the possibility of resolving a wide range of problems that would
258 otherwise be extremely challenging.
260 Tracing is often compared to _logging_. However, tracers and loggers are
261 two different tools, serving two different purposes. Tracers are
262 designed to record much lower-level events that occur much more
263 frequently than log messages, often in the range of thousands per
264 second, with very little execution overhead. Logging is more appropriate
265 for a very high-level analysis of less frequent events: user accesses,
266 exceptional conditions (errors and warnings, for example), database
267 transactions, instant messaging communications, and such. Simply put,
268 logging is one of the many use cases that can be satisfied with tracing.
270 The list of recorded events inside a trace file can be read manually
271 like a log file for the maximum level of detail, but it is generally
272 much more interesting to perform application-specific analyses to
273 produce reduced statistics and graphs that are useful to resolve a
274 given problem. Trace viewers and analyzers are specialized tools
277 In the end, this is what LTTng is: a powerful, open source set of
278 tools to trace the Linux kernel and user applications at the same time.
279 LTTng is composed of several components actively maintained and
280 developed by its link:/community/#where[community].
283 [[lttng-alternatives]]
284 === Alternatives to noch:{LTTng}
286 Excluding proprietary solutions, a few competing software tracers
289 * https://github.com/dtrace4linux/linux[dtrace4linux] is a port of
290 Sun Microsystems's DTrace to Linux. The cmd:dtrace tool interprets
291 user scripts and is responsible for loading code into the
292 Linux kernel for further execution and collecting the outputted data.
293 * https://en.wikipedia.org/wiki/Berkeley_Packet_Filter[eBPF] is a
294 subsystem in the Linux kernel in which a virtual machine can execute
295 programs passed from the user space to the kernel. You can attach
296 such programs to tracepoints and KProbes thanks to a system call, and
297 they can output data to the user space when executed thanks to
298 different mechanisms (pipe, VM register values, and eBPF maps, to name
300 * https://www.kernel.org/doc/Documentation/trace/ftrace.txt[ftrace]
301 is the de facto function tracer of the Linux kernel. Its user
302 interface is a set of special files in sysfs.
303 * https://perf.wiki.kernel.org/[perf] is
304 a performance analyzing tool for Linux which supports hardware
305 performance counters, tracepoints, as well as other counters and
306 types of probes. perf's controlling utility is the cmd:perf command
308 * http://linux.die.net/man/1/strace[strace]
309 is a command-line utility which records system calls made by a
310 user process, as well as signal deliveries and changes of process
311 state. strace makes use of https://en.wikipedia.org/wiki/Ptrace[ptrace]
312 to fulfill its function.
313 * http://www.sysdig.org/[sysdig], like SystemTap, uses scripts to
314 analyze Linux kernel events. You write scripts, or _chisels_ in
315 sysdig's jargon, in Lua and sysdig executes them while the system is
316 being traced or afterwards. sysdig's interface is the cmd:sysdig
317 command-line tool as well as the curses-based cmd:csysdig tool.
318 * https://sourceware.org/systemtap/[SystemTap] is a Linux kernel and
319 user space tracer which uses custom user scripts to produce plain text
320 traces. SystemTap converts the scripts to the C language, and then
321 compiles them as Linux kernel modules which are loaded to produce
322 trace data. SystemTap's primary user interface is the cmd:stap
325 The main distinctive features of LTTng is that it produces correlated
326 kernel and user space traces, as well as doing so with the lowest
327 overhead amongst other solutions. It produces trace files in the
328 http://diamon.org/ctf[CTF] format, a file format optimized
329 for the production and analyses of multi-gigabyte data.
331 LTTng is the result of more than 10 years of active open source
332 development by a community of passionate developers.
333 LTTng{nbsp}{revision} is currently available on major desktop and server
336 The main interface for tracing control is a single command-line tool
337 named cmd:lttng. The latter can create several tracing sessions, enable
338 and disable events on the fly, filter events efficiently with custom
339 user expressions, start and stop tracing, and much more. LTTng can
340 record the traces on the file system or send them over the network, and
341 keep them totally or partially. You can view the traces once tracing
342 becomes inactive or in real-time.
344 <<installing-lttng,Install LTTng now>> and
345 <<getting-started,start tracing>>!
351 **LTTng** is a set of software <<plumbing,components>> which interact to
352 <<instrumenting,instrument>> the Linux kernel and user applications, and
353 to <<controlling-tracing,control tracing>> (start and stop
354 tracing, enable and disable event rules, and the rest). Those
355 components are bundled into the following packages:
357 * **LTTng-tools**: Libraries and command-line interface to
359 * **LTTng-modules**: Linux kernel modules to instrument and
361 * **LTTng-UST**: Libraries and Java/Python packages to instrument and
362 trace user applications.
364 Most distributions mark the LTTng-modules and LTTng-UST packages as
365 optional when installing LTTng-tools (which is always required). In the
366 following sections, we always provide the steps to install all three,
369 * You only need to install LTTng-modules if you intend to trace the
371 * You only need to install LTTng-UST if you intend to trace user
375 .Availability of LTTng{nbsp}{revision} for major Linux distributions.
377 |Distribution |Available in releases |Alternatives
380 |<<ubuntu,Ubuntu{nbsp}16.10 _Yakkety Yak_>>
381 |LTTng{nbsp}{revision} for Ubuntu{nbsp}14.04 _Trusty Tahr_
382 and Ubuntu{nbsp}16.04 _Xenial Xerus_:
383 <<ubuntu-ppa,use the LTTng Stable{nbsp}{revision} PPA>>.
385 <<building-from-source,Build LTTng{nbsp}{revision} from source>> for
386 other Ubuntu releases.
390 |LTTng-tools{nbsp}{revision} and LTTng-UST{nbsp}{revision} for
391 Fedora{nbsp}25 and Fedora{nbsp}26 (both are not released yet).
393 <<building-from-source,Build LTTng-modules{nbsp}{revision} from
396 <<building-from-source,Build LTTng{nbsp}{revision} from source>> for
397 other Fedora releases.
400 |<<debian,Debian "stretch" (testing)>>
402 <<building-from-source,Build LTTng{nbsp}{revision} from source>> for
403 previous Debian releases.
407 |LTTng{nbsp}2.7 for openSUSE Leap{nbsp}42.1.
409 <<building-from-source,Build LTTng{nbsp}{revision} from source>> for
410 other openSUSE releases.
413 |Latest AUR packages.
417 |<<alpine-linux,Alpine Linux "edge">>
418 |LTTng{nbsp}{revision} for Alpine Linux{nbsp}3.5 (not released yet).
420 <<building-from-source,Build LTTng{nbsp}{revision} from source>> for
421 other Alpine Linux releases.
424 |See http://packages.efficios.com/[EfficiOS Enterprise Packages].
429 |LTTng{nbsp}{revision} for Buildroot{nbsp}2016.11 (not released yet).
431 LTTng{nbsp}2.7 for Buildroot{nbsp}2016.02, Buildroot{nbsp}2016.05,
432 and Buildroot{nbsp}2016.08.
434 <<building-from-source,Build LTTng{nbsp}{revision} from source>> for
435 other Buildroot releases.
437 |OpenEmbedded and Yocto
438 |<<oe-yocto,Yocto Project{nbsp}2.2 _Morty_>> (`openembedded-core` layer)
439 |<<building-from-source,Build LTTng{nbsp}{revision} from source>> for
440 other OpenEmbedded releases.
445 === [[ubuntu-official-repositories]]Ubuntu
447 LTTng{nbsp}{revision} is available on Ubuntu{nbsp}16.10 _Yakkety Yak_.
448 For previous releases of Ubuntu, <<ubuntu-ppa,use the LTTng
449 Stable{nbsp}{revision} PPA>>.
451 To install LTTng{nbsp}{revision} on Ubuntu 16.10{nbsp}_Yakkety Yak_:
453 . Install the main LTTng{nbsp}{revision} packages:
458 sudo apt-get install lttng-tools
459 sudo apt-get install lttng-modules-dkms
460 sudo apt-get install liblttng-ust-dev
464 . **If you need to instrument and trace
465 <<java-application,Java applications>>**, install the LTTng-UST
471 sudo apt-get install liblttng-ust-agent-java
475 . **If you need to instrument and trace
476 <<python-application,Python applications>>**, install the
477 LTTng-UST Python agent:
482 sudo apt-get install python3-lttngust
488 ==== noch:{LTTng} Stable {revision} PPA
490 The https://launchpad.net/~lttng/+archive/ubuntu/stable-{revision}[LTTng
491 Stable{nbsp}{revision} PPA] offers the latest stable
492 LTTng{nbsp}{revision} packages for:
494 * Ubuntu{nbsp}14.04 _Trusty Tahr_
495 * Ubuntu{nbsp}16.04 _Xenial Xerus_
497 To install LTTng{nbsp}{revision} from the LTTng Stable{nbsp}{revision} PPA:
499 . Add the LTTng Stable{nbsp}{revision} PPA repository and update the
505 sudo apt-add-repository ppa:lttng/stable-2.8
510 . Install the main LTTng{nbsp}{revision} packages:
515 sudo apt-get install lttng-tools
516 sudo apt-get install lttng-modules-dkms
517 sudo apt-get install liblttng-ust-dev
521 . **If you need to instrument and trace
522 <<java-application,Java applications>>**, install the LTTng-UST
528 sudo apt-get install liblttng-ust-agent-java
532 . **If you need to instrument and trace
533 <<python-application,Python applications>>**, install the
534 LTTng-UST Python agent:
539 sudo apt-get install python3-lttngust
547 To install LTTng{nbsp}{revision} on Debian "stretch" (testing):
549 . Install the main LTTng{nbsp}{revision} packages:
554 sudo apt-get install lttng-modules-dkms
555 sudo apt-get install liblttng-ust-dev
556 sudo apt-get install lttng-tools
560 . **If you need to instrument and trace <<java-application,Java
561 applications>>**, install the LTTng-UST Java agent:
566 sudo apt-get install liblttng-ust-agent-java
570 . **If you need to instrument and trace <<python-application,Python
571 applications>>**, install the LTTng-UST Python agent:
576 sudo apt-get install python3-lttngust
584 To install LTTng{nbsp}{revision} (tracing control and user space
585 tracing) on Alpine Linux "edge":
587 . Make sure your system is
588 https://wiki.alpinelinux.org/wiki/Edge[configured for "edge"].
589 . Enable the _testing_ repository by uncommenting the corresponding
590 line in path:{/etc/apk/repositories}.
591 . Add the LTTng packages:
596 sudo apk add lttng-tools
597 sudo apk add lttng-ust-dev
601 To install LTTng-modules{nbsp}{revision} (Linux kernel tracing support)
602 on Alpine Linux "edge":
604 . Add the vanilla Linux kernel:
609 apk add linux-vanilla linux-vanilla-dev
613 . Reboot with the vanilla Linux kernel.
614 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
620 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.8.tar.bz2 &&
621 tar -xf lttng-modules-latest-2.8.tar.bz2 &&
622 cd lttng-modules-2.8.* &&
624 sudo make modules_install &&
631 === OpenEmbedded and Yocto
633 LTTng{nbsp}{revision} recipes are available in the
634 http://layers.openembedded.org/layerindex/branch/master/layer/openembedded-core/[`openembedded-core`]
635 layer for Yocto Project{nbsp}2.2 _Morty_ under the following names:
641 With BitBake, the simplest way to include LTTng recipes in your target
642 image is to add them to `IMAGE_INSTALL_append` in path:{conf/local.conf}:
645 IMAGE_INSTALL_append = " lttng-tools lttng-modules lttng-ust"
650 . Select a machine and an image recipe.
651 . Click **Edit image recipe**.
652 . Under the **All recipes** tab, search for **lttng**.
653 . Check the desired LTTng recipes.
656 .Java and Python application instrumentation and tracing
658 If you need to instrument and trace <<java-application,Java
659 applications>> on openSUSE, you need to build and install
660 LTTng-UST{nbsp}{revision} <<building-from-source,from source>> and pass
661 the `--enable-java-agent-jul`, `--enable-java-agent-log4j`, or
662 `--enable-java-agent-all` options to the `configure` script, depending
663 on which Java logging framework you use.
665 If you need to instrument and trace <<python-application,Python
666 applications>> on openSUSE, you need to build and install
667 LTTng-UST{nbsp}{revision} from source and pass the
668 `--enable-python-agent` option to the `configure` script.
672 [[enterprise-distributions]]
673 === RHEL, SUSE, and other enterprise distributions
675 To install LTTng on enterprise Linux distributions, such as Red Hat
676 Enterprise Linux (RHEL) and SUSE Linux Enterprise Server (SUSE), please
677 see http://packages.efficios.com/[EfficiOS Enterprise Packages].
680 [[building-from-source]]
681 === Build from source
683 To build and install LTTng{nbsp}{revision} from source:
685 . Using your distribution's package manager, or from source, install
686 the following dependencies of LTTng-tools and LTTng-UST:
689 * https://sourceforge.net/projects/libuuid/[libuuid]
690 * http://directory.fsf.org/wiki/Popt[popt]
691 * http://liburcu.org/[Userspace RCU]
692 * http://www.xmlsoft.org/[libxml2]
695 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
701 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.8.tar.bz2 &&
702 tar -xf lttng-modules-latest-2.8.tar.bz2 &&
703 cd lttng-modules-2.8.* &&
705 sudo make modules_install &&
710 . Download, build, and install the latest LTTng-UST{nbsp}{revision}:
716 wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.8.tar.bz2 &&
717 tar -xf lttng-ust-latest-2.8.tar.bz2 &&
718 cd lttng-ust-2.8.* &&
728 .Java and Python application tracing
730 If you need to instrument and trace <<java-application,Java
731 applications>>, pass the `--enable-java-agent-jul`,
732 `--enable-java-agent-log4j`, or `--enable-java-agent-all` options to the
733 `configure` script, depending on which Java logging framework you use.
735 If you need to instrument and trace <<python-application,Python
736 applications>>, pass the `--enable-python-agent` option to the
737 `configure` script. You can set the `PYTHON` environment variable to the
738 path to the Python interpreter for which to install the LTTng-UST Python
746 By default, LTTng-UST libraries are installed to
747 dir:{/usr/local/lib}, which is the de facto directory in which to
748 keep self-compiled and third-party libraries.
750 When <<building-tracepoint-providers-and-user-application,linking an
751 instrumented user application with `liblttng-ust`>>:
753 * Append `/usr/local/lib` to the env:LD_LIBRARY_PATH environment
755 * Pass the `-L/usr/local/lib` and `-Wl,-rpath,/usr/local/lib` options to
756 man:gcc(1), man:g++(1), or man:clang(1).
760 . Download, build, and install the latest LTTng-tools{nbsp}{revision}:
766 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.8.tar.bz2 &&
767 tar -xf lttng-tools-latest-2.8.tar.bz2 &&
768 cd lttng-tools-2.8.* &&
776 TIP: The https://github.com/eepp/vlttng[vlttng tool] can do all the
777 previous steps automatically for a given version of LTTng and confine
778 the installed files in a specific directory. This can be useful to test
779 LTTng without installing it on your system.
785 This is a short guide to get started quickly with LTTng kernel and user
788 Before you follow this guide, make sure to <<installing-lttng,install>>
791 This tutorial walks you through the steps to:
793 . <<tracing-the-linux-kernel,Trace the Linux kernel>>.
794 . <<tracing-your-own-user-application,Trace a user application>> written
796 . <<viewing-and-analyzing-your-traces,View and analyze the
800 [[tracing-the-linux-kernel]]
801 === Trace the Linux kernel
803 The following command lines start with cmd:sudo because you need root
804 privileges to trace the Linux kernel. You can avoid using cmd:sudo if
805 your Unix user is a member of the <<lttng-sessiond,tracing group>>.
807 . Create a <<tracing-session,tracing session>>:
812 sudo lttng create my-kernel-session
816 . List the available kernel tracepoints and system calls:
825 . Create an <<event,event rule>> which matches the desired event names,
826 for example `sched_switch` and `sched_process_fork`:
831 sudo lttng enable-event --kernel sched_switch,sched_process_fork
835 You can also create an event rule which _matches_ all the Linux kernel
836 tracepoints (this will generate a lot of data when tracing):
841 sudo lttng enable-event --kernel --all
854 . Do some operation on your system for a few seconds. For example,
855 load a website, or list the files of a directory.
856 . Stop tracing and destroy the tracing session:
866 The man:lttng-destroy(1) command does not destroy the trace data; it
867 only destroys the state of the tracing session.
869 By default, LTTng saves the traces in
870 +$LTTNG_HOME/lttng-traces/__name__-__date__-__time__+,
871 where +__name__+ is the tracing session name. Note that the
872 env:LTTNG_HOME environment variable defaults to `$HOME` if not set.
874 See <<viewing-and-analyzing-your-traces,View and analyze the
875 recorded events>> to view the recorded events.
878 [[tracing-your-own-user-application]]
879 === Trace a user application
881 This section steps you through a simple example to trace a
882 _Hello world_ program written in C.
884 To create the traceable user application:
886 . Create the tracepoint provider header file, which defines the
887 tracepoints and the events they can generate:
893 #undef TRACEPOINT_PROVIDER
894 #define TRACEPOINT_PROVIDER hello_world
896 #undef TRACEPOINT_INCLUDE
897 #define TRACEPOINT_INCLUDE "./hello-tp.h"
899 #if !defined(_HELLO_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
902 #include <lttng/tracepoint.h>
912 ctf_string(my_string_field, my_string_arg)
913 ctf_integer(int, my_integer_field, my_integer_arg)
917 #endif /* _HELLO_TP_H */
919 #include <lttng/tracepoint-event.h>
923 . Create the tracepoint provider package source file:
929 #define TRACEPOINT_CREATE_PROBES
930 #define TRACEPOINT_DEFINE
932 #include "hello-tp.h"
936 . Build the tracepoint provider package:
941 gcc -c -I. hello-tp.c
945 . Create the _Hello World_ application source file:
952 #include "hello-tp.h"
954 int main(int argc, char *argv[])
958 puts("Hello, World!\nPress Enter to continue...");
961 * The following getchar() call is only placed here for the purpose
962 * of this demonstration, to pause the application in order for
963 * you to have time to list its tracepoints. It is not
969 * A tracepoint() call.
971 * Arguments, as defined in hello-tp.h:
973 * 1. Tracepoint provider name (required)
974 * 2. Tracepoint name (required)
975 * 3. my_integer_arg (first user-defined argument)
976 * 4. my_string_arg (second user-defined argument)
978 * Notice the tracepoint provider and tracepoint names are
979 * NOT strings: they are in fact parts of variables that the
980 * macros in hello-tp.h create.
982 tracepoint(hello_world, my_first_tracepoint, 23, "hi there!");
984 for (x = 0; x < argc; ++x) {
985 tracepoint(hello_world, my_first_tracepoint, x, argv[x]);
988 puts("Quitting now!");
989 tracepoint(hello_world, my_first_tracepoint, x * x, "x^2");
996 . Build the application:
1005 . Link the application with the tracepoint provider package,
1006 `liblttng-ust`, and `libdl`:
1011 gcc -o hello hello.o hello-tp.o -llttng-ust -ldl
1015 Here's the whole build process:
1018 .User space tracing tutorial's build steps.
1019 image::ust-flow.png[]
1021 To trace the user application:
1023 . Run the application with a few arguments:
1028 ./hello world and beyond
1037 Press Enter to continue...
1041 . Start an LTTng <<lttng-sessiond,session daemon>>:
1046 lttng-sessiond --daemonize
1050 Note that a session daemon might already be running, for example as
1051 a service that the distribution's service manager started.
1053 . List the available user space tracepoints:
1058 lttng list --userspace
1062 You see the `hello_world:my_first_tracepoint` tracepoint listed
1063 under the `./hello` process.
1065 . Create a <<tracing-session,tracing session>>:
1070 lttng create my-user-space-session
1074 . Create an <<event,event rule>> which matches the
1075 `hello_world:my_first_tracepoint` event name:
1080 lttng enable-event --userspace hello_world:my_first_tracepoint
1093 . Go back to the running `hello` application and press Enter. The
1094 program executes all `tracepoint()` instrumentation points and exits.
1095 . Stop tracing and destroy the tracing session:
1105 The man:lttng-destroy(1) command does not destroy the trace data; it
1106 only destroys the state of the tracing session.
1108 By default, LTTng saves the traces in
1109 +$LTTNG_HOME/lttng-traces/__name__-__date__-__time__+,
1110 where +__name__+ is the tracing session name. Note that the
1111 env:LTTNG_HOME environment variable defaults to `$HOME` if not set.
1113 See <<viewing-and-analyzing-your-traces,View and analyze the
1114 recorded events>> to view the recorded events.
1117 [[viewing-and-analyzing-your-traces]]
1118 === View and analyze the recorded events
1120 Once you have completed the <<tracing-the-linux-kernel,Trace the Linux
1121 kernel>> and <<tracing-your-own-user-application,Trace a user
1122 application>> tutorials, you can inspect the recorded events.
1124 Many tools are available to read LTTng traces:
1126 * **cmd:babeltrace** is a command-line utility which converts trace
1127 formats; it supports the format that LTTng produces, CTF, as well as a
1128 basic text output which can be ++grep++ed. The cmd:babeltrace command
1129 is part of the http://diamon.org/babeltrace[Babeltrace] project.
1130 * Babeltrace also includes
1131 **https://www.python.org/[Python] bindings** so
1132 that you can easily open and read an LTTng trace with your own script,
1133 benefiting from the power of Python.
