1 The LTTng Documentation
2 =======================
3 Philippe Proulx <pproulx@efficios.com>
4 v2.11, 19 November 2018
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{nbsp}{revision}?
77 LTTng{nbsp}{revision} bears the name _Lafontaine_. This modern
78 https://en.wikipedia.org/wiki/Saison[saison] from the
79 https://oshlag.com/[Oshlag] microbrewery is a refreshing--zesty--rice
80 beer with hints of fruit and spices. Some even say it makes for a great
81 https://en.wikipedia.org/wiki/Somaek[Somaek] when mixed with
82 Chamisul Soju, not that we've tried!
84 New features and changes in LTTng{nbsp}{revision}:
86 * Just like you can typically perform
87 https://en.wikipedia.org/wiki/Log_rotation[log rotation], you can
88 now <<session-rotation,_rotate_ a tracing session>>, that
89 is, according to man:lttng-rotate(1), archive the current trace
90 chunk (all the tracing session's trace data since the last rotation
91 or since its inception) so that LTTng does not manage it anymore.
93 Once LTTng archives a trace chunk, you are free to read it, modify it,
94 move it, or remove it.
96 You can rotate a tracing session immediately or set a rotation schedule
97 to automate rotations.
99 * When you <<enabling-disabling-events,create an event rule>>, the
100 filter expression syntax now supports the following new operators:
104 ** `<<` (bitwise left shift)
105 ** `>>` (bitwise right shift)
111 The syntax also supports array indexing with the usual square brackets:
114 regs[3][1] & 0xff7 == 0x240
117 There are peculiarities for both the new operators and the array
118 indexing brackets, like a custom precedence table and implicit casting.
119 See man:lttng-enable-event(1) to get all the details about the filter
122 * You can now dynamically instrument any application's or library's
123 function entry by symbol name thanks to the new
124 opt:lttng-enable-event(1):--userspace-probe option of
125 the `lttng enable-event` command:
129 $ lttng enable-event --kernel \
130 --userspace-probe=/usr/lib/libc.so.6:malloc libc_malloc
133 The option also supports tracing existing
134 https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[SystemTap
135 Statically Defined Tracing] (SDT) probe (DTrace-style marker). For example,
136 given the following probe:
140 DTRACE_PROBE2("server", "accept-request", request_id, ip_addr);
143 You can trace this probe with:
146 $ lttng enable-event --kernel \
147 --userspace-probe=sdt:/path/to/server:server:accept-request \
148 server_accept_request
151 This feature makes use of Linux's
152 https://www.kernel.org/doc/Documentation/trace/uprobetracer.txt[uprobe]
153 mechanism, therefore you must use the `--userspace-probe`
154 instrumentation option with the opt:lttng-enable-event(1):--kernel
157 NOTE: As of LTTng{nbsp}{revision}, LTTng does not record function
158 parameters with the opt:lttng-enable-event(1):--userspace-probe option.
160 * Two new <<adding-context,context>> fields are available for Linux
161 kernel <<channel,channels>>:
164 ** `callstack-kernel`
168 Thanks to those, you can record the Linux kernel and user call stacks
169 when a kernel event occurs. For example:
173 $ lttng enable-event --kernel --syscall read
174 $ lttng add-context --kernel --type=callstack-kernel --type=callstack-user
177 When an man:open(2) system call occurs, LTTng attaches the kernel and
178 user call stacks to the recorded event.
180 NOTE: LTTng cannot always sample the user space call stack reliably.
181 For instance, LTTng cannot sample the call stack of user applications
182 and libraries compiled with the
183 https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html[`-fomit-frame pointer`]
184 option. In such a case, the tracing is not affected, but the sampled
185 user space call stack may only contain the user call stack's topmost
188 * The <<lttng-relayd,relay daemon>> is more efficient and presents fewer
189 connectivity issues, especially when a large number of targets send
190 trace data to a given relay daemon.
192 * User applications and libraries instrumented with
193 <<lttng-ust,LTTng-UST>> can now safely unload (man:dlclose(3)) a
195 <<building-tracepoint-providers-and-user-application,tracepoint
202 What is LTTng? As its name suggests, the _Linux Trace Toolkit: next
203 generation_ is a modern toolkit for tracing Linux systems and
204 applications. So your first question might be:
211 As the history of software engineering progressed and led to what
212 we now take for granted--complex, numerous and
213 interdependent software applications running in parallel on
214 sophisticated operating systems like Linux--the authors of such
215 components, software developers, began feeling a natural
216 urge to have tools that would ensure the robustness and good performance
217 of their masterpieces.
219 One major achievement in this field is, inarguably, the
220 https://www.gnu.org/software/gdb/[GNU debugger (GDB)],
221 an essential tool for developers to find and fix bugs. But even the best
222 debugger won't help make your software run faster, and nowadays, faster
223 software means either more work done by the same hardware, or cheaper
224 hardware for the same work.
226 A _profiler_ is often the tool of choice to identify performance
227 bottlenecks. Profiling is suitable to identify _where_ performance is
228 lost in a given software. The profiler outputs a profile, a statistical
229 summary of observed events, which you may use to discover which
230 functions took the most time to execute. However, a profiler won't
231 report _why_ some identified functions are the bottleneck. Bottlenecks
232 might only occur when specific conditions are met, conditions that are
233 sometimes impossible to capture by a statistical profiler, or impossible
234 to reproduce with an application altered by the overhead of an
235 event-based profiler. For a thorough investigation of software
236 performance issues, a history of execution is essential, with the
237 recorded values of variables and context fields you choose, and
238 with as little influence as possible on the instrumented software. This
239 is where tracing comes in handy.
241 _Tracing_ is a technique used to understand what goes on in a running
242 software system. The software used for tracing is called a _tracer_,
243 which is conceptually similar to a tape recorder. When recording,
244 specific instrumentation points placed in the software source code
245 generate events that are saved on a giant tape: a _trace_ file. You
246 can trace user applications and the operating system at the same time,
247 opening the possibility of resolving a wide range of problems that would
248 otherwise be extremely challenging.
250 Tracing is often compared to _logging_. However, tracers and loggers are
251 two different tools, serving two different purposes. Tracers are
252 designed to record much lower-level events that occur much more
253 frequently than log messages, often in the range of thousands per
254 second, with very little execution overhead. Logging is more appropriate
255 for a very high-level analysis of less frequent events: user accesses,
256 exceptional conditions (errors and warnings, for example), database
257 transactions, instant messaging communications, and such. Simply put,
258 logging is one of the many use cases that can be satisfied with tracing.
260 The list of recorded events inside a trace file can be read manually
261 like a log file for the maximum level of detail, but it is generally
262 much more interesting to perform application-specific analyses to
263 produce reduced statistics and graphs that are useful to resolve a
264 given problem. Trace viewers and analyzers are specialized tools
267 In the end, this is what LTTng is: a powerful, open source set of
268 tools to trace the Linux kernel and user applications at the same time.
269 LTTng is composed of several components actively maintained and
270 developed by its link:/community/#where[community].
273 [[lttng-alternatives]]
274 === Alternatives to noch:{LTTng}
276 Excluding proprietary solutions, a few competing software tracers
279 * https://github.com/dtrace4linux/linux[dtrace4linux] is a port of
280 Sun Microsystems's DTrace to Linux. The cmd:dtrace tool interprets
281 user scripts and is responsible for loading code into the
282 Linux kernel for further execution and collecting the outputted data.
283 * https://en.wikipedia.org/wiki/Berkeley_Packet_Filter[eBPF] is a
284 subsystem in the Linux kernel in which a virtual machine can execute
285 programs passed from the user space to the kernel. You can attach
286 such programs to tracepoints and kprobes thanks to a system call, and
287 they can output data to the user space when executed thanks to
288 different mechanisms (pipe, VM register values, and eBPF maps, to name
290 * https://www.kernel.org/doc/Documentation/trace/ftrace.txt[ftrace]
291 is the de facto function tracer of the Linux kernel. Its user
292 interface is a set of special files in sysfs.
293 * https://perf.wiki.kernel.org/[perf] is
294 a performance analyzing tool for Linux which supports hardware
295 performance counters, tracepoints, as well as other counters and
296 types of probes. perf's controlling utility is the cmd:perf command
298 * http://linux.die.net/man/1/strace[strace]
299 is a command-line utility which records system calls made by a
300 user process, as well as signal deliveries and changes of process
301 state. strace makes use of https://en.wikipedia.org/wiki/Ptrace[ptrace]
302 to fulfill its function.
303 * http://www.sysdig.org/[sysdig], like SystemTap, uses scripts to
304 analyze Linux kernel events. You write scripts, or _chisels_ in
305 sysdig's jargon, in Lua and sysdig executes them while it traces the
306 system or afterwards. sysdig's interface is the cmd:sysdig
307 command-line tool as well as the curses-based cmd:csysdig tool.
308 * https://sourceware.org/systemtap/[SystemTap] is a Linux kernel and
309 user space tracer which uses custom user scripts to produce plain text
310 traces. SystemTap converts the scripts to the C language, and then
311 compiles them as Linux kernel modules which are loaded to produce
312 trace data. SystemTap's primary user interface is the cmd:stap
315 The main distinctive features of LTTng is that it produces correlated
316 kernel and user space traces, as well as doing so with the lowest
317 overhead amongst other solutions. It produces trace files in the
318 http://diamon.org/ctf[CTF] format, a file format optimized
319 for the production and analyses of multi-gigabyte data.
321 LTTng is the result of more than 10{nbsp}years of active open source
322 development by a community of passionate developers.
323 LTTng{nbsp}{revision} is currently available on major desktop and server
326 The main interface for tracing control is a single command-line tool
327 named cmd:lttng. The latter can create several tracing sessions, enable
328 and disable events on the fly, filter events efficiently with custom
329 user expressions, start and stop tracing, and much more. LTTng can
330 record the traces on the file system or send them over the network, and
331 keep them totally or partially. You can view the traces once tracing
332 becomes inactive or in real-time.
334 <<installing-lttng,Install LTTng now>> and
335 <<getting-started,start tracing>>!
341 **LTTng** is a set of software <<plumbing,components>> which interact to
342 <<instrumenting,instrument>> the Linux kernel and user applications, and
343 to <<controlling-tracing,control tracing>> (start and stop
344 tracing, enable and disable event rules, and the rest). Those
345 components are bundled into the following packages:
347 * **LTTng-tools**: Libraries and command-line interface to
349 * **LTTng-modules**: Linux kernel modules to instrument and
351 * **LTTng-UST**: Libraries and Java/Python packages to instrument and
352 trace user applications.
354 Most distributions mark the LTTng-modules and LTTng-UST packages as
355 optional when installing LTTng-tools (which is always required). In the
356 following sections, we always provide the steps to install all three,
359 * You only need to install LTTng-modules if you intend to trace the
361 * You only need to install LTTng-UST if you intend to trace user
365 .Availability of LTTng{nbsp}{revision} for major Linux distributions as of 12 November 2018.
367 |Distribution |Available in releases |Alternatives
369 |https://www.ubuntu.com/[Ubuntu]
370 |Ubuntu{nbsp}14.04 _Trusty Tahr_, Ubuntu{nbsp}16.04 _Xenial Xerus_,
371 and Ubuntu{nbsp}18.04 _Bionic Beaver_:
372 <<ubuntu-ppa,use the LTTng Stable{nbsp}{revision} PPA>>.
373 |<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
375 |https://getfedora.org/[Fedora]
378 link:/docs/v2.10#doc-fedora[LTTng{nbsp}2.10 for Fedora{nbsp}27,
379 Fedora{nbsp}28, and Fedora{nbsp}29].
381 <<building-from-source,Build LTTng{nbsp}{revision} from source>>.
383 |https://www.debian.org/[Debian]
385 |link:/docs/v2.10#doc-debian[LTTng{nbsp}2.10 for Debian "buster" (testing)].
387 <<building-from-source,Build LTTng{nbsp}{revision} from source>>.
389 |https://www.archlinux.org/[Arch Linux]
391 |<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
393 |https://alpinelinux.org/[Alpine Linux]
396 link:/docs/v2.10#doc-alpine-linux[LTTng{nbsp}2.10 for
397 Alpine Linux{nbsp}3.7 and Alpine Linux{nbsp}3.8].
399 <<building-from-source,Build LTTng{nbsp}{revision} from source>>.
401 |https://www.opensuse.org/[openSUSE]
403 |link:/docs/v2.10#doc-opensuse[LTTng{nbsp}2.10 for openSUSE Leap{nbsp}15.0].
405 <<building-from-source,Build LTTng{nbsp}{revision} from source>>.
407 |https://www.redhat.com/[RHEL] and https://www.suse.com/[SLES]
408 |See http://packages.efficios.com/[EfficiOS Enterprise Packages].
411 |https://buildroot.org/[Buildroot]
414 link:/docs/v2.10#doc-buildroot[LTTng{nbsp}2.10 for Buildroot{nbsp}2018.02,
415 Buildroot{nbsp}2018.05, Buildroot{nbsp}2018.08, and Buildroot{nbsp}2018.11].
417 <<building-from-source,Build LTTng{nbsp}{revision} from source>>.
419 |http://www.openembedded.org/wiki/Main_Page[OpenEmbedded] and
420 https://www.yoctoproject.org/[Yocto]
422 |<<building-from-source,Build LTTng{nbsp}{revision} from source>>.
427 === Ubuntu: noch:{LTTng} Stable {revision} PPA
429 The https://launchpad.net/~lttng/+archive/ubuntu/stable-{revision}[LTTng
430 Stable{nbsp}{revision} PPA] offers the latest stable
431 LTTng{nbsp}{revision} packages for:
433 * Ubuntu{nbsp}14.04 _Trusty Tahr_
434 * Ubuntu{nbsp}16.04 _Xenial Xerus_
435 * Ubuntu{nbsp}18.04 _Bionic Beaver_
437 To install LTTng{nbsp}{revision} from the LTTng Stable{nbsp}{revision} PPA:
439 . Add the LTTng Stable{nbsp}{revision} PPA repository and update the
445 # apt-add-repository ppa:lttng/stable-2.11
450 . Install the main LTTng{nbsp}{revision} packages:
455 # apt-get install lttng-tools
456 # apt-get install lttng-modules-dkms
457 # apt-get install liblttng-ust-dev
461 . **If you need to instrument and trace
462 <<java-application,Java applications>>**, install the LTTng-UST
468 # apt-get install liblttng-ust-agent-java
472 . **If you need to instrument and trace
473 <<python-application,Python{nbsp}3 applications>>**, install the
474 LTTng-UST Python agent:
479 # apt-get install python3-lttngust
484 [[enterprise-distributions]]
485 === RHEL, SUSE, and other enterprise distributions
487 To install LTTng on enterprise Linux distributions, such as Red Hat
488 Enterprise Linux (RHEL) and SUSE Linux Enterprise Server (SUSE), please
489 see http://packages.efficios.com/[EfficiOS Enterprise Packages].
492 [[building-from-source]]
493 === Build from source
495 To build and install LTTng{nbsp}{revision} from source:
497 . Using your distribution's package manager, or from source, install
498 the following dependencies of LTTng-tools and LTTng-UST:
501 * https://sourceforge.net/projects/libuuid/[libuuid]
502 * http://directory.fsf.org/wiki/Popt[popt]
503 * http://liburcu.org/[Userspace RCU]
504 * http://www.xmlsoft.org/[libxml2]
505 * **Optional**: https://github.com/numactl/numactl[numactl]
508 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
514 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.11.tar.bz2 &&
515 tar -xf lttng-modules-latest-2.11.tar.bz2 &&
516 cd lttng-modules-2.11.* &&
518 sudo make modules_install &&
523 . Download, build, and install the latest LTTng-UST{nbsp}{revision}:
529 wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.11.tar.bz2 &&
530 tar -xf lttng-ust-latest-2.11.tar.bz2 &&
531 cd lttng-ust-2.11.* &&
539 Add `--disable-numa` to `./configure` if you don't have
540 https://github.com/numactl/numactl[numactl].
544 .Java and Python application tracing
546 If you need to instrument and trace <<java-application,Java
547 applications>>, pass the `--enable-java-agent-jul`,
548 `--enable-java-agent-log4j`, or `--enable-java-agent-all` options to the
549 `configure` script, depending on which Java logging framework you use.
551 If you need to instrument and trace <<python-application,Python
552 applications>>, pass the `--enable-python-agent` option to the
553 `configure` script. You can set the `PYTHON` environment variable to the
554 path to the Python interpreter for which to install the LTTng-UST Python
562 By default, LTTng-UST libraries are installed to
563 dir:{/usr/local/lib}, which is the de facto directory in which to
564 keep self-compiled and third-party libraries.
566 When <<building-tracepoint-providers-and-user-application,linking an
567 instrumented user application with `liblttng-ust`>>:
569 * Append `/usr/local/lib` to the env:LD_LIBRARY_PATH environment
571 * Pass the `-L/usr/local/lib` and `-Wl,-rpath,/usr/local/lib` options to
572 man:gcc(1), man:g++(1), or man:clang(1).
576 . Download, build, and install the latest LTTng-tools{nbsp}{revision}:
582 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.11.tar.bz2 &&
583 tar -xf lttng-tools-latest-2.11.tar.bz2 &&
584 cd lttng-tools-2.11.* &&
592 TIP: The https://github.com/eepp/vlttng[vlttng tool] can do all the
593 previous steps automatically for a given version of LTTng and confine
594 the installed files in a specific directory. This can be useful to test
595 LTTng without installing it on your system.
601 This is a short guide to get started quickly with LTTng kernel and user
604 Before you follow this guide, make sure to <<installing-lttng,install>>
607 This tutorial walks you through the steps to:
609 . <<tracing-the-linux-kernel,Trace the Linux kernel>>.
610 . <<tracing-your-own-user-application,Trace a user application>> written
612 . <<viewing-and-analyzing-your-traces,View and analyze the
616 [[tracing-the-linux-kernel]]
617 === Trace the Linux kernel
619 The following command lines start with the `#` prompt because you need
620 root privileges to trace the Linux kernel. You can also trace the kernel
621 as a regular user if your Unix user is a member of the
622 <<tracing-group,tracing group>>.
624 . Create a <<tracing-session,tracing session>> which writes its traces
625 to dir:{/tmp/my-kernel-trace}:
630 # lttng create my-kernel-session --output=/tmp/my-kernel-trace
634 . List the available kernel tracepoints and system calls:
639 # lttng list --kernel
640 # lttng list --kernel --syscall
644 . Create <<event,event rules>> which match the desired instrumentation
645 point names, for example the `sched_switch` and `sched_process_fork`
646 tracepoints, and the man:open(2) and man:close(2) system calls:
651 # lttng enable-event --kernel sched_switch,sched_process_fork
652 # lttng enable-event --kernel --syscall open,close
656 You can also create an event rule which matches _all_ the Linux kernel
657 tracepoints (this will generate a lot of data when tracing):
662 # lttng enable-event --kernel --all
666 . <<basic-tracing-session-control,Start tracing>>:
675 . Do some operation on your system for a few seconds. For example,
676 load a website, or list the files of a directory.
677 . <<basic-tracing-session-control,Stop tracing>> and destroy the
688 You need to stop tracing to make LTTng flush the remaining trace data
689 and make the trace readable.
691 The man:lttng-destroy(1) command does not destroy the trace data; it
692 only destroys the state of the tracing session.
694 . For the sake of this example, make the recorded trace accessible to
700 # chown -R $(whoami) /tmp/my-kernel-trace
704 See <<viewing-and-analyzing-your-traces,View and analyze the
705 recorded events>> to view the recorded events.
708 [[tracing-your-own-user-application]]
709 === Trace a user application
711 This section steps you through a simple example to trace a
712 _Hello world_ program written in C.
714 To create the traceable user application:
716 . Create the tracepoint provider header file, which defines the
717 tracepoints and the events they can generate:
723 #undef TRACEPOINT_PROVIDER
724 #define TRACEPOINT_PROVIDER hello_world
726 #undef TRACEPOINT_INCLUDE
727 #define TRACEPOINT_INCLUDE "./hello-tp.h"
729 #if !defined(_HELLO_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
732 #include <lttng/tracepoint.h>
742 ctf_string(my_string_field, my_string_arg)
743 ctf_integer(int, my_integer_field, my_integer_arg)
747 #endif /* _HELLO_TP_H */
749 #include <lttng/tracepoint-event.h>
753 . Create the tracepoint provider package source file:
759 #define TRACEPOINT_CREATE_PROBES
760 #define TRACEPOINT_DEFINE
762 #include "hello-tp.h"
766 . Build the tracepoint provider package:
771 $ gcc -c -I. hello-tp.c
775 . Create the _Hello World_ application source file:
782 #include "hello-tp.h"
784 int main(int argc, char *argv[])
788 puts("Hello, World!\nPress Enter to continue...");
791 * The following getchar() call is only placed here for the purpose
792 * of this demonstration, to pause the application in order for
793 * you to have time to list its tracepoints. It is not
799 * A tracepoint() call.
801 * Arguments, as defined in hello-tp.h:
803 * 1. Tracepoint provider name (required)
804 * 2. Tracepoint name (required)
805 * 3. my_integer_arg (first user-defined argument)
806 * 4. my_string_arg (second user-defined argument)
808 * Notice the tracepoint provider and tracepoint names are
809 * NOT strings: they are in fact parts of variables that the
810 * macros in hello-tp.h create.
812 tracepoint(hello_world, my_first_tracepoint, 23, "hi there!");
814 for (x = 0; x < argc; ++x) {
815 tracepoint(hello_world, my_first_tracepoint, x, argv[x]);
818 puts("Quitting now!");
819 tracepoint(hello_world, my_first_tracepoint, x * x, "x^2");
826 . Build the application:
835 . Link the application with the tracepoint provider package,
836 `liblttng-ust`, and `libdl`:
841 $ gcc -o hello hello.o hello-tp.o -llttng-ust -ldl
845 Here's the whole build process:
848 .User space tracing tutorial's build steps.
849 image::ust-flow.png[]
851 To trace the user application:
853 . Run the application with a few arguments:
858 $ ./hello world and beyond
867 Press Enter to continue...
871 . Start an LTTng <<lttng-sessiond,session daemon>>:
876 $ lttng-sessiond --daemonize
880 Note that a session daemon might already be running, for example as
881 a service that the distribution's service manager started.
883 . List the available user space tracepoints:
888 $ lttng list --userspace
892 You see the `hello_world:my_first_tracepoint` tracepoint listed
893 under the `./hello` process.
895 . Create a <<tracing-session,tracing session>>:
900 $ lttng create my-user-space-session
904 . Create an <<event,event rule>> which matches the
905 `hello_world:my_first_tracepoint` event name:
910 $ lttng enable-event --userspace hello_world:my_first_tracepoint
914 . <<basic-tracing-session-control,Start tracing>>:
923 . Go back to the running `hello` application and press Enter. The
924 program executes all `tracepoint()` instrumentation points and exits.
925 . <<basic-tracing-session-control,Stop tracing>> and destroy the
936 You need to stop tracing to make LTTng flush the remaining trace data
937 and make the trace readable.
939 The man:lttng-destroy(1) command does not destroy the trace data; it
940 only destroys the state of the tracing session.
942 By default, LTTng saves the traces in
943 +$LTTNG_HOME/lttng-traces/__name__-__date__-__time__+,
944 where +__name__+ is the tracing session name. The
945 env:LTTNG_HOME environment variable defaults to `$HOME` if not set.
947 See <<viewing-and-analyzing-your-traces,View and analyze the
948 recorded events>> to view the recorded events.
951 [[viewing-and-analyzing-your-traces]]
952 === View and analyze the recorded events
954 Once you have completed the <<tracing-the-linux-kernel,Trace the Linux
955 kernel>> and <<tracing-your-own-user-application,Trace a user
956 application>> tutorials, you can inspect the recorded events.
958 Many tools are available to read LTTng traces:
960 * **cmd:babeltrace** is a command-line utility which converts trace
961 formats; it supports the format that LTTng produces, CTF, as well as a
962 basic text output which can be ++grep++ed. The cmd:babeltrace command
963 is part of the http://diamon.org/babeltrace[Babeltrace] project.
964 * Babeltrace also includes
965 **https://www.python.org/[Python] bindings** so
966 that you can easily open and read an LTTng trace with your own script,
967 benefiting from the power of Python.
968 * http://tracecompass.org/[**Trace Compass**]
969 is a graphical user interface for viewing and analyzing any type of
970 logs or traces, including LTTng's.
971 * https://github.com/lttng/lttng-analyses[**LTTng analyses**] is a
972 project which includes many high-level analyses of LTTng kernel
973 traces, like scheduling statistics, interrupt frequency distribution,
974 top CPU usage, and more.
976 NOTE: This section assumes that LTTng saved the traces it recorded
977 during the previous tutorials to their default location, in the
978 dir:{$LTTNG_HOME/lttng-traces} directory. The env:LTTNG_HOME
979 environment variable defaults to `$HOME` if not set.
982 [[viewing-and-analyzing-your-traces-bt]]
983 ==== Use the cmd:babeltrace command-line tool
985 The simplest way to list all the recorded events of a trace is to pass
986 its path to cmd:babeltrace with no options:
990 $ babeltrace ~/lttng-traces/my-user-space-session*
993 cmd:babeltrace finds all traces recursively within the given path and
994 prints all their events, merging them in chronological order.
996 You can pipe the output of cmd:babeltrace into a tool like man:grep(1) for
1001 $ babeltrace /tmp/my-kernel-trace | grep _switch
1004 You can pipe the output of cmd:babeltrace into a tool like man:wc(1) to
1005 count the recorded events:
1009 $ babeltrace /tmp/my-kernel-trace | grep _open | wc --lines
1013 [[viewing-and-analyzing-your-traces-bt-python]]
1014 ==== Use the Babeltrace Python bindings
1016 The <<viewing-and-analyzing-your-traces-bt,text output of cmd:babeltrace>>
1017 is useful to isolate events by simple matching using man:grep(1) and
1018 similar utilities. However, more elaborate filters, such as keeping only
1019 event records with a field value falling within a specific range, are
1020 not trivial to write using a shell. Moreover, reductions and even the
1021 most basic computations involving multiple event records are virtually
1022 impossible to implement.
1024 Fortunately, Babeltrace ships with Python{nbsp}3 bindings which makes it
1025 easy to read the event records of an LTTng trace sequentially and
1026 compute the desired information.
1028 The following script accepts an LTTng Linux kernel trace path as its
1029 first argument and prints the short names of the top five running
1030 processes on CPU{nbsp}0 during the whole trace:
1035 from collections import Counter
1041 if len(sys.argv) != 2:
1042 msg = 'Usage: python3 {} TRACEPATH'.format(sys.argv[0])
1043 print(msg, file=sys.stderr)
1046 # A trace collection contains one or more traces
1047 col = babeltrace.TraceCollection()
1049 # Add the trace provided by the user (LTTng traces always have
1051 if col.add_trace(sys.argv[1], 'ctf') is None:
1052 raise RuntimeError('Cannot add trace')
1054 # This counter dict contains execution times:
1056 # task command name -> total execution time (ns)
1057 exec_times = Counter()
1059 # This contains the last `sched_switch` timestamp
1063 for event in col.events:
1064 # Keep only `sched_switch` events
1065 if event.name != 'sched_switch':
1068 # Keep only events which happened on CPU 0
1069 if event['cpu_id'] != 0:
1073 cur_ts = event.timestamp
1079 # Previous task command (short) name
1080 prev_comm = event['prev_comm']
1082 # Initialize entry in our dict if not yet done
1083 if prev_comm not in exec_times:
1084 exec_times[prev_comm] = 0
1086 # Compute previous command execution time
1087 diff = cur_ts - last_ts
1089 # Update execution time of this command
1090 exec_times[prev_comm] += diff
1092 # Update last timestamp
1096 for name, ns in exec_times.most_common(5):
1098 print('{:20}{} s'.format(name, s))
1103 if __name__ == '__main__':
1104 sys.exit(0 if top5proc() else 1)
1111 $ python3 top5proc.py /tmp/my-kernel-trace/kernel
1117 swapper/0 48.607245889 s
1118 chromium 7.192738188 s
1119 pavucontrol 0.709894415 s
1120 Compositor 0.660867933 s
1121 Xorg.bin 0.616753786 s
1124 Note that `swapper/0` is the "idle" process of CPU{nbsp}0 on Linux;
1125 since we weren't using the CPU that much when tracing, its first
1126 position in the list makes sense.
1130 == [[understanding-lttng]]Core concepts
1132 From a user's perspective, the LTTng system is built on a few concepts,
1133 or objects, on which the <<lttng-cli,cmd:lttng command-line tool>>
1134 operates by sending commands to the <<lttng-sessiond,session daemon>>.
1135 Understanding how those objects relate to eachother is key in mastering
1138 The core concepts are:
1140 * <<tracing-session,Tracing session>>
1141 * <<domain,Tracing domain>>
1142 * <<channel,Channel and ring buffer>>
1143 * <<"event","Instrumentation point, event rule, event, and event record">>
1149 A _tracing session_ is a stateful dialogue between you and
1150 a <<lttng-sessiond,session daemon>>. You can
1151 <<creating-destroying-tracing-sessions,create a new tracing
1152 session>> with the `lttng create` command.
1154 Anything that you do when you control LTTng tracers happens within a
1155 tracing session. In particular, a tracing session:
1158 * Has its own set of trace files.
1159 * Has its own state of activity (started or stopped).
1160 * Has its own <<tracing-session-mode,mode>> (local, network streaming,
1162 * Has its own <<channel,channels>> to which are associated their own
1163 <<event,event rules>>.
1166 .A _tracing session_ contains <<channel,channels>> that are members of <<domain,tracing domains>> and contain <<event,event rules>>.
1167 image::concepts.png[]
1169 Those attributes and objects are completely isolated between different
1172 A tracing session is analogous to a cash machine session:
1173 the operations you do on the banking system through the cash machine do
1174 not alter the data of other users of the same system. In the case of
1175 the cash machine, a session lasts as long as your bank card is inside.
1176 In the case of LTTng, a tracing session lasts from the `lttng create`
1177 command to the `lttng destroy` command.
1180 .Each Unix user has its own set of tracing sessions.
1181 image::many-sessions.png[]
1184 [[tracing-session-mode]]
1185 ==== Tracing session mode
1187 LTTng can send the generated trace data to different locations. The
1188 _tracing session mode_ dictates where to send it. The following modes
1189 are available in LTTng{nbsp}{revision}:
1192 LTTng writes the traces to the file system of the machine it traces
1195 Network streaming mode::
1196 LTTng sends the traces over the network to a
1197 <<lttng-relayd,relay daemon>> running on a remote system.
1200 LTTng does not write the traces by default. Instead, you can request
1201 LTTng to <<taking-a-snapshot,take a snapshot>>, that is, a copy of the
1202 tracing session's current sub-buffers, and to write it to the
1203 target's file system or to send it over the network to a
1204 <<lttng-relayd,relay daemon>> running on a remote system.
1207 This mode is similar to the network streaming mode, but a live
1208 trace viewer can connect to the distant relay daemon to
1209 <<lttng-live,view event records as LTTng generates them>>.
1215 A _tracing domain_ is a namespace for event sources. A tracing domain
1216 has its own properties and features.
1218 There are currently five available tracing domains:
1222 * `java.util.logging` (JUL)
1226 You must specify a tracing domain when using some commands to avoid
1227 ambiguity. For example, since all the domains support named tracepoints
1228 as event sources (instrumentation points that you manually insert in the
1229 source code), you need to specify a tracing domain when
1230 <<enabling-disabling-events,creating an event rule>> because all the
1231 tracing domains could have tracepoints with the same names.
1233 Some features are reserved to specific tracing domains. Dynamic function
1234 entry and return instrumentation points, for example, are currently only
1235 supported in the Linux kernel tracing domain, but support for other
1236 tracing domains could be added in the future.
1238 You can create <<channel,channels>> in the Linux kernel and user space
1239 tracing domains. The other tracing domains have a single default
1244 === Channel and ring buffer
1246 A _channel_ is an object which is responsible for a set of ring buffers.
1247 Each ring buffer is divided into multiple sub-buffers. When an LTTng
1248 tracer emits an event, it can record it to one or more
1249 sub-buffers. The attributes of a channel determine what to do when
1250 there's no space left for a new event record because all sub-buffers
1251 are full, where to send a full sub-buffer, and other behaviours.
1253 A channel is always associated to a <<domain,tracing domain>>. The
1254 `java.util.logging` (JUL), log4j, and Python tracing domains each have
1255 a default channel which you cannot configure.
1257 A channel also owns <<event,event rules>>. When an LTTng tracer emits
1258 an event, it records it to the sub-buffers of all
1259 the enabled channels with a satisfied event rule, as long as those
1260 channels are part of active <<tracing-session,tracing sessions>>.
1263 [[channel-buffering-schemes]]
1264 ==== Per-user vs. per-process buffering schemes
1266 A channel has at least one ring buffer _per CPU_. LTTng always
1267 records an event to the ring buffer associated to the CPU on which it
1270 Two _buffering schemes_ are available when you
1271 <<enabling-disabling-channels,create a channel>> in the
1272 user space <<domain,tracing domain>>:
1274 Per-user buffering::
1275 Allocate one set of ring buffers--one per CPU--shared by all the
1276 instrumented processes of each Unix user.
1280 .Per-user buffering scheme.
1281 image::per-user-buffering.png[]
1284 Per-process buffering::
1285 Allocate one set of ring buffers--one per CPU--for each
1286 instrumented process.
1290 .Per-process buffering scheme.
1291 image::per-process-buffering.png[]
1294 The per-process buffering scheme tends to consume more memory than the
1295 per-user option because systems generally have more instrumented
1296 processes than Unix users running instrumented processes. However, the
1297 per-process buffering scheme ensures that one process having a high
1298 event throughput won't fill all the shared sub-buffers of the same
1301 The Linux kernel tracing domain has only one available buffering scheme
1302 which is to allocate a single set of ring buffers for the whole system.
1303 This scheme is similar to the per-user option, but with a single, global
1304 user "running" the kernel.
1307 [[channel-overwrite-mode-vs-discard-mode]]
1308 ==== Overwrite vs. discard event record loss modes
1310 When an event occurs, LTTng records it to a specific sub-buffer (yellow
1311 arc in the following animations) of a specific channel's ring buffer.
1312 When there's no space left in a sub-buffer, the tracer marks it as
1313 consumable (red) and another, empty sub-buffer starts receiving the
1314 following event records. A <<lttng-consumerd,consumer daemon>>
1315 eventually consumes the marked sub-buffer (returns to white).
1318 [role="docsvg-channel-subbuf-anim"]
1323 In an ideal world, sub-buffers are consumed faster than they are filled,
1324 as it is the case in the previous animation. In the real world,
1325 however, all sub-buffers can be full at some point, leaving no space to
1326 record the following events.
1328 By default, LTTng-modules and LTTng-UST are _non-blocking_ tracers: when
1329 no empty sub-buffer is available, it is acceptable to lose event records
1330 when the alternative would be to cause substantial delays in the
1331 instrumented application's execution. LTTng privileges performance over
1332 integrity; it aims at perturbing the target system as little as possible
1333 in order to make tracing of subtle race conditions and rare interrupt
1336 Since LTTng{nbsp}2.10, the LTTng user space tracer, LTTng-UST, supports
1337 a _blocking mode_. See the <<blocking-timeout-example,blocking timeout
1338 example>> to learn how to use the blocking mode.
1340 When it comes to losing event records because no empty sub-buffer is
1341 available, or because the <<opt-blocking-timeout,blocking timeout>> is
1342 reached, the channel's _event record loss mode_ determines what to do.
1343 The available event record loss modes are:
1346 Drop the newest event records until a the tracer releases a
1349 This is the only available mode when you specify a
1350 <<opt-blocking-timeout,blocking timeout>>.
1353 Clear the sub-buffer containing the oldest event records and start
1354 writing the newest event records there.
1356 This mode is sometimes called _flight recorder mode_ because it's
1358 https://en.wikipedia.org/wiki/Flight_recorder[flight recorder]:
1359 always keep a fixed amount of the latest data.
1361 Which mechanism you should choose depends on your context: prioritize
1362 the newest or the oldest event records in the ring buffer?
1364 Beware that, in overwrite mode, the tracer abandons a _whole sub-buffer_
1365 as soon as a there's no space left for a new event record, whereas in
1366 discard mode, the tracer only discards the event record that doesn't
1369 In discard mode, LTTng increments a count of lost event records when an
1370 event record is lost and saves this count to the trace. In overwrite
1371 mode, since LTTng{nbsp}2.8, LTTng increments a count of lost sub-buffers
1372 when a sub-buffer is lost and saves this count to the trace. In this
1373 mode, LTTng does not write to the trace the exact number of lost event
1374 records in those lost sub-buffers. Trace analyses can use the trace's
1375 saved discarded event record and sub-buffer counts to decide whether or
1376 not to perform the analyses even if trace data is known to be missing.