1134 * http://tracecompass.org/[**Trace Compass**]
1135 is a graphical user interface for viewing and analyzing any type of
1136 logs or traces, including LTTng's.
1137 * https://github.com/lttng/lttng-analyses[**LTTng analyses**] is a
1138 project which includes many high-level analyses of LTTng kernel
1139 traces, like scheduling statistics, interrupt frequency distribution,
1140 top CPU usage, and more.
1142 NOTE: This section assumes that the traces recorded during the previous
1143 tutorials were saved to their default location, in the
1144 dir:{$LTTNG_HOME/lttng-traces} directory. Note that the env:LTTNG_HOME
1145 environment variable defaults to `$HOME` if not set.
1148 [[viewing-and-analyzing-your-traces-bt]]
1149 ==== Use the cmd:babeltrace command-line tool
1151 The simplest way to list all the recorded events of a trace is to pass
1152 its path to cmd:babeltrace with no options:
1156 babeltrace ~/lttng-traces/my-user-space-session*
1159 cmd:babeltrace finds all traces recursively within the given path and
1160 prints all their events, merging them in chronological order.
1162 You can pipe the output of cmd:babeltrace into a tool like man:grep(1) for
1167 babeltrace ~/lttng-traces/my-kernel-session* | grep sys_
1170 You can pipe the output of cmd:babeltrace into a tool like man:wc(1) to
1171 count the recorded events:
1175 babeltrace ~/lttng-traces/my-kernel-session* | grep sys_read | wc --lines
1179 [[viewing-and-analyzing-your-traces-bt-python]]
1180 ==== Use the Babeltrace Python bindings
1182 The <<viewing-and-analyzing-your-traces-bt,text output of cmd:babeltrace>>
1183 is useful to isolate events by simple matching using man:grep(1) and
1184 similar utilities. However, more elaborate filters, such as keeping only
1185 event records with a field value falling within a specific range, are
1186 not trivial to write using a shell. Moreover, reductions and even the
1187 most basic computations involving multiple event records are virtually
1188 impossible to implement.
1190 Fortunately, Babeltrace ships with Python 3 bindings which makes it easy
1191 to read the event records of an LTTng trace sequentially and compute the
1192 desired information.
1194 The following script accepts an LTTng Linux kernel trace path as its
1195 first argument and prints the short names of the top 5 running processes
1196 on CPU 0 during the whole trace:
1201 from collections import Counter
1207 if len(sys.argv) != 2:
1208 msg = 'Usage: python3 {} TRACEPATH'.format(sys.argv[0])
1209 print(msg, file=sys.stderr)
1212 # A trace collection contains one or more traces
1213 col = babeltrace.TraceCollection()
1215 # Add the trace provided by the user (LTTng traces always have
1217 if col.add_trace(sys.argv[1], 'ctf') is None:
1218 raise RuntimeError('Cannot add trace')
1220 # This counter dict contains execution times:
1222 # task command name -> total execution time (ns)
1223 exec_times = Counter()
1225 # This contains the last `sched_switch` timestamp
1229 for event in col.events:
1230 # Keep only `sched_switch` events
1231 if event.name != 'sched_switch':
1234 # Keep only events which happened on CPU 0
1235 if event['cpu_id'] != 0:
1239 cur_ts = event.timestamp
1245 # Previous task command (short) name
1246 prev_comm = event['prev_comm']
1248 # Initialize entry in our dict if not yet done
1249 if prev_comm not in exec_times:
1250 exec_times[prev_comm] = 0
1252 # Compute previous command execution time
1253 diff = cur_ts - last_ts
1255 # Update execution time of this command
1256 exec_times[prev_comm] += diff
1258 # Update last timestamp
1262 for name, ns in exec_times.most_common(5):
1264 print('{:20}{} s'.format(name, s))
1269 if __name__ == '__main__':
1270 sys.exit(0 if top5proc() else 1)
1277 python3 top5proc.py ~/lttng-traces/my-kernel-session*/kernel
1283 swapper/0 48.607245889 s
1284 chromium 7.192738188 s
1285 pavucontrol 0.709894415 s
1286 Compositor 0.660867933 s
1287 Xorg.bin 0.616753786 s
1290 Note that `swapper/0` is the "idle" process of CPU 0 on Linux; since we
1291 weren't using the CPU that much when tracing, its first position in the
1296 == [[understanding-lttng]]Core concepts
1298 From a user's perspective, the LTTng system is built on a few concepts,
1299 or objects, on which the <<lttng-cli,cmd:lttng command-line tool>>
1300 operates by sending commands to the <<lttng-sessiond,session daemon>>.
1301 Understanding how those objects relate to eachother is key in mastering
1304 The core concepts are:
1306 * <<tracing-session,Tracing session>>
1307 * <<domain,Tracing domain>>
1308 * <<channel,Channel and ring buffer>>
1309 * <<"event","Instrumentation point, event rule, event, and event record">>
1315 A _tracing session_ is a stateful dialogue between you and
1316 a <<lttng-sessiond,session daemon>>. You can
1317 <<creating-destroying-tracing-sessions,create a new tracing
1318 session>> with the `lttng create` command.
1320 Anything that you do when you control LTTng tracers happens within a
1321 tracing session. In particular, a tracing session:
1324 * Has its own set of trace files.
1325 * Has its own state of activity (started or stopped).
1326 * Has its own <<tracing-session-mode,mode>> (local, network streaming,
1328 * Has its own <<channel,channels>> which have their own
1329 <<event,event rules>>.
1332 .A _tracing session_ contains <<channel,channels>> that are members of <<domain,tracing domains>> and contain <<event,event rules>>.
1333 image::concepts.png[]
1335 Those attributes and objects are completely isolated between different
1338 A tracing session is analogous to a cash machine session:
1339 the operations you do on the banking system through the cash machine do
1340 not alter the data of other users of the same system. In the case of
1341 the cash machine, a session lasts as long as your bank card is inside.
1342 In the case of LTTng, a tracing session lasts from the `lttng create`
1343 command to the `lttng destroy` command.
1346 .Each Unix user has its own set of tracing sessions.
1347 image::many-sessions.png[]
1350 [[tracing-session-mode]]
1351 ==== Tracing session mode
1353 LTTng can send the generated trace data to different locations. The
1354 _tracing session mode_ dictates where to send it. The following modes
1355 are available in LTTng{nbsp}{revision}:
1358 LTTng writes the traces to the file system of the machine being traced
1361 Network streaming mode::
1362 LTTng sends the traces over the network to a
1363 <<lttng-relayd,relay daemon>> running on a remote system.
1366 LTTng does not write the traces by default. Instead, you can request
1367 LTTng to <<taking-a-snapshot,take a snapshot>>, that is, a copy of the
1368 current tracing buffers, and to write it to the target's file system
1369 or to send it over the network to a <<lttng-relayd,relay daemon>>
1370 running on a remote system.
1373 This mode is similar to the network streaming mode, but a live
1374 trace viewer can connect to the distant relay daemon to
1375 <<lttng-live,view event records as LTTng generates them>> by
1382 A _tracing domain_ is a namespace for event sources. A tracing domain
1383 has its own properties and features.
1385 There are currently five available tracing domains:
1389 * `java.util.logging` (JUL)
1393 You must specify a tracing domain when using some commands to avoid
1394 ambiguity. For example, since all the domains support named tracepoints
1395 as event sources (instrumentation points that you manually insert in the
1396 source code), you need to specify a tracing domain when
1397 <<enabling-disabling-events,creating an event rule>> because all the
1398 tracing domains could have tracepoints with the same names.
1400 Some features are reserved to specific tracing domains. Dynamic function
1401 entry and return instrumentation points, for example, are currently only
1402 supported in the Linux kernel tracing domain, but support for other
1403 tracing domains could be added in the future.
1405 You can create <<channel,channels>> in the Linux kernel and user space
1406 tracing domains. The other tracing domains have a single default
1411 === Channel and ring buffer
1413 A _channel_ is an object which is responsible for a set of ring buffers.
1414 Each ring buffer is divided into multiple sub-buffers. When an LTTng
1415 tracer emits an event, it can record it to one or more
1416 sub-buffers. The attributes of a channel determine what to do when
1417 there's no space left for a new event record because all sub-buffers
1418 are full, where to send a full sub-buffer, and other behaviours.
1420 A channel is always associated to a <<domain,tracing domain>>. The
1421 `java.util.logging` (JUL), log4j, and Python tracing domains each have
1422 a default channel which you cannot configure.
1424 A channel also owns <<event,event rules>>. When an LTTng tracer emits
1425 an event, it records it to the sub-buffers of all
1426 the enabled channels with a satisfied event rule, as long as those
1427 channels are part of active <<tracing-session,tracing sessions>>.
1430 [[channel-buffering-schemes]]
1431 ==== Per-user vs. per-process buffering schemes
1433 A channel has at least one ring buffer _per CPU_. LTTng always
1434 records an event to the ring buffer associated to the CPU on which it
1437 Two _buffering schemes_ are available when you
1438 <<enabling-disabling-channels,create a channel>> in the
1439 user space <<domain,tracing domain>>:
1441 Per-user buffering::
1442 Allocate one set of ring buffers--one per CPU--shared by all the
1443 instrumented processes of each Unix user.
1447 .Per-user buffering scheme.
1448 image::per-user-buffering.png[]
1451 Per-process buffering::
1452 Allocate one set of ring buffers--one per CPU--for each
1453 instrumented process.
1457 .Per-process buffering scheme.
1458 image::per-process-buffering.png[]
1461 The per-process buffering scheme tends to consume more memory than the
1462 per-user option because systems generally have more instrumented
1463 processes than Unix users running instrumented processes. However, the
1464 per-process buffering scheme ensures that one process having a high
1465 event throughput won't fill all the shared sub-buffers of the same
1468 The Linux kernel tracing domain has only one available buffering scheme
1469 which is to allocate a single set of ring buffers for the whole system.
1470 This scheme is similar to the per-user option, but with a single, global
1471 user "running" the kernel.
1474 [[channel-overwrite-mode-vs-discard-mode]]
1475 ==== Overwrite vs. discard event loss modes
1477 When an event occurs, LTTng records it to a specific sub-buffer (yellow
1478 arc in the following animation) of a specific channel's ring buffer.
1479 When there's no space left in a sub-buffer, the tracer marks it as
1480 consumable (red) and another, empty sub-buffer starts receiving the
1481 following event records. A <<lttng-consumerd,consumer daemon>>
1482 eventually consumes the marked sub-buffer (returns to white).
1485 [role="docsvg-channel-subbuf-anim"]
1490 In an ideal world, sub-buffers are consumed faster than they are filled,
1491 as is the case in the previous animation. In the real world,
1492 however, all sub-buffers can be full at some point, leaving no space to
1493 record the following events.
1495 By design, LTTng is a _non-blocking_ tracer: when no empty sub-buffer is
1496 available, it is acceptable to lose event records when the alternative
1497 would be to cause substantial delays in the instrumented application's
1498 execution. LTTng privileges performance over integrity; it aims at
1499 perturbing the traced system as little as possible in order to make
1500 tracing of subtle race conditions and rare interrupt cascades possible.
1502 When it comes to losing event records because no empty sub-buffer is
1503 available, the channel's _event loss mode_ determines what to do. The
1504 available event loss modes are:
1507 Drop the newest event records until a the tracer
1508 releases a sub-buffer.
1511 Clear the sub-buffer containing the oldest event records and start
1512 writing the newest event records there.
1514 This mode is sometimes called _flight recorder mode_ because it's
1516 https://en.wikipedia.org/wiki/Flight_recorder[flight recorder]:
1517 always keep a fixed amount of the latest data.
1519 Which mechanism you should choose depends on your context: prioritize
1520 the newest or the oldest event records in the ring buffer?
1522 Beware that, in overwrite mode, the tracer abandons a whole sub-buffer
1523 as soon as a there's no space left for a new event record, whereas in
1524 discard mode, the tracer only discards the event record that doesn't
1527 In discard mode, LTTng increments a count of lost event records when
1528 an event record is lost and saves this count to the trace. In
1529 overwrite mode, LTTng keeps no information when it overwrites a
1530 sub-buffer before consuming it.
1532 There are a few ways to decrease your probability of losing event
1534 <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>> shows
1535 how you can fine-une the sub-buffer count and size of a channel to
1536 virtually stop losing event records, though at the cost of greater
1540 [[channel-subbuf-size-vs-subbuf-count]]
1541 ==== Sub-buffer count and size
1543 When you <<enabling-disabling-channels,create a channel>>, you can
1544 set its number of sub-buffers and their size.
1546 Note that there is noticeable CPU overhead introduced when
1547 switching sub-buffers (marking a full one as consumable and switching
1548 to an empty one for the following events to be recorded). Knowing this,
1549 the following list presents a few practical situations along with how
1550 to configure the sub-buffer count and size for them:
1552 * **High event throughput**: In general, prefer bigger sub-buffers to
1553 lower the risk of losing event records.
1555 Having bigger sub-buffers also ensures a lower sub-buffer switching
1558 The number of sub-buffers is only meaningful if you create the channel
1559 in overwrite mode: in this case, if a sub-buffer overwrite happens, the
1560 other sub-buffers are left unaltered.
1562 * **Low event throughput**: In general, prefer smaller sub-buffers
1563 since the risk of losing event records is low.
1565 Because events occur less frequently, the sub-buffer switching frequency
1566 should remain low and thus the tracer's overhead should not be a
1569 * **Low memory system**: If your target system has a low memory
1570 limit, prefer fewer first, then smaller sub-buffers.
1572 Even if the system is limited in memory, you want to keep the
1573 sub-buffers as big as possible to avoid a high sub-buffer switching
1576 Note that LTTng uses http://diamon.org/ctf/[CTF] as its trace format,
1577 which means event data is very compact. For example, the average
1578 LTTng kernel event record weights about 32{nbsp}bytes. Thus, a
1579 sub-buffer size of 1{nbsp}MiB is considered big.
1581 The previous situations highlight the major trade-off between a few big
1582 sub-buffers and more, smaller sub-buffers: sub-buffer switching
1583 frequency vs. how much data is lost in overwrite mode. Assuming a
1584 constant event throughput and using the overwrite mode, the two
1585 following configurations have the same ring buffer total size:
1588 [role="docsvg-channel-subbuf-size-vs-count-anim"]
1593 * **2 sub-buffers of 4{nbsp}MiB each**: Expect a very low sub-buffer
1594 switching frequency, but if a sub-buffer overwrite happens, half of
1595 the event records so far (4{nbsp}MiB) are definitely lost.
1596 * **8 sub-buffers of 1{nbsp}MiB each**: Expect 4{nbsp}times the tracer's
1597 overhead as the previous configuration, but if a sub-buffer
1598 overwrite happens, only the eighth of event records so far are
1601 In discard mode, the sub-buffers count parameter is pointless: use two
1602 sub-buffers and set their size according to the requirements of your
1606 [[channel-switch-timer]]
1607 ==== Switch timer period
1609 The _switch timer period_ is an important configurable attribute of
1610 a channel to ensure periodic sub-buffer flushing.
1612 When the _switch timer_ expires, a sub-buffer switch happens. You can
1613 set the switch timer period attribute when you
1614 <<enabling-disabling-channels,create a channel>> to ensure that event
1615 data is consumed and committed to trace files or to a distant relay
1616 daemon periodically in case of a low event throughput.
1619 [role="docsvg-channel-switch-timer"]
1624 This attribute is also convenient when you use big sub-buffers to cope
1625 with a sporadic high event throughput, even if the throughput is
1629 [[channel-read-timer]]
1630 ==== Read timer period
1632 By default, the LTTng tracers use a notification mechanism to signal a
1633 full sub-buffer so that a consumer daemon can consume it. When such
1634 notifications must be avoided, for example in real-time applications,
1635 you can use the channel's _read timer_ instead. When the read timer
1636 fires, the <<lttng-consumerd,consumer daemon>> checks for full,
1637 consumable sub-buffers.
1640 [[tracefile-rotation]]
1641 ==== Trace file count and size
1643 By default, trace files can grow as large as needed. You can set the
1644 maximum size of each trace file that a channel writes when you
1645 <<enabling-disabling-channels,create a channel>>. When the size of
1646 a trace file reaches the channel's fixed maximum size, LTTng creates
1647 another file to contain the next event records. LTTng appends a file
1648 count to each trace file name in this case.
1650 If you set the trace file size attribute when you create a channel, the
1651 maximum number of trace files that LTTng creates is _unlimited_ by
1652 default. To limit them, you can also set a maximum number of trace
1653 files. When the number of trace files reaches the channel's fixed
1654 maximum count, the oldest trace file is overwritten. This mechanism is
1655 called _trace file rotation_.
1659 === Instrumentation point, event rule, event, and event record
1661 An _event rule_ is a set of conditions which must be **all** satisfied
1662 for LTTng to record an occuring event.
1664 You set the conditions when you <<enabling-disabling-events,create
1667 You always attach an event rule to <<channel,channel>> when you create
1670 When an event passes the conditions of an event rule, LTTng records it
1671 in one of the attached channel's sub-buffers.
1673 The available conditions, as of LTTng{nbsp}{revision}, are:
1675 * The event rule _is enabled_.
1676 * The instrumentation point's type _is{nbsp}T_.
1677 * The instrumentation point's name (sometimes called _event name_)
1678 _matches{nbsp}N_, but _is not{nbsp}E_.
1679 * The instrumentation point's log level _is as severe as{nbsp}L_, or
1680 _is exactly{nbsp}L_.
1681 * The fields of the event's payload _satisfy_ a filter
1682 expression{nbsp}__F__.
1684 As you can see, all the conditions but the dynamic filter are related to
1685 the event rule's status or to the instrumentation point, not to the
1686 occurring events. This is why, without a filter, checking if an event
1687 passes an event rule is not a dynamic task: when you create or modify an
1688 event rule, all the tracers of its tracing domain enable or disable the
1689 instrumentation points themselves once. This is possible because the
1690 attributes of an instrumentation point (type, name, and log level) are
1691 defined statically. In other words, without a dynamic filter, the tracer
1692 _does not evaluate_ the arguments of an instrumentation point unless it
1693 matches an enabled event rule.
1695 Note that, for LTTng to record an event, the <<channel,channel>> to
1696 which a matching event rule is attached must also be enabled, and the
1697 tracing session owning this channel must be active.
1700 .Logical path from an instrumentation point to an event record.
1701 image::event-rule.png[]
1703 .Event, event record, or event rule?
1705 With so many similar terms, it's easy to get confused.
1707 An **event** is the consequence of the execution of an _instrumentation
1708 point_, like a tracepoint that you manually place in some source code,
1709 or a Linux kernel KProbe. An event is said to _occur_ at a specific
1710 time. Different actions can be taken upon the occurance of an event,
1711 like record the event's payload to a buffer.
1713 An **event record** is the representation of an event in a sub-buffer. A
1714 tracer is responsible for capturing the payload of an event, current
1715 context variables, the event's ID, and the event's timestamp. LTTng
1716 can append this sub-buffer to a trace file.
1718 An **event rule** is a set of conditions which must all be satisfied for
1719 LTTng to record an occuring event. Events still occur without
1720 satisfying event rules, but LTTng does not record them.
1725 == Components of noch:{LTTng}
1727 The second _T_ in _LTTng_ stands for _toolkit_: it would be wrong
1728 to call LTTng a simple _tool_ since it is composed of multiple
1729 interacting components. This section describes those components,
1730 explains their respective roles, and shows how they connect together to
1731 form the LTTng ecosystem.
1733 The following diagram shows how the most important components of LTTng
1734 interact with user applications, the Linux kernel, and you:
1737 .Control and trace data paths between LTTng components.
1738 image::plumbing.png[]
1740 The LTTng project incorporates:
1742 * **LTTng-tools**: Libraries and command-line interface to
1743 control tracing sessions.
1744 ** <<lttng-sessiond,Session daemon>> (man:lttng-sessiond(8)).
1745 ** <<lttng-consumerd,Consumer daemon>> (man:lttng-consumerd(8)).
1746 ** <<lttng-relayd,Relay daemon>> (man:lttng-relayd(8)).
1747 ** <<liblttng-ctl-lttng,Tracing control library>> (`liblttng-ctl`).
1748 ** <<lttng-cli,Tracing control command-line tool>> (man:lttng(1)).
1749 * **LTTng-UST**: Libraries and Java/Python packages to trace user
1751 ** <<lttng-ust,User space tracing library>> (`liblttng-ust`) and its
1752 headers to instrument and trace any native user application.
1753 ** <<prebuilt-ust-helpers,Preloadable user space tracing helpers>>:
1754 *** `liblttng-ust-libc-wrapper`
1755 *** `liblttng-ust-pthread-wrapper`
1756 *** `liblttng-ust-cyg-profile`
1757 *** `liblttng-ust-cyg-profile-fast`
1758 *** `liblttng-ust-dl`
1759 ** User space tracepoint provider source files generator command-line
1760 tool (man:lttng-gen-tp(1)).
1761 ** <<lttng-ust-agents,LTTng-UST Java agent>> to instrument and trace
1762 Java applications using `java.util.logging` or
1763 Apache log4j 1.2 logging.
1764 ** <<lttng-ust-agents,LTTng-UST Python agent>> to instrument
1765 Python applications using the standard `logging` package.
1766 * **LTTng-modules**: <<lttng-modules,Linux kernel modules>> to trace
1768 ** LTTng kernel tracer module.