1378 There are a few ways to decrease your probability of losing event
1380 <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>> shows
1381 how you can fine-tune the sub-buffer count and size of a channel to
1382 virtually stop losing event records, though at the cost of greater
1386 [[channel-subbuf-size-vs-subbuf-count]]
1387 ==== Sub-buffer count and size
1389 When you <<enabling-disabling-channels,create a channel>>, you can
1390 set its number of sub-buffers and their size.
1392 Note that there is noticeable CPU overhead introduced when
1393 switching sub-buffers (marking a full one as consumable and switching
1394 to an empty one for the following events to be recorded). Knowing this,
1395 the following list presents a few practical situations along with how
1396 to configure the sub-buffer count and size for them:
1398 * **High event throughput**: In general, prefer bigger sub-buffers to
1399 lower the risk of losing event records.
1401 Having bigger sub-buffers also ensures a lower
1402 <<channel-switch-timer,sub-buffer switching frequency>>.
1404 The number of sub-buffers is only meaningful if you create the channel
1405 in overwrite mode: in this case, if a sub-buffer overwrite happens, the
1406 other sub-buffers are left unaltered.
1408 * **Low event throughput**: In general, prefer smaller sub-buffers
1409 since the risk of losing event records is low.
1411 Because events occur less frequently, the sub-buffer switching frequency
1412 should remain low and thus the tracer's overhead should not be a
1415 * **Low memory system**: If your target system has a low memory
1416 limit, prefer fewer first, then smaller sub-buffers.
1418 Even if the system is limited in memory, you want to keep the
1419 sub-buffers as big as possible to avoid a high sub-buffer switching
1422 Note that LTTng uses http://diamon.org/ctf/[CTF] as its trace format,
1423 which means event data is very compact. For example, the average
1424 LTTng kernel event record weights about 32{nbsp}bytes. Thus, a
1425 sub-buffer size of 1{nbsp}MiB is considered big.
1427 The previous situations highlight the major trade-off between a few big
1428 sub-buffers and more, smaller sub-buffers: sub-buffer switching
1429 frequency vs. how much data is lost in overwrite mode. Assuming a
1430 constant event throughput and using the overwrite mode, the two
1431 following configurations have the same ring buffer total size:
1434 [role="docsvg-channel-subbuf-size-vs-count-anim"]
1439 * **Two sub-buffers of 4{nbsp}MiB each**: Expect a very low sub-buffer
1440 switching frequency, but if a sub-buffer overwrite happens, half of
1441 the event records so far (4{nbsp}MiB) are definitely lost.
1442 * **Eight sub-buffers of 1{nbsp}MiB each**: Expect four times the tracer's
1443 overhead as the previous configuration, but if a sub-buffer
1444 overwrite happens, only the eighth of event records so far are
1447 In discard mode, the sub-buffers count parameter is pointless: use two
1448 sub-buffers and set their size according to the requirements of your
1452 [[channel-switch-timer]]
1453 ==== Switch timer period
1455 The _switch timer period_ is an important configurable attribute of
1456 a channel to ensure periodic sub-buffer flushing.
1458 When the _switch timer_ expires, a sub-buffer switch happens. You can
1459 set the switch timer period attribute when you
1460 <<enabling-disabling-channels,create a channel>> to ensure that LTTng
1461 consumes and commits trace data to trace files or to a distant relay
1462 daemon periodically in case of a low event throughput.
1465 [role="docsvg-channel-switch-timer"]
1470 This attribute is also convenient when you use big sub-buffers to cope
1471 with a sporadic high event throughput, even if the throughput is
1475 [[channel-read-timer]]
1476 ==== Read timer period
1478 By default, the LTTng tracers use a notification mechanism to signal a
1479 full sub-buffer so that a consumer daemon can consume it. When such
1480 notifications must be avoided, for example in real-time applications,
1481 you can use the channel's _read timer_ instead. When the read timer
1482 fires, the <<lttng-consumerd,consumer daemon>> checks for full,
1483 consumable sub-buffers.
1486 [[tracefile-rotation]]
1487 ==== Trace file count and size
1489 By default, trace files can grow as large as needed. You can set the
1490 maximum size of each trace file that a channel writes when you
1491 <<enabling-disabling-channels,create a channel>>. When the size of
1492 a trace file reaches the channel's fixed maximum size, LTTng creates
1493 another file to contain the next event records. LTTng appends a file
1494 count to each trace file name in this case.
1496 If you set the trace file size attribute when you create a channel, the
1497 maximum number of trace files that LTTng creates is _unlimited_ by
1498 default. To limit them, you can also set a maximum number of trace
1499 files. When the number of trace files reaches the channel's fixed
1500 maximum count, the oldest trace file is overwritten. This mechanism is
1501 called _trace file rotation_.
1503 Even if you don't limit the trace file count, you cannot assume that
1504 LTTng doesn't manage any trace file. In other words, there is no safe
1505 way to know if LTTng still holds a given trace file open with the trace
1506 file rotation feature. The only way to obtain an unmanaged,
1507 self-contained LTTng trace before you
1508 <<creating-destroying-tracing-sessions,destroy>> the tracing session is
1509 with the <<session-rotation,tracing session rotation>> feature
1510 (available since LTTng{nbsp}2.11).
1514 === Instrumentation point, event rule, event, and event record
1516 An _event rule_ is a set of conditions which must be **all** satisfied
1517 for LTTng to record an occuring event.
1519 You set the conditions when you <<enabling-disabling-events,create
1522 You always attach an event rule to <<channel,channel>> when you create
1525 When an event passes the conditions of an event rule, LTTng records it
1526 in one of the attached channel's sub-buffers.
1528 The available conditions, as of LTTng{nbsp}{revision}, are:
1530 * The event rule _is enabled_.
1531 * The instrumentation point's type _is{nbsp}T_.
1532 * The instrumentation point's name (sometimes called _event name_)
1533 _matches{nbsp}N_, but _is not{nbsp}E_.
1534 * The instrumentation point's log level _is as severe as{nbsp}L_, or
1535 _is exactly{nbsp}L_.
1536 * The fields of the event's payload _satisfy_ a filter
1537 expression{nbsp}__F__.
1539 As you can see, all the conditions but the dynamic filter are related to
1540 the event rule's status or to the instrumentation point, not to the
1541 occurring events. This is why, without a filter, checking if an event
1542 passes an event rule is not a dynamic task: when you create or modify an
1543 event rule, all the tracers of its tracing domain enable or disable the
1544 instrumentation points themselves once. This is possible because the
1545 attributes of an instrumentation point (type, name, and log level) are
1546 defined statically. In other words, without a dynamic filter, the tracer
1547 _does not evaluate_ the arguments of an instrumentation point unless it
1548 matches an enabled event rule.
1550 Note that, for LTTng to record an event, the <<channel,channel>> to
1551 which a matching event rule is attached must also be enabled, and the
1552 <<tracing-session,tracing session>> owning this channel must be active
1556 .Logical path from an instrumentation point to an event record.
1557 image::event-rule.png[]
1559 .Event, event record, or event rule?
1561 With so many similar terms, it's easy to get confused.
1563 An **event** is the consequence of the execution of an _instrumentation
1564 point_, like a tracepoint that you manually place in some source code,
1565 or a Linux kernel kprobe. An event is said to _occur_ at a specific
1566 time. Different actions can be taken upon the occurrence of an event,
1567 like record the event's payload to a buffer.
1569 An **event record** is the representation of an event in a sub-buffer. A
1570 tracer is responsible for capturing the payload of an event, current
1571 context variables, the event's ID, and the event's timestamp. LTTng
1572 can append this sub-buffer to a trace file.
1574 An **event rule** is a set of conditions which must _all_ be satisfied
1575 for LTTng to record an occuring event. Events still occur without
1576 satisfying event rules, but LTTng does not record them.
1581 == Components of noch:{LTTng}
1583 The second _T_ in _LTTng_ stands for _toolkit_: it would be wrong
1584 to call LTTng a simple _tool_ since it is composed of multiple
1585 interacting components. This section describes those components,
1586 explains their respective roles, and shows how they connect together to
1587 form the LTTng ecosystem.
1589 The following diagram shows how the most important components of LTTng
1590 interact with user applications, the Linux kernel, and you:
1593 .Control and trace data paths between LTTng components.
1594 image::plumbing.png[]
1596 The LTTng project incorporates:
1598 * **LTTng-tools**: Libraries and command-line interface to
1599 control tracing sessions.
1600 ** <<lttng-sessiond,Session daemon>> (man:lttng-sessiond(8)).
1601 ** <<lttng-consumerd,Consumer daemon>> (cmd:lttng-consumerd).
1602 ** <<lttng-relayd,Relay daemon>> (man:lttng-relayd(8)).
1603 ** <<liblttng-ctl-lttng,Tracing control library>> (`liblttng-ctl`).
1604 ** <<lttng-cli,Tracing control command-line tool>> (man:lttng(1)).
1605 * **LTTng-UST**: Libraries and Java/Python packages to trace user
1607 ** <<lttng-ust,User space tracing library>> (`liblttng-ust`) and its
1608 headers to instrument and trace any native user application.
1609 ** <<prebuilt-ust-helpers,Preloadable user space tracing helpers>>:
1610 *** `liblttng-ust-libc-wrapper`
1611 *** `liblttng-ust-pthread-wrapper`
1612 *** `liblttng-ust-cyg-profile`
1613 *** `liblttng-ust-cyg-profile-fast`
1614 *** `liblttng-ust-dl`
1615 ** User space tracepoint provider source files generator command-line
1616 tool (man:lttng-gen-tp(1)).
1617 ** <<lttng-ust-agents,LTTng-UST Java agent>> to instrument and trace
1618 Java applications using `java.util.logging` or
1619 Apache log4j{nbsp}1.2 logging.
1620 ** <<lttng-ust-agents,LTTng-UST Python agent>> to instrument
1621 Python applications using the standard `logging` package.
1622 * **LTTng-modules**: <<lttng-modules,Linux kernel modules>> to trace
1624 ** LTTng kernel tracer module.
1625 ** Tracing ring buffer kernel modules.
1626 ** Probe kernel modules.
1627 ** LTTng logger kernel module.
1631 === Tracing control command-line interface
1634 .The tracing control command-line interface.
1635 image::plumbing-lttng-cli.png[]
1637 The _man:lttng(1) command-line tool_ is the standard user interface to
1638 control LTTng <<tracing-session,tracing sessions>>. The cmd:lttng tool
1639 is part of LTTng-tools.
1641 The cmd:lttng tool is linked with
1642 <<liblttng-ctl-lttng,`liblttng-ctl`>> to communicate with
1643 one or more <<lttng-sessiond,session daemons>> behind the scenes.
1645 The cmd:lttng tool has a Git-like interface:
1649 $ lttng <GENERAL OPTIONS> <COMMAND> <COMMAND OPTIONS>
1652 The <<controlling-tracing,Tracing control>> section explores the
1653 available features of LTTng using the cmd:lttng tool.
1656 [[liblttng-ctl-lttng]]
1657 === Tracing control library
1660 .The tracing control library.
1661 image::plumbing-liblttng-ctl.png[]
1663 The _LTTng control library_, `liblttng-ctl`, is used to communicate
1664 with a <<lttng-sessiond,session daemon>> using a C API that hides the
1665 underlying protocol's details. `liblttng-ctl` is part of LTTng-tools.
1667 The <<lttng-cli,cmd:lttng command-line tool>>
1668 is linked with `liblttng-ctl`.
1670 You can use `liblttng-ctl` in C or $$C++$$ source code by including its
1675 #include <lttng/lttng.h>
1678 Some objects are referenced by name (C string), such as tracing
1679 sessions, but most of them require to create a handle first using
1680 `lttng_create_handle()`.
1682 The best available developer documentation for `liblttng-ctl` is, as of
1683 LTTng{nbsp}{revision}, its installed header files. Every function and
1684 structure is thoroughly documented.
1688 === User space tracing library
1691 .The user space tracing library.
1692 image::plumbing-liblttng-ust.png[]
1694 The _user space tracing library_, `liblttng-ust` (see man:lttng-ust(3)),
1695 is the LTTng user space tracer. It receives commands from a
1696 <<lttng-sessiond,session daemon>>, for example to
1697 enable and disable specific instrumentation points, and writes event
1698 records to ring buffers shared with a
1699 <<lttng-consumerd,consumer daemon>>.
1700 `liblttng-ust` is part of LTTng-UST.
1702 Public C header files are installed beside `liblttng-ust` to
1703 instrument any <<c-application,C or $$C++$$ application>>.
1705 <<lttng-ust-agents,LTTng-UST agents>>, which are regular Java and Python
1706 packages, use their own library providing tracepoints which is
1707 linked with `liblttng-ust`.
1709 An application or library does not have to initialize `liblttng-ust`
1710 manually: its constructor does the necessary tasks to properly register
1711 to a session daemon. The initialization phase also enables the
1712 instrumentation points matching the <<event,event rules>> that you
1716 [[lttng-ust-agents]]
1717 === User space tracing agents
1720 .The user space tracing agents.
1721 image::plumbing-lttng-ust-agents.png[]
1723 The _LTTng-UST Java and Python agents_ are regular Java and Python
1724 packages which add LTTng tracing capabilities to the
1725 native logging frameworks. The LTTng-UST agents are part of LTTng-UST.
1727 In the case of Java, the
1728 https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[`java.util.logging`
1729 core logging facilities] and
1730 https://logging.apache.org/log4j/1.2/[Apache log4j{nbsp}1.2] are supported.
1731 Note that Apache Log4{nbsp}2 is not supported.
1733 In the case of Python, the standard
1734 https://docs.python.org/3/library/logging.html[`logging`] package
1735 is supported. Both Python{nbsp}2 and Python{nbsp}3 modules can import the
1736 LTTng-UST Python agent package.
1738 The applications using the LTTng-UST agents are in the
1739 `java.util.logging` (JUL),
1740 log4j, and Python <<domain,tracing domains>>.
1742 Both agents use the same mechanism to trace the log statements. When an
1743 agent initializes, it creates a log handler that attaches to the root
1744 logger. The agent also registers to a <<lttng-sessiond,session daemon>>.
1745 When the application executes a log statement, the root logger passes it
1746 to the agent's log handler. The agent's log handler calls a native
1747 function in a tracepoint provider package shared library linked with
1748 <<lttng-ust,`liblttng-ust`>>, passing the formatted log message and
1749 other fields, like its logger name and its log level. This native
1750 function contains a user space instrumentation point, hence tracing the
1753 The log level condition of an
1754 <<event,event rule>> is considered when tracing
1755 a Java or a Python application, and it's compatible with the standard
1756 JUL, log4j, and Python log levels.
1760 === LTTng kernel modules
1763 .The LTTng kernel modules.
1764 image::plumbing-lttng-modules.png[]
1766 The _LTTng kernel modules_ are a set of Linux kernel modules
1767 which implement the kernel tracer of the LTTng project. The LTTng
1768 kernel modules are part of LTTng-modules.
1770 The LTTng kernel modules include:
1772 * A set of _probe_ modules.
1774 Each module attaches to a specific subsystem
1775 of the Linux kernel using its tracepoint instrument points. There are
1776 also modules to attach to the entry and return points of the Linux
1777 system call functions.
1779 * _Ring buffer_ modules.
1781 A ring buffer implementation is provided as kernel modules. The LTTng
1782 kernel tracer writes to the ring buffer; a
1783 <<lttng-consumerd,consumer daemon>> reads from the ring buffer.
1785 * The _LTTng kernel tracer_ module.
1786 * The _LTTng logger_ module.
1788 The LTTng logger module implements the special path:{/proc/lttng-logger}
1789 file so that any executable can generate LTTng events by opening and
1790 writing to this file.
1792 See <<proc-lttng-logger-abi,LTTng logger>>.
1794 Generally, you do not have to load the LTTng kernel modules manually
1795 (using man:modprobe(8), for example): a root <<lttng-sessiond,session
1796 daemon>> loads the necessary modules when starting. If you have extra
1797 probe modules, you can specify to load them to the session daemon on
1800 The LTTng kernel modules are installed in
1801 +/usr/lib/modules/__release__/extra+ by default, where +__release__+ is
1802 the kernel release (see `uname --kernel-release`).
1809 .The session daemon.
1810 image::plumbing-sessiond.png[]
1812 The _session daemon_, man:lttng-sessiond(8), is a daemon responsible for
1813 managing tracing sessions and for controlling the various components of
1814 LTTng. The session daemon is part of LTTng-tools.
1816 The session daemon sends control requests to and receives control
1819 * The <<lttng-ust,user space tracing library>>.
1821 Any instance of the user space tracing library first registers to
1822 a session daemon. Then, the session daemon can send requests to
1823 this instance, such as:
1826 ** Get the list of tracepoints.
1827 ** Share an <<event,event rule>> so that the user space tracing library
1828 can enable or disable tracepoints. Amongst the possible conditions
1829 of an event rule is a filter expression which `liblttng-ust` evalutes
1830 when an event occurs.
1831 ** Share <<channel,channel>> attributes and ring buffer locations.
1834 The session daemon and the user space tracing library use a Unix
1835 domain socket for their communication.
1837 * The <<lttng-ust-agents,user space tracing agents>>.
1839 Any instance of a user space tracing agent first registers to
1840 a session daemon. Then, the session daemon can send requests to
1841 this instance, such as:
1844 ** Get the list of loggers.
1845 ** Enable or disable a specific logger.
1848 The session daemon and the user space tracing agent use a TCP connection
1849 for their communication.
1851 * The <<lttng-modules,LTTng kernel tracer>>.
1852 * The <<lttng-consumerd,consumer daemon>>.
1854 The session daemon sends requests to the consumer daemon to instruct
1855 it where to send the trace data streams, amongst other information.
1857 * The <<lttng-relayd,relay daemon>>.
1859 The session daemon receives commands from the
1860 <<liblttng-ctl-lttng,tracing control library>>.
1862 The root session daemon loads the appropriate
1863 <<lttng-modules,LTTng kernel modules>> on startup. It also spawns
1864 a <<lttng-consumerd,consumer daemon>> as soon as you create
1865 an <<event,event rule>>.
1867 The session daemon does not send and receive trace data: this is the
1868 role of the <<lttng-consumerd,consumer daemon>> and
1869 <<lttng-relayd,relay daemon>>. It does, however, generate the
1870 http://diamon.org/ctf/[CTF] metadata stream.
1872 Each Unix user can have its own session daemon instance. The
1873 tracing sessions which different session daemons manage are completely
1876 The root user's session daemon is the only one which is
1877 allowed to control the LTTng kernel tracer, and its spawned consumer
1878 daemon is the only one which is allowed to consume trace data from the
1879 LTTng kernel tracer. Note, however, that any Unix user which is a member
1880 of the <<tracing-group,tracing group>> is allowed
1881 to create <<channel,channels>> in the
1882 Linux kernel <<domain,tracing domain>>, and thus to trace the Linux
1885 The <<lttng-cli,cmd:lttng command-line tool>> automatically starts a
1886 session daemon when using its `create` command if none is currently
1887 running. You can also start the session daemon manually.
1894 .The consumer daemon.
1895 image::plumbing-consumerd.png[]
1897 The _consumer daemon_, cmd:lttng-consumerd, is a daemon which shares
1898 ring buffers with user applications or with the LTTng kernel modules to
1899 collect trace data and send it to some location (on disk or to a
1900 <<lttng-relayd,relay daemon>> over the network). The consumer daemon
1901 is part of LTTng-tools.
1903 You do not start a consumer daemon manually: a consumer daemon is always
1904 spawned by a <<lttng-sessiond,session daemon>> as soon as you create an
1905 <<event,event rule>>, that is, before you start tracing. When you kill
1906 its owner session daemon, the consumer daemon also exits because it is
1907 the session daemon's child process. Command-line options of
1908 man:lttng-sessiond(8) target the consumer daemon process.
1910 There are up to two running consumer daemons per Unix user, whereas only
1911 one session daemon can run per user. This is because each process can be
1912 either 32-bit or 64-bit: if the target system runs a mixture of 32-bit
1913 and 64-bit processes, it is more efficient to have separate
1914 corresponding 32-bit and 64-bit consumer daemons. The root user is an
1915 exception: it can have up to _three_ running consumer daemons: 32-bit
1916 and 64-bit instances for its user applications, and one more
1917 reserved for collecting kernel trace data.
1925 image::plumbing-relayd.png[]
1927 The _relay daemon_, man:lttng-relayd(8), is a daemon acting as a bridge
1928 between remote session and consumer daemons, local trace files, and a
1929 remote live trace viewer. The relay daemon is part of LTTng-tools.
1931 The main purpose of the relay daemon is to implement a receiver of
1932 <<sending-trace-data-over-the-network,trace data over the network>>.
1933 This is useful when the target system does not have much file system
1934 space to record trace files locally.
1936 The relay daemon is also a server to which a
1937 <<lttng-live,live trace viewer>> can
1938 connect. The live trace viewer sends requests to the relay daemon to
1939 receive trace data as the target system emits events. The
1940 communication protocol is named _LTTng live_; it is used over TCP
1943 Note that you can start the relay daemon on the target system directly.
1944 This is the setup of choice when the use case is to view events as
1945 the target system emits them without the need of a remote system.
1949 == [[using-lttng]]Instrumentation
1951 There are many examples of tracing and monitoring in our everyday life:
1953 * You have access to real-time and historical weather reports and
1954 forecasts thanks to weather stations installed around the country.
1955 * You know your heart is safe thanks to an electrocardiogram.
1956 * You make sure not to drive your car too fast and to have enough fuel
1957 to reach your destination thanks to gauges visible on your dashboard.
1959 All the previous examples have something in common: they rely on
1960 **instruments**. Without the electrodes attached to the surface of your
1961 body's skin, cardiac monitoring is futile.
1963 LTTng, as a tracer, is no different from those real life examples. If
1964 you're about to trace a software system or, in other words, record its
1965 history of execution, you better have **instrumentation points** in the
1966 subject you're tracing, that is, the actual software.
1968 Various ways were developed to instrument a piece of software for LTTng
1969 tracing. The most straightforward one is to manually place
1970 instrumentation points, called _tracepoints_, in the software's source
1971 code. It is also possible to add instrumentation points dynamically in
1972 the Linux kernel <<domain,tracing domain>>.
1974 If you're only interested in tracing the Linux kernel, your
1975 instrumentation needs are probably already covered by LTTng's built-in
1976 <<lttng-modules,Linux kernel tracepoints>>. You may also wish to trace a
1977 user application which is already instrumented for LTTng tracing.
1978 In such cases, you can skip this whole section and read the topics of
1979 the <<controlling-tracing,Tracing control>> section.
1981 Many methods are available to instrument a piece of software for LTTng
1984 * <<c-application,User space instrumentation for C and $$C++$$
1986 * <<prebuilt-ust-helpers,Prebuilt user space tracing helpers>>.
1987 * <<java-application,User space Java agent>>.
1988 * <<python-application,User space Python agent>>.
1989 * <<proc-lttng-logger-abi,LTTng logger>>.
1990 * <<instrumenting-linux-kernel,LTTng kernel tracepoints>>.
1994 === [[cxx-application]]User space instrumentation for C and $$C++$$ applications
1996 The procedure to instrument a C or $$C++$$ user application with
1997 the <<lttng-ust,LTTng user space tracing library>>, `liblttng-ust`, is:
1999 . <<tracepoint-provider,Create the source files of a tracepoint provider
2001 . <<probing-the-application-source-code,Add tracepoints to
2002 the application's source code>>.
2003 . <<building-tracepoint-providers-and-user-application,Build and link
2004 a tracepoint provider package and the user application>>.
2006 If you need quick, man:printf(3)-like instrumentation, you can skip
2007 those steps and use <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>>
2010 IMPORTANT: You need to <<installing-lttng,install>> LTTng-UST to
2011 instrument a user application with `liblttng-ust`.
2014 [[tracepoint-provider]]
2015 ==== Create the source files of a tracepoint provider package
2017 A _tracepoint provider_ is a set of compiled functions which provide
2018 **tracepoints** to an application, the type of instrumentation point
2019 supported by LTTng-UST. Those functions can emit events with
2020 user-defined fields and serialize those events as event records to one
2021 or more LTTng-UST <<channel,channel>> sub-buffers. The `tracepoint()`
2022 macro, which you <<probing-the-application-source-code,insert in a user
2023 application's source code>>, calls those functions.
2025 A _tracepoint provider package_ is an object file (`.o`) or a shared
2026 library (`.so`) which contains one or more tracepoint providers.
2027 Its source files are:
2029 * One or more <<tpp-header,tracepoint provider header>> (`.h`).
2030 * A <<tpp-source,tracepoint provider package source>> (`.c`).
2032 A tracepoint provider package is dynamically linked with `liblttng-ust`,
2033 the LTTng user space tracer, at run time.
2036 .User application linked with `liblttng-ust` and containing a tracepoint provider.
2037 image::ust-app.png[]
2039 NOTE: If you need quick, man:printf(3)-like instrumentation, you can
2040 skip creating and using a tracepoint provider and use
2041 <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>> instead.
2045 ===== Create a tracepoint provider header file template
2047 A _tracepoint provider header file_ contains the tracepoint
2048 definitions of a tracepoint provider.
2050 To create a tracepoint provider header file:
2052 . Start from this template:
2056 .Tracepoint provider header file template (`.h` file extension).
2058 #undef TRACEPOINT_PROVIDER
2059 #define TRACEPOINT_PROVIDER provider_name
2061 #undef TRACEPOINT_INCLUDE
2062 #define TRACEPOINT_INCLUDE "./tp.h"
2064 #if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
2067 #include <lttng/tracepoint.h>
2070 * Use TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(),
2071 * TRACEPOINT_EVENT_INSTANCE(), and TRACEPOINT_LOGLEVEL() here.
2076 #include <lttng/tracepoint-event.h>
2082 * `provider_name` with the name of your tracepoint provider.
2083 * `"tp.h"` with the name of your tracepoint provider header file.
2085 . Below the `#include <lttng/tracepoint.h>` line, put your
2086 <<defining-tracepoints,tracepoint definitions>>.
2088 Your tracepoint provider name must be unique amongst all the possible
2089 tracepoint provider names used on the same target system. We
2090 suggest to include the name of your project or company in the name,
2091 for example, `org_lttng_my_project_tpp`.
2093 TIP: [[lttng-gen-tp]]You can use the man:lttng-gen-tp(1) tool to create
2094 this boilerplate for you. When using cmd:lttng-gen-tp, all you need to
2095 write are the <<defining-tracepoints,tracepoint definitions>>.
2098 [[defining-tracepoints]]
2099 ===== Create a tracepoint definition
2101 A _tracepoint definition_ defines, for a given tracepoint:
2103 * Its **input arguments**. They are the macro parameters that the
2104 `tracepoint()` macro accepts for this particular tracepoint
2105 in the user application's source code.
2106 * Its **output event fields**. They are the sources of event fields
2107 that form the payload of any event that the execution of the
2108 `tracepoint()` macro emits for this particular tracepoint.
2110 You can create a tracepoint definition by using the
2111 `TRACEPOINT_EVENT()` macro below the `#include <lttng/tracepoint.h>`
2113 <<tpp-header,tracepoint provider header file template>>.
2115 The syntax of the `TRACEPOINT_EVENT()` macro is:
2118 .`TRACEPOINT_EVENT()` macro syntax.
2121 /* Tracepoint provider name */
2124 /* Tracepoint name */
2127 /* Input arguments */
2132 /* Output event fields */
2141 * `provider_name` with your tracepoint provider name.
2142 * `tracepoint_name` with your tracepoint name.
2143 * `arguments` with the <<tpp-def-input-args,input arguments>>.
2144 * `fields` with the <<tpp-def-output-fields,output event field>>
2147 This tracepoint emits events named `provider_name:tracepoint_name`.
2150 .Event name's length limitation
2152 The concatenation of the tracepoint provider name and the
2153 tracepoint name must not exceed **254{nbsp}characters**. If it does, the
2154 instrumented application compiles and runs, but LTTng throws multiple
2155 warnings and you could experience serious issues.
2158 [[tpp-def-input-args]]The syntax of the `TP_ARGS()` macro is:
2161 .`TP_ARGS()` macro syntax.
2170 * `type` with the C type of the argument.
2171 * `arg_name` with the argument name.
2173 You can repeat `type` and `arg_name` up to 10{nbsp}times to have
2174 more than one argument.
2176 .`TP_ARGS()` usage with three arguments.
2188 The `TP_ARGS()` and `TP_ARGS(void)` forms are valid to create a
2189 tracepoint definition with no input arguments.
2191 [[tpp-def-output-fields]]The `TP_FIELDS()` macro contains a list of
2192 `ctf_*()` macros. Each `ctf_*()` macro defines one event field. See
2193 man:lttng-ust(3) for a complete description of the available `ctf_*()`
2194 macros. A `ctf_*()` macro specifies the type, size, and byte order of
2197 Each `ctf_*()` macro takes an _argument expression_ parameter. This is a
2198 C expression that the tracer evalutes at the `tracepoint()` macro site
2199 in the application's source code. This expression provides a field's
2200 source of data. The argument expression can include input argument names
2201 listed in the `TP_ARGS()` macro.
2203 Each `ctf_*()` macro also takes a _field name_ parameter. Field names
2204 must be unique within a given tracepoint definition.
2206 Here's a complete tracepoint definition example:
2208 .Tracepoint definition.
2210 The following tracepoint definition defines a tracepoint which takes
2211 three input arguments and has four output event fields.
2215 #include "my-custom-structure.h"
2221 const struct my_custom_structure*, my_custom_structure,
2226 ctf_string(query_field, query)
2227 ctf_float(double, ratio_field, ratio)
2228 ctf_integer(int, recv_size, my_custom_structure->recv_size)
2229 ctf_integer(int, send_size, my_custom_structure->send_size)
2234 You can refer to this tracepoint definition with the `tracepoint()`
2235 macro in your application's source code like this:
2239 tracepoint(my_provider, my_tracepoint,
2240 my_structure, some_ratio, the_query);
2244 NOTE: The LTTng tracer only evaluates tracepoint arguments at run time
2245 if they satisfy an enabled <<event,event rule>>.
2248 [[using-tracepoint-classes]]
2249 ===== Use a tracepoint class
2251 A _tracepoint class_ is a class of tracepoints which share the same
2252 output event field definitions. A _tracepoint instance_ is one
2253 instance of such a defined tracepoint class, with its own tracepoint
2256 The <<defining-tracepoints,`TRACEPOINT_EVENT()` macro>> is actually a
2257 shorthand which defines both a tracepoint class and a tracepoint
2258 instance at the same time.
2260 When you build a tracepoint provider package, the C or $$C++$$ compiler
2261 creates one serialization function for each **tracepoint class**. A
2262 serialization function is responsible for serializing the event fields
2263 of a tracepoint to a sub-buffer when tracing.
2265 For various performance reasons, when your situation requires multiple
2266 tracepoint definitions with different names, but with the same event
2267 fields, we recommend that you manually create a tracepoint class
2268 and instantiate as many tracepoint instances as needed. One positive
2269 effect of such a design, amongst other advantages, is that all
2270 tracepoint instances of the same tracepoint class reuse the same
2271 serialization function, thus reducing
2272 https://en.wikipedia.org/wiki/Cache_pollution[cache pollution].
2274 .Use a tracepoint class and tracepoint instances.
2276 Consider the following three tracepoint definitions:
2288 ctf_integer(int, userid, userid)
2289 ctf_integer(size_t, len, len)
2301 ctf_integer(int, userid, userid)
2302 ctf_integer(size_t, len, len)
2314 ctf_integer(int, userid, userid)
2315 ctf_integer(size_t, len, len)
2320 In this case, we create three tracepoint classes, with one implicit
2321 tracepoint instance for each of them: `get_account`, `get_settings`, and
2322 `get_transaction`. However, they all share the same event field names
2323 and types. Hence three identical, yet independent serialization
2324 functions are created when you build the tracepoint provider package.
2326 A better design choice is to define a single tracepoint class and three
2327 tracepoint instances:
2331 /* The tracepoint class */
2332 TRACEPOINT_EVENT_CLASS(
2333 /* Tracepoint provider name */
2336 /* Tracepoint class name */
2339 /* Input arguments */
2345 /* Output event fields */
2347 ctf_integer(int, userid, userid)
2348 ctf_integer(size_t, len, len)
2352 /* The tracepoint instances */
2353 TRACEPOINT_EVENT_INSTANCE(
2354 /* Tracepoint provider name */
2357 /* Tracepoint class name */
2360 /* Tracepoint name */
2363 /* Input arguments */
2369 TRACEPOINT_EVENT_INSTANCE(
2378 TRACEPOINT_EVENT_INSTANCE(
2391 [[assigning-log-levels]]
2392 ===== Assign a log level to a tracepoint definition
2394 You can assign an optional _log level_ to a
2395 <<defining-tracepoints,tracepoint definition>>.
2397 Assigning different levels of severity to tracepoint definitions can
2398 be useful: when you <<enabling-disabling-events,create an event rule>>,
2399 you can target tracepoints having a log level as severe as a specific
2402 The concept of LTTng-UST log levels is similar to the levels found
2403 in typical logging frameworks:
2405 * In a logging framework, the log level is given by the function
2406 or method name you use at the log statement site: `debug()`,
2407 `info()`, `warn()`, `error()`, and so on.
2408 * In LTTng-UST, you statically assign the log level to a tracepoint
2409 definition; any `tracepoint()` macro invocation which refers to
2410 this definition has this log level.
2412 You can assign a log level to a tracepoint definition with the
2413 `TRACEPOINT_LOGLEVEL()` macro. You must use this macro _after_ the
2414 <<defining-tracepoints,`TRACEPOINT_EVENT()`>> or
2415 <<using-tracepoint-classes,`TRACEPOINT_INSTANCE()`>> macro for a given
2418 The syntax of the `TRACEPOINT_LOGLEVEL()` macro is:
2421 .`TRACEPOINT_LOGLEVEL()` macro syntax.
2423 TRACEPOINT_LOGLEVEL(provider_name, tracepoint_name, log_level)
2428 * `provider_name` with the tracepoint provider name.
2429 * `tracepoint_name` with the tracepoint name.
2430 * `log_level` with the log level to assign to the tracepoint
2431 definition named `tracepoint_name` in the `provider_name`
2432 tracepoint provider.
2434 See man:lttng-ust(3) for a list of available log level names.
2436 .Assign the `TRACE_DEBUG_UNIT` log level to a tracepoint definition.
2440 /* Tracepoint definition */
2449 ctf_integer(int, userid, userid)
2450 ctf_integer(size_t, len, len)
2454 /* Log level assignment */
2455 TRACEPOINT_LOGLEVEL(my_app, get_transaction, TRACE_DEBUG_UNIT)
2461 ===== Create a tracepoint provider package source file
2463 A _tracepoint provider package source file_ is a C source file which
2464 includes a <<tpp-header,tracepoint provider header file>> to expand its
2465 macros into event serialization and other functions.
2467 You can always use the following tracepoint provider package source
2471 .Tracepoint provider package source file template.
2473 #define TRACEPOINT_CREATE_PROBES
2478 Replace `tp.h` with the name of your <<tpp-header,tracepoint provider
2479 header file>> name. You may also include more than one tracepoint
2480 provider header file here to create a tracepoint provider package
2481 holding more than one tracepoint providers.
2484 [[probing-the-application-source-code]]
2485 ==== Add tracepoints to an application's source code
2487 Once you <<tpp-header,create a tracepoint provider header file>>, you
2488 can use the `tracepoint()` macro in your application's
2489 source code to insert the tracepoints that this header
2490 <<defining-tracepoints,defines>>.
2492 The `tracepoint()` macro takes at least two parameters: the tracepoint
2493 provider name and the tracepoint name. The corresponding tracepoint
2494 definition defines the other parameters.
2496 .`tracepoint()` usage.
2498 The following <<defining-tracepoints,tracepoint definition>> defines a
2499 tracepoint which takes two input arguments and has two output event
2503 .Tracepoint provider header file.
2505 #include "my-custom-structure.h"
2512 const char*, cmd_name
2515 ctf_string(cmd_name, cmd_name)
2516 ctf_integer(int, number_of_args, argc)
2521 You can refer to this tracepoint definition with the `tracepoint()`
2522 macro in your application's source code like this:
2525 .Application's source file.
2529 int main(int argc, char* argv[])
2531 tracepoint(my_provider, my_tracepoint, argc, argv[0]);
2537 Note how the application's source code includes
2538 the tracepoint provider header file containing the tracepoint
2539 definitions to use, path:{tp.h}.
2542 .`tracepoint()` usage with a complex tracepoint definition.