1769 ** Tracing ring buffer kernel modules.
1770 ** Probe kernel modules.
1771 ** LTTng logger kernel module.
1775 === Tracing control command-line interface
1778 .The tracing control command-line interface.
1779 image::plumbing-lttng-cli.png[]
1781 The _man:lttng(1) command-line tool_ is the standard user interface to
1782 control LTTng <<tracing-session,tracing sessions>>. The cmd:lttng tool
1783 is part of LTTng-tools.
1785 The cmd:lttng tool is linked with
1786 <<liblttng-ctl-lttng,`liblttng-ctl`>> to communicate with
1787 one or more <<lttng-sessiond,session daemons>> behind the scenes.
1789 The cmd:lttng tool has a Git-like interface:
1793 lttng <general options> <command> <command options>
1796 The <<controlling-tracing,Tracing control>> section explores the
1797 available features of LTTng using the cmd:lttng tool.
1800 [[liblttng-ctl-lttng]]
1801 === Tracing control library
1804 .The tracing control library.
1805 image::plumbing-liblttng-ctl.png[]
1807 The _LTTng control library_, `liblttng-ctl`, is used to communicate
1808 with a <<lttng-sessiond,session daemon>> using a C API that hides the
1809 underlying protocol's details. `liblttng-ctl` is part of LTTng-tools.
1811 The <<lttng-cli,cmd:lttng command-line tool>>
1812 is linked with `liblttng-ctl`.
1814 You can use `liblttng-ctl` in C or $$C++$$ source code by including its
1819 #include <lttng/lttng.h>
1822 Some objects are referenced by name (C string), such as tracing
1823 sessions, but most of them require to create a handle first using
1824 `lttng_create_handle()`.
1826 The best available developer documentation for `liblttng-ctl` is, as of
1827 LTTng{nbsp}{revision}, its installed header files. Every function and
1828 structure is thoroughly documented.
1832 === User space tracing library
1835 .The user space tracing library.
1836 image::plumbing-liblttng-ust.png[]
1838 The _user space tracing library_, `liblttng-ust` (see man:lttng-ust(3)),
1839 is the LTTng user space tracer. It receives commands from a
1840 <<lttng-sessiond,session daemon>>, for example to
1841 enable and disable specific instrumentation points, and writes event
1842 records to ring buffers shared with a
1843 <<lttng-consumerd,consumer daemon>>.
1844 `liblttng-ust` is part of LTTng-UST.
1846 Public C header files are installed beside `liblttng-ust` to
1847 instrument any <<c-application,C or $$C++$$ application>>.
1849 <<lttng-ust-agents,LTTng-UST agents>>, which are regular Java and Python
1850 packages, use their own library providing tracepoints which is
1851 linked with `liblttng-ust`.
1853 An application or library does not have to initialize `liblttng-ust`
1854 manually: its constructor does the necessary tasks to properly register
1855 to a session daemon. The initialization phase also enables the
1856 instrumentation points matching the <<event,event rules>> that you
1860 [[lttng-ust-agents]]
1861 === User space tracing agents
1864 .The user space tracing agents.
1865 image::plumbing-lttng-ust-agents.png[]
1867 The _LTTng-UST Java and Python agents_ are regular Java and Python
1868 packages which add LTTng tracing capabilities to the
1869 native logging frameworks. The LTTng-UST agents are part of LTTng-UST.
1871 In the case of Java, the
1872 https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[`java.util.logging`
1873 core logging facilities] and
1874 https://logging.apache.org/log4j/1.2/[Apache log4j 1.2] are supported.
1875 Note that Apache Log4{nbsp}2 is not supported.
1877 In the case of Python, the standard
1878 https://docs.python.org/3/library/logging.html[`logging`] package
1879 is supported. Both Python 2 and Python 3 modules can import the
1880 LTTng-UST Python agent package.
1882 The applications using the LTTng-UST agents are in the
1883 `java.util.logging` (JUL),
1884 log4j, and Python <<domain,tracing domains>>.
1886 Both agents use the same mechanism to trace the log statements. When an
1887 agent is initialized, it creates a log handler that attaches to the root
1888 logger. The agent also registers to a <<lttng-sessiond,session daemon>>.
1889 When the application executes a log statement, it is passed to the
1890 agent's log handler by the root logger. The agent's log handler calls a
1891 native function in a tracepoint provider package shared library linked
1892 with <<lttng-ust,`liblttng-ust`>>, passing the formatted log message and
1893 other fields, like its logger name and its log level. This native
1894 function contains a user space instrumentation point, hence tracing the
1897 The log level condition of an
1898 <<event,event rule>> is considered when tracing
1899 a Java or a Python application, and it's compatible with the standard
1900 JUL, log4j, and Python log levels.
1904 === LTTng kernel modules
1907 .The LTTng kernel modules.
1908 image::plumbing-lttng-modules.png[]
1910 The _LTTng kernel modules_ are a set of Linux kernel modules
1911 which implement the kernel tracer of the LTTng project. The LTTng
1912 kernel modules are part of LTTng-modules.
1914 The LTTng kernel modules include:
1916 * A set of _probe_ modules.
1918 Each module attaches to a specific subsystem
1919 of the Linux kernel using its tracepoint instrument points. There are
1920 also modules to attach to the entry and return points of the Linux
1921 system call functions.
1923 * _Ring buffer_ modules.
1925 A ring buffer implementation is provided as kernel modules. The LTTng
1926 kernel tracer writes to the ring buffer; a
1927 <<lttng-consumerd,consumer daemon>> reads from the ring buffer.
1929 * The _LTTng kernel tracer_ module.
1930 * The _LTTng logger_ module.
1932 The LTTng logger module implements the special path:{/proc/lttng-logger}
1933 file so that any executable can generate LTTng events by opening and
1934 writing to this file.
1936 See <<proc-lttng-logger-abi,LTTng logger>>.
1938 Generally, you do not have to load the LTTng kernel modules manually
1939 (using man:modprobe(8), for example): a root <<lttng-sessiond,session
1940 daemon>> loads the necessary modules when starting. If you have extra
1941 probe modules, you can specify to load them to the session daemon on
1944 The LTTng kernel modules are installed in
1945 +/usr/lib/modules/__release__/extra+ by default, where +__release__+ is
1946 the kernel release (see `uname --kernel-release`).
1953 .The session daemon.
1954 image::plumbing-sessiond.png[]
1956 The _session daemon_, man:lttng-sessiond(8), is a daemon responsible for
1957 managing tracing sessions and for controlling the various components of
1958 LTTng. The session daemon is part of LTTng-tools.
1960 The session daemon sends control requests to and receives control
1963 * The <<lttng-ust,user space tracing library>>.
1965 Any instance of the user space tracing library first registers to
1966 a session daemon. Then, the session daemon can send requests to
1967 this instance, such as:
1970 ** Get the list of tracepoints.
1971 ** Share an <<event,event rule>> so that the user space tracing library
1972 can enable or disable tracepoints. Amongst the possible conditions
1973 of an event rule is a filter expression which `liblttng-ust` evalutes
1974 when an event occurs.
1975 ** Share <<channel,channel>> attributes and ring buffer locations.
1978 The session daemon and the user space tracing library use a Unix
1979 domain socket for their communication.
1981 * The <<lttng-ust-agents,user space tracing agents>>.
1983 Any instance of a user space tracing agent first registers to
1984 a session daemon. Then, the session daemon can send requests to
1985 this instance, such as:
1988 ** Get the list of loggers.
1989 ** Enable or disable a specific logger.
1992 The session daemon and the user space tracing agent use a TCP connection
1993 for their communication.
1995 * The <<lttng-modules,LTTng kernel tracer>>.
1996 * The <<lttng-consumerd,consumer daemon>>.
1998 The session daemon sends requests to the consumer daemon to instruct
1999 it where to send the trace data streams, amongst other information.
2001 * The <<lttng-relayd,relay daemon>>.
2003 The session daemon receives commands from the
2004 <<liblttng-ctl-lttng,tracing control library>>.
2006 The root session daemon loads the appropriate
2007 <<lttng-modules,LTTng kernel modules>> on startup. It also spawns
2008 a <<lttng-consumerd,consumer daemon>> as soon as you create
2009 an <<event,event rule>>.
2011 The session daemon does not send and receive trace data: this is the
2012 role of the <<lttng-consumerd,consumer daemon>> and
2013 <<lttng-relayd,relay daemon>>. It does, however, generate the
2014 http://diamon.org/ctf/[CTF] metadata stream.
2016 Each Unix user can have its own session daemon instance. The
2017 tracing sessions managed by different session daemons are completely
2020 The root user's session daemon is the only one which is
2021 allowed to control the LTTng kernel tracer, and its spawned consumer
2022 daemon is the only one which is allowed to consume trace data from the
2023 LTTng kernel tracer. Note, however, that any Unix user which is a member
2024 of the <<tracing-group,tracing group>> is allowed
2025 to create <<channel,channels>> in the
2026 Linux kernel <<domain,tracing domain>>, and thus to trace the Linux
2029 The <<lttng-cli,cmd:lttng command-line tool>> automatically starts a
2030 session daemon when using its `create` command if none is currently
2031 running. You can also start the session daemon manually.
2038 .The consumer daemon.
2039 image::plumbing-consumerd.png[]
2041 The _consumer daemon_, man:lttng-consumerd(8), is a daemon which shares
2042 ring buffers with user applications or with the LTTng kernel modules to
2043 collect trace data and send it to some location (on disk or to a
2044 <<lttng-relayd,relay daemon>> over the network). The consumer daemon
2045 is part of LTTng-tools.
2047 You do not start a consumer daemon manually: a consumer daemon is always
2048 spawned by a <<lttng-sessiond,session daemon>> as soon as you create an
2049 <<event,event rule>>, that is, before you start tracing. When you kill
2050 its owner session daemon, the consumer daemon also exits because it is
2051 the session daemon's child process. Command-line options of
2052 man:lttng-sessiond(8) target the consumer daemon process.
2054 There are up to two running consumer daemons per Unix user, whereas only
2055 one session daemon can run per user. This is because each process can be
2056 either 32-bit or 64-bit: if the target system runs a mixture of 32-bit
2057 and 64-bit processes, it is more efficient to have separate
2058 corresponding 32-bit and 64-bit consumer daemons. The root user is an
2059 exception: it can have up to _three_ running consumer daemons: 32-bit
2060 and 64-bit instances for its user applications, and one more
2061 reserved for collecting kernel trace data.
2069 image::plumbing-relayd.png[]
2071 The _relay daemon_, man:lttng-relayd(8), is a daemon acting as a bridge
2072 between remote session and consumer daemons, local trace files, and a
2073 remote live trace viewer. The relay daemon is part of LTTng-tools.
2075 The main purpose of the relay daemon is to implement a receiver of
2076 <<sending-trace-data-over-the-network,trace data over the network>>.
2077 This is useful when the target system does not have much file system
2078 space to record trace files locally.
2080 The relay daemon is also a server to which a
2081 <<lttng-live,live trace viewer>> can
2082 connect. The live trace viewer sends requests to the relay daemon to
2083 receive trace data as the target system emits events. The
2084 communication protocol is named _LTTng live_; it is used over TCP
2087 Note that you can start the relay daemon on the target system directly.
2088 This is the setup of choice when the use case is to view events as
2089 the target system emits them without the need of a remote system.
2093 == [[using-lttng]]Instrumentation
2095 There are many examples of tracing and monitoring in our everyday life:
2097 * You have access to real-time and historical weather reports and
2098 forecasts thanks to weather stations installed around the country.
2099 * You know your heart is safe thanks to an electrocardiogram.
2100 * You make sure not to drive your car too fast and to have enough fuel
2101 to reach your destination thanks to gauges visible on your dashboard.
2103 All the previous examples have something in common: they rely on
2104 **instruments**. Without the electrodes attached to the surface of your
2105 body's skin, cardiac monitoring is futile.
2107 LTTng, as a tracer, is no different from those real life examples. If
2108 you're about to trace a software system or, in other words, record its
2109 history of execution, you better have **instrumentation points** in the
2110 subject you're tracing, that is, the actual software.
2112 Various ways were developed to instrument a piece of software for LTTng
2113 tracing. The most straightforward one is to manually place
2114 instrumentation points, called _tracepoints_, in the software's source
2115 code. It is also possible to add instrumentation points dynamically in
2116 the Linux kernel <<domain,tracing domain>>.
2118 If you're only interested in tracing the Linux kernel, your
2119 instrumentation needs are probably already covered by LTTng's built-in
2120 <<lttng-modules,Linux kernel tracepoints>>. You may also wish to trace a
2121 user application which is already instrumented for LTTng tracing.
2122 In such cases, you can skip this whole section and read the topics of
2123 the <<controlling-tracing,Tracing control>> section.
2125 Many methods are available to instrument a piece of software for LTTng
2128 * <<c-application,User space instrumentation for C and $$C++$$
2130 * <<prebuilt-ust-helpers,Prebuilt user space tracing helpers>>.
2131 * <<java-application,User space Java agent>>.
2132 * <<python-application,User space Python agent>>.
2133 * <<proc-lttng-logger-abi,LTTng logger>>.
2134 * <<instrumenting-linux-kernel,LTTng kernel tracepoints>>.
2138 === [[cxx-application]]User space instrumentation for C and $$C++$$ applications
2140 The procedure to instrument a C or $$C++$$ user application with
2141 the <<lttng-ust,LTTng user space tracing library>>, `liblttng-ust`, is:
2143 . <<tracepoint-provider,Create the source files of a tracepoint provider
2145 . <<probing-the-application-source-code,Add tracepoints to
2146 the application's source code>>.
2147 . <<building-tracepoint-providers-and-user-application,Build and link
2148 a tracepoint provider package and the user application>>.
2150 If you need quick, man:printf(3)-like instrumentation, you can skip
2151 those steps and use <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>>
2154 IMPORTANT: You need to <<installing-lttng,install>> LTTng-UST to
2155 instrument a user application with `liblttng-ust`.
2158 [[tracepoint-provider]]
2159 ==== Create the source files of a tracepoint provider package
2161 A _tracepoint provider_ is a set of compiled functions which provide
2162 **tracepoints** to an application, the type of instrumentation point
2163 supported by LTTng-UST. Those functions can emit events with
2164 user-defined fields and serialize those events as event records to one
2165 or more LTTng-UST <<channel,channel>> sub-buffers. The `tracepoint()`
2166 macro, which you <<probing-the-application-source-code,insert in a user
2167 application's source code>>, calls those functions.
2169 A _tracepoint provider package_ is an object file (`.o`) or a shared
2170 library (`.so`) which contains one or more tracepoint providers.
2171 Its source files are:
2173 * One or more <<tpp-header,tracepoint provider header>> (`.h`).
2174 * A <<tpp-source,tracepoint provider package source>> (`.c`).
2176 A tracepoint provider package is dynamically linked with `liblttng-ust`,
2177 the LTTng user space tracer, at run time.
2180 .User application linked with `liblttng-ust` and containing a tracepoint provider.
2181 image::ust-app.png[]
2183 NOTE: If you need quick, man:printf(3)-like instrumentation, you can
2184 skip creating and using a tracepoint provider and use
2185 <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>> instead.
2189 ===== Create a tracepoint provider header file template
2191 A _tracepoint provider header file_ contains the tracepoint
2192 definitions of a tracepoint provider.
2194 To create a tracepoint provider header file:
2196 . Start from this template:
2200 .Tracepoint provider header file template (`.h` file extension).
2202 #undef TRACEPOINT_PROVIDER
2203 #define TRACEPOINT_PROVIDER provider_name
2205 #undef TRACEPOINT_INCLUDE
2206 #define TRACEPOINT_INCLUDE "./tp.h"
2208 #if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
2211 #include <lttng/tracepoint.h>
2214 * Use TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(),
2215 * TRACEPOINT_EVENT_INSTANCE(), and TRACEPOINT_LOGLEVEL() here.
2220 #include <lttng/tracepoint-event.h>
2226 * `provider_name` with the name of your tracepoint provider.
2227 * `"tp.h"` with the name of your tracepoint provider header file.
2229 . Below the `#include <lttng/tracepoint.h>` line, put your
2230 <<defining-tracepoints,tracepoint definitions>>.
2232 Your tracepoint provider name must be unique amongst all the possible
2233 tracepoint provider names used on the same target system. We
2234 suggest to include the name of your project or company in the name,
2235 for example, `org_lttng_my_project_tpp`.
2237 TIP: [[lttng-gen-tp]]You can use the man:lttng-gen-tp(1) tool to create
2238 this boilerplate for you. When using cmd:lttng-gen-tp, all you need to
2239 write are the <<defining-tracepoints,tracepoint definitions>>.
2242 [[defining-tracepoints]]
2243 ===== Create a tracepoint definition
2245 A _tracepoint definition_ defines, for a given tracepoint:
2247 * Its **input arguments**. They are the macro parameters that the
2248 `tracepoint()` macro accepts for this particular tracepoint
2249 in the user application's source code.
2250 * Its **output event fields**. They are the sources of event fields
2251 that form the payload of any event that the execution of the
2252 `tracepoint()` macro emits for this particular tracepoint.
2254 You can create a tracepoint definition by using the
2255 `TRACEPOINT_EVENT()` macro below the `#include <lttng/tracepoint.h>`
2257 <<tpp-header,tracepoint provider header file template>>.
2259 The syntax of the `TRACEPOINT_EVENT()` macro is:
2262 .`TRACEPOINT_EVENT()` macro syntax.
2265 /* Tracepoint provider name */
2268 /* Tracepoint name */
2271 /* Input arguments */
2276 /* Output event fields */
2285 * `provider_name` with your tracepoint provider name.
2286 * `tracepoint_name` with your tracepoint name.
2287 * `arguments` with the <<tpp-def-input-args,input arguments>>.
2288 * `fields` with the <<tpp-def-output-fields,output event field>>
2291 This tracepoint emits events named `provider_name:tracepoint_name`.
2294 .Event name's length limitation
2296 The concatenation of the tracepoint provider name and the
2297 tracepoint name must not exceed **254 characters**. If it does, the
2298 instrumented application compiles and runs, but LTTng throws multiple
2299 warnings and you could experience serious issues.
2302 [[tpp-def-input-args]]The syntax of the `TP_ARGS()` macro is:
2305 .`TP_ARGS()` macro syntax.
2314 * `type` with the C type of the argument.
2315 * `arg_name` with the argument name.
2317 You can repeat `type` and `arg_name` up to 10 times to have
2318 more than one argument.
2320 .`TP_ARGS()` usage with three arguments.
2332 The `TP_ARGS()` and `TP_ARGS(void)` forms are valid to create a
2333 tracepoint definition with no input arguments.
2335 [[tpp-def-output-fields]]The `TP_FIELDS()` macro contains a list of
2336 `ctf_*()` macros. Each `ctf_*()` macro defines one event field. See
2337 man:lttng-ust(3) for a complete description of the available `ctf_*()`
2338 macros. A `ctf_*()` macro specifies the type, size, and byte order of
2341 Each `ctf_*()` macro takes an _argument expression_ parameter. This is a
2342 C expression that the tracer evalutes at the `tracepoint()` macro site
2343 in the application's source code. This expression provides a field's
2344 source of data. The argument expression can include input argument names
2345 listed in the `TP_ARGS()` macro.
2347 Each `ctf_*()` macro also takes a _field name_ parameter. Field names
2348 must be unique within a given tracepoint definition.
2350 Here's a complete tracepoint definition example:
2352 .Tracepoint definition.
2354 The following tracepoint definition defines a tracepoint which takes
2355 three input arguments and has four output event fields.
2359 #include "my-custom-structure.h"
2365 const struct my_custom_structure*, my_custom_structure,
2370 ctf_string(query_field, query)
2371 ctf_float(double, ratio_field, ratio)
2372 ctf_integer(int, recv_size, my_custom_structure->recv_size)
2373 ctf_integer(int, send_size, my_custom_structure->send_size)
2378 You can refer to this tracepoint definition with the `tracepoint()`
2379 macro in your application's source code like this:
2383 tracepoint(my_provider, my_tracepoint,
2384 my_structure, some_ratio, the_query);
2388 NOTE: The LTTng tracer only evaluates tracepoint arguments at run time
2389 if they satisfy an enabled <<event,event rule>>.
2392 [[using-tracepoint-classes]]
2393 ===== Use a tracepoint class
2395 A _tracepoint class_ is a class of tracepoints which share the same
2396 output event field definitions. A _tracepoint instance_ is one
2397 instance of such a defined tracepoint class, with its own tracepoint
2400 The <<defining-tracepoints,`TRACEPOINT_EVENT()` macro>> is actually a
2401 shorthand which defines both a tracepoint class and a tracepoint
2402 instance at the same time.
2404 When you build a tracepoint provider package, the C or $$C++$$ compiler
2405 creates one serialization function for each **tracepoint class**. A
2406 serialization function is responsible for serializing the event fields
2407 of a tracepoint to a sub-buffer when tracing.
2409 For various performance reasons, when your situation requires multiple
2410 tracepoint definitions with different names, but with the same event
2411 fields, we recommend that you manually create a tracepoint class
2412 and instantiate as many tracepoint instances as needed. One positive
2413 effect of such a design, amongst other advantages, is that all
2414 tracepoint instances of the same tracepoint class reuse the same
2415 serialization function, thus reducing
2416 https://en.wikipedia.org/wiki/Cache_pollution[cache pollution].
2418 .Use a tracepoint class and tracepoint instances.
2420 Consider the following three tracepoint definitions:
2432 ctf_integer(int, userid, userid)
2433 ctf_integer(size_t, len, len)
2445 ctf_integer(int, userid, userid)
2446 ctf_integer(size_t, len, len)
2458 ctf_integer(int, userid, userid)
2459 ctf_integer(size_t, len, len)
2464 In this case, we create three tracepoint classes, with one implicit
2465 tracepoint instance for each of them: `get_account`, `get_settings`, and
2466 `get_transaction`. However, they all share the same event field names
2467 and types. Hence three identical, yet independent serialization
2468 functions are created when you build the tracepoint provider package.
2470 A better design choice is to define a single tracepoint class and three
2471 tracepoint instances:
2475 /* The tracepoint class */
2476 TRACEPOINT_EVENT_CLASS(
2477 /* Tracepoint provider name */
2480 /* Tracepoint class name */
2483 /* Input arguments */
2489 /* Output event fields */
2491 ctf_integer(int, userid, userid)
2492 ctf_integer(size_t, len, len)
2496 /* The tracepoint instances */
2497 TRACEPOINT_EVENT_INSTANCE(
2498 /* Tracepoint provider name */
2501 /* Tracepoint class name */
2504 /* Tracepoint name */
2507 /* Input arguments */
2513 TRACEPOINT_EVENT_INSTANCE(
2522 TRACEPOINT_EVENT_INSTANCE(
2535 [[assigning-log-levels]]
2536 ===== Assign a log level to a tracepoint definition
2538 You can assign an optional _log level_ to a
2539 <<defining-tracepoints,tracepoint definition>>.
2541 Assigning different levels of severity to tracepoint definitions can
2542 be useful: when you <<enabling-disabling-events,create an event rule>>,
2543 you can target tracepoints having a log level as severe as a specific
2546 The concept of LTTng-UST log levels is similar to the levels found
2547 in typical logging frameworks:
2549 * In a logging framework, the log level is given by the function
2550 or method name you use at the log statement site: `debug()`,
2551 `info()`, `warn()`, `error()`, and so on.
2552 * In LTTng-UST, you statically assign the log level to a tracepoint
2553 definition; any `tracepoint()` macro invocation which refers to
2554 this definition has this log level.
2556 You can assign a log level to a tracepoint definition with the
2557 `TRACEPOINT_LOGLEVEL()` macro. You must use this macro _after_ the
2558 <<defining-tracepoints,`TRACEPOINT_EVENT()`>> or
2559 <<using-tracepoint-classes,`TRACEPOINT_INSTANCE()`>> macro for a given
2562 The syntax of the `TRACEPOINT_LOGLEVEL()` macro is:
2565 .`TRACEPOINT_LOGLEVEL()` macro syntax.
2567 TRACEPOINT_LOGLEVEL(provider_name, tracepoint_name, log_level)
2572 * `provider_name` with the tracepoint provider name.
2573 * `tracepoint_name` with the tracepoint name.
2574 * `log_level` with the log level to assign to the tracepoint
2575 definition named `tracepoint_name` in the `provider_name`
2576 tracepoint provider.
2578 See man:lttng-ust(3) for a list of available log level names.
2580 .Assign the `TRACE_DEBUG_UNIT` log level to a tracepoint definition.
2584 /* Tracepoint definition */
2593 ctf_integer(int, userid, userid)
2594 ctf_integer(size_t, len, len)
2598 /* Log level assignment */
2599 TRACEPOINT_LOGLEVEL(my_app, get_transaction, TRACE_DEBUG_UNIT)
2605 ===== Create a tracepoint provider package source file
2607 A _tracepoint provider package source file_ is a C source file which
2608 includes a <<tpp-header,tracepoint provider header file>> to expand its
2609 macros into event serialization and other functions.
2611 You can always use the following tracepoint provider package source
2615 .Tracepoint provider package source file template.
2617 #define TRACEPOINT_CREATE_PROBES
2622 Replace `tp.h` with the name of your <<tpp-header,tracepoint provider
2623 header file>> name. You may also include more than one tracepoint
2624 provider header file here to create a tracepoint provider package
2625 holding more than one tracepoint providers.
2628 [[probing-the-application-source-code]]
2629 ==== Add tracepoints to an application's source code
2631 Once you <<tpp-header,create a tracepoint provider header file>>, you
2632 can use the `tracepoint()` macro in your application's
2633 source code to insert the tracepoints that this header
2634 <<defining-tracepoints,defines>>.
2636 The `tracepoint()` macro takes at least two parameters: the tracepoint
2637 provider name and the tracepoint name. The corresponding tracepoint
2638 definition defines the other parameters.
2640 .`tracepoint()` usage.
2642 The following <<defining-tracepoints,tracepoint definition>> defines a
2643 tracepoint which takes two input arguments and has two output event
2647 .Tracepoint provider header file.
2649 #include "my-custom-structure.h"
2656 const char*, cmd_name
2659 ctf_string(cmd_name, cmd_name)
2660 ctf_integer(int, number_of_args, argc)
2665 You can refer to this tracepoint definition with the `tracepoint()`
2666 macro in your application's source code like this:
2669 .Application's source file.
2673 int main(int argc, char* argv[])
2675 tracepoint(my_provider, my_tracepoint, argc, argv[0]);
2681 Note how the application's source code includes
2682 the tracepoint provider header file containing the tracepoint
2683 definitions to use, path:{tp.h}.
2686 .`tracepoint()` usage with a complex tracepoint definition.
2688 Consider this complex tracepoint definition, where multiple event
2689 fields refer to the same input arguments in their argument expression
2693 .Tracepoint provider header file.
2695 /* For `struct stat` */
2696 #include <sys/types.h>
2697 #include <sys/stat.h>
2709 ctf_integer(int, my_constant_field, 23 + 17)
2710 ctf_integer(int, my_int_arg_field, my_int_arg)
2711 ctf_integer(int, my_int_arg_field2, my_int_arg * my_int_arg)
2712 ctf_integer(int, sum4_field, my_str_arg[0] + my_str_arg[1] +
2713 my_str_arg[2] + my_str_arg[3])
2714 ctf_string(my_str_arg_field, my_str_arg)
2715 ctf_integer_hex(off_t, size_field, st->st_size)
2716 ctf_float(double, size_dbl_field, (double) st->st_size)
2717 ctf_sequence_text(char, half_my_str_arg_field, my_str_arg,
2718 size_t, strlen(my_str_arg) / 2)
2723 You can refer to this tracepoint definition with the `tracepoint()`
2724 macro in your application's source code like this:
2727 .Application's source file.
2729 #define TRACEPOINT_DEFINE
2736 stat("/etc/fstab", &s);
2737 tracepoint(my_provider, my_tracepoint, 23, "Hello, World!", &s);
2743 If you look at the event record that LTTng writes when tracing this
2744 program, assuming the file size of path:{/etc/fstab} is 301{nbsp}bytes,
2745 it should look like this:
2747 .Event record fields
2749 |Field's name |Field's value
2750 |`my_constant_field` |40
2751 |`my_int_arg_field` |23
2752 |`my_int_arg_field2` |529
2754 |`my_str_arg_field` |`Hello, World!`
2755 |`size_field` |0x12d
2756 |`size_dbl_field` |301.0
2757 |`half_my_str_arg_field` |`Hello,`
2761 Sometimes, the arguments you pass to `tracepoint()` are expensive to
2762 compute--they use the call stack, for example. To avoid this
2763 computation when the tracepoint is disabled, you can use the
2764 `tracepoint_enabled()` and `do_tracepoint()` macros.
2766 The syntax of the `tracepoint_enabled()` and `do_tracepoint()` macros
2770 .`tracepoint_enabled()` and `do_tracepoint()` macros syntax.
2772 tracepoint_enabled(provider_name, tracepoint_name)
2773 do_tracepoint(provider_name, tracepoint_name, ...)
2778 * `provider_name` with the tracepoint provider name.
2779 * `tracepoint_name` with the tracepoint name.
2781 `tracepoint_enabled()` returns a non-zero value if the tracepoint named
2782 `tracepoint_name` from the provider named `provider_name` is enabled
2785 `do_tracepoint()` is like `tracepoint()`, except that it doesn't check
2786 if the tracepoint is enabled. Using `tracepoint()` with
2787 `tracepoint_enabled()` is dangerous since `tracepoint()` also contains
2788 the `tracepoint_enabled()` check, thus a race condition is
2789 possible in this situation:
2792 .Possible race condition when using `tracepoint_enabled()` with `tracepoint()`.
2794 if (tracepoint_enabled(my_provider, my_tracepoint)) {
2795 stuff = prepare_stuff();
2798 tracepoint(my_provider, my_tracepoint, stuff);
2801 If the tracepoint is enabled after the condition, then `stuff` is not
2802 prepared: the emitted event will either contain wrong data, or the whole
2803 application could crash (segmentation fault, for example).
2805 NOTE: Neither `tracepoint_enabled()` nor `do_tracepoint()` have an
2806 `STAP_PROBEV()` call. If you need it, you must emit
2810 [[building-tracepoint-providers-and-user-application]]
2811 ==== Build and link a tracepoint provider package and an application
2813 Once you have one or more <<tpp-header,tracepoint provider header
2814 files>> and a <<tpp-source,tracepoint provider package source file>>,
2815 you can create the tracepoint provider package by compiling its source
2816 file. From here, multiple build and run scenarios are possible. The
2817 following table shows common application and library configurations
2818 along with the required command lines to achieve them.
2820 In the following diagrams, we use the following file names:
2823 Executable application.
2826 Application's object file.
2829 Tracepoint provider package object file.
2832 Tracepoint provider package archive file.
2835 Tracepoint provider package shared object file.
2838 User library object file.
2841 User library shared object file.
2843 We use the following symbols in the diagrams of table below:
2846 .Symbols used in the build scenario diagrams.
2847 image::ust-sit-symbols.png[]
2849 We assume that path:{.} is part of the env:LD_LIBRARY_PATH environment
2850 variable in the following instructions.
2852 [role="growable ust-scenarios",cols="asciidoc,asciidoc"]
2853 .Common tracepoint provider package scenarios.
2855 |Scenario |Instructions
2858 The instrumented application is statically linked with
2859 the tracepoint provider package object.
2861 image::ust-sit+app-linked-with-tp-o+app-instrumented.png[]
2864 include::../common/ust-sit-step-tp-o.txt[]
2866 To build the instrumented application:
2868 . In path:{app.c}, before including path:{tpp.h}, add the following line:
2873 #define TRACEPOINT_DEFINE
2877 . Compile the application source file:
2886 . Build the application:
2891 gcc -o app app.o tpp.o -llttng-ust -ldl
2895 To run the instrumented application:
2897 * Start the application:
2907 The instrumented application is statically linked with the
2908 tracepoint provider package archive file.
2910 image::ust-sit+app-linked-with-tp-a+app-instrumented.png[]
2913 To create the tracepoint provider package archive file:
2915 . Compile the <<tpp-source,tracepoint provider package source file>>:
2924 . Create the tracepoint provider package archive file:
2933 To build the instrumented application:
2935 . In path:{app.c}, before including path:{tpp.h}, add the following line:
2940 #define TRACEPOINT_DEFINE
2944 . Compile the application source file:
2953 . Build the application:
2958 gcc -o app app.o tpp.a -llttng-ust -ldl
2962 To run the instrumented application:
2964 * Start the application:
2974 The instrumented application is linked with the tracepoint provider
2975 package shared object.
2977 image::ust-sit+app-linked-with-tp-so+app-instrumented.png[]
2980 include::../common/ust-sit-step-tp-so.txt[]
2982 To build the instrumented application:
2984 . In path:{app.c}, before including path:{tpp.h}, add the following line:
2989 #define TRACEPOINT_DEFINE
2993 . Compile the application source file:
3002 . Build the application:
3007 gcc -o app app.o -ldl -L. -ltpp
3011 To run the instrumented application:
3013 * Start the application:
3023 The tracepoint provider package shared object is preloaded before the
3024 instrumented application starts.
3026 image::ust-sit+tp-so-preloaded+app-instrumented.png[]
3029 include::../common/ust-sit-step-tp-so.txt[]
3031 To build the instrumented application:
3033 . In path:{app.c}, before including path:{tpp.h}, add the
3039 #define TRACEPOINT_DEFINE
3040 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3044 . Compile the application source file:
3053 . Build the application:
3058 gcc -o app app.o -ldl
3062 To run the instrumented application with tracing support:
3064 * Preload the tracepoint provider package shared object and
3065 start the application:
3070 LD_PRELOAD=./libtpp.so ./app
3074 To run the instrumented application without tracing support:
3076 * Start the application:
3086 The instrumented application dynamically loads the tracepoint provider
3087 package shared object.
3089 See the <<dlclose-warning,warning about `dlclose()`>>.
3091 image::ust-sit+app-dlopens-tp-so+app-instrumented.png[]
3094 include::../common/ust-sit-step-tp-so.txt[]
3096 To build the instrumented application:
3098 . In path:{app.c}, before including path:{tpp.h}, add the
3104 #define TRACEPOINT_DEFINE
3105 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3109 . Compile the application source file:
3118 . Build the application:
3123 gcc -o app app.o -ldl
3127 To run the instrumented application:
3129 * Start the application:
3139 The application is linked with the instrumented user library.
3141 The instrumented user library is statically linked with the tracepoint
3142 provider package object file.
3144 image::ust-sit+app-linked-with-lib+lib-linked-with-tp-o+lib-instrumented.png[]
3147 include::../common/ust-sit-step-tp-o-fpic.txt[]
3149 To build the instrumented user library:
3151 . In path:{emon.c}, before including path:{tpp.h}, add the
3157 #define TRACEPOINT_DEFINE
3161 . Compile the user library source file:
3166 gcc -I. -fpic -c emon.c
3170 . Build the user library shared object:
3175 gcc -shared -o libemon.so emon.o tpp.o -llttng-ust -ldl
3179 To build the application:
3181 . Compile the application source file:
3190 . Build the application:
3195 gcc -o app app.o -L. -lemon
3199 To run the application:
3201 * Start the application:
3211 The application is linked with the instrumented user library.
3213 The instrumented user library is linked with the tracepoint provider
3214 package shared object.
3216 image::ust-sit+app-linked-with-lib+lib-linked-with-tp-so+lib-instrumented.png[]
3219 include::../common/ust-sit-step-tp-so.txt[]
3221 To build the instrumented user library:
3223 . In path:{emon.c}, before including path:{tpp.h}, add the
3229 #define TRACEPOINT_DEFINE
3233 . Compile the user library source file:
3238 gcc -I. -fpic -c emon.c
3242 . Build the user library shared object:
3247 gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
3251 To build the application:
3253 . Compile the application source file:
3262 . Build the application:
3267 gcc -o app app.o -L. -lemon
3271 To run the application:
3273 * Start the application:
3283 The tracepoint provider package shared object is preloaded before the
3286 The application is linked with the instrumented user library.
3288 image::ust-sit+tp-so-preloaded+app-linked-with-lib+lib-instrumented.png[]
3291 include::../common/ust-sit-step-tp-so.txt[]
3293 To build the instrumented user library:
3295 . In path:{emon.c}, before including path:{tpp.h}, add the
3301 #define TRACEPOINT_DEFINE
3302 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3306 . Compile the user library source file:
3311 gcc -I. -fpic -c emon.c
3315 . Build the user library shared object:
3320 gcc -shared -o libemon.so emon.o -ldl
3324 To build the application:
3326 . Compile the application source file:
3335 . Build the application:
3340 gcc -o app app.o -L. -lemon
3344 To run the application with tracing support:
3346 * Preload the tracepoint provider package shared object and
3347 start the application:
3352 LD_PRELOAD=./libtpp.so ./app
3356 To run the application without tracing support:
3358 * Start the application:
3368 The application is linked with the instrumented user library.
3370 The instrumented user library dynamically loads the tracepoint provider
3371 package shared object.
3373 See the <<dlclose-warning,warning about `dlclose()`>>.
3375 image::ust-sit+app-linked-with-lib+lib-dlopens-tp-so+lib-instrumented.png[]
3378 include::../common/ust-sit-step-tp-so.txt[]
3380 To build the instrumented user library:
3382 . In path:{emon.c}, before including path:{tpp.h}, add the
3388 #define TRACEPOINT_DEFINE
3389 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3393 . Compile the user library source file:
3398 gcc -I. -fpic -c emon.c
3402 . Build the user library shared object:
3407 gcc -shared -o libemon.so emon.o -ldl
3411 To build the application:
3413 . Compile the application source file:
3422 . Build the application:
3427 gcc -o app app.o -L. -lemon
3431 To run the application:
3433 * Start the application:
3443 The application dynamically loads the instrumented user library.
3445 The instrumented user library is linked with the tracepoint provider
3446 package shared object.
3448 See the <<dlclose-warning,warning about `dlclose()`>>.
3450 image::ust-sit+app-dlopens-lib+lib-linked-with-tp-so+lib-instrumented.png[]
3453 include::../common/ust-sit-step-tp-so.txt[]
3455 To build the instrumented user library:
3457 . In path:{emon.c}, before including path:{tpp.h}, add the
3463 #define TRACEPOINT_DEFINE
3467 . Compile the user library source file:
3472 gcc -I. -fpic -c emon.c
3476 . Build the user library shared object:
3481 gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
3485 To build the application:
3487 . Compile the application source file:
3496 . Build the application:
3501 gcc -o app app.o -ldl -L. -lemon
3505 To run the application:
3507 * Start the application:
3517 The application dynamically loads the instrumented user library.
3519 The instrumented user library dynamically loads the tracepoint provider
3520 package shared object.
3522 See the <<dlclose-warning,warning about `dlclose()`>>.
3524 image::ust-sit+app-dlopens-lib+lib-dlopens-tp-so+lib-instrumented.png[]
3527 include::../common/ust-sit-step-tp-so.txt[]
3529 To build the instrumented user library:
3531 . In path:{emon.c}, before including path:{tpp.h}, add the
3537 #define TRACEPOINT_DEFINE
3538 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3542 . Compile the user library source file:
3547 gcc -I. -fpic -c emon.c
3551 . Build the user library shared object:
3556 gcc -shared -o libemon.so emon.o -ldl
3560 To build the application:
3562 . Compile the application source file:
3571 . Build the application:
3576 gcc -o app app.o -ldl -L. -lemon
3580 To run the application:
3582 * Start the application:
3592 The tracepoint provider package shared object is preloaded before the
3595 The application dynamically loads the instrumented user library.
3597 image::ust-sit+tp-so-preloaded+app-dlopens-lib+lib-instrumented.png[]
3600 include::../common/ust-sit-step-tp-so.txt[]
3602 To build the instrumented user library:
3604 . In path:{emon.c}, before including path:{tpp.h}, add the
3610 #define TRACEPOINT_DEFINE
3611 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3615 . Compile the user library source file:
3620 gcc -I. -fpic -c emon.c
3624 . Build the user library shared object:
3629 gcc -shared -o libemon.so emon.o -ldl
3633 To build the application:
3635 . Compile the application source file:
3644 . Build the application:
3649 gcc -o app app.o -L. -lemon
3653 To run the application with tracing support:
3655 * Preload the tracepoint provider package shared object and
3656 start the application:
3661 LD_PRELOAD=./libtpp.so ./app
3665 To run the application without tracing support:
3667 * Start the application:
3677 The application is statically linked with the tracepoint provider
3678 package object file.
3680 The application is linked with the instrumented user library.
3682 image::ust-sit+app-linked-with-tp-o+app-linked-with-lib+lib-instrumented.png[]
3685 include::../common/ust-sit-step-tp-o.txt[]
3687 To build the instrumented user library:
3689 . In path:{emon.c}, before including path:{tpp.h}, add the
3695 #define TRACEPOINT_DEFINE
3699 . Compile the user library source file:
3704 gcc -I. -fpic -c emon.c
3708 . Build the user library shared object:
3713 gcc -shared -o libemon.so emon.o
3717 To build the application:
3719 . Compile the application source file:
3728 . Build the application:
3733 gcc -o app app.o tpp.o -llttng-ust -ldl -L. -lemon
3737 To run the instrumented application:
3739 * Start the application:
3749 The application is statically linked with the tracepoint provider
3750 package object file.
3752 The application dynamically loads the instrumented user library.
3754 image::ust-sit+app-linked-with-tp-o+app-dlopens-lib+lib-instrumented.png[]
3757 include::../common/ust-sit-step-tp-o.txt[]
3759 To build the application:
3761 . In path:{app.c}, before including path:{tpp.h}, add the following line:
3766 #define TRACEPOINT_DEFINE
3770 . Compile the application source file:
3779 . Build the application:
3784 gcc -Wl,--export-dynamic -o app app.o tpp.o \
3789 The `--export-dynamic` option passed to the linker is necessary for the
3790 dynamically loaded library to ``see'' the tracepoint symbols defined in
3793 To build the instrumented user library:
3795 . Compile the user library source file:
3800 gcc -I. -fpic -c emon.c
3804 . Build the user library shared object:
3809 gcc -shared -o libemon.so emon.o
3813 To run the application:
3815 * Start the application:
3827 .Do not use man:dlclose(3) on a tracepoint provider package
3829 Never use man:dlclose(3) on any shared object which:
3831 * Is linked with, statically or dynamically, a tracepoint provider
3833 * Calls man:dlopen(3) itself to dynamically open a tracepoint provider
3834 package shared object.