2544 Consider this complex tracepoint definition, where multiple event
2545 fields refer to the same input arguments in their argument expression
2549 .Tracepoint provider header file.
2551 /* For `struct stat` */
2552 #include <sys/types.h>
2553 #include <sys/stat.h>
2565 ctf_integer(int, my_constant_field, 23 + 17)
2566 ctf_integer(int, my_int_arg_field, my_int_arg)
2567 ctf_integer(int, my_int_arg_field2, my_int_arg * my_int_arg)
2568 ctf_integer(int, sum4_field, my_str_arg[0] + my_str_arg[1] +
2569 my_str_arg[2] + my_str_arg[3])
2570 ctf_string(my_str_arg_field, my_str_arg)
2571 ctf_integer_hex(off_t, size_field, st->st_size)
2572 ctf_float(double, size_dbl_field, (double) st->st_size)
2573 ctf_sequence_text(char, half_my_str_arg_field, my_str_arg,
2574 size_t, strlen(my_str_arg) / 2)
2579 You can refer to this tracepoint definition with the `tracepoint()`
2580 macro in your application's source code like this:
2583 .Application's source file.
2585 #define TRACEPOINT_DEFINE
2592 stat("/etc/fstab", &s);
2593 tracepoint(my_provider, my_tracepoint, 23, "Hello, World!", &s);
2599 If you look at the event record that LTTng writes when tracing this
2600 program, assuming the file size of path:{/etc/fstab} is 301{nbsp}bytes,
2601 it should look like this:
2603 .Event record fields
2605 |Field's name |Field's value
2606 |`my_constant_field` |40
2607 |`my_int_arg_field` |23
2608 |`my_int_arg_field2` |529
2610 |`my_str_arg_field` |`Hello, World!`
2611 |`size_field` |0x12d
2612 |`size_dbl_field` |301.0
2613 |`half_my_str_arg_field` |`Hello,`
2617 Sometimes, the arguments you pass to `tracepoint()` are expensive to
2618 compute--they use the call stack, for example. To avoid this
2619 computation when the tracepoint is disabled, you can use the
2620 `tracepoint_enabled()` and `do_tracepoint()` macros.
2622 The syntax of the `tracepoint_enabled()` and `do_tracepoint()` macros
2626 .`tracepoint_enabled()` and `do_tracepoint()` macros syntax.
2628 tracepoint_enabled(provider_name, tracepoint_name)
2629 do_tracepoint(provider_name, tracepoint_name, ...)
2634 * `provider_name` with the tracepoint provider name.
2635 * `tracepoint_name` with the tracepoint name.
2637 `tracepoint_enabled()` returns a non-zero value if the tracepoint named
2638 `tracepoint_name` from the provider named `provider_name` is enabled
2641 `do_tracepoint()` is like `tracepoint()`, except that it doesn't check
2642 if the tracepoint is enabled. Using `tracepoint()` with
2643 `tracepoint_enabled()` is dangerous since `tracepoint()` also contains
2644 the `tracepoint_enabled()` check, thus a race condition is
2645 possible in this situation:
2648 .Possible race condition when using `tracepoint_enabled()` with `tracepoint()`.
2650 if (tracepoint_enabled(my_provider, my_tracepoint)) {
2651 stuff = prepare_stuff();
2654 tracepoint(my_provider, my_tracepoint, stuff);
2657 If the tracepoint is enabled after the condition, then `stuff` is not
2658 prepared: the emitted event will either contain wrong data, or the whole
2659 application could crash (segmentation fault, for example).
2661 NOTE: Neither `tracepoint_enabled()` nor `do_tracepoint()` have an
2662 `STAP_PROBEV()` call. If you need it, you must emit
2666 [[building-tracepoint-providers-and-user-application]]
2667 ==== Build and link a tracepoint provider package and an application
2669 Once you have one or more <<tpp-header,tracepoint provider header
2670 files>> and a <<tpp-source,tracepoint provider package source file>>,
2671 you can create the tracepoint provider package by compiling its source
2672 file. From here, multiple build and run scenarios are possible. The
2673 following table shows common application and library configurations
2674 along with the required command lines to achieve them.
2676 In the following diagrams, we use the following file names:
2679 Executable application.
2682 Application's object file.
2685 Tracepoint provider package object file.
2688 Tracepoint provider package archive file.
2691 Tracepoint provider package shared object file.
2694 User library object file.
2697 User library shared object file.
2699 We use the following symbols in the diagrams of table below:
2702 .Symbols used in the build scenario diagrams.
2703 image::ust-sit-symbols.png[]
2705 We assume that path:{.} is part of the env:LD_LIBRARY_PATH environment
2706 variable in the following instructions.
2708 [role="growable ust-scenarios",cols="asciidoc,asciidoc"]
2709 .Common tracepoint provider package scenarios.
2711 |Scenario |Instructions
2714 The instrumented application is statically linked with
2715 the tracepoint provider package object.
2717 image::ust-sit+app-linked-with-tp-o+app-instrumented.png[]
2720 include::../common/ust-sit-step-tp-o.txt[]
2722 To build the instrumented application:
2724 . In path:{app.c}, before including path:{tpp.h}, add the following line:
2729 #define TRACEPOINT_DEFINE
2733 . Compile the application source file:
2742 . Build the application:
2747 $ gcc -o app app.o tpp.o -llttng-ust -ldl
2751 To run the instrumented application:
2753 * Start the application:
2763 The instrumented application is statically linked with the
2764 tracepoint provider package archive file.
2766 image::ust-sit+app-linked-with-tp-a+app-instrumented.png[]
2769 To create the tracepoint provider package archive file:
2771 . Compile the <<tpp-source,tracepoint provider package source file>>:
2780 . Create the tracepoint provider package archive file:
2785 $ ar rcs tpp.a tpp.o
2789 To build the instrumented application:
2791 . In path:{app.c}, before including path:{tpp.h}, add the following line:
2796 #define TRACEPOINT_DEFINE
2800 . Compile the application source file:
2809 . Build the application:
2814 $ gcc -o app app.o tpp.a -llttng-ust -ldl
2818 To run the instrumented application:
2820 * Start the application:
2830 The instrumented application is linked with the tracepoint provider
2831 package shared object.
2833 image::ust-sit+app-linked-with-tp-so+app-instrumented.png[]
2836 include::../common/ust-sit-step-tp-so.txt[]
2838 To build the instrumented application:
2840 . In path:{app.c}, before including path:{tpp.h}, add the following line:
2845 #define TRACEPOINT_DEFINE
2849 . Compile the application source file:
2858 . Build the application:
2863 $ gcc -o app app.o -ldl -L. -ltpp
2867 To run the instrumented application:
2869 * Start the application:
2879 The tracepoint provider package shared object is preloaded before the
2880 instrumented application starts.
2882 image::ust-sit+tp-so-preloaded+app-instrumented.png[]
2885 include::../common/ust-sit-step-tp-so.txt[]
2887 To build the instrumented application:
2889 . In path:{app.c}, before including path:{tpp.h}, add the
2895 #define TRACEPOINT_DEFINE
2896 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
2900 . Compile the application source file:
2909 . Build the application:
2914 $ gcc -o app app.o -ldl
2918 To run the instrumented application with tracing support:
2920 * Preload the tracepoint provider package shared object and
2921 start the application:
2926 $ LD_PRELOAD=./libtpp.so ./app
2930 To run the instrumented application without tracing support:
2932 * Start the application:
2942 The instrumented application dynamically loads the tracepoint provider
2943 package shared object.
2945 image::ust-sit+app-dlopens-tp-so+app-instrumented.png[]
2948 include::../common/ust-sit-step-tp-so.txt[]
2950 To build the instrumented application:
2952 . In path:{app.c}, before including path:{tpp.h}, add the
2958 #define TRACEPOINT_DEFINE
2959 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
2963 . Compile the application source file:
2972 . Build the application:
2977 $ gcc -o app app.o -ldl
2981 To run the instrumented application:
2983 * Start the application:
2993 The application is linked with the instrumented user library.
2995 The instrumented user library is statically linked with the tracepoint
2996 provider package object file.
2998 image::ust-sit+app-linked-with-lib+lib-linked-with-tp-o+lib-instrumented.png[]
3001 include::../common/ust-sit-step-tp-o-fpic.txt[]
3003 To build the instrumented user library:
3005 . In path:{emon.c}, before including path:{tpp.h}, add the
3011 #define TRACEPOINT_DEFINE
3015 . Compile the user library source file:
3020 $ gcc -I. -fpic -c emon.c
3024 . Build the user library shared object:
3029 $ gcc -shared -o libemon.so emon.o tpp.o -llttng-ust -ldl
3033 To build the application:
3035 . Compile the application source file:
3044 . Build the application:
3049 $ gcc -o app app.o -L. -lemon
3053 To run the application:
3055 * Start the application:
3065 The application is linked with the instrumented user library.
3067 The instrumented user library is linked with the tracepoint provider
3068 package shared object.
3070 image::ust-sit+app-linked-with-lib+lib-linked-with-tp-so+lib-instrumented.png[]
3073 include::../common/ust-sit-step-tp-so.txt[]
3075 To build the instrumented user library:
3077 . In path:{emon.c}, before including path:{tpp.h}, add the
3083 #define TRACEPOINT_DEFINE
3087 . Compile the user library source file:
3092 $ gcc -I. -fpic -c emon.c
3096 . Build the user library shared object:
3101 $ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
3105 To build the application:
3107 . Compile the application source file:
3116 . Build the application:
3121 $ gcc -o app app.o -L. -lemon
3125 To run the application:
3127 * Start the application:
3137 The tracepoint provider package shared object is preloaded before the
3140 The application is linked with the instrumented user library.
3142 image::ust-sit+tp-so-preloaded+app-linked-with-lib+lib-instrumented.png[]
3145 include::../common/ust-sit-step-tp-so.txt[]
3147 To build the instrumented user library:
3149 . In path:{emon.c}, before including path:{tpp.h}, add the
3155 #define TRACEPOINT_DEFINE
3156 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3160 . Compile the user library source file:
3165 $ gcc -I. -fpic -c emon.c
3169 . Build the user library shared object:
3174 $ gcc -shared -o libemon.so emon.o -ldl
3178 To build the application:
3180 . Compile the application source file:
3189 . Build the application:
3194 $ gcc -o app app.o -L. -lemon
3198 To run the application with tracing support:
3200 * Preload the tracepoint provider package shared object and
3201 start the application:
3206 $ LD_PRELOAD=./libtpp.so ./app
3210 To run the application without tracing support:
3212 * Start the application:
3222 The application is linked with the instrumented user library.
3224 The instrumented user library dynamically loads the tracepoint provider
3225 package shared object.
3227 image::ust-sit+app-linked-with-lib+lib-dlopens-tp-so+lib-instrumented.png[]
3230 include::../common/ust-sit-step-tp-so.txt[]
3232 To build the instrumented user library:
3234 . In path:{emon.c}, before including path:{tpp.h}, add the
3240 #define TRACEPOINT_DEFINE
3241 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3245 . Compile the user library source file:
3250 $ gcc -I. -fpic -c emon.c
3254 . Build the user library shared object:
3259 $ gcc -shared -o libemon.so emon.o -ldl
3263 To build the application:
3265 . Compile the application source file:
3274 . Build the application:
3279 $ gcc -o app app.o -L. -lemon
3283 To run the application:
3285 * Start the application:
3295 The application dynamically loads the instrumented user library.
3297 The instrumented user library is linked with the tracepoint provider
3298 package shared object.
3300 image::ust-sit+app-dlopens-lib+lib-linked-with-tp-so+lib-instrumented.png[]
3303 include::../common/ust-sit-step-tp-so.txt[]
3305 To build the instrumented user library:
3307 . In path:{emon.c}, before including path:{tpp.h}, add the
3313 #define TRACEPOINT_DEFINE
3317 . Compile the user library source file:
3322 $ gcc -I. -fpic -c emon.c
3326 . Build the user library shared object:
3331 $ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
3335 To build the application:
3337 . Compile the application source file:
3346 . Build the application:
3351 $ gcc -o app app.o -ldl -L. -lemon
3355 To run the application:
3357 * Start the application:
3367 The application dynamically loads the instrumented user library.
3369 The instrumented user library dynamically loads the tracepoint provider
3370 package shared object.
3372 image::ust-sit+app-dlopens-lib+lib-dlopens-tp-so+lib-instrumented.png[]
3375 include::../common/ust-sit-step-tp-so.txt[]
3377 To build the instrumented user library:
3379 . In path:{emon.c}, before including path:{tpp.h}, add the
3385 #define TRACEPOINT_DEFINE
3386 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3390 . Compile the user library source file:
3395 $ gcc -I. -fpic -c emon.c
3399 . Build the user library shared object:
3404 $ gcc -shared -o libemon.so emon.o -ldl
3408 To build the application:
3410 . Compile the application source file:
3419 . Build the application:
3424 $ gcc -o app app.o -ldl -L. -lemon
3428 To run the application:
3430 * Start the application:
3440 The tracepoint provider package shared object is preloaded before the
3443 The application dynamically loads the instrumented user library.
3445 image::ust-sit+tp-so-preloaded+app-dlopens-lib+lib-instrumented.png[]
3448 include::../common/ust-sit-step-tp-so.txt[]
3450 To build the instrumented user library:
3452 . In path:{emon.c}, before including path:{tpp.h}, add the
3458 #define TRACEPOINT_DEFINE
3459 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3463 . Compile the user library source file:
3468 $ gcc -I. -fpic -c emon.c
3472 . Build the user library shared object:
3477 $ gcc -shared -o libemon.so emon.o -ldl
3481 To build the application:
3483 . Compile the application source file:
3492 . Build the application:
3497 $ gcc -o app app.o -L. -lemon
3501 To run the application with tracing support:
3503 * Preload the tracepoint provider package shared object and
3504 start the application:
3509 $ LD_PRELOAD=./libtpp.so ./app
3513 To run the application without tracing support:
3515 * Start the application:
3525 The application is statically linked with the tracepoint provider
3526 package object file.
3528 The application is linked with the instrumented user library.
3530 image::ust-sit+app-linked-with-tp-o+app-linked-with-lib+lib-instrumented.png[]
3533 include::../common/ust-sit-step-tp-o.txt[]
3535 To build the instrumented user library:
3537 . In path:{emon.c}, before including path:{tpp.h}, add the
3543 #define TRACEPOINT_DEFINE
3547 . Compile the user library source file:
3552 $ gcc -I. -fpic -c emon.c
3556 . Build the user library shared object:
3561 $ gcc -shared -o libemon.so emon.o
3565 To build the application:
3567 . Compile the application source file:
3576 . Build the application:
3581 $ gcc -o app app.o tpp.o -llttng-ust -ldl -L. -lemon
3585 To run the instrumented application:
3587 * Start the application:
3597 The application is statically linked with the tracepoint provider
3598 package object file.
3600 The application dynamically loads the instrumented user library.
3602 image::ust-sit+app-linked-with-tp-o+app-dlopens-lib+lib-instrumented.png[]
3605 include::../common/ust-sit-step-tp-o.txt[]
3607 To build the application:
3609 . In path:{app.c}, before including path:{tpp.h}, add the following line:
3614 #define TRACEPOINT_DEFINE
3618 . Compile the application source file:
3627 . Build the application:
3632 $ gcc -Wl,--export-dynamic -o app app.o tpp.o \
3637 The `--export-dynamic` option passed to the linker is necessary for the
3638 dynamically loaded library to ``see'' the tracepoint symbols defined in
3641 To build the instrumented user library:
3643 . Compile the user library source file:
3648 $ gcc -I. -fpic -c emon.c
3652 . Build the user library shared object:
3657 $ gcc -shared -o libemon.so emon.o
3661 To run the application:
3663 * Start the application:
3674 [[using-lttng-ust-with-daemons]]
3675 ===== Use noch:{LTTng-UST} with daemons
3677 If your instrumented application calls man:fork(2), man:clone(2),
3678 or BSD's man:rfork(2), without a following man:exec(3)-family
3679 system call, you must preload the path:{liblttng-ust-fork.so} shared
3680 object when you start the application.
3684 $ LD_PRELOAD=liblttng-ust-fork.so ./my-app
3687 If your tracepoint provider package is
3688 a shared library which you also preload, you must put both
3689 shared objects in env:LD_PRELOAD:
3693 $ LD_PRELOAD=liblttng-ust-fork.so:/path/to/tp.so ./my-app
3699 ===== Use noch:{LTTng-UST} with applications which close file descriptors that don't belong to them
3701 If your instrumented application closes one or more file descriptors
3702 which it did not open itself, you must preload the
3703 path:{liblttng-ust-fd.so} shared object when you start the application:
3707 $ LD_PRELOAD=liblttng-ust-fd.so ./my-app
3710 Typical use cases include closing all the file descriptors after
3711 man:fork(2) or man:rfork(2) and buggy applications doing
3715 [[lttng-ust-pkg-config]]
3716 ===== Use noch:{pkg-config}
3718 On some distributions, LTTng-UST ships with a
3719 https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config]
3720 metadata file. If this is your case, then you can use cmd:pkg-config to
3721 build an application on the command line:
3725 $ gcc -o my-app my-app.o tp.o $(pkg-config --cflags --libs lttng-ust)
3729 [[instrumenting-32-bit-app-on-64-bit-system]]
3730 ===== [[advanced-instrumenting-techniques]]Build a 32-bit instrumented application for a 64-bit target system
3732 In order to trace a 32-bit application running on a 64-bit system,
3733 LTTng must use a dedicated 32-bit
3734 <<lttng-consumerd,consumer daemon>>.
3736 The following steps show how to build and install a 32-bit consumer
3737 daemon, which is _not_ part of the default 64-bit LTTng build, how to
3738 build and install the 32-bit LTTng-UST libraries, and how to build and
3739 link an instrumented 32-bit application in that context.