3836 This is currently considered **unsafe** due to a lack of reference
3837 counting from LTTng-UST to the shared object.
3839 A known workaround (available since glibc 2.2) is to use the
3840 `RTLD_NODELETE` flag when calling man:dlopen(3) initially. This has the
3841 effect of not unloading the loaded shared object, even if man:dlclose(3)
3844 You can also preload the tracepoint provider package shared object with
3845 the env:LD_PRELOAD environment variable to overcome this limitation.
3849 [[using-lttng-ust-with-daemons]]
3850 ===== Use noch:{LTTng-UST} with daemons
3852 If your instrumented application calls man:fork(2), man:clone(2),
3853 or BSD's man:rfork(2), without a following man:exec(3)-family
3854 system call, you must preload the path:{liblttng-ust-fork.so} shared
3855 object when starting the application.
3859 LD_PRELOAD=liblttng-ust-fork.so ./my-app
3862 If your tracepoint provider package is
3863 a shared library which you also preload, you must put both
3864 shared objects in env:LD_PRELOAD:
3868 LD_PRELOAD=liblttng-ust-fork.so:/path/to/tp.so ./my-app
3872 [[lttng-ust-pkg-config]]
3873 ===== Use noch:{pkg-config}
3875 On some distributions, LTTng-UST ships with a
3876 https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config]
3877 metadata file. If this is your case, then you can use cmd:pkg-config to
3878 build an application on the command line:
3882 gcc -o my-app my-app.o tp.o $(pkg-config --cflags --libs lttng-ust)
3886 [[instrumenting-32-bit-app-on-64-bit-system]]
3887 ===== [[advanced-instrumenting-techniques]]Build a 32-bit instrumented application for a 64-bit target system
3889 In order to trace a 32-bit application running on a 64-bit system,
3890 LTTng must use a dedicated 32-bit
3891 <<lttng-consumerd,consumer daemon>>.
3893 The following steps show how to build and install a 32-bit consumer
3894 daemon, which is _not_ part of the default 64-bit LTTng build, how to
3895 build and install the 32-bit LTTng-UST libraries, and how to build and
3896 link an instrumented 32-bit application in that context.
3898 To build a 32-bit instrumented application for a 64-bit target system,
3899 assuming you have a fresh target system with no installed Userspace RCU
3902 . Download, build, and install a 32-bit version of Userspace RCU:
3908 wget http://lttng.org/files/urcu/userspace-rcu-latest-0.9.tar.bz2 &&
3909 tar -xf userspace-rcu-latest-0.9.tar.bz2 &&
3910 cd userspace-rcu-0.9.* &&
3911 ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 &&
3913 sudo make install &&
3918 . Using your distribution's package manager, or from source, install
3919 the following 32-bit versions of the following dependencies of
3920 LTTng-tools and LTTng-UST:
3923 * https://sourceforge.net/projects/libuuid/[libuuid]
3924 * http://directory.fsf.org/wiki/Popt[popt]
3925 * http://www.xmlsoft.org/[libxml2]
3928 . Download, build, and install a 32-bit version of the latest
3929 LTTng-UST{nbsp}{revision}:
3935 wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.8.tar.bz2 &&
3936 tar -xf lttng-ust-latest-2.8.tar.bz2 &&
3937 cd lttng-ust-2.8.* &&
3938 ./configure --libdir=/usr/local/lib32 \
3939 CFLAGS=-m32 CXXFLAGS=-m32 \
3940 LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' &&
3942 sudo make install &&
3949 Depending on your distribution,
3950 32-bit libraries could be installed at a different location than
3951 `/usr/lib32`. For example, Debian is known to install
3952 some 32-bit libraries in `/usr/lib/i386-linux-gnu`.
3954 In this case, make sure to set `LDFLAGS` to all the
3955 relevant 32-bit library paths, for example:
3959 LDFLAGS='-L/usr/lib/i386-linux-gnu -L/usr/lib32'
3963 . Download the latest LTTng-tools{nbsp}{revision}, build, and install
3964 the 32-bit consumer daemon:
3970 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.8.tar.bz2 &&
3971 tar -xf lttng-tools-latest-2.8.tar.bz2 &&
3972 cd lttng-tools-2.8.* &&
3973 ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \
3974 LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' &&
3976 cd src/bin/lttng-consumerd &&
3977 sudo make install &&
3982 . From your distribution or from source,
3983 <<installing-lttng,install>> the 64-bit versions of
3984 LTTng-UST and Userspace RCU.
3985 . Download, build, and install the 64-bit version of the
3986 latest LTTng-tools{nbsp}{revision}:
3992 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.8.tar.bz2 &&
3993 tar -xf lttng-tools-latest-2.8.tar.bz2 &&
3994 cd lttng-tools-2.8.* &&
3995 ./configure --with-consumerd32-libdir=/usr/local/lib32 \
3996 --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd &&
3998 sudo make install &&
4003 . Pass the following options to man:gcc(1), man:g++(1), or man:clang(1)
4004 when linking your 32-bit application:
4007 -m32 -L/usr/lib32 -L/usr/local/lib32 \
4008 -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32
4011 For example, let's rebuild the quick start example in
4012 <<tracing-your-own-user-application,Trace a user application>> as an
4013 instrumented 32-bit application:
4018 gcc -m32 -c -I. hello-tp.c
4020 gcc -m32 -o hello hello.o hello-tp.o \
4021 -L/usr/lib32 -L/usr/local/lib32 \
4022 -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 \
4027 No special action is required to execute the 32-bit application and
4028 to trace it: use the command-line man:lttng(1) tool as usual.
4035 man:tracef(3) is a small LTTng-UST API designed for quick,
4036 man:printf(3)-like instrumentation without the burden of
4037 <<tracepoint-provider,creating>> and
4038 <<building-tracepoint-providers-and-user-application,building>>
4039 a tracepoint provider package.
4041 To use `tracef()` in your application:
4043 . In the C or C++ source files where you need to use `tracef()`,
4044 include `<lttng/tracef.h>`:
4049 #include <lttng/tracef.h>
4053 . In the application's source code, use `tracef()` like you would use
4061 tracef("my message: %d (%s)", my_integer, my_string);
4067 . Link your application with `liblttng-ust`:
4072 gcc -o app app.c -llttng-ust
4076 To trace the events that `tracef()` calls emit:
4078 * <<enabling-disabling-events,Create an event rule>> which matches the
4079 `lttng_ust_tracef:*` event name:
4084 lttng enable-event --userspace 'lttng_ust_tracef:*'
4089 .Limitations of `tracef()`
4091 The `tracef()` utility function was developed to make user space tracing
4092 super simple, albeit with notable disadvantages compared to
4093 <<defining-tracepoints,user-defined tracepoints>>:
4095 * All the emitted events have the same tracepoint provider and
4096 tracepoint names, respectively `lttng_ust_tracef` and `event`.
4097 * There is no static type checking.
4098 * The only event record field you actually get, named `msg`, is a string
4099 potentially containing the values you passed to `tracef()`
4100 using your own format string. This also means that you cannot filter
4101 events with a custom expression at run time because there are no
4103 * Since `tracef()` uses the C standard library's man:vasprintf(3)
4104 function behind the scenes to format the strings at run time, its
4105 expected performance is lower than with user-defined tracepoints,
4106 which do not require a conversion to a string.
4108 Taking this into consideration, `tracef()` is useful for some quick
4109 prototyping and debugging, but you should not consider it for any
4110 permanent and serious applicative instrumentation.
4116 ==== Use `tracelog()`
4118 The man:tracelog(3) API is very similar to <<tracef,`tracef()`>>, with
4119 the difference that it accepts an additional log level parameter.
4121 The goal of `tracelog()` is to ease the migration from logging to
4124 To use `tracelog()` in your application:
4126 . In the C or C++ source files where you need to use `tracelog()`,
4127 include `<lttng/tracelog.h>`:
4132 #include <lttng/tracelog.h>
4136 . In the application's source code, use `tracelog()` like you would use
4137 man:printf(3), except for the first parameter which is the log
4145 tracelog(TRACE_WARNING, "my message: %d (%s)",
4146 my_integer, my_string);
4152 See man:lttng-ust(3) for a list of available log level names.
4154 . Link your application with `liblttng-ust`:
4159 gcc -o app app.c -llttng-ust
4163 To trace the events that `tracelog()` calls emit with a log level
4164 _as severe as_ a specific log level:
4166 * <<enabling-disabling-events,Create an event rule>> which matches the
4167 `lttng_ust_tracelog:*` event name and a minimum level
4173 lttng enable-event --userspace 'lttng_ust_tracelog:*'
4174 --loglevel=TRACE_WARNING
4178 To trace the events that `tracelog()` calls emit with a
4179 _specific log level_:
4181 * Create an event rule which matches the `lttng_ust_tracelog:*`
4182 event name and a specific log level:
4187 lttng enable-event --userspace 'lttng_ust_tracelog:*'
4188 --loglevel-only=TRACE_INFO
4193 [[prebuilt-ust-helpers]]
4194 === Prebuilt user space tracing helpers
4196 The LTTng-UST package provides a few helpers in the form or preloadable
4197 shared objects which automatically instrument system functions and
4200 The helper shared objects are normally found in dir:{/usr/lib}. If you
4201 built LTTng-UST <<building-from-source,from source>>, they are probably
4202 located in dir:{/usr/local/lib}.
4204 The installed user space tracing helpers in LTTng-UST{nbsp}{revision}
4207 path:{liblttng-ust-libc-wrapper.so}::
4208 path:{liblttng-ust-pthread-wrapper.so}::
4209 <<liblttng-ust-libc-pthread-wrapper,C{nbsp}standard library
4210 memory and POSIX threads function tracing>>.
4212 path:{liblttng-ust-cyg-profile.so}::
4213 path:{liblttng-ust-cyg-profile-fast.so}::
4214 <<liblttng-ust-cyg-profile,Function entry and exit tracing>>.
4216 path:{liblttng-ust-dl.so}::
4217 <<liblttng-ust-dl,Dynamic linker tracing>>.
4219 To use a user space tracing helper with any user application:
4221 * Preload the helper shared object when you start the application:
4226 LD_PRELOAD=liblttng-ust-libc-wrapper.so my-app
4230 You can preload more than one helper:
4235 LD_PRELOAD=liblttng-ust-libc-wrapper.so:liblttng-ust-dl.so my-app
4241 [[liblttng-ust-libc-pthread-wrapper]]
4242 ==== Instrument C standard library memory and POSIX threads functions
4244 The path:{liblttng-ust-libc-wrapper.so} and
4245 path:{liblttng-ust-pthread-wrapper.so} helpers
4246 add instrumentation to some C standard library and POSIX
4250 .Functions instrumented by preloading path:{liblttng-ust-libc-wrapper.so}.
4252 |TP provider name |TP name |Instrumented function
4254 .6+|`lttng_ust_libc` |`malloc` |man:malloc(3)
4255 |`calloc` |man:calloc(3)
4256 |`realloc` |man:realloc(3)
4257 |`free` |man:free(3)
4258 |`memalign` |man:memalign(3)
4259 |`posix_memalign` |man:posix_memalign(3)
4263 .Functions instrumented by preloading path:{liblttng-ust-pthread-wrapper.so}.
4265 |TP provider name |TP name |Instrumented function
4267 .4+|`lttng_ust_pthread` |`pthread_mutex_lock_req` |man:pthread_mutex_lock(3p) (request time)
4268 |`pthread_mutex_lock_acq` |man:pthread_mutex_lock(3p) (acquire time)
4269 |`pthread_mutex_trylock` |man:pthread_mutex_trylock(3p)
4270 |`pthread_mutex_unlock` |man:pthread_mutex_unlock(3p)
4273 When you preload the shared object, it replaces the functions listed
4274 in the previous tables by wrappers which contain tracepoints and call
4275 the replaced functions.
4278 [[liblttng-ust-cyg-profile]]
4279 ==== Instrument function entry and exit
4281 The path:{liblttng-ust-cyg-profile*.so} helpers can add instrumentation
4282 to the entry and exit points of functions.
4284 man:gcc(1) and man:clang(1) have an option named
4285 https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html[`-finstrument-functions`]
4286 which generates instrumentation calls for entry and exit to functions.
4287 The LTTng-UST function tracing helpers,
4288 path:{liblttng-ust-cyg-profile.so} and
4289 path:{liblttng-ust-cyg-profile-fast.so}, take advantage of this feature
4290 to add tracepoints to the two generated functions (which contain
4291 `cyg_profile` in their names, hence the helper's name).
4293 To use the LTTng-UST function tracing helper, the source files to
4294 instrument must be built using the `-finstrument-functions` compiler
4297 There are two versions of the LTTng-UST function tracing helper:
4299 * **path:{liblttng-ust-cyg-profile-fast.so}** is a lightweight variant
4300 that you should only use when it can be _guaranteed_ that the
4301 complete event stream is recorded without any lost event record.
4302 Any kind of duplicate information is left out.
4304 Assuming no event record is lost, having only the function addresses on
4305 entry is enough to create a call graph, since an event record always
4306 contains the ID of the CPU that generated it.
4308 You can use a tool like man:addr2line(1) to convert function addresses
4309 back to source file names and line numbers.
4311 * **path:{liblttng-ust-cyg-profile.so}** is a more robust variant
4312 which also works in use cases where event records might get discarded or
4313 not recorded from application startup.
4314 In these cases, the trace analyzer needs more information to be
4315 able to reconstruct the program flow.
4317 See man:lttng-ust-cyg-profile(3) to learn more about the instrumentation
4318 points of this helper.
4320 All the tracepoints that this helper provides have the
4321 log level `TRACE_DEBUG_FUNCTION` (see man:lttng-ust(3)).
4323 TIP: It's sometimes a good idea to limit the number of source files that
4324 you compile with the `-finstrument-functions` option to prevent LTTng
4325 from writing an excessive amount of trace data at run time. When using
4326 man:gcc(1), you can use the
4327 `-finstrument-functions-exclude-function-list` option to avoid
4328 instrument entries and exits of specific function names.
4333 ==== Instrument the dynamic linker
4335 The path:{liblttng-ust-dl.so} helper adds instrumentation to the
4336 man:dlopen(3) and man:dlclose(3) function calls.
4338 See man:lttng-ust-dl(3) to learn more about the instrumentation points
4343 [[java-application]]
4344 === User space Java agent
4346 You can instrument any Java application which uses one of the following
4349 * The https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[**`java.util.logging`**]
4350 (JUL) core logging facilities.
4351 * http://logging.apache.org/log4j/1.2/[**Apache log4j 1.2**], since
4352 LTTng 2.6. Note that Apache Log4j{nbsp}2 is not supported.
4355 .LTTng-UST Java agent imported by a Java application.
4356 image::java-app.png[]
4358 Note that the methods described below are new in LTTng{nbsp}{revision}.
4359 Previous LTTng versions use another technique.
4361 NOTE: We use http://openjdk.java.net/[OpenJDK]{nbsp}8 for development
4362 and https://ci.lttng.org/[continuous integration], thus this version is
4363 directly supported. However, the LTTng-UST Java agent is also tested
4364 with OpenJDK{nbsp}7.
4369 ==== Use the LTTng-UST Java agent for `java.util.logging`
4371 To use the LTTng-UST Java agent in a Java application which uses
4372 `java.util.logging` (JUL):
4374 . In the Java application's source code, import the LTTng-UST
4375 log handler package for `java.util.logging`:
4380 import org.lttng.ust.agent.jul.LttngLogHandler;
4384 . Create an LTTng-UST JUL log handler:
4389 Handler lttngUstLogHandler = new LttngLogHandler();
4393 . Add this handler to the JUL loggers which should emit LTTng events:
4398 Logger myLogger = Logger.getLogger("some-logger");
4400 myLogger.addHandler(lttngUstLogHandler);
4404 . Use `java.util.logging` log statements and configuration as usual.
4405 The loggers with an attached LTTng-UST log handler can emit
4408 . Before exiting the application, remove the LTTng-UST log handler from
4409 the loggers attached to it and call its `close()` method:
4414 myLogger.removeHandler(lttngUstLogHandler);
4415 lttngUstLogHandler.close();
4419 This is not strictly necessary, but it is recommended for a clean
4420 disposal of the handler's resources.
4422 . Include the LTTng-UST Java agent's common and JUL-specific JAR files,
4423 path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-jul.jar},
4425 https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
4426 path] when you build the Java application.
4428 The JAR files are typically located in dir:{/usr/share/java}.
4430 IMPORTANT: The LTTng-UST Java agent must be
4431 <<installing-lttng,installed>> for the logging framework your
4434 .Use the LTTng-UST Java agent for `java.util.logging`.
4439 import java.io.IOException;
4440 import java.util.logging.Handler;
4441 import java.util.logging.Logger;
4442 import org.lttng.ust.agent.jul.LttngLogHandler;
4446 private static final int answer = 42;
4448 public static void main(String[] argv) throws Exception
4451 Logger logger = Logger.getLogger("jello");
4453 // Create an LTTng-UST log handler
4454 Handler lttngUstLogHandler = new LttngLogHandler();
4456 // Add the LTTng-UST log handler to our logger
4457 logger.addHandler(lttngUstLogHandler);
4460 logger.info("some info");
4461 logger.warning("some warning");
4463 logger.finer("finer information; the answer is " + answer);
4465 logger.severe("error!");
4467 // Not mandatory, but cleaner
4468 logger.removeHandler(lttngUstLogHandler);
4469 lttngUstLogHandler.close();
4478 javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
4481 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4482 <<enabling-disabling-events,create an event rule>> matching the
4483 `jello` JUL logger, and <<basic-tracing-session-control,start tracing>>:
4488 lttng enable-event --jul jello
4492 Run the compiled class:
4496 java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
4499 <<basic-tracing-session-control,Stop tracing>> and inspect the
4509 You can use the opt:lttng-enable-event(1):--loglevel or
4510 opt:lttng-enable-event(1):--loglevel-only option of the
4511 man:lttng-enable-event(1) command to target a range of JUL log levels
4512 or a specific JUL log level.
4517 ==== Use the LTTng-UST Java agent for Apache log4j
4519 To use the LTTng-UST Java agent in a Java application which uses
4522 . In the Java application's source code, import the LTTng-UST
4523 log appender package for Apache log4j:
4528 import org.lttng.ust.agent.log4j.LttngLogAppender;
4532 . Create an LTTng-UST log4j log appender:
4537 Appender lttngUstLogAppender = new LttngLogAppender();
4541 . Add this appender to the log4j loggers which should emit LTTng events:
4546 Logger myLogger = Logger.getLogger("some-logger");
4548 myLogger.addAppender(lttngUstLogAppender);
4552 . Use Apache log4j log statements and configuration as usual. The
4553 loggers with an attached LTTng-UST log appender can emit LTTng events.
4555 . Before exiting the application, remove the LTTng-UST log appender from
4556 the loggers attached to it and call its `close()` method:
4561 myLogger.removeAppender(lttngUstLogAppender);
4562 lttngUstLogAppender.close();
4566 This is not strictly necessary, but it is recommended for a clean
4567 disposal of the appender's resources.
4569 . Include the LTTng-UST Java agent's common and log4j-specific JAR
4570 files, path:{lttng-ust-agent-common.jar} and
4571 path:{lttng-ust-agent-log4j.jar}, in the
4572 https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
4573 path] when you build the Java application.
4575 The JAR files are typically located in dir:{/usr/share/java}.
4577 IMPORTANT: The LTTng-UST Java agent must be
4578 <<installing-lttng,installed>> for the logging framework your
4581 .Use the LTTng-UST Java agent for Apache log4j.
4586 import org.apache.log4j.Appender;
4587 import org.apache.log4j.Logger;
4588 import org.lttng.ust.agent.log4j.LttngLogAppender;
4592 private static final int answer = 42;
4594 public static void main(String[] argv) throws Exception
4597 Logger logger = Logger.getLogger("jello");
4599 // Create an LTTng-UST log appender
4600 Appender lttngUstLogAppender = new LttngLogAppender();
4602 // Add the LTTng-UST log appender to our logger
4603 logger.addAppender(lttngUstLogAppender);
4606 logger.info("some info");
4607 logger.warn("some warning");
4609 logger.debug("debug information; the answer is " + answer);
4611 logger.fatal("error!");
4613 // Not mandatory, but cleaner
4614 logger.removeAppender(lttngUstLogAppender);
4615 lttngUstLogAppender.close();
4621 Build this example (`$LOG4JPATH` is the path to the Apache log4j JAR
4626 javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java
4629 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4630 <<enabling-disabling-events,create an event rule>> matching the
4631 `jello` log4j logger, and <<basic-tracing-session-control,start tracing>>:
4636 lttng enable-event --log4j jello
4640 Run the compiled class:
4644 java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH:. Test
4647 <<basic-tracing-session-control,Stop tracing>> and inspect the
4657 You can use the opt:lttng-enable-event(1):--loglevel or
4658 opt:lttng-enable-event(1):--loglevel-only option of the
4659 man:lttng-enable-event(1) command to target a range of Apache log4j log levels
4660 or a specific log4j log level.
4664 [[java-application-context]]
4665 ==== Provide application-specific context fields in a Java application
4667 A Java application-specific context field is a piece of state provided
4668 by the application which <<adding-context,you can add>>, using the
4669 man:lttng-add-context(1) command, to each <<event,event record>>
4670 produced by the log statements of this application.
4672 For example, a given object might have a current request ID variable.
4673 You can create a context information retriever for this object and
4674 assign a name to this current request ID. You can then, using the
4675 man:lttng-add-context(1) command, add this context field by name to
4676 the JUL or log4j <<channel,channel>>.
4678 To provide application-specific context fields in a Java application:
4680 . In the Java application's source code, import the LTTng-UST
4681 Java agent context classes and interfaces:
4686 import org.lttng.ust.agent.context.ContextInfoManager;
4687 import org.lttng.ust.agent.context.IContextInfoRetriever;
4691 . Create a context information retriever class, that is, a class which
4692 implements the `IContextInfoRetriever` interface:
4697 class MyContextInfoRetriever implements IContextInfoRetriever
4700 public Object retrieveContextInfo(String key)
4702 if (key.equals("intCtx")) {
4704 } else if (key.equals("strContext")) {
4705 return "context value!";
4714 This `retrieveContextInfo()` method is the only member of the
4715 `IContextInfoRetriever` interface. Its role is to return the current
4716 value of a state by name to create a context field. The names of the
4717 context fields and which state variables they return depends on your
4720 All primitive types and objects are supported as context fields.
4721 When `retrieveContextInfo()` returns an object, the context field
4722 serializer calls its `toString()` method to add a string field to
4723 event records. The method can also return `null`, which means that
4724 no context field is available for the required name.
4726 . Register an instance of your context information retriever class to
4727 the context information manager singleton:
4732 IContextInfoRetriever cir = new MyContextInfoRetriever();
4733 ContextInfoManager cim = ContextInfoManager.getInstance();
4734 cim.registerContextInfoRetriever("retrieverName", cir);
4738 . Before exiting the application, remove your context information
4739 retriever from the context information manager singleton:
4744 ContextInfoManager cim = ContextInfoManager.getInstance();
4745 cim.unregisterContextInfoRetriever("retrieverName");
4749 This is not strictly necessary, but it is recommended for a clean
4750 disposal of some manager's resources.
4752 . Build your Java application with LTTng-UST Java agent support as
4753 usual, following the procedure for either the <<jul,JUL>> or
4754 <<log4j,Apache log4j>> framework.
4757 .Provide application-specific context fields in a Java application.
4762 import java.util.logging.Handler;
4763 import java.util.logging.Logger;
4764 import org.lttng.ust.agent.jul.LttngLogHandler;
4765 import org.lttng.ust.agent.context.ContextInfoManager;
4766 import org.lttng.ust.agent.context.IContextInfoRetriever;
4770 // Our context information retriever class
4771 private static class MyContextInfoRetriever
4772 implements IContextInfoRetriever
4775 public Object retrieveContextInfo(String key) {
4776 if (key.equals("intCtx")) {
4778 } else if (key.equals("strContext")) {
4779 return "context value!";
4786 private static final int answer = 42;
4788 public static void main(String args[]) throws Exception
4790 // Get the context information manager instance
4791 ContextInfoManager cim = ContextInfoManager.getInstance();
4793 // Create and register our context information retriever
4794 IContextInfoRetriever cir = new MyContextInfoRetriever();
4795 cim.registerContextInfoRetriever("myRetriever", cir);
4798 Logger logger = Logger.getLogger("jello");
4800 // Create an LTTng-UST log handler
4801 Handler lttngUstLogHandler = new LttngLogHandler();
4803 // Add the LTTng-UST log handler to our logger
4804 logger.addHandler(lttngUstLogHandler);
4807 logger.info("some info");
4808 logger.warning("some warning");
4810 logger.finer("finer information; the answer is " + answer);
4812 logger.severe("error!");
4814 // Not mandatory, but cleaner
4815 logger.removeHandler(lttngUstLogHandler);
4816 lttngUstLogHandler.close();
4817 cim.unregisterContextInfoRetriever("myRetriever");
4826 javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
4829 <<creating-destroying-tracing-sessions,Create a tracing session>>
4830 and <<enabling-disabling-events,create an event rule>> matching the
4836 lttng enable-event --jul jello
4839 <<adding-context,Add the application-specific context fields>> to the
4844 lttng add-context --jul --type='$app.myRetriever:intCtx'
4845 lttng add-context --jul --type='$app.myRetriever:strContext'
4848 <<basic-tracing-session-control,Start tracing>>:
4855 Run the compiled class:
4859 java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
4862 <<basic-tracing-session-control,Stop tracing>> and inspect the
4874 [[python-application]]
4875 === User space Python agent
4877 You can instrument a Python 2 or Python 3 application which uses the
4878 standard https://docs.python.org/3/library/logging.html[`logging`]
4881 Each log statement emits an LTTng event once the
4882 application module imports the
4883 <<lttng-ust-agents,LTTng-UST Python agent>> package.
4886 .A Python application importing the LTTng-UST Python agent.
4887 image::python-app.png[]
4889 To use the LTTng-UST Python agent:
4891 . In the Python application's source code, import the LTTng-UST Python
4901 The LTTng-UST Python agent automatically adds its logging handler to the
4902 root logger at import time.
4904 Any log statement that the application executes before this import does
4905 not emit an LTTng event.
4907 IMPORTANT: The LTTng-UST Python agent must be
4908 <<installing-lttng,installed>>.
4910 . Use log statements and logging configuration as usual.
4911 Since the LTTng-UST Python agent adds a handler to the _root_
4912 logger, you can trace any log statement from any logger.
4914 .Use the LTTng-UST Python agent.
4925 logging.basicConfig()
4926 logger = logging.getLogger('my-logger')
4929 logger.debug('debug message')
4930 logger.info('info message')
4931 logger.warn('warn message')
4932 logger.error('error message')
4933 logger.critical('critical message')
4937 if __name__ == '__main__':
4941 NOTE: `logging.basicConfig()`, which adds to the root logger a basic
4942 logging handler which prints to the standard error stream, is not
4943 strictly required for LTTng-UST tracing to work, but in versions of
4944 Python preceding 3.2, you could see a warning message which indicates
4945 that no handler exists for the logger `my-logger`.
4947 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4948 <<enabling-disabling-events,create an event rule>> matching the
4949 `my-logger` Python logger, and <<basic-tracing-session-control,start
4955 lttng enable-event --python my-logger
4959 Run the Python script:
4966 <<basic-tracing-session-control,Stop tracing>> and inspect the recorded
4976 You can use the opt:lttng-enable-event(1):--loglevel or
4977 opt:lttng-enable-event(1):--loglevel-only option of the
4978 man:lttng-enable-event(1) command to target a range of Python log levels
4979 or a specific Python log level.
4981 When an application imports the LTTng-UST Python agent, the agent tries
4982 to register to a <<lttng-sessiond,session daemon>>. Note that you must
4983 <<start-sessiond,start the session daemon>> _before_ you run the Python
4984 application. If a session daemon is found, the agent tries to register
4985 to it during 5{nbsp}seconds, after which the application continues
4986 without LTTng tracing support. You can override this timeout value with
4987 the env:LTTNG_UST_PYTHON_REGISTER_TIMEOUT environment variable
4990 If the session daemon stops while a Python application with an imported
4991 LTTng-UST Python agent runs, the agent retries to connect and to
4992 register to a session daemon every 3{nbsp}seconds. You can override this
4993 delay with the env:LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment
4998 [[proc-lttng-logger-abi]]
5001 The `lttng-tracer` Linux kernel module, part of
5002 <<lttng-modules,LTTng-modules>>, creates the special LTTng logger file
5003 path:{/proc/lttng-logger} when it's loaded. Any application can write
5004 text data to this file to emit an LTTng event.
5007 .An application writes to the LTTng logger file to emit an LTTng event.
5008 image::lttng-logger.png[]
5010 The LTTng logger is the quickest method--not the most efficient,
5011 however--to add instrumentation to an application. It is designed
5012 mostly to instrument shell scripts:
5016 echo "Some message, some $variable" > /proc/lttng-logger
5019 Any event that the LTTng logger emits is named `lttng_logger` and
5020 belongs to the Linux kernel <<domain,tracing domain>>. However, unlike
5021 other instrumentation points in the kernel tracing domain, **any Unix
5022 user** can <<enabling-disabling-events,create an event rule>> which
5023 matches its event name, not only the root user or users in the tracing
5026 To use the LTTng logger:
5028 * From any application, write text data to the path:{/proc/lttng-logger}
5031 The `msg` field of `lttng_logger` event records contains the
5034 NOTE: The maximum message length of an LTTng logger event is
5035 1024{nbsp}bytes. Writing more than this makes the LTTng logger emit more
5036 than one event to contain the remaining data.
5038 You should not use the LTTng logger to trace a user application which
5039 can be instrumented in a more efficient way, namely:
5041 * <<c-application,C and $$C++$$ applications>>.
5042 * <<java-application,Java applications>>.
5043 * <<python-application,Python applications>>.
5045 .Use the LTTng logger.
5050 echo 'Hello, World!' > /proc/lttng-logger
5052 df --human-readable --print-type / > /proc/lttng-logger
5055 <<creating-destroying-tracing-sessions,Create a tracing session>>,
5056 <<enabling-disabling-events,create an event rule>> matching the
5057 `lttng_logger` Linux kernel tracepoint, and
5058 <<basic-tracing-session-control,start tracing>>:
5063 lttng enable-event --kernel lttng_logger
5067 Run the Bash script:
5074 <<basic-tracing-session-control,Stop tracing>> and inspect the recorded
5085 [[instrumenting-linux-kernel]]
5086 === LTTng kernel tracepoints
5088 NOTE: This section shows how to _add_ instrumentation points to the
5089 Linux kernel. The kernel's subsystems are already thoroughly
5090 instrumented at strategic places for LTTng when you
5091 <<installing-lttng,install>> the <<lttng-modules,LTTng-modules>>
5095 There are two methods to instrument the Linux kernel:
5097 . <<linux-add-lttng-layer,Add an LTTng layer>> over an existing ftrace
5098 tracepoint which uses the `TRACE_EVENT()` API.
5100 Choose this if you want to instrumentation a Linux kernel tree with an
5101 instrumentation point compatible with ftrace, perf, and SystemTap.
5103 . Use an <<linux-lttng-tracepoint-event,LTTng-only approach>> to
5104 instrument an out-of-tree kernel module.
5106 Choose this if you don't need ftrace, perf, or SystemTap support.
5110 [[linux-add-lttng-layer]]
5111 ==== [[instrumenting-linux-kernel-itself]][[mainline-trace-event]][[lttng-adaptation-layer]]Add an LTTng layer to an existing ftrace tracepoint
5113 This section shows how to add an LTTng layer to existing ftrace
5114 instrumentation using the `TRACE_EVENT()` API.
5116 This section does not document the `TRACE_EVENT()` macro. You can
5117 read the following articles to learn more about this API:
5119 * http://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part 1)]
5120 * http://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part 2)]
5121 * http://lwn.net/Articles/383362/[Using the TRACE_EVENT() macro (Part 3)]
5123 The following procedure assumes that your ftrace tracepoints are
5124 correctly defined in their own header and that they are created in
5125 one source file using the `CREATE_TRACE_POINTS` definition.
5127 To add an LTTng layer over an existing ftrace tracepoint:
5129 . Make sure the following kernel configuration options are
5135 * `CONFIG_HIGH_RES_TIMERS`
5136 * `CONFIG_TRACEPOINTS`
5139 . Build the Linux source tree with your custom ftrace tracepoints.
5140 . Boot the resulting Linux image on your target system.
5142 Confirm that the tracepoints exist by looking for their names in the
5143 dir:{/sys/kernel/debug/tracing/events/subsys} directory, where `subsys`
5144 is your subsystem's name.
5146 . Get a copy of the latest LTTng-modules{nbsp}{revision}:
5152 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.8.tar.bz2 &&
5153 tar -xf lttng-modules-latest-2.8.tar.bz2 &&
5154 cd lttng-modules-2.8.*
5158 . In dir:{instrumentation/events/lttng-module}, relative to the root
5159 of the LTTng-modules source tree, create a header file named
5160 +__subsys__.h+ for your custom subsystem +__subsys__+ and write your
5161 LTTng-modules tracepoint definitions using the LTTng-modules
5164 Start with this template:
5168 .path:{instrumentation/events/lttng-module/my_subsys.h}
5171 #define TRACE_SYSTEM my_subsys
5173 #if !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ)
5174 #define _LTTNG_MY_SUBSYS_H
5176 #include "../../../probes/lttng-tracepoint-event.h"
5177 #include <linux/tracepoint.h>
5179 LTTNG_TRACEPOINT_EVENT(
5181 * Format is identical to TRACE_EVENT()'s version for the three
5182 * following macro parameters:
5185 TP_PROTO(int my_int, const char *my_string),
5186 TP_ARGS(my_int, my_string),
5188 /* LTTng-modules specific macros */
5190 ctf_integer(int, my_int_field, my_int)
5191 ctf_string(my_bar_field, my_bar)
5195 #endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */
5197 #include "../../../probes/define_trace.h"
5201 The entries in the `TP_FIELDS()` section are the list of fields for the
5202 LTTng tracepoint. This is similar to the `TP_STRUCT__entry()` part of
5203 ftrace's `TRACE_EVENT()` macro.
5205 See <<lttng-modules-tp-fields,Tracepoint fields macros>> for a
5206 complete description of the available `ctf_*()` macros.
5208 . Create the LTTng-modules probe's kernel module C source file,
5209 +probes/lttng-probe-__subsys__.c+, where +__subsys__+ is your
5214 .path:{probes/lttng-probe-my-subsys.c}
5216 #include <linux/module.h>
5217 #include "../lttng-tracer.h"
5220 * Build-time verification of mismatch between mainline
5221 * TRACE_EVENT() arguments and the LTTng-modules adaptation
5222 * layer LTTNG_TRACEPOINT_EVENT() arguments.
5224 #include <trace/events/my_subsys.h>
5226 /* Create LTTng tracepoint probes */
5227 #define LTTNG_PACKAGE_BUILD
5228 #define CREATE_TRACE_POINTS
5229 #define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module
5231 #include "../instrumentation/events/lttng-module/my_subsys.h"
5233 MODULE_LICENSE("GPL and additional rights");
5234 MODULE_AUTHOR("Your name <your-email>");
5235 MODULE_DESCRIPTION("LTTng my_subsys probes");
5236 MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "."
5237 __stringify(LTTNG_MODULES_MINOR_VERSION) "."
5238 __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION)
5239 LTTNG_MODULES_EXTRAVERSION);
5243 . Edit path:{probes/Makefile} and add your new kernel module object
5244 next to the existing ones:
5248 .path:{probes/Makefile}
5252 obj-m += lttng-probe-module.o
5253 obj-m += lttng-probe-power.o
5255 obj-m += lttng-probe-my-subsys.o
5261 . Build and install the LTTng kernel modules:
5266 make KERNELDIR=/path/to/linux
5267 sudo make modules_install
5271 Replace `/path/to/linux` with the path to the Linux source tree where
5272 you defined and used tracepoints with ftrace's `TRACE_EVENT()` macro.
5274 Note that you can also use the
5275 <<lttng-tracepoint-event-code,`LTTNG_TRACEPOINT_EVENT_CODE()` macro>>
5276 instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and
5277 C code that need to be executed before the event fields are recorded.
5279 The best way to learn how to use the previous LTTng-modules macros is to
5280 inspect the existing LTTng-modules tracepoint definitions in the
5281 dir:{instrumentation/events/lttng-module} header files. Compare them
5282 with the Linux kernel mainline versions in the
5283 dir:{include/trace/events} directory of the Linux source tree.
5287 [[lttng-tracepoint-event-code]]
5288 ===== Use custom C code to access the data for tracepoint fields
5290 Although we recommended to always use the
5291 <<lttng-adaptation-layer,`LTTNG_TRACEPOINT_EVENT()`>> macro to describe
5292 the arguments and fields of an LTTng-modules tracepoint when possible,
5293 sometimes you need a more complex process to access the data that the
5294 tracer records as event record fields. In other words, you need local
5295 variables and multiple C{nbsp}statements instead of simple
5296 argument-based expressions that you pass to the
5297 <<lttng-modules-tp-fields,`ctf_*()` macros of `TP_FIELDS()`>>.
5299 You can use the `LTTNG_TRACEPOINT_EVENT_CODE()` macro instead of
5300 `LTTNG_TRACEPOINT_EVENT()` to declare custom local variables and define
5301 a block of C{nbsp}code to be executed before LTTng records the fields.
5302 The structure of this macro is:
5305 .`LTTNG_TRACEPOINT_EVENT_CODE()` macro syntax.
5307 LTTNG_TRACEPOINT_EVENT_CODE(
5309 * Format identical to the LTTNG_TRACEPOINT_EVENT()
5310 * version for the following three macro parameters:
5313 TP_PROTO(int my_int, const char *my_string),
5314 TP_ARGS(my_int, my_string),
5316 /* Declarations of custom local variables */
5319 unsigned long b = 0;
5320 const char *name = "(undefined)";
5321 struct my_struct *my_struct;
5325 * Custom code which uses both tracepoint arguments
5326 * (in TP_ARGS()) and local variables (in TP_locvar()).
5328 * Local variables are actually members of a structure pointed
5329 * to by the special variable tp_locvar.
5333 tp_locvar->a = my_int + 17;
5334 tp_locvar->my_struct = get_my_struct_at(tp_locvar->a);
5335 tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct);
5336 tp_locvar->name = my_struct_get_name(tp_locvar->my_struct);
5337 put_my_struct(tp_locvar->my_struct);
5346 * Format identical to the LTTNG_TRACEPOINT_EVENT()
5347 * version for this, except that tp_locvar members can be
5348 * used in the argument expression parameters of
5349 * the ctf_*() macros.
5352 ctf_integer(unsigned long, my_struct_b, tp_locvar->b)
5353 ctf_integer(int, my_struct_a, tp_locvar->a)
5354 ctf_string(my_string_field, my_string)
5355 ctf_string(my_struct_name, tp_locvar->name)
5360 IMPORTANT: The C code defined in `TP_code()` must not have any side
5361 effects when executed. In particular, the code must not allocate
5362 memory or get resources without deallocating this memory or putting
5363 those resources afterwards.
5366 [[instrumenting-linux-kernel-tracing]]
5367 ==== Load and unload a custom probe kernel module
5369 You must load a <<lttng-adaptation-layer,created LTTng-modules probe
5370 kernel module>> in the kernel before it can emit LTTng events.
5372 To load the default probe kernel modules and a custom probe kernel
5375 * Use the opt:lttng-sessiond(8):--extra-kmod-probes option to give extra
5376 probe modules to load when starting a root <<lttng-sessiond,session
5380 .Load the `my_subsys`, `usb`, and the default probe modules.
5384 sudo lttng-sessiond --extra-kmod-probes=my_subsys,usb
5389 You only need to pass the subsystem name, not the whole kernel module
5392 To load _only_ a given custom probe kernel module:
5394 * Use the opt:lttng-sessiond(8):--kmod-probes option to give the probe
5395 modules to load when starting a root session daemon:
5398 .Load only the `my_subsys` and `usb` probe modules.
5402 sudo lttng-sessiond --kmod-probes=my_subsys,usb
5407 To confirm that a probe module is loaded:
5414 lsmod | grep lttng_probe_usb
5418 To unload the loaded probe modules:
5420 * Kill the session daemon with `SIGTERM`:
5425 sudo pkill lttng-sessiond
5429 You can also use man:modprobe(8)'s `--remove` option if the session
5430 daemon terminates abnormally.
5433 [[controlling-tracing]]
5436 Once an application or a Linux kernel is
5437 <<instrumenting,instrumented>> for LTTng tracing,
5440 This section is divided in topics on how to use the various
5441 <<plumbing,components of LTTng>>, in particular the <<lttng-cli,cmd:lttng
5442 command-line tool>>, to _control_ the LTTng daemons and tracers.
5444 NOTE: In the following subsections, we refer to an man:lttng(1) command
5445 using its man page name. For example, instead of _Run the `create`
5446 command to..._, we use _Run the man:lttng-create(1) command to..._.
5450 === Start a session daemon
5452 In some situations, you need to run a <<lttng-sessiond,session daemon>>
5453 (man:lttng-sessiond(8)) _before_ you can use the man:lttng(1)
5456 You will see the following error when you run a command while no session
5460 Error: No session daemon is available
5463 The only command that automatically runs a session daemon is
5464 man:lttng-create(1), which you use to
5465 <<creating-destroying-tracing-sessions,create a tracing session>>. While
5466 this is most of the time the first operation that you do, sometimes it's
5467 not. Some examples are:
5469 * <<list-instrumentation-points,List the available instrumentation points>>.
5470 * <<saving-loading-tracing-session,Load a tracing session configuration>>.
5472 [[tracing-group]] Each Unix user must have its own running session
5473 daemon to trace user applications. The session daemon that the root user
5474 starts is the only one allowed to control the LTTng kernel tracer. Users
5475 that are part of the _tracing group_ can control the root session
5476 daemon. The default tracing group name is `tracing`; you can set it to
5477 something else with the opt:lttng-sessiond(8):--group option when you
5478 start the root session daemon.
5480 To start a user session daemon:
5482 * Run man:lttng-sessiond(8):
5487 lttng-sessiond --daemonize
5491 To start the root session daemon:
5493 * Run man:lttng-sessiond(8) as the root user:
5498 sudo lttng-sessiond --daemonize
5502 In both cases, remove the opt:lttng-sessiond(8):--daemonize option to
5503 start the session daemon in foreground.