3741 To build a 32-bit instrumented application for a 64-bit target system,
3742 assuming you have a fresh target system with no installed Userspace RCU
3745 . Download, build, and install a 32-bit version of Userspace RCU:
3750 $ cd $(mktemp -d) &&
3751 wget http://lttng.org/files/urcu/userspace-rcu-latest-0.9.tar.bz2 &&
3752 tar -xf userspace-rcu-latest-0.9.tar.bz2 &&
3753 cd userspace-rcu-0.9.* &&
3754 ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 &&
3756 sudo make install &&
3761 . Using your distribution's package manager, or from source, install
3762 the following 32-bit versions of the following dependencies of
3763 LTTng-tools and LTTng-UST:
3766 * https://sourceforge.net/projects/libuuid/[libuuid]
3767 * http://directory.fsf.org/wiki/Popt[popt]
3768 * http://www.xmlsoft.org/[libxml2]
3771 . Download, build, and install a 32-bit version of the latest
3772 LTTng-UST{nbsp}{revision}:
3777 $ cd $(mktemp -d) &&
3778 wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.11.tar.bz2 &&
3779 tar -xf lttng-ust-latest-2.11.tar.bz2 &&
3780 cd lttng-ust-2.11.* &&
3781 ./configure --libdir=/usr/local/lib32 \
3782 CFLAGS=-m32 CXXFLAGS=-m32 \
3783 LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' &&
3785 sudo make install &&
3792 Depending on your distribution,
3793 32-bit libraries could be installed at a different location than
3794 `/usr/lib32`. For example, Debian is known to install
3795 some 32-bit libraries in `/usr/lib/i386-linux-gnu`.
3797 In this case, make sure to set `LDFLAGS` to all the
3798 relevant 32-bit library paths, for example:
3802 $ LDFLAGS='-L/usr/lib/i386-linux-gnu -L/usr/lib32'
3806 . Download the latest LTTng-tools{nbsp}{revision}, build, and install
3807 the 32-bit consumer daemon:
3812 $ cd $(mktemp -d) &&
3813 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.11.tar.bz2 &&
3814 tar -xf lttng-tools-latest-2.11.tar.bz2 &&
3815 cd lttng-tools-2.11.* &&
3816 ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \
3817 LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' \
3818 --disable-bin-lttng --disable-bin-lttng-crash \
3819 --disable-bin-lttng-relayd --disable-bin-lttng-sessiond &&
3821 cd src/bin/lttng-consumerd &&
3822 sudo make install &&
3827 . From your distribution or from source,
3828 <<installing-lttng,install>> the 64-bit versions of
3829 LTTng-UST and Userspace RCU.
3830 . Download, build, and install the 64-bit version of the
3831 latest LTTng-tools{nbsp}{revision}:
3836 $ cd $(mktemp -d) &&
3837 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.11.tar.bz2 &&
3838 tar -xf lttng-tools-latest-2.11.tar.bz2 &&
3839 cd lttng-tools-2.11.* &&
3840 ./configure --with-consumerd32-libdir=/usr/local/lib32 \
3841 --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd &&
3843 sudo make install &&
3848 . Pass the following options to man:gcc(1), man:g++(1), or man:clang(1)
3849 when linking your 32-bit application:
3852 -m32 -L/usr/lib32 -L/usr/local/lib32 \
3853 -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32
3856 For example, let's rebuild the quick start example in
3857 <<tracing-your-own-user-application,Trace a user application>> as an
3858 instrumented 32-bit application:
3863 $ gcc -m32 -c -I. hello-tp.c
3864 $ gcc -m32 -c hello.c
3865 $ gcc -m32 -o hello hello.o hello-tp.o \
3866 -L/usr/lib32 -L/usr/local/lib32 \
3867 -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 \
3872 No special action is required to execute the 32-bit application and
3873 to trace it: use the command-line man:lttng(1) tool as usual.
3880 man:tracef(3) is a small LTTng-UST API designed for quick,
3881 man:printf(3)-like instrumentation without the burden of
3882 <<tracepoint-provider,creating>> and
3883 <<building-tracepoint-providers-and-user-application,building>>
3884 a tracepoint provider package.
3886 To use `tracef()` in your application:
3888 . In the C or C++ source files where you need to use `tracef()`,
3889 include `<lttng/tracef.h>`:
3894 #include <lttng/tracef.h>
3898 . In the application's source code, use `tracef()` like you would use
3906 tracef("my message: %d (%s)", my_integer, my_string);
3912 . Link your application with `liblttng-ust`:
3917 $ gcc -o app app.c -llttng-ust
3921 To trace the events that `tracef()` calls emit:
3923 * <<enabling-disabling-events,Create an event rule>> which matches the
3924 `lttng_ust_tracef:*` event name:
3929 $ lttng enable-event --userspace 'lttng_ust_tracef:*'
3934 .Limitations of `tracef()`
3936 The `tracef()` utility function was developed to make user space tracing
3937 super simple, albeit with notable disadvantages compared to
3938 <<defining-tracepoints,user-defined tracepoints>>:
3940 * All the emitted events have the same tracepoint provider and
3941 tracepoint names, respectively `lttng_ust_tracef` and `event`.
3942 * There is no static type checking.
3943 * The only event record field you actually get, named `msg`, is a string
3944 potentially containing the values you passed to `tracef()`
3945 using your own format string. This also means that you cannot filter
3946 events with a custom expression at run time because there are no
3948 * Since `tracef()` uses the C standard library's man:vasprintf(3)
3949 function behind the scenes to format the strings at run time, its
3950 expected performance is lower than with user-defined tracepoints,
3951 which do not require a conversion to a string.
3953 Taking this into consideration, `tracef()` is useful for some quick
3954 prototyping and debugging, but you should not consider it for any
3955 permanent and serious applicative instrumentation.
3961 ==== Use `tracelog()`
3963 The man:tracelog(3) API is very similar to <<tracef,`tracef()`>>, with
3964 the difference that it accepts an additional log level parameter.
3966 The goal of `tracelog()` is to ease the migration from logging to
3969 To use `tracelog()` in your application:
3971 . In the C or C++ source files where you need to use `tracelog()`,
3972 include `<lttng/tracelog.h>`:
3977 #include <lttng/tracelog.h>
3981 . In the application's source code, use `tracelog()` like you would use
3982 man:printf(3), except for the first parameter which is the log
3990 tracelog(TRACE_WARNING, "my message: %d (%s)",
3991 my_integer, my_string);
3997 See man:lttng-ust(3) for a list of available log level names.
3999 . Link your application with `liblttng-ust`:
4004 $ gcc -o app app.c -llttng-ust
4008 To trace the events that `tracelog()` calls emit with a log level
4009 _as severe as_ a specific log level:
4011 * <<enabling-disabling-events,Create an event rule>> which matches the
4012 `lttng_ust_tracelog:*` event name and a minimum level
4018 $ lttng enable-event --userspace 'lttng_ust_tracelog:*'
4019 --loglevel=TRACE_WARNING
4023 To trace the events that `tracelog()` calls emit with a
4024 _specific log level_:
4026 * Create an event rule which matches the `lttng_ust_tracelog:*`
4027 event name and a specific log level:
4032 $ lttng enable-event --userspace 'lttng_ust_tracelog:*'
4033 --loglevel-only=TRACE_INFO
4038 [[prebuilt-ust-helpers]]
4039 === Prebuilt user space tracing helpers
4041 The LTTng-UST package provides a few helpers in the form or preloadable
4042 shared objects which automatically instrument system functions and
4045 The helper shared objects are normally found in dir:{/usr/lib}. If you
4046 built LTTng-UST <<building-from-source,from source>>, they are probably
4047 located in dir:{/usr/local/lib}.
4049 The installed user space tracing helpers in LTTng-UST{nbsp}{revision}
4052 path:{liblttng-ust-libc-wrapper.so}::
4053 path:{liblttng-ust-pthread-wrapper.so}::
4054 <<liblttng-ust-libc-pthread-wrapper,C{nbsp}standard library
4055 memory and POSIX threads function tracing>>.
4057 path:{liblttng-ust-cyg-profile.so}::
4058 path:{liblttng-ust-cyg-profile-fast.so}::
4059 <<liblttng-ust-cyg-profile,Function entry and exit tracing>>.
4061 path:{liblttng-ust-dl.so}::
4062 <<liblttng-ust-dl,Dynamic linker tracing>>.
4064 To use a user space tracing helper with any user application:
4066 * Preload the helper shared object when you start the application:
4071 $ LD_PRELOAD=liblttng-ust-libc-wrapper.so my-app
4075 You can preload more than one helper:
4080 $ LD_PRELOAD=liblttng-ust-libc-wrapper.so:liblttng-ust-dl.so my-app
4086 [[liblttng-ust-libc-pthread-wrapper]]
4087 ==== Instrument C standard library memory and POSIX threads functions
4089 The path:{liblttng-ust-libc-wrapper.so} and
4090 path:{liblttng-ust-pthread-wrapper.so} helpers
4091 add instrumentation to some C standard library and POSIX
4095 .Functions instrumented by preloading path:{liblttng-ust-libc-wrapper.so}.
4097 |TP provider name |TP name |Instrumented function
4099 .6+|`lttng_ust_libc` |`malloc` |man:malloc(3)
4100 |`calloc` |man:calloc(3)
4101 |`realloc` |man:realloc(3)
4102 |`free` |man:free(3)
4103 |`memalign` |man:memalign(3)
4104 |`posix_memalign` |man:posix_memalign(3)
4108 .Functions instrumented by preloading path:{liblttng-ust-pthread-wrapper.so}.
4110 |TP provider name |TP name |Instrumented function
4112 .4+|`lttng_ust_pthread` |`pthread_mutex_lock_req` |man:pthread_mutex_lock(3p) (request time)
4113 |`pthread_mutex_lock_acq` |man:pthread_mutex_lock(3p) (acquire time)
4114 |`pthread_mutex_trylock` |man:pthread_mutex_trylock(3p)
4115 |`pthread_mutex_unlock` |man:pthread_mutex_unlock(3p)
4118 When you preload the shared object, it replaces the functions listed
4119 in the previous tables by wrappers which contain tracepoints and call
4120 the replaced functions.
4123 [[liblttng-ust-cyg-profile]]
4124 ==== Instrument function entry and exit
4126 The path:{liblttng-ust-cyg-profile*.so} helpers can add instrumentation
4127 to the entry and exit points of functions.
4129 man:gcc(1) and man:clang(1) have an option named
4130 https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html[`-finstrument-functions`]
4131 which generates instrumentation calls for entry and exit to functions.
4132 The LTTng-UST function tracing helpers,
4133 path:{liblttng-ust-cyg-profile.so} and
4134 path:{liblttng-ust-cyg-profile-fast.so}, take advantage of this feature
4135 to add tracepoints to the two generated functions (which contain
4136 `cyg_profile` in their names, hence the helper's name).
4138 To use the LTTng-UST function tracing helper, the source files to
4139 instrument must be built using the `-finstrument-functions` compiler
4142 There are two versions of the LTTng-UST function tracing helper:
4144 * **path:{liblttng-ust-cyg-profile-fast.so}** is a lightweight variant
4145 that you should only use when it can be _guaranteed_ that the
4146 complete event stream is recorded without any lost event record.
4147 Any kind of duplicate information is left out.
4149 Assuming no event record is lost, having only the function addresses on
4150 entry is enough to create a call graph, since an event record always
4151 contains the ID of the CPU that generated it.
4153 You can use a tool like man:addr2line(1) to convert function addresses
4154 back to source file names and line numbers.
4156 * **path:{liblttng-ust-cyg-profile.so}** is a more robust variant
4157 which also works in use cases where event records might get discarded or
4158 not recorded from application startup.
4159 In these cases, the trace analyzer needs more information to be
4160 able to reconstruct the program flow.
4162 See man:lttng-ust-cyg-profile(3) to learn more about the instrumentation
4163 points of this helper.
4165 All the tracepoints that this helper provides have the
4166 log level `TRACE_DEBUG_FUNCTION` (see man:lttng-ust(3)).
4168 TIP: It's sometimes a good idea to limit the number of source files that
4169 you compile with the `-finstrument-functions` option to prevent LTTng
4170 from writing an excessive amount of trace data at run time. When using
4171 man:gcc(1), you can use the
4172 `-finstrument-functions-exclude-function-list` option to avoid
4173 instrument entries and exits of specific function names.
4178 ==== Instrument the dynamic linker
4180 The path:{liblttng-ust-dl.so} helper adds instrumentation to the
4181 man:dlopen(3) and man:dlclose(3) function calls.
4183 See man:lttng-ust-dl(3) to learn more about the instrumentation points
4188 [[java-application]]
4189 === User space Java agent
4191 You can instrument any Java application which uses one of the following
4194 * The https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[**`java.util.logging`**]
4195 (JUL) core logging facilities.
4196 * http://logging.apache.org/log4j/1.2/[**Apache log4j{nbsp}1.2**], since
4197 LTTng{nbsp}2.6. Note that Apache Log4j{nbsp}2 is not supported.
4200 .LTTng-UST Java agent imported by a Java application.
4201 image::java-app.png[]
4203 Note that the methods described below are new in LTTng{nbsp}{revision}.
4204 Previous LTTng versions use another technique.
4206 NOTE: We use http://openjdk.java.net/[OpenJDK]{nbsp}8 for development
4207 and https://ci.lttng.org/[continuous integration], thus this version is
4208 directly supported. However, the LTTng-UST Java agent is also tested
4209 with OpenJDK{nbsp}7.
4214 ==== Use the LTTng-UST Java agent for `java.util.logging`
4216 To use the LTTng-UST Java agent in a Java application which uses
4217 `java.util.logging` (JUL):
4219 . In the Java application's source code, import the LTTng-UST
4220 log handler package for `java.util.logging`:
4225 import org.lttng.ust.agent.jul.LttngLogHandler;
4229 . Create an LTTng-UST JUL log handler:
4234 Handler lttngUstLogHandler = new LttngLogHandler();
4238 . Add this handler to the JUL loggers which should emit LTTng events:
4243 Logger myLogger = Logger.getLogger("some-logger");
4245 myLogger.addHandler(lttngUstLogHandler);
4249 . Use `java.util.logging` log statements and configuration as usual.
4250 The loggers with an attached LTTng-UST log handler can emit
4253 . Before exiting the application, remove the LTTng-UST log handler from
4254 the loggers attached to it and call its `close()` method:
4259 myLogger.removeHandler(lttngUstLogHandler);
4260 lttngUstLogHandler.close();
4264 This is not strictly necessary, but it is recommended for a clean
4265 disposal of the handler's resources.
4267 . Include the LTTng-UST Java agent's common and JUL-specific JAR files,
4268 path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-jul.jar},
4270 https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
4271 path] when you build the Java application.
4273 The JAR files are typically located in dir:{/usr/share/java}.
4275 IMPORTANT: The LTTng-UST Java agent must be
4276 <<installing-lttng,installed>> for the logging framework your
4279 .Use the LTTng-UST Java agent for `java.util.logging`.
4284 import java.io.IOException;
4285 import java.util.logging.Handler;
4286 import java.util.logging.Logger;
4287 import org.lttng.ust.agent.jul.LttngLogHandler;
4291 private static final int answer = 42;
4293 public static void main(String[] argv) throws Exception
4296 Logger logger = Logger.getLogger("jello");
4298 // Create an LTTng-UST log handler
4299 Handler lttngUstLogHandler = new LttngLogHandler();
4301 // Add the LTTng-UST log handler to our logger
4302 logger.addHandler(lttngUstLogHandler);
4305 logger.info("some info");
4306 logger.warning("some warning");
4308 logger.finer("finer information; the answer is " + answer);
4310 logger.severe("error!");
4312 // Not mandatory, but cleaner
4313 logger.removeHandler(lttngUstLogHandler);
4314 lttngUstLogHandler.close();
4323 $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
4326 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4327 <<enabling-disabling-events,create an event rule>> matching the
4328 `jello` JUL logger, and <<basic-tracing-session-control,start tracing>>:
4333 $ lttng enable-event --jul jello
4337 Run the compiled class:
4341 $ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
4344 <<basic-tracing-session-control,Stop tracing>> and inspect the
4354 In the resulting trace, an <<event,event record>> generated by a Java
4355 application using `java.util.logging` is named `lttng_jul:event` and
4356 has the following fields:
4359 Log record's message.
4365 Name of the class in which the log statement was executed.
4368 Name of the method in which the log statement was executed.
4371 Logging time (timestamp in milliseconds).
4374 Log level integer value.
4377 ID of the thread in which the log statement was executed.
4379 You can use the opt:lttng-enable-event(1):--loglevel or
4380 opt:lttng-enable-event(1):--loglevel-only option of the
4381 man:lttng-enable-event(1) command to target a range of JUL log levels
4382 or a specific JUL log level.
4387 ==== Use the LTTng-UST Java agent for Apache log4j
4389 To use the LTTng-UST Java agent in a Java application which uses
4390 Apache log4j{nbsp}1.2:
4392 . In the Java application's source code, import the LTTng-UST
4393 log appender package for Apache log4j:
4398 import org.lttng.ust.agent.log4j.LttngLogAppender;
4402 . Create an LTTng-UST log4j log appender:
4407 Appender lttngUstLogAppender = new LttngLogAppender();
4411 . Add this appender to the log4j loggers which should emit LTTng events:
4416 Logger myLogger = Logger.getLogger("some-logger");
4418 myLogger.addAppender(lttngUstLogAppender);
4422 . Use Apache log4j log statements and configuration as usual. The
4423 loggers with an attached LTTng-UST log appender can emit LTTng events.
4425 . Before exiting the application, remove the LTTng-UST log appender from
4426 the loggers attached to it and call its `close()` method:
4431 myLogger.removeAppender(lttngUstLogAppender);
4432 lttngUstLogAppender.close();
4436 This is not strictly necessary, but it is recommended for a clean
4437 disposal of the appender's resources.
4439 . Include the LTTng-UST Java agent's common and log4j-specific JAR
4440 files, path:{lttng-ust-agent-common.jar} and
4441 path:{lttng-ust-agent-log4j.jar}, in the
4442 https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
4443 path] when you build the Java application.
4445 The JAR files are typically located in dir:{/usr/share/java}.
4447 IMPORTANT: The LTTng-UST Java agent must be
4448 <<installing-lttng,installed>> for the logging framework your
4451 .Use the LTTng-UST Java agent for Apache log4j.
4456 import org.apache.log4j.Appender;
4457 import org.apache.log4j.Logger;
4458 import org.lttng.ust.agent.log4j.LttngLogAppender;
4462 private static final int answer = 42;
4464 public static void main(String[] argv) throws Exception
4467 Logger logger = Logger.getLogger("jello");
4469 // Create an LTTng-UST log appender
4470 Appender lttngUstLogAppender = new LttngLogAppender();
4472 // Add the LTTng-UST log appender to our logger
4473 logger.addAppender(lttngUstLogAppender);
4476 logger.info("some info");
4477 logger.warn("some warning");
4479 logger.debug("debug information; the answer is " + answer);
4481 logger.fatal("error!");
4483 // Not mandatory, but cleaner
4484 logger.removeAppender(lttngUstLogAppender);
4485 lttngUstLogAppender.close();
4491 Build this example (`$LOG4JPATH` is the path to the Apache log4j JAR
4496 $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java
4499 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4500 <<enabling-disabling-events,create an event rule>> matching the
4501 `jello` log4j logger, and <<basic-tracing-session-control,start tracing>>:
4506 $ lttng enable-event --log4j jello
4510 Run the compiled class:
4514 $ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH:. Test
4517 <<basic-tracing-session-control,Stop tracing>> and inspect the
4527 In the resulting trace, an <<event,event record>> generated by a Java
4528 application using log4j is named `lttng_log4j:event` and
4529 has the following fields:
4532 Log record's message.
4538 Name of the class in which the log statement was executed.
4541 Name of the method in which the log statement was executed.
4544 Name of the file in which the executed log statement is located.
4547 Line number at which the log statement was executed.
4553 Log level integer value.
4556 Name of the Java thread in which the log statement was executed.
4558 You can use the opt:lttng-enable-event(1):--loglevel or
4559 opt:lttng-enable-event(1):--loglevel-only option of the
4560 man:lttng-enable-event(1) command to target a range of Apache log4j log levels
4561 or a specific log4j log level.
4565 [[java-application-context]]
4566 ==== Provide application-specific context fields in a Java application
4568 A Java application-specific context field is a piece of state provided
4569 by the application which <<adding-context,you can add>>, using the
4570 man:lttng-add-context(1) command, to each <<event,event record>>
4571 produced by the log statements of this application.
4573 For example, a given object might have a current request ID variable.
4574 You can create a context information retriever for this object and
4575 assign a name to this current request ID. You can then, using the
4576 man:lttng-add-context(1) command, add this context field by name to
4577 the JUL or log4j <<channel,channel>>.
4579 To provide application-specific context fields in a Java application:
4581 . In the Java application's source code, import the LTTng-UST
4582 Java agent context classes and interfaces:
4587 import org.lttng.ust.agent.context.ContextInfoManager;
4588 import org.lttng.ust.agent.context.IContextInfoRetriever;
4592 . Create a context information retriever class, that is, a class which
4593 implements the `IContextInfoRetriever` interface:
4598 class MyContextInfoRetriever implements IContextInfoRetriever
4601 public Object retrieveContextInfo(String key)
4603 if (key.equals("intCtx")) {
4605 } else if (key.equals("strContext")) {
4606 return "context value!";
4615 This `retrieveContextInfo()` method is the only member of the
4616 `IContextInfoRetriever` interface. Its role is to return the current
4617 value of a state by name to create a context field. The names of the
4618 context fields and which state variables they return depends on your
4621 All primitive types and objects are supported as context fields.
4622 When `retrieveContextInfo()` returns an object, the context field
4623 serializer calls its `toString()` method to add a string field to
4624 event records. The method can also return `null`, which means that
4625 no context field is available for the required name.
4627 . Register an instance of your context information retriever class to
4628 the context information manager singleton:
4633 IContextInfoRetriever cir = new MyContextInfoRetriever();
4634 ContextInfoManager cim = ContextInfoManager.getInstance();
4635 cim.registerContextInfoRetriever("retrieverName", cir);
4639 . Before exiting the application, remove your context information
4640 retriever from the context information manager singleton:
4645 ContextInfoManager cim = ContextInfoManager.getInstance();
4646 cim.unregisterContextInfoRetriever("retrieverName");
4650 This is not strictly necessary, but it is recommended for a clean
4651 disposal of some manager's resources.
4653 . Build your Java application with LTTng-UST Java agent support as
4654 usual, following the procedure for either the <<jul,JUL>> or
4655 <<log4j,Apache log4j>> framework.
4658 .Provide application-specific context fields in a Java application.
4663 import java.util.logging.Handler;
4664 import java.util.logging.Logger;
4665 import org.lttng.ust.agent.jul.LttngLogHandler;
4666 import org.lttng.ust.agent.context.ContextInfoManager;
4667 import org.lttng.ust.agent.context.IContextInfoRetriever;
4671 // Our context information retriever class
4672 private static class MyContextInfoRetriever
4673 implements IContextInfoRetriever
4676 public Object retrieveContextInfo(String key) {
4677 if (key.equals("intCtx")) {
4679 } else if (key.equals("strContext")) {
4680 return "context value!";
4687 private static final int answer = 42;
4689 public static void main(String args[]) throws Exception
4691 // Get the context information manager instance
4692 ContextInfoManager cim = ContextInfoManager.getInstance();
4694 // Create and register our context information retriever
4695 IContextInfoRetriever cir = new MyContextInfoRetriever();
4696 cim.registerContextInfoRetriever("myRetriever", cir);
4699 Logger logger = Logger.getLogger("jello");
4701 // Create an LTTng-UST log handler
4702 Handler lttngUstLogHandler = new LttngLogHandler();
4704 // Add the LTTng-UST log handler to our logger
4705 logger.addHandler(lttngUstLogHandler);
4708 logger.info("some info");
4709 logger.warning("some warning");
4711 logger.finer("finer information; the answer is " + answer);
4713 logger.severe("error!");
4715 // Not mandatory, but cleaner
4716 logger.removeHandler(lttngUstLogHandler);
4717 lttngUstLogHandler.close();
4718 cim.unregisterContextInfoRetriever("myRetriever");
4727 $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
4730 <<creating-destroying-tracing-sessions,Create a tracing session>>
4731 and <<enabling-disabling-events,create an event rule>> matching the
4737 $ lttng enable-event --jul jello
4740 <<adding-context,Add the application-specific context fields>> to the
4745 $ lttng add-context --jul --type='$app.myRetriever:intCtx'
4746 $ lttng add-context --jul --type='$app.myRetriever:strContext'
4749 <<basic-tracing-session-control,Start tracing>>:
4756 Run the compiled class:
4760 $ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
4763 <<basic-tracing-session-control,Stop tracing>> and inspect the
4775 [[python-application]]
4776 === User space Python agent
4778 You can instrument a Python{nbsp}2 or Python{nbsp}3 application which
4780 https://docs.python.org/3/library/logging.html[`logging`] package.
4782 Each log statement emits an LTTng event once the
4783 application module imports the
4784 <<lttng-ust-agents,LTTng-UST Python agent>> package.
4787 .A Python application importing the LTTng-UST Python agent.
4788 image::python-app.png[]
4790 To use the LTTng-UST Python agent:
4792 . In the Python application's source code, import the LTTng-UST Python
4802 The LTTng-UST Python agent automatically adds its logging handler to the
4803 root logger at import time.
4805 Any log statement that the application executes before this import does
4806 not emit an LTTng event.
4808 IMPORTANT: The LTTng-UST Python agent must be
4809 <<installing-lttng,installed>>.
4811 . Use log statements and logging configuration as usual.
4812 Since the LTTng-UST Python agent adds a handler to the _root_
4813 logger, you can trace any log statement from any logger.
4815 .Use the LTTng-UST Python agent.
4826 logging.basicConfig()
4827 logger = logging.getLogger('my-logger')
4830 logger.debug('debug message')
4831 logger.info('info message')
4832 logger.warn('warn message')
4833 logger.error('error message')
4834 logger.critical('critical message')
4838 if __name__ == '__main__':
4842 NOTE: `logging.basicConfig()`, which adds to the root logger a basic
4843 logging handler which prints to the standard error stream, is not
4844 strictly required for LTTng-UST tracing to work, but in versions of
4845 Python preceding{nbsp}3.2, you could see a warning message which indicates
4846 that no handler exists for the logger `my-logger`.
4848 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4849 <<enabling-disabling-events,create an event rule>> matching the
4850 `my-logger` Python logger, and <<basic-tracing-session-control,start
4856 $ lttng enable-event --python my-logger
4860 Run the Python script:
4867 <<basic-tracing-session-control,Stop tracing>> and inspect the recorded
4877 In the resulting trace, an <<event,event record>> generated by a Python
4878 application is named `lttng_python:event` and has the following fields:
4881 Logging time (string).
4884 Log record's message.
4890 Name of the function in which the log statement was executed.
4893 Line number at which the log statement was executed.
4896 Log level integer value.
4899 ID of the Python thread in which the log statement was executed.
4902 Name of the Python thread in which the log statement was executed.
4904 You can use the opt:lttng-enable-event(1):--loglevel or
4905 opt:lttng-enable-event(1):--loglevel-only option of the
4906 man:lttng-enable-event(1) command to target a range of Python log levels
4907 or a specific Python log level.
4909 When an application imports the LTTng-UST Python agent, the agent tries
4910 to register to a <<lttng-sessiond,session daemon>>. Note that you must
4911 <<start-sessiond,start the session daemon>> _before_ you run the Python
4912 application. If a session daemon is found, the agent tries to register
4913 to it during five seconds, after which the application continues
4914 without LTTng tracing support. You can override this timeout value with
4915 the env:LTTNG_UST_PYTHON_REGISTER_TIMEOUT environment variable
4918 If the session daemon stops while a Python application with an imported
4919 LTTng-UST Python agent runs, the agent retries to connect and to
4920 register to a session daemon every three seconds. You can override this
4921 delay with the env:LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment
4926 [[proc-lttng-logger-abi]]
4929 The `lttng-tracer` Linux kernel module, part of
4930 <<lttng-modules,LTTng-modules>>, creates the special LTTng logger file
4931 path:{/proc/lttng-logger} when it's loaded. Any application can write
4932 text data to this file to emit an LTTng event.
4935 .An application writes to the LTTng logger file to emit an LTTng event.
4936 image::lttng-logger.png[]
4938 The LTTng logger is the quickest method--not the most efficient,
4939 however--to add instrumentation to an application. It is designed
4940 mostly to instrument shell scripts:
4944 $ echo "Some message, some $variable" > /proc/lttng-logger
4947 Any event that the LTTng logger emits is named `lttng_logger` and
4948 belongs to the Linux kernel <<domain,tracing domain>>. However, unlike
4949 other instrumentation points in the kernel tracing domain, **any Unix
4950 user** can <<enabling-disabling-events,create an event rule>> which
4951 matches its event name, not only the root user or users in the
4952 <<tracing-group,tracing group>>.
4954 To use the LTTng logger:
4956 * From any application, write text data to the path:{/proc/lttng-logger}
4959 The `msg` field of `lttng_logger` event records contains the
4962 NOTE: The maximum message length of an LTTng logger event is
4963 1024{nbsp}bytes. Writing more than this makes the LTTng logger emit more
4964 than one event to contain the remaining data.
4966 You should not use the LTTng logger to trace a user application which
4967 can be instrumented in a more efficient way, namely:
4969 * <<c-application,C and $$C++$$ applications>>.
4970 * <<java-application,Java applications>>.
4971 * <<python-application,Python applications>>.
4973 .Use the LTTng logger.
4978 echo 'Hello, World!' > /proc/lttng-logger
4980 df --human-readable --print-type / > /proc/lttng-logger
4983 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4984 <<enabling-disabling-events,create an event rule>> matching the
4985 `lttng_logger` Linux kernel tracepoint, and
4986 <<basic-tracing-session-control,start tracing>>:
4991 $ lttng enable-event --kernel lttng_logger
4995 Run the Bash script:
5002 <<basic-tracing-session-control,Stop tracing>> and inspect the recorded
5013 [[instrumenting-linux-kernel]]
5014 === LTTng kernel tracepoints
5016 NOTE: This section shows how to _add_ instrumentation points to the
5017 Linux kernel. The kernel's subsystems are already thoroughly
5018 instrumented at strategic places for LTTng when you
5019 <<installing-lttng,install>> the <<lttng-modules,LTTng-modules>>
5023 There are two methods to instrument the Linux kernel:
5025 . <<linux-add-lttng-layer,Add an LTTng layer>> over an existing ftrace
5026 tracepoint which uses the `TRACE_EVENT()` API.
5028 Choose this if you want to instrumentation a Linux kernel tree with an
5029 instrumentation point compatible with ftrace, perf, and SystemTap.
5031 . Use an <<linux-lttng-tracepoint-event,LTTng-only approach>> to
5032 instrument an out-of-tree kernel module.
5034 Choose this if you don't need ftrace, perf, or SystemTap support.
5038 [[linux-add-lttng-layer]]
5039 ==== [[instrumenting-linux-kernel-itself]][[mainline-trace-event]][[lttng-adaptation-layer]]Add an LTTng layer to an existing ftrace tracepoint
5041 This section shows how to add an LTTng layer to existing ftrace
5042 instrumentation using the `TRACE_EVENT()` API.
5044 This section does not document the `TRACE_EVENT()` macro. You can
5045 read the following articles to learn more about this API:
5047 * http://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part{nbsp}1)]
5048 * http://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part{nbsp}2)]
5049 * http://lwn.net/Articles/383362/[Using the TRACE_EVENT() macro (Part{nbsp}3)]
5051 The following procedure assumes that your ftrace tracepoints are
5052 correctly defined in their own header and that they are created in
5053 one source file using the `CREATE_TRACE_POINTS` definition.
5055 To add an LTTng layer over an existing ftrace tracepoint:
5057 . Make sure the following kernel configuration options are
5063 * `CONFIG_HIGH_RES_TIMERS`
5064 * `CONFIG_TRACEPOINTS`
5067 . Build the Linux source tree with your custom ftrace tracepoints.
5068 . Boot the resulting Linux image on your target system.
5070 Confirm that the tracepoints exist by looking for their names in the
5071 dir:{/sys/kernel/debug/tracing/events/subsys} directory, where `subsys`
5072 is your subsystem's name.
5074 . Get a copy of the latest LTTng-modules{nbsp}{revision}:
5079 $ cd $(mktemp -d) &&
5080 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.11.tar.bz2 &&
5081 tar -xf lttng-modules-latest-2.11.tar.bz2 &&
5082 cd lttng-modules-2.11.*
5086 . In dir:{instrumentation/events/lttng-module}, relative to the root
5087 of the LTTng-modules source tree, create a header file named
5088 +__subsys__.h+ for your custom subsystem +__subsys__+ and write your
5089 LTTng-modules tracepoint definitions using the LTTng-modules
5092 Start with this template:
5096 .path:{instrumentation/events/lttng-module/my_subsys.h}
5099 #define TRACE_SYSTEM my_subsys
5101 #if !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ)
5102 #define _LTTNG_MY_SUBSYS_H
5104 #include "../../../probes/lttng-tracepoint-event.h"
5105 #include <linux/tracepoint.h>
5107 LTTNG_TRACEPOINT_EVENT(
5109 * Format is identical to TRACE_EVENT()'s version for the three
5110 * following macro parameters:
5113 TP_PROTO(int my_int, const char *my_string),
5114 TP_ARGS(my_int, my_string),
5116 /* LTTng-modules specific macros */
5118 ctf_integer(int, my_int_field, my_int)
5119 ctf_string(my_bar_field, my_bar)
5123 #endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */
5125 #include "../../../probes/define_trace.h"
5129 The entries in the `TP_FIELDS()` section are the list of fields for the
5130 LTTng tracepoint. This is similar to the `TP_STRUCT__entry()` part of
5131 ftrace's `TRACE_EVENT()` macro.
5133 See <<lttng-modules-tp-fields,Tracepoint fields macros>> for a
5134 complete description of the available `ctf_*()` macros.
5136 . Create the LTTng-modules probe's kernel module C source file,
5137 +probes/lttng-probe-__subsys__.c+, where +__subsys__+ is your
5142 .path:{probes/lttng-probe-my-subsys.c}
5144 #include <linux/module.h>
5145 #include "../lttng-tracer.h"
5148 * Build-time verification of mismatch between mainline
5149 * TRACE_EVENT() arguments and the LTTng-modules adaptation
5150 * layer LTTNG_TRACEPOINT_EVENT() arguments.
5152 #include <trace/events/my_subsys.h>
5154 /* Create LTTng tracepoint probes */
5155 #define LTTNG_PACKAGE_BUILD
5156 #define CREATE_TRACE_POINTS
5157 #define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module
5159 #include "../instrumentation/events/lttng-module/my_subsys.h"
5161 MODULE_LICENSE("GPL and additional rights");
5162 MODULE_AUTHOR("Your name <your-email>");
5163 MODULE_DESCRIPTION("LTTng my_subsys probes");
5164 MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "."
5165 __stringify(LTTNG_MODULES_MINOR_VERSION) "."
5166 __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION)
5167 LTTNG_MODULES_EXTRAVERSION);
5171 . Edit path:{probes/KBuild} and add your new kernel module object
5172 next to the existing ones:
5176 .path:{probes/KBuild}
5180 obj-m += lttng-probe-module.o
5181 obj-m += lttng-probe-power.o
5183 obj-m += lttng-probe-my-subsys.o
5189 . Build and install the LTTng kernel modules:
5194 $ make KERNELDIR=/path/to/linux
5195 # make modules_install && depmod -a
5199 Replace `/path/to/linux` with the path to the Linux source tree where
5200 you defined and used tracepoints with ftrace's `TRACE_EVENT()` macro.
5202 Note that you can also use the
5203 <<lttng-tracepoint-event-code,`LTTNG_TRACEPOINT_EVENT_CODE()` macro>>
5204 instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and
5205 C code that need to be executed before the event fields are recorded.
5207 The best way to learn how to use the previous LTTng-modules macros is to
5208 inspect the existing LTTng-modules tracepoint definitions in the
5209 dir:{instrumentation/events/lttng-module} header files. Compare them
5210 with the Linux kernel mainline versions in the
5211 dir:{include/trace/events} directory of the Linux source tree.
5215 [[lttng-tracepoint-event-code]]
5216 ===== Use custom C code to access the data for tracepoint fields
5218 Although we recommended to always use the
5219 <<lttng-adaptation-layer,`LTTNG_TRACEPOINT_EVENT()`>> macro to describe
5220 the arguments and fields of an LTTng-modules tracepoint when possible,
5221 sometimes you need a more complex process to access the data that the
5222 tracer records as event record fields. In other words, you need local
5223 variables and multiple C{nbsp}statements instead of simple
5224 argument-based expressions that you pass to the
5225 <<lttng-modules-tp-fields,`ctf_*()` macros of `TP_FIELDS()`>>.
5227 You can use the `LTTNG_TRACEPOINT_EVENT_CODE()` macro instead of
5228 `LTTNG_TRACEPOINT_EVENT()` to declare custom local variables and define
5229 a block of C{nbsp}code to be executed before LTTng records the fields.
5230 The structure of this macro is:
5233 .`LTTNG_TRACEPOINT_EVENT_CODE()` macro syntax.