5505 To stop a session daemon, use man:kill(1) on its process ID (standard
5508 Note that some Linux distributions could manage the LTTng session daemon
5509 as a service. In this case, you should use the service manager to
5510 start, restart, and stop session daemons.
5513 [[creating-destroying-tracing-sessions]]
5514 === Create and destroy a tracing session
5516 Almost all the LTTng control operations happen in the scope of
5517 a <<tracing-session,tracing session>>, which is the dialogue between the
5518 <<lttng-sessiond,session daemon>> and you.
5520 To create a tracing session with a generated name:
5522 * Use the man:lttng-create(1) command:
5531 The created tracing session's name is `auto` followed by the
5534 To create a tracing session with a specific name:
5536 * Use the optional argument of the man:lttng-create(1) command:
5541 lttng create my-session
5545 Replace `my-session` with the specific tracing session name.
5547 LTTng appends the creation date to the created tracing session's name.
5549 LTTng writes the traces of a tracing session in
5550 +$LTTNG_HOME/lttng-trace/__name__+ by default, where +__name__+ is the
5551 name of the tracing session. Note that the env:LTTNG_HOME environment
5552 variable defaults to `$HOME` if not set.
5554 To output LTTng traces to a non-default location:
5556 * Use the opt:lttng-create(1):--output option of the man:lttng-create(1) command:
5561 lttng create --output=/tmp/some-directory my-session
5565 You may create as many tracing sessions as you wish.
5567 To list all the existing tracing sessions for your Unix user:
5569 * Use the man:lttng-list(1) command:
5578 When you create a tracing session, it is set as the _current tracing
5579 session_. The following man:lttng(1) commands operate on the current
5580 tracing session when you don't specify one:
5582 [role="list-3-cols"]
5598 To change the current tracing session:
5600 * Use the man:lttng-set-session(1) command:
5605 lttng set-session new-session
5609 Replace `new-session` by the name of the new current tracing session.
5611 When you are done tracing in a given tracing session, you can destroy
5612 it. This operation frees the resources taken by the tracing session
5613 to destroy; it does not destroy the trace data that LTTng wrote for
5614 this tracing session.
5616 To destroy the current tracing session:
5618 * Use the man:lttng-destroy(1) command:
5628 [[list-instrumentation-points]]
5629 === List the available instrumentation points
5631 The <<lttng-sessiond,session daemon>> can query the running instrumented
5632 user applications and the Linux kernel to get a list of available
5633 instrumentation points. For the Linux kernel <<domain,tracing domain>>,
5634 they are tracepoints and system calls. For the user space tracing
5635 domain, they are tracepoints. For the other tracing domains, they are
5638 To list the available instrumentation points:
5640 * Use the man:lttng-list(1) command with the requested tracing domain's
5644 * opt:lttng-list(1):--kernel: Linux kernel tracepoints (your Unix user
5645 must be a root user, or it must be a member of the
5646 <<tracing-group,tracing group>>).
5647 * opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall: Linux
5648 kernel system calls (your Unix user must be a root user, or it must be
5649 a member of the tracing group).
5650 * opt:lttng-list(1):--userspace: user space tracepoints.
5651 * opt:lttng-list(1):--jul: `java.util.logging` loggers.
5652 * opt:lttng-list(1):--log4j: Apache log4j loggers.
5653 * opt:lttng-list(1):--python: Python loggers.
5656 .List the available user space tracepoints.
5660 lttng list --userspace
5664 .List the available Linux kernel system call tracepoints.
5668 lttng list --kernel --syscall
5673 [[enabling-disabling-events]]
5674 === Create and enable an event rule
5676 Once you <<creating-destroying-tracing-sessions,create a tracing
5677 session>>, you can create <<event,event rules>> with the
5678 man:lttng-enable-event(1) command.
5680 You specify each condition with a command-line option. The available
5681 condition options are shown in the following table.
5683 [role="growable",cols="asciidoc,asciidoc,default"]
5684 .Condition command-line options for the man:lttng-enable-event(1) command.
5686 |Option |Description |Applicable tracing domains
5692 . +--probe=__ADDR__+
5693 . +--function=__ADDR__+
5696 Instead of using the default _tracepoint_ instrumentation type, use:
5698 . A Linux system call.
5699 . A Linux https://lwn.net/Articles/132196/[KProbe] (symbol or address).
5700 . The entry and return points of a Linux function (symbol or address).
5704 |First positional argument.
5707 Tracepoint or system call name. In the case of a Linux KProbe or
5708 function, this is a custom name given to the event rule. With the
5709 JUL, log4j, and Python domains, this is a logger name.
5711 With a tracepoint, logger, or system call name, the last character
5712 can be `*` to match anything that remains.
5719 . +--loglevel=__LEVEL__+
5720 . +--loglevel-only=__LEVEL__+
5723 . Match only tracepoints or log statements with a logging level at
5724 least as severe as +__LEVEL__+.
5725 . Match only tracepoints or log statements with a logging level
5726 equal to +__LEVEL__+.
5728 See man:lttng-enable-event(1) for the list of available logging level
5731 |User space, JUL, log4j, and Python.
5733 |+--exclude=__EXCLUSIONS__+
5736 When you use a `*` character at the end of the tracepoint or logger
5737 name (first positional argument), exclude the specific names in the
5738 comma-delimited list +__EXCLUSIONS__+.
5741 User space, JUL, log4j, and Python.
5743 |+--filter=__EXPR__+
5746 Match only events which satisfy the expression +__EXPR__+.
5748 See man:lttng-enable-event(1) to learn more about the syntax of a
5755 You attach an event rule to a <<channel,channel>> on creation. If you do
5756 not specify the channel with the opt:lttng-enable-event(1):--channel
5757 option, and if the event rule to create is the first in its
5758 <<domain,tracing domain>> for a given tracing session, then LTTng
5759 creates a _default channel_ for you. This default channel is reused in
5760 subsequent invocations of the man:lttng-enable-event(1) command for the
5761 same tracing domain.
5763 An event rule is always enabled at creation time.
5765 The following examples show how you can combine the previous
5766 command-line options to create simple to more complex event rules.
5768 .Create an event rule targetting a Linux kernel tracepoint (default channel).
5772 lttng enable-event --kernel sched_switch
5776 .Create an event rule matching four Linux kernel system calls (default channel).
5780 lttng enable-event --kernel --syscall open,write,read,close
5784 .Create event rules matching tracepoints with filter expressions (default channel).
5788 lttng enable-event --kernel sched_switch --filter='prev_comm == "bash"'
5793 lttng enable-event --kernel --all \
5794 --filter='$ctx.tid == 1988 || $ctx.tid == 1534'
5799 lttng enable-event --jul my_logger \
5800 --filter='$app.retriever:cur_msg_id > 3'
5803 IMPORTANT: Make sure to always quote the filter string when you
5804 use man:lttng(1) from a shell.
5807 .Create an event rule matching any user space tracepoint of a given tracepoint provider with a log level range (default channel).
5811 lttng enable-event --userspace my_app:'*' --loglevel=TRACE_INFO
5814 IMPORTANT: Make sure to always quote the wildcard character when you
5815 use man:lttng(1) from a shell.
5818 .Create an event rule matching multiple Python loggers with a wildcard and with exclusions (default channel).
5822 lttng enable-event --python my-app.'*' \
5823 --exclude='my-app.module,my-app.hello'
5827 .Create an event rule matching any Apache log4j logger with a specific log level (default channel).
5831 lttng enable-event --log4j --all --loglevel-only=LOG4J_WARN
5835 .Create an event rule attached to a specific channel matching a specific user space tracepoint provider and tracepoint.
5839 lttng enable-event --userspace my_app:my_tracepoint --channel=my-channel
5843 The event rules of a given channel form a whitelist: as soon as an
5844 emitted event passes one of them, LTTng can record the event. For
5845 example, an event named `my_app:my_tracepoint` emitted from a user space
5846 tracepoint with a `TRACE_ERROR` log level passes both of the following
5851 lttng enable-event --userspace my_app:my_tracepoint
5852 lttng enable-event --userspace my_app:my_tracepoint \
5853 --loglevel=TRACE_INFO
5856 The second event rule is redundant: the first one includes
5860 [[disable-event-rule]]
5861 === Disable an event rule
5863 To disable an event rule that you <<enabling-disabling-events,created>>
5864 previously, use the man:lttng-disable-event(1) command. This command
5865 disables _all_ the event rules (of a given tracing domain and channel)
5866 which match an instrumentation point. The other conditions are not
5867 supported as of LTTng{nbsp}{revision}.
5869 The LTTng tracer does not record an emitted event which passes
5870 a _disabled_ event rule.
5872 .Disable an event rule matching a Python logger (default channel).
5876 lttng disable-event --python my-logger
5880 .Disable an event rule matching all `java.util.logging` loggers (default channel).
5884 lttng disable-event --jul '*'
5888 .Disable _all_ the event rules of the default channel.
5890 The opt:lttng-disable-event(1):--all-events option is not, like the
5891 opt:lttng-enable-event(1):--all option of man:lttng-enable-event(1), the
5892 equivalent of the event name `*` (wildcard): it disables _all_ the event
5893 rules of a given channel.
5897 lttng disable-event --jul --all-events
5901 NOTE: You cannot delete an event rule once you create it.
5905 === Get the status of a tracing session
5907 To get the status of the current tracing session, that is, its
5908 parameters, its channels, event rules, and their attributes:
5910 * Use the man:lttng-status(1) command:
5920 To get the status of any tracing session:
5922 * Use the man:lttng-list(1) command with the tracing session's name:
5927 lttng list my-session
5931 Replace `my-session` with the desired tracing session's name.
5934 [[basic-tracing-session-control]]
5935 === Start and stop a tracing session
5937 Once you <<creating-destroying-tracing-sessions,create a tracing
5939 <<enabling-disabling-events,create one or more event rules>>,
5940 you can start and stop the tracers for this tracing session.
5942 To start tracing in the current tracing session:
5944 * Use the man:lttng-start(1) command:
5953 LTTng is very flexible: you can launch user applications before
5954 or after the you start the tracers. The tracers only record the events
5955 if they pass enabled event rules and if they occur while the tracers are
5958 To stop tracing in the current tracing session:
5960 * Use the man:lttng-stop(1) command:
5969 If there were <<channel-overwrite-mode-vs-discard-mode,lost event
5970 records>> or lost sub-buffers since the last time you ran
5971 man:lttng-start(1), warnings are printed when you run the
5972 man:lttng-stop(1) command.
5975 [[enabling-disabling-channels]]
5976 === Create a channel
5978 Once you create a tracing session, you can create a <<channel,channel>>
5979 with the man:lttng-enable-channel(1) command.
5981 Note that LTTng automatically creates a default channel when, for a
5982 given <<domain,tracing domain>>, no channels exist and you
5983 <<enabling-disabling-events,create>> the first event rule. This default
5984 channel is named `channel0` and its attributes are set to reasonable
5985 values. Therefore, you only need to create a channel when you need
5986 non-default attributes.
5988 You specify each non-default channel attribute with a command-line
5989 option when you use the man:lttng-enable-channel(1) command. The
5990 available command-line options are:
5992 [role="growable",cols="asciidoc,asciidoc"]
5993 .Command-line options for the man:lttng-enable-channel(1) command.
5995 |Option |Description
6001 <<channel-overwrite-mode-vs-discard-mode,event loss mode>> instead of
6002 the default _discard_ mode.
6004 |`--buffers-pid` (user space tracing domain only)
6007 Use the per-process <<channel-buffering-schemes,buffering scheme>>
6008 instead of the default per-user buffering scheme.
6010 |+--subbuf-size=__SIZE__+
6013 Allocate sub-buffers of +__SIZE__+ bytes (power of two), for each CPU,
6014 either for each Unix user (default), or for each instrumented process.
6016 See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
6018 |+--num-subbuf=__COUNT__+
6021 Allocate +__COUNT__+ sub-buffers (power of two), for each CPU, either
6022 for each Unix user (default), or for each instrumented process.
6024 See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
6026 |+--tracefile-size=__SIZE__+
6029 Set the maximum size of each trace file that this channel writes within
6030 a stream to +__SIZE__+ bytes instead of no maximum.
6032 See <<tracefile-rotation,Trace file count and size>>.
6034 |+--tracefile-count=__COUNT__+
6037 Limit the number of trace files that this channel creates to
6038 +__COUNT__+ channels instead of no limit.
6040 See <<tracefile-rotation,Trace file count and size>>.
6042 |+--switch-timer=__PERIODUS__+
6045 Set the <<channel-switch-timer,switch timer period>>
6046 to +__PERIODUS__+{nbsp}µs.
6048 |+--read-timer=__PERIODUS__+
6051 Set the <<channel-read-timer,read timer period>>
6052 to +__PERIODUS__+{nbsp}µs.
6054 |+--output=__TYPE__+ (Linux kernel tracing domain only)
6057 Set the channel's output type to +__TYPE__+, either `mmap` or `splice`.
6061 You can only create a channel in the Linux kernel and user space
6062 <<domain,tracing domains>>: other tracing domains have their own channel
6063 created on the fly when <<enabling-disabling-events,creating event
6068 Because of a current LTTng limitation, you must create all channels
6069 _before_ you <<basic-tracing-session-control,start tracing>> in a given
6070 tracing session, that is, before the first time you run
6073 Since LTTng automatically creates a default channel when you use the
6074 man:lttng-enable-event(1) command with a specific tracing domain, you
6075 cannot, for example, create a Linux kernel event rule, start tracing,
6076 and then create a user space event rule, because no user space channel
6077 exists yet and it's too late to create one.
6079 For this reason, make sure to configure your channels properly
6080 before starting the tracers for the first time!
6083 The following examples show how you can combine the previous
6084 command-line options to create simple to more complex channels.
6086 .Create a Linux kernel channel with default attributes.
6090 lttng enable-channel --kernel my-channel
6094 .Create a user space channel with 4 sub-buffers or 1{nbsp}MiB each, per CPU, per instrumented process.
6098 lttng enable-channel --userspace --num-subbuf=4 --subbuf-size=1M \
6099 --buffers-pid my-channel
6103 .Create a Linux kernel channel which rotates 8 trace files of 4{nbsp}MiB each for each stream
6107 lttng enable-channel --kernel --tracefile-count=8 \
6108 --tracefile-size=4194304 my-channel
6112 .Create a user space channel in overwrite (or _flight recorder_) mode.
6116 lttng enable-channel --userspace --overwrite my-channel
6120 You can <<enabling-disabling-events,create>> the same event rule in
6121 two different channels:
6125 lttng enable-event --userspace --channel=my-channel app:tp
6126 lttng enable-event --userspace --channel=other-channel app:tp
6129 If both channels are enabled, when a tracepoint named `app:tp` is
6130 reached, LTTng records two events, one for each channel.
6134 === Disable a channel
6136 To disable a specific channel that you <<enabling-disabling-channels,created>>
6137 previously, use the man:lttng-disable-channel(1) command.
6139 .Disable a specific Linux kernel channel.
6143 lttng disable-channel --kernel my-channel
6147 The state of a channel precedes the individual states of event rules
6148 attached to it: event rules which belong to a disabled channel, even if
6149 they are enabled, are also considered disabled.
6153 === Add context fields to a channel
6155 Event record fields in trace files provide important information about
6156 events that occured previously, but sometimes some external context may
6157 help you solve a problem faster. Examples of context fields are:
6159 * The **process ID**, **thread ID**, **process name**, and
6160 **process priority** of the thread in which the event occurs.
6161 * The **hostname** of the system on which the event occurs.
6162 * The current values of many possible **performance counters** using
6164 ** CPU cycles, stalled cycles, idle cycles, and the other cycle types.
6166 ** Branch instructions, misses, and loads.
6168 * Any context defined at the application level (supported for the
6169 JUL and log4j <<domain,tracing domains>>).
6171 To get the full list of available context fields, see
6172 `lttng add-context --list`. Some context fields are reserved for a
6173 specific <<domain,tracing domain>> (Linux kernel or user space).
6175 You add context fields to <<channel,channels>>. All the events
6176 that a channel with added context fields records contain those fields.
6178 To add context fields to one or all the channels of a given tracing
6181 * Use the man:lttng-add-context(1) command.
6183 .Add context fields to all the channels of the current tracing session.
6185 The following command line adds the virtual process identifier and
6186 the per-thread CPU cycles count fields to all the user space channels
6187 of the current tracing session.
6191 lttng add-context --userspace --type=vpid --type=perf:thread:cpu-cycles
6195 .Add a context field to a specific channel.
6197 The following command line adds the thread identifier context field
6198 to the Linux kernel channel named `my-channel` in the current
6203 lttng add-context --kernel --channel=my-channel --type=tid
6207 .Add an application-specific context field to a specific channel.
6209 The following command line adds the `cur_msg_id` context field of the
6210 `retriever` context retriever for all the instrumented
6211 <<java-application,Java applications>> recording <<event,event records>>
6212 in the channel named `my-channel`:
6216 lttng add-context --kernel --channel=my-channel \
6217 --type='$app:retriever:cur_msg_id'
6220 IMPORTANT: Make sure to always quote the `$` character when you
6221 use man:lttng-add-context(1) from a shell.
6224 NOTE: You cannot remove context fields from a channel once you add it.
6229 === Track process IDs
6231 It's often useful to allow only specific process IDs (PIDs) to emit
6232 events. For example, you may wish to record all the system calls made by
6233 a given process (à la http://linux.die.net/man/1/strace[strace]).
6235 The man:lttng-track(1) and man:lttng-untrack(1) commands serve this
6236 purpose. Both commands operate on a whitelist of process IDs. You _add_
6237 entries to this whitelist with the man:lttng-track(1) command and remove
6238 entries with the man:lttng-untrack(1) command. Any process which has one
6239 of the PIDs in the whitelist is allowed to emit LTTng events which pass
6240 an enabled <<event,event rule>>.
6242 NOTE: The PID tracker tracks the _numeric process IDs_. Should a
6243 process with a given tracked ID exit and another process be given this
6244 ID, then the latter would also be allowed to emit events.
6246 .Track and untrack process IDs.
6248 For the sake of the following example, assume the target system has 16
6252 <<creating-destroying-tracing-sessions,create a tracing session>>,
6253 the whitelist contains all the possible PIDs:
6256 .All PIDs are tracked.
6257 image::track-all.png[]
6259 When the whitelist is full and you use the man:lttng-track(1) command to
6260 specify some PIDs to track, LTTng first clears the whitelist, then it
6261 tracks the specific PIDs. After:
6265 lttng track --pid=3,4,7,10,13
6271 .PIDs 3, 4, 7, 10, and 13 are tracked.
6272 image::track-3-4-7-10-13.png[]
6274 You can add more PIDs to the whitelist afterwards:
6278 lttng track --pid=1,15,16
6284 .PIDs 1, 15, and 16 are added to the whitelist.
6285 image::track-1-3-4-7-10-13-15-16.png[]
6287 The man:lttng-untrack(1) command removes entries from the PID tracker's
6288 whitelist. Given the previous example, the following command:
6292 lttng untrack --pid=3,7,10,13
6295 leads to this whitelist:
6298 .PIDs 3, 7, 10, and 13 are removed from the whitelist.
6299 image::track-1-4-15-16.png[]
6301 LTTng can track all possible PIDs again using the opt:track(1):--all
6306 lttng track --pid --all
6309 The result is, again:
6312 .All PIDs are tracked.
6313 image::track-all.png[]
6316 .Track only specific PIDs
6318 A very typical use case with PID tracking is to start with an empty
6319 whitelist, then <<basic-tracing-session-control,start the tracers>>, and
6320 then add PIDs manually while tracers are active. You can accomplish this
6321 by using the opt:lttng-untrack(1):--all option of the
6322 man:lttng-untrack(1) command to clear the whitelist after you
6323 <<creating-destroying-tracing-sessions,create a tracing session>>:
6327 lttng untrack --pid --all
6333 .No PIDs are tracked.
6334 image::untrack-all.png[]
6336 If you trace with this whitelist configuration, the tracer records no
6337 events for this <<domain,tracing domain>> because no processes are
6338 tracked. You can use the man:lttng-track(1) command as usual to track
6339 specific PIDs, for example:
6343 lttng track --pid=6,11
6349 .PIDs 6 and 11 are tracked.
6350 image::track-6-11.png[]
6355 [[saving-loading-tracing-session]]
6356 === Save and load tracing session configurations
6358 Configuring a <<tracing-session,tracing session>> can be long. Some of
6359 the tasks involved are:
6361 * <<enabling-disabling-channels,Create channels>> with
6362 specific attributes.
6363 * <<adding-context,Add context fields>> to specific channels.
6364 * <<enabling-disabling-events,Create event rules>> with specific log
6365 level and filter conditions.
6367 If you use LTTng to solve real world problems, chances are you have to
6368 record events using the same tracing session setup over and over,
6369 modifying a few variables each time in your instrumented program
6370 or environment. To avoid constant tracing session reconfiguration,
6371 the man:lttng(1) command-line tool can save and load tracing session
6372 configurations to/from XML files.
6374 To save a given tracing session configuration:
6376 * Use the man:lttng-save(1) command:
6381 lttng save my-session
6385 Replace `my-session` with the name of the tracing session to save.
6387 LTTng saves tracing session configurations to
6388 dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the
6389 env:LTTNG_HOME environment variable defaults to `$HOME` if not set. Use
6390 the opt:lttng-save(1):--output-path option to change this destination
6393 LTTng saves all configuration parameters, for example:
6395 * The tracing session name.