5235 LTTNG_TRACEPOINT_EVENT_CODE(
5237 * Format identical to the LTTNG_TRACEPOINT_EVENT()
5238 * version for the following three macro parameters:
5241 TP_PROTO(int my_int, const char *my_string),
5242 TP_ARGS(my_int, my_string),
5244 /* Declarations of custom local variables */
5247 unsigned long b = 0;
5248 const char *name = "(undefined)";
5249 struct my_struct *my_struct;
5253 * Custom code which uses both tracepoint arguments
5254 * (in TP_ARGS()) and local variables (in TP_locvar()).
5256 * Local variables are actually members of a structure pointed
5257 * to by the special variable tp_locvar.
5261 tp_locvar->a = my_int + 17;
5262 tp_locvar->my_struct = get_my_struct_at(tp_locvar->a);
5263 tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct);
5264 tp_locvar->name = my_struct_get_name(tp_locvar->my_struct);
5265 put_my_struct(tp_locvar->my_struct);
5274 * Format identical to the LTTNG_TRACEPOINT_EVENT()
5275 * version for this, except that tp_locvar members can be
5276 * used in the argument expression parameters of
5277 * the ctf_*() macros.
5280 ctf_integer(unsigned long, my_struct_b, tp_locvar->b)
5281 ctf_integer(int, my_struct_a, tp_locvar->a)
5282 ctf_string(my_string_field, my_string)
5283 ctf_string(my_struct_name, tp_locvar->name)
5288 IMPORTANT: The C code defined in `TP_code()` must not have any side
5289 effects when executed. In particular, the code must not allocate
5290 memory or get resources without deallocating this memory or putting
5291 those resources afterwards.
5294 [[instrumenting-linux-kernel-tracing]]
5295 ==== Load and unload a custom probe kernel module
5297 You must load a <<lttng-adaptation-layer,created LTTng-modules probe
5298 kernel module>> in the kernel before it can emit LTTng events.
5300 To load the default probe kernel modules and a custom probe kernel
5303 * Use the opt:lttng-sessiond(8):--extra-kmod-probes option to give extra
5304 probe modules to load when starting a root <<lttng-sessiond,session
5308 .Load the `my_subsys`, `usb`, and the default probe modules.
5312 # lttng-sessiond --extra-kmod-probes=my_subsys,usb
5317 You only need to pass the subsystem name, not the whole kernel module
5320 To load _only_ a given custom probe kernel module:
5322 * Use the opt:lttng-sessiond(8):--kmod-probes option to give the probe
5323 modules to load when starting a root session daemon:
5326 .Load only the `my_subsys` and `usb` probe modules.
5330 # lttng-sessiond --kmod-probes=my_subsys,usb
5335 To confirm that a probe module is loaded:
5342 $ lsmod | grep lttng_probe_usb
5346 To unload the loaded probe modules:
5348 * Kill the session daemon with `SIGTERM`:
5353 # pkill lttng-sessiond
5357 You can also use man:modprobe(8)'s `--remove` option if the session
5358 daemon terminates abnormally.
5361 [[controlling-tracing]]
5364 Once an application or a Linux kernel is
5365 <<instrumenting,instrumented>> for LTTng tracing,
5368 This section is divided in topics on how to use the various
5369 <<plumbing,components of LTTng>>, in particular the <<lttng-cli,cmd:lttng
5370 command-line tool>>, to _control_ the LTTng daemons and tracers.
5372 NOTE: In the following subsections, we refer to an man:lttng(1) command
5373 using its man page name. For example, instead of _Run the `create`
5374 command to..._, we use _Run the man:lttng-create(1) command to..._.
5378 === Start a session daemon
5380 In some situations, you need to run a <<lttng-sessiond,session daemon>>
5381 (man:lttng-sessiond(8)) _before_ you can use the man:lttng(1)
5384 You will see the following error when you run a command while no session
5388 Error: No session daemon is available
5391 The only command that automatically runs a session daemon is
5392 man:lttng-create(1), which you use to
5393 <<creating-destroying-tracing-sessions,create a tracing session>>. While
5394 this is most of the time the first operation that you do, sometimes it's
5395 not. Some examples are:
5397 * <<list-instrumentation-points,List the available instrumentation points>>.
5398 * <<saving-loading-tracing-session,Load a tracing session configuration>>.
5400 [[tracing-group]] Each Unix user must have its own running session
5401 daemon to trace user applications. The session daemon that the root user
5402 starts is the only one allowed to control the LTTng kernel tracer. Users
5403 that are part of the _tracing group_ can control the root session
5404 daemon. The default tracing group name is `tracing`; you can set it to
5405 something else with the opt:lttng-sessiond(8):--group option when you
5406 start the root session daemon.
5408 To start a user session daemon:
5410 * Run man:lttng-sessiond(8):
5415 $ lttng-sessiond --daemonize
5419 To start the root session daemon:
5421 * Run man:lttng-sessiond(8) as the root user:
5426 # lttng-sessiond --daemonize
5430 In both cases, remove the opt:lttng-sessiond(8):--daemonize option to
5431 start the session daemon in foreground.
5433 To stop a session daemon, use man:kill(1) on its process ID (standard
5436 Note that some Linux distributions could manage the LTTng session daemon
5437 as a service. In this case, you should use the service manager to
5438 start, restart, and stop session daemons.
5441 [[creating-destroying-tracing-sessions]]
5442 === Create and destroy a tracing session
5444 Almost all the LTTng control operations happen in the scope of
5445 a <<tracing-session,tracing session>>, which is the dialogue between the
5446 <<lttng-sessiond,session daemon>> and you.
5448 To create a tracing session with a generated name:
5450 * Use the man:lttng-create(1) command:
5459 The created tracing session's name is `auto` followed by the
5462 To create a tracing session with a specific name:
5464 * Use the optional argument of the man:lttng-create(1) command:
5469 $ lttng create my-session
5473 Replace `my-session` with the specific tracing session name.
5475 LTTng appends the creation date to the created tracing session's name.
5477 LTTng writes the traces of a tracing session in
5478 +$LTTNG_HOME/lttng-trace/__name__+ by default, where +__name__+ is the
5479 name of the tracing session. Note that the env:LTTNG_HOME environment
5480 variable defaults to `$HOME` if not set.
5482 To output LTTng traces to a non-default location:
5484 * Use the opt:lttng-create(1):--output option of the man:lttng-create(1) command:
5489 $ lttng create my-session --output=/tmp/some-directory
5493 You may create as many tracing sessions as you wish.
5495 To list all the existing tracing sessions for your Unix user:
5497 * Use the man:lttng-list(1) command:
5506 When you create a tracing session, it is set as the _current tracing
5507 session_. The following man:lttng(1) commands operate on the current
5508 tracing session when you don't specify one:
5510 [role="list-3-cols"]
5511 * man:lttng-add-context(1)
5512 * man:lttng-destroy(1)
5513 * man:lttng-disable-channel(1)
5514 * man:lttng-disable-event(1)
5515 * man:lttng-disable-rotation(1)
5516 * man:lttng-enable-channel(1)
5517 * man:lttng-enable-event(1)
5518 * man:lttng-enable-rotation(1)
5520 * man:lttng-regenerate(1)
5521 * man:lttng-rotate(1)
5523 * man:lttng-snapshot(1)
5524 * man:lttng-start(1)
5525 * man:lttng-status(1)
5527 * man:lttng-track(1)
5528 * man:lttng-untrack(1)
5531 To change the current tracing session:
5533 * Use the man:lttng-set-session(1) command:
5538 $ lttng set-session new-session
5542 Replace `new-session` by the name of the new current tracing session.
5544 When you are done tracing in a given tracing session, you can destroy
5545 it. This operation frees the resources taken by the tracing session
5546 to destroy; it does not destroy the trace data that LTTng wrote for
5547 this tracing session.
5549 To destroy the current tracing session:
5551 * Use the man:lttng-destroy(1) command:
5561 [[list-instrumentation-points]]
5562 === List the available instrumentation points
5564 The <<lttng-sessiond,session daemon>> can query the running instrumented
5565 user applications and the Linux kernel to get a list of available
5566 instrumentation points. For the Linux kernel <<domain,tracing domain>>,
5567 they are tracepoints and system calls. For the user space tracing
5568 domain, they are tracepoints. For the other tracing domains, they are
5571 To list the available instrumentation points:
5573 * Use the man:lttng-list(1) command with the requested tracing domain's
5577 * opt:lttng-list(1):--kernel: Linux kernel tracepoints (your Unix user
5578 must be a root user, or it must be a member of the
5579 <<tracing-group,tracing group>>).
5580 * opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall: Linux
5581 kernel system calls (your Unix user must be a root user, or it must be
5582 a member of the tracing group).
5583 * opt:lttng-list(1):--userspace: user space tracepoints.
5584 * opt:lttng-list(1):--jul: `java.util.logging` loggers.
5585 * opt:lttng-list(1):--log4j: Apache log4j loggers.
5586 * opt:lttng-list(1):--python: Python loggers.
5589 .List the available user space tracepoints.
5593 $ lttng list --userspace
5597 .List the available Linux kernel system call tracepoints.
5601 $ lttng list --kernel --syscall
5606 [[enabling-disabling-events]]
5607 === Create and enable an event rule
5609 Once you <<creating-destroying-tracing-sessions,create a tracing
5610 session>>, you can create <<event,event rules>> with the
5611 man:lttng-enable-event(1) command.
5613 You specify each condition with a command-line option. The available
5614 condition arguments are shown in the following table.
5616 [role="growable",cols="asciidoc,asciidoc,default"]
5617 .Condition command-line arguments for the man:lttng-enable-event(1) command.
5619 |Argument |Description |Applicable tracing domains
5625 . +--probe=__ADDR__+
5626 . +--function=__ADDR__+
5627 . +--userspace-probe=__PATH__:__SYMBOL__+
5628 . +--userspace-probe=sdt:__PATH__:__PROVIDER__:__NAME__+
5631 Instead of using the default _tracepoint_ instrumentation type, use:
5633 . A Linux system call (entry and exit).
5634 . A Linux https://lwn.net/Articles/132196/[kprobe] (symbol or address).
5635 . The entry and return points of a Linux function (symbol or address).
5636 . The entry point of a user application or library function (path to
5637 application/library and symbol).
5638 . A https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[SystemTap
5639 Statically Defined Tracing] (SDT) probe (path to application/library,
5640 provider and probe names).
5644 |First positional argument.
5647 Tracepoint or system call name.
5649 With the opt:lttng-enable-event(1):--probe,
5650 opt:lttng-enable-event(1):--function, and
5651 opt:lttng-enable-event(1):--userspace-probe options, this is a custom
5652 name given to the event rule. With the JUL, log4j, and Python domains,
5653 this is a logger name.
5655 With a tracepoint, logger, or system call name, you can use the special
5656 `*` globbing character to match anything (for example, `sched_*`,
5664 . +--loglevel=__LEVEL__+
5665 . +--loglevel-only=__LEVEL__+
5668 . Match only tracepoints or log statements with a logging level at
5669 least as severe as +__LEVEL__+.
5670 . Match only tracepoints or log statements with a logging level
5671 equal to +__LEVEL__+.
5673 See man:lttng-enable-event(1) for the list of available logging level
5676 |User space, JUL, log4j, and Python.
5678 |+--exclude=__EXCLUSIONS__+
5681 When you use a `*` character at the end of the tracepoint or logger
5682 name (first positional argument), exclude the specific names in the
5683 comma-delimited list +__EXCLUSIONS__+.
5686 User space, JUL, log4j, and Python.
5688 |+--filter=__EXPR__+
5691 Match only events which satisfy the expression +__EXPR__+.
5693 See man:lttng-enable-event(1) to learn more about the syntax of a
5700 You attach an event rule to a <<channel,channel>> on creation. If you do
5701 not specify the channel with the opt:lttng-enable-event(1):--channel
5702 option, and if the event rule to create is the first in its
5703 <<domain,tracing domain>> for a given tracing session, then LTTng
5704 creates a _default channel_ for you. This default channel is reused in
5705 subsequent invocations of the man:lttng-enable-event(1) command for the
5706 same tracing domain.
5708 An event rule is always enabled at creation time.
5710 The following examples show how you can combine the previous
5711 command-line options to create simple to more complex event rules.
5713 .Create an event rule targetting a Linux kernel tracepoint (default channel).
5717 $ lttng enable-event --kernel sched_switch
5721 .Create an event rule matching four Linux kernel system calls (default channel).
5725 $ lttng enable-event --kernel --syscall open,write,read,close
5729 .Create event rules matching tracepoints with filter expressions (default channel).
5733 $ lttng enable-event --kernel sched_switch --filter='prev_comm == "bash"'
5738 $ lttng enable-event --kernel --all \
5739 --filter='$ctx.tid == 1988 || $ctx.tid == 1534'
5744 $ lttng enable-event --jul my_logger \
5745 --filter='$app.retriever:cur_msg_id > 3'
5748 IMPORTANT: Make sure to always quote the filter string when you
5749 use man:lttng(1) from a shell.
5752 .Create an event rule matching any user space tracepoint of a given tracepoint provider with a log level range (default channel).
5756 $ lttng enable-event --userspace my_app:'*' --loglevel=TRACE_INFO
5759 IMPORTANT: Make sure to always quote the wildcard character when you
5760 use man:lttng(1) from a shell.
5763 .Create an event rule matching multiple Python loggers with a wildcard and with exclusions (default channel).
5767 $ lttng enable-event --python my-app.'*' \
5768 --exclude='my-app.module,my-app.hello'
5772 .Create an event rule matching any Apache log4j logger with a specific log level (default channel).
5776 $ lttng enable-event --log4j --all --loglevel-only=LOG4J_WARN
5780 .Create an event rule attached to a specific channel matching a specific user space tracepoint provider and tracepoint.
5784 $ lttng enable-event --userspace my_app:my_tracepoint --channel=my-channel
5788 .Create an event rule matching the `malloc` function entry in path:{/usr/lib/libc.so.6}:
5792 $ lttng enable-event --kernel --userspace-probe=/usr/lib/libc.so.6:malloc \
5797 .Create an event rule matching the `server`/`accept_request` https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[SDT probe] in path:{/usr/bin/serv}:
5801 $ lttng enable-event --kernel --userspace-probe=sdt:serv:server:accept_request \
5802 server_accept_request
5806 The event rules of a given channel form a whitelist: as soon as an
5807 emitted event passes one of them, LTTng can record the event. For
5808 example, an event named `my_app:my_tracepoint` emitted from a user space
5809 tracepoint with a `TRACE_ERROR` log level passes both of the following
5814 $ lttng enable-event --userspace my_app:my_tracepoint
5815 $ lttng enable-event --userspace my_app:my_tracepoint \
5816 --loglevel=TRACE_INFO
5819 The second event rule is redundant: the first one includes
5823 [[disable-event-rule]]
5824 === Disable an event rule
5826 To disable an event rule that you <<enabling-disabling-events,created>>
5827 previously, use the man:lttng-disable-event(1) command. This command
5828 disables _all_ the event rules (of a given tracing domain and channel)
5829 which match an instrumentation point. The other conditions are not
5830 supported as of LTTng{nbsp}{revision}.
5832 The LTTng tracer does not record an emitted event which passes
5833 a _disabled_ event rule.
5835 .Disable an event rule matching a Python logger (default channel).
5839 $ lttng disable-event --python my-logger
5843 .Disable an event rule matching all `java.util.logging` loggers (default channel).
5847 $ lttng disable-event --jul '*'
5851 .Disable _all_ the event rules of the default channel.
5853 The opt:lttng-disable-event(1):--all-events option is not, like the
5854 opt:lttng-enable-event(1):--all option of man:lttng-enable-event(1), the
5855 equivalent of the event name `*` (wildcard): it disables _all_ the event
5856 rules of a given channel.
5860 $ lttng disable-event --jul --all-events
5864 NOTE: You cannot delete an event rule once you create it.
5868 === Get the status of a tracing session
5870 To get the status of the current tracing session, that is, its
5871 parameters, its channels, event rules, and their attributes:
5873 * Use the man:lttng-status(1) command:
5883 To get the status of any tracing session:
5885 * Use the man:lttng-list(1) command with the tracing session's name:
5890 $ lttng list my-session
5894 Replace `my-session` with the desired tracing session's name.
5897 [[basic-tracing-session-control]]
5898 === Start and stop a tracing session
5900 Once you <<creating-destroying-tracing-sessions,create a tracing
5902 <<enabling-disabling-events,create one or more event rules>>,
5903 you can start and stop the tracers for this tracing session.
5905 To start tracing in the current tracing session:
5907 * Use the man:lttng-start(1) command:
5916 LTTng is very flexible: you can launch user applications before
5917 or after the you start the tracers. The tracers only record the events
5918 if they pass enabled event rules and if they occur while the tracers are
5921 To stop tracing in the current tracing session:
5923 * Use the man:lttng-stop(1) command:
5932 If there were <<channel-overwrite-mode-vs-discard-mode,lost event
5933 records>> or lost sub-buffers since the last time you ran
5934 man:lttng-start(1), warnings are printed when you run the
5935 man:lttng-stop(1) command.
5937 IMPORTANT: You need to stop tracing to make LTTng flush the remaining
5938 trace data and make the trace readable.
5941 [[enabling-disabling-channels]]
5942 === Create a channel
5944 Once you create a tracing session, you can create a <<channel,channel>>
5945 with the man:lttng-enable-channel(1) command.
5947 Note that LTTng automatically creates a default channel when, for a
5948 given <<domain,tracing domain>>, no channels exist and you
5949 <<enabling-disabling-events,create>> the first event rule. This default
5950 channel is named `channel0` and its attributes are set to reasonable
5951 values. Therefore, you only need to create a channel when you need
5952 non-default attributes.
5954 You specify each non-default channel attribute with a command-line
5955 option when you use the man:lttng-enable-channel(1) command. The
5956 available command-line options are:
5958 [role="growable",cols="asciidoc,asciidoc"]
5959 .Command-line options for the man:lttng-enable-channel(1) command.
5961 |Option |Description
5967 <<channel-overwrite-mode-vs-discard-mode,event record loss mode>> instead
5968 of the default _discard_ mode.
5970 |`--buffers-pid` (user space tracing domain only)
5973 Use the per-process <<channel-buffering-schemes,buffering scheme>>
5974 instead of the default per-user buffering scheme.
5976 |+--subbuf-size=__SIZE__+
5979 Allocate sub-buffers of +__SIZE__+ bytes (power of two), for each CPU,
5980 either for each Unix user (default), or for each instrumented process.
5982 See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
5984 |+--num-subbuf=__COUNT__+
5987 Allocate +__COUNT__+ sub-buffers (power of two), for each CPU, either