6396 * The trace data output path.
6397 * The channels with their state and all their attributes.
6398 * The context fields you added to channels.
6399 * The event rules with their state, log level and filter conditions.
6401 To load a tracing session:
6403 * Use the man:lttng-load(1) command:
6408 lttng load my-session
6412 Replace `my-session` with the name of the tracing session to load.
6414 When LTTng loads a configuration, it restores your saved tracing session
6415 as if you just configured it manually.
6417 See man:lttng(1) for the complete list of command-line options. You
6418 can also save and load all many sessions at a time, and decide in which
6419 directory to output the XML files.
6422 [[sending-trace-data-over-the-network]]
6423 === Send trace data over the network
6425 LTTng can send the recorded trace data to a remote system over the
6426 network instead of writing it to the local file system.
6428 To send the trace data over the network:
6430 . On the _remote_ system (which can also be the target system),
6431 start an LTTng <<lttng-relayd,relay daemon>> (man:lttng-relayd(8)):
6440 . On the _target_ system, create a tracing session configured to
6441 send trace data over the network:
6446 lttng create my-session --set-url=net://remote-system
6450 Replace `remote-system` by the host name or IP address of the
6451 remote system. See man:lttng-create(1) for the exact URL format.
6453 . On the target system, use the man:lttng(1) command-line tool as usual.
6454 When tracing is active, the target's consumer daemon sends sub-buffers
6455 to the relay daemon running on the remote system intead of flushing
6456 them to the local file system. The relay daemon writes the received
6457 packets to the local file system.
6459 The relay daemon writes trace files to
6460 +$LTTNG_HOME/lttng-traces/__hostname__/__session__+ by default, where
6461 +__hostname__+ is the host name of the target system and +__session__+
6462 is the tracing session name. Note that the env:LTTNG_HOME environment
6463 variable defaults to `$HOME` if not set. Use the
6464 opt:lttng-relayd(8):--output option of man:lttng-relayd(8) to write
6465 trace files to another base directory.
6470 === View events as LTTng emits them (noch:{LTTng} live)
6472 LTTng live is a network protocol implemented by the <<lttng-relayd,relay
6473 daemon>> (man:lttng-relayd(8)) to allow compatible trace viewers to
6474 display events as LTTng emits them on the target system while tracing is
6477 The relay daemon creates a _tee_: it forwards the trace data to both
6478 the local file system and to connected live viewers:
6481 .The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live viewer.
6486 . On the _target system_, create a <<tracing-session,tracing session>>
6492 lttng create --live my-session
6496 This spawns a local relay daemon.
6498 . Start the live viewer and configure it to connect to the relay
6499 daemon. For example, with http://diamon.org/babeltrace[Babeltrace]:
6504 babeltrace --input-format=lttng-live net://localhost/host/hostname/my-session
6511 * `hostname` with the host name of the target system.
6512 * `my-session` with the name of the tracing session to view.
6515 . Configure the tracing session as usual with the man:lttng(1)
6516 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6518 You can list the available live tracing sessions with Babeltrace:
6522 babeltrace --input-format=lttng-live net://localhost
6525 You can start the relay daemon on another system. In this case, you need
6526 to specify the relay daemon's URL when you create the tracing session
6527 with the opt:lttng-create(1):--set-url option. You also need to replace
6528 `localhost` in the procedure above with the host name of the system on
6529 which the relay daemon is running.
6531 See man:lttng-create(1) and man:lttng-relayd(8) for the complete list of
6532 command-line options.
6536 [[taking-a-snapshot]]
6537 === Take a snapshot of the current sub-buffers of a tracing session
6539 The normal behavior of LTTng is to append full sub-buffers to growing
6540 trace data files. This is ideal to keep a full history of the events
6541 that occurred on the target system, but it can
6542 represent too much data in some situations. For example, you may wish
6543 to trace your application continuously until some critical situation
6544 happens, in which case you only need the latest few recorded
6545 events to perform the desired analysis, not multi-gigabyte trace files.
6547 With the man:lttng-snapshot(1) command, you can take a snapshot of the
6548 current sub-buffers of a given <<tracing-session,tracing session>>.
6549 LTTng can write the snapshot to the local file system or send it over
6554 . Create a tracing session in _snapshot mode_:
6559 lttng create --snapshot my-session
6563 The <<channel-overwrite-mode-vs-discard-mode,event loss mode>> of
6564 <<channel,channels>> created in this mode is automatically set to
6565 _overwrite_ (flight recorder mode).
6567 . Configure the tracing session as usual with the man:lttng(1)
6568 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6570 . **Optional**: When you need to take a snapshot,
6571 <<basic-tracing-session-control,stop tracing>>.
6573 You can take a snapshot when the tracers are active, but if you stop
6574 them first, you are sure that the data in the sub-buffers does not
6575 change before you actually take the snapshot.
6582 lttng snapshot record --name=my-first-snapshot
6586 LTTng writes the current sub-buffers of all the current tracing
6587 session's channels to trace files on the local file system. Those trace
6588 files have `my-first-snapshot` in their name.
6590 There is no difference between the format of a normal trace file and the
6591 format of a snapshot: viewers of LTTng traces also support LTTng
6594 By default, LTTng writes snapshot files to the path shown by
6595 `lttng snapshot list-output`. You can change this path or decide to send
6596 snapshots over the network using either:
6598 . An output path or URL that you specify when you create the
6600 . An snapshot output path or URL that you add using
6601 `lttng snapshot add-output`
6602 . An output path or URL that you provide directly to the
6603 `lttng snapshot record` command.
6605 Method 3 overrides method 2, which overrides method 1. When you
6606 specify a URL, a relay daemon must listen on a remote system (see
6607 <<sending-trace-data-over-the-network,Send trace data over the network>>).
6612 === Use the machine interface
6614 With any command of the man:lttng(1) command-line tool, you can set the
6615 opt:lttng(1):--mi option to `xml` (before the command name) to get an
6616 XML machine interface output, for example:
6620 lttng --mi=xml enable-event --kernel --syscall open
6623 A schema definition (XSD) is
6624 https://github.com/lttng/lttng-tools/blob/stable-2.8/src/common/mi-lttng-3.0.xsd[available]
6625 to ease the integration with external tools as much as possible.
6629 [[metadata-regenerate]]
6630 === Regenerate the metadata of an LTTng trace
6632 An LTTng trace, which is a http://diamon.org/ctf[CTF] trace, has both
6633 data stream files and a metadata file. This metadata file contains,
6634 amongst other things, information about the offset of the clock sources
6635 used to timestamp <<event,event records>> when tracing.
6637 If, once a <<tracing-session,tracing session>> is
6638 <<basic-tracing-session-control,started>>, a major
6639 https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction
6640 happens, the trace's clock offset also needs to be updated. You
6641 can use the man:lttng-metadata(1) command to do so.
6643 The main use case of this command is to allow a system to boot with
6644 an incorrect wall time and trace it with LTTng before its wall time
6645 is corrected. Once the system is known to be in a state where its
6646 wall time is correct, it can run `lttng metadata regenerate`.
6648 To regenerate the metadata of an LTTng trace:
6650 * Use the `regenerate` action of the man:lttng-metadata(1) command:
6655 lttng metadata regenerate
6661 `lttng metadata regenerate` has the following limitations:
6663 * Tracing session <<creating-destroying-tracing-sessions,created>>
6665 * User space <<channel,channels>>, if any, using
6666 <<channel-buffering-schemes,per-user buffering>>.
6671 [[persistent-memory-file-systems]]
6672 === Record trace data on persistent memory file systems
6674 https://en.wikipedia.org/wiki/Non-volatile_random-access_memory[Non-volatile random-access memory]
6675 (NVRAM) is random-access memory that retains its information when power
6676 is turned off (non-volatile). Systems with such memory can store data
6677 structures in RAM and retrieve them after a reboot, without flushing
6678 to typical _storage_.
6680 Linux supports NVRAM file systems thanks to either
6681 http://pramfs.sourceforge.net/[PRAMFS] or
6682 https://www.kernel.org/doc/Documentation/filesystems/dax.txt[DAX]{nbsp}+{nbsp}http://lkml.iu.edu/hypermail/linux/kernel/1504.1/03463.html[pmem]
6683 (requires Linux 4.1+).
6685 This section does not describe how to operate such file systems;
6686 we assume that you have a working persistent memory file system.
6688 When you create a <<tracing-session,tracing session>>, you can specify
6689 the path of the shared memory holding the sub-buffers. If you specify a
6690 location on an NVRAM file system, then you can retrieve the latest
6691 recorded trace data when the system reboots after a crash.
6693 To record trace data on a persistent memory file system and retrieve the
6694 trace data after a system crash:
6696 . Create a tracing session with a sub-buffer shared memory path located
6697 on an NVRAM file system:
6702 lttng create --shm-path=/path/to/shm
6706 . Configure the tracing session as usual with the man:lttng(1)
6707 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6709 . After a system crash, use the man:lttng-crash(1) command-line tool to
6710 view the trace data recorded on the NVRAM file system:
6715 lttng-crash /path/to/shm
6719 The binary layout of the ring buffer files is not exactly the same as
6720 the trace files layout. This is why you need to use man:lttng-crash(1)
6721 instead of your preferred trace viewer directly.
6723 To convert the ring buffer files to LTTng trace files:
6725 * Use the opt:lttng-crash(1):--extract option of man:lttng-crash(1):
6730 lttng-crash --extract=/path/to/trace /path/to/shm
6738 [[lttng-modules-ref]]
6739 === noch:{LTTng-modules}
6742 [[lttng-modules-tp-fields]]
6743 ==== Tracepoint fields macros (for `TP_FIELDS()`)
6745 [[tp-fast-assign]][[tp-struct-entry]]The available macros to define
6746 tracepoint fields, which must be listed within `TP_FIELDS()` in
6747 `LTTNG_TRACEPOINT_EVENT()`, are:
6749 [role="func-desc growable",cols="asciidoc,asciidoc"]
6750 .Available macros to define LTTng-modules tracepoint fields
6752 |Macro |Description and parameters
6755 +ctf_integer(__t__, __n__, __e__)+
6757 +ctf_integer_nowrite(__t__, __n__, __e__)+
6759 +ctf_user_integer(__t__, __n__, __e__)+
6761 +ctf_user_integer_nowrite(__t__, __n__, __e__)+
6763 Standard integer, displayed in base 10.
6766 Integer C type (`int`, `long`, `size_t`, ...).
6772 Argument expression.
6775 +ctf_integer_hex(__t__, __n__, __e__)+
6777 +ctf_user_integer_hex(__t__, __n__, __e__)+
6779 Standard integer, displayed in base 16.
6788 Argument expression.
6790 |+ctf_integer_oct(__t__, __n__, __e__)+
6792 Standard integer, displayed in base 8.
6801 Argument expression.
6804 +ctf_integer_network(__t__, __n__, __e__)+
6806 +ctf_user_integer_network(__t__, __n__, __e__)+
6808 Integer in network byte order (big-endian), displayed in base 10.
6817 Argument expression.
6820 +ctf_integer_network_hex(__t__, __n__, __e__)+
6822 +ctf_user_integer_network_hex(__t__, __n__, __e__)+
6824 Integer in network byte order, displayed in base 16.
6833 Argument expression.
6836 +ctf_string(__n__, __e__)+
6838 +ctf_string_nowrite(__n__, __e__)+
6840 +ctf_user_string(__n__, __e__)+
6842 +ctf_user_string_nowrite(__n__, __e__)+
6844 Null-terminated string; undefined behavior if +__e__+ is `NULL`.
6850 Argument expression.
6853 +ctf_array(__t__, __n__, __e__, __s__)+
6855 +ctf_array_nowrite(__t__, __n__, __e__, __s__)+
6857 +ctf_user_array(__t__, __n__, __e__, __s__)+
6859 +ctf_user_array_nowrite(__t__, __n__, __e__, __s__)+
6861 Statically-sized array of integers.
6864 Array element C type.
6870 Argument expression.
6876 +ctf_array_bitfield(__t__, __n__, __e__, __s__)+
6878 +ctf_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
6880 +ctf_user_array_bitfield(__t__, __n__, __e__, __s__)+
6882 +ctf_user_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
6884 Statically-sized array of bits.
6886 The type of +__e__+ must be an integer type. +__s__+ is the number
6887 of elements of such type in +__e__+, not the number of bits.
6890 Array element C type.
6896 Argument expression.
6902 +ctf_array_text(__t__, __n__, __e__, __s__)+
6904 +ctf_array_text_nowrite(__t__, __n__, __e__, __s__)+
6906 +ctf_user_array_text(__t__, __n__, __e__, __s__)+
6908 +ctf_user_array_text_nowrite(__t__, __n__, __e__, __s__)+
6910 Statically-sized array, printed as text.
6912 The string does not need to be null-terminated.
6915 Array element C type (always `char`).
6921 Argument expression.
6927 +ctf_sequence(__t__, __n__, __e__, __T__, __E__)+
6929 +ctf_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
6931 +ctf_user_sequence(__t__, __n__, __e__, __T__, __E__)+
6933 +ctf_user_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
6935 Dynamically-sized array of integers.
6937 The type of +__E__+ must be unsigned.
6940 Array element C type.
6946 Argument expression.
6949 Length expression C type.
6955 +ctf_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
6957 +ctf_user_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
6959 Dynamically-sized array of integers, displayed in base 16.
6961 The type of +__E__+ must be unsigned.
6964 Array element C type.
6970 Argument expression.
6973 Length expression C type.
6978 |+ctf_sequence_network(__t__, __n__, __e__, __T__, __E__)+
6980 Dynamically-sized array of integers in network byte order (big-endian),
6981 displayed in base 10.
6983 The type of +__E__+ must be unsigned.
6986 Array element C type.
6992 Argument expression.
6995 Length expression C type.
7001 +ctf_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
7003 +ctf_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
7005 +ctf_user_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
7007 +ctf_user_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
7009 Dynamically-sized array of bits.
7011 The type of +__e__+ must be an integer type. +__s__+ is the number
7012 of elements of such type in +__e__+, not the number of bits.
7014 The type of +__E__+ must be unsigned.
7017 Array element C type.
7023 Argument expression.
7026 Length expression C type.
7032 +ctf_sequence_text(__t__, __n__, __e__, __T__, __E__)+
7034 +ctf_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
7036 +ctf_user_sequence_text(__t__, __n__, __e__, __T__, __E__)+
7038 +ctf_user_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
7040 Dynamically-sized array, displayed as text.
7042 The string does not need to be null-terminated.
7044 The type of +__E__+ must be unsigned.
7046 The behaviour is undefined if +__e__+ is `NULL`.
7049 Sequence element C type (always `char`).
7055 Argument expression.
7058 Length expression C type.
7064 Use the `_user` versions when the argument expression, `e`, is
7065 a user space address. In the cases of `ctf_user_integer*()` and
7066 `ctf_user_float*()`, `&e` must be a user space address, thus `e` must
7069 The `_nowrite` versions omit themselves from the session trace, but are
7070 otherwise identical. This means the `_nowrite` fields won't be written
7071 in the recorded trace. Their primary purpose is to make some
7072 of the event context available to the
7073 <<enabling-disabling-events,event filters>> without having to
7074 commit the data to sub-buffers.
7080 Terms related to LTTng and to tracing in general:
7083 The http://diamon.org/babeltrace[Babeltrace] project, which includes
7084 the cmd:babeltrace command, some libraries, and Python bindings.
7086 <<channel-buffering-schemes,buffering scheme>>::
7087 A layout of sub-buffers applied to a given channel.
7089 <<channel,channel>>::
7090 An entity which is responsible for a set of ring buffers.
7092 <<event,Event rules>> are always attached to a specific channel.
7095 A reference of time for a tracer.
7097 <<lttng-consumerd,consumer daemon>>::
7098 A process which is responsible for consuming the full sub-buffers
7099 and write them to a file system or send them over the network.
7101 <<channel-overwrite-mode-vs-discard-mode,discard mode>>:: The event loss
7102 mode in which the tracer _discards_ new event records when there's no
7103 sub-buffer space left to store them.
7106 The consequence of the execution of an instrumentation
7107 point, like a tracepoint that you manually place in some source code,
7108 or a Linux kernel KProbe.
7110 An event is said to _occur_ at a specific time. Different actions can
7111 be taken upon the occurance of an event, like record the event's payload
7114 <<channel-overwrite-mode-vs-discard-mode,event loss mode>>::
7115 The mechanism by which event records of a given channel are lost
7116 (not recorded) when there is no sub-buffer space left to store them.
7118 [[def-event-name]]event name::
7119 The name of an event, which is also the name of the event record.
7120 This is also called the _instrumentation point name_.
7123 A record, in a trace, of the payload of an event which occured.
7125 <<event,event rule>>::
7126 Set of conditions which must be satisfied for one or more occuring
7127 events to be recorded.
7129 `java.util.logging`::
7131 https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities].
7133 <<instrumenting,instrumentation>>::
7134 The use of LTTng probes to make a piece of software traceable.
7136 instrumentation point::
7137 A point in the execution path of a piece of software that, when
7138 reached by this execution, can emit an event.
7140 instrumentation point name::
7141 See _<<def-event-name,event name>>_.
7144 A http://logging.apache.org/log4j/1.2/[logging library] for Java
7145 developed by the Apache Software Foundation.
7148 Level of severity of a log statement or user space
7149 instrumentation point.
7152 The _Linux Trace Toolkit: next generation_ project.
7154 <<lttng-cli,cmd:lttng>>::
7155 A command-line tool provided by the LTTng-tools project which you
7156 can use to send and receive control messages to and from a
7160 The https://github.com/lttng/lttng-analyses[LTTng analyses] project,
7161 which is a set of analyzing programs that are used to obtain a
7162 higher level view of an LTTng trace.
7164 cmd:lttng-consumerd::
7165 The name of the consumer daemon program.
7168 A utility provided by the LTTng-tools project which can convert
7169 ring buffer files (usually
7170 <<persistent-memory-file-systems,saved on a persistent memory file system>>)
7173 LTTng Documentation::
7176 <<lttng-live,LTTng live>>::
7177 A communication protocol between the relay daemon and live viewers
7178 which makes it possible to see events "live", as they are received by
7181 <<lttng-modules,LTTng-modules>>::
7182 The https://github.com/lttng/lttng-modules[LTTng-modules] project,
7183 which contains the Linux kernel modules to make the Linux kernel
7184 instrumentation points available for LTTng tracing.
7187 The name of the relay daemon program.
7189 cmd:lttng-sessiond::
7190 The name of the session daemon program.
7193 The https://github.com/lttng/lttng-tools[LTTng-tools] project, which
7194 contains the various programs and libraries used to
7195 <<controlling-tracing,control tracing>>.
7197 <<lttng-ust,LTTng-UST>>::
7198 The https://github.com/lttng/lttng-ust[LTTng-UST] project, which
7199 contains libraries to instrument user applications.
7201 <<lttng-ust-agents,LTTng-UST Java agent>>::
7202 A Java package provided by the LTTng-UST project to allow the
7203 LTTng instrumentation of `java.util.logging` and Apache log4j 1.2
7206 <<lttng-ust-agents,LTTng-UST Python agent>>::
7207 A Python package provided by the LTTng-UST project to allow the
7208 LTTng instrumentation of Python logging statements.
7210 <<channel-overwrite-mode-vs-discard-mode,overwrite mode>>::
7211 The event loss mode in which new event records overwrite older
7212 event records when there's no sub-buffer space left to store them.
7214 <<channel-buffering-schemes,per-process buffering>>::
7215 A buffering scheme in which each instrumented process has its own
7216 sub-buffers for a given user space channel.
7218 <<channel-buffering-schemes,per-user buffering>>::
7219 A buffering scheme in which all the processes of a Unix user share the
7220 same sub-buffer for a given user space channel.
7222 <<lttng-relayd,relay daemon>>::
7223 A process which is responsible for receiving the trace data sent by
7224 a distant consumer daemon.
7227 A set of sub-buffers.
7229 <<lttng-sessiond,session daemon>>::
7230 A process which receives control commands from you and orchestrates
7231 the tracers and various LTTng daemons.
7233 <<taking-a-snapshot,snapshot>>::
7234 A copy of the current data of all the sub-buffers of a given tracing
7235 session, saved as trace files.
7238 One part of an LTTng ring buffer which contains event records.
7241 The time information attached to an event when it is emitted.
7244 A set of files which are the concatenations of one or more
7245 flushed sub-buffers.
7248 The action of recording the events emitted by an application
7249 or by a system, or to initiate such recording by controlling
7253 The http://tracecompass.org[Trace Compass] project and application.
7256 An instrumentation point using the tracepoint mechanism of the Linux
7257 kernel or of LTTng-UST.
7259 tracepoint definition::
7260 The definition of a single tracepoint.
7263 The name of a tracepoint.
7265 tracepoint provider::
7266 A set of functions providing tracepoints to an instrumented user
7269 Not to be confused with a _tracepoint provider package_: many tracepoint
7270 providers can exist within a tracepoint provider package.
7272 tracepoint provider package::
7273 One or more tracepoint providers compiled as an object file or as
7277 A software which records emitted events.
7279 <<domain,tracing domain>>::
7280 A namespace for event sources.
7283 The Unix group in which a Unix user can be to be allowed to trace the
7286 <<tracing-session,tracing session>>::
7287 A stateful dialogue between you and a <<lttng-sessiond,session
7291 An application running in user space, as opposed to a Linux kernel
7292 module, for example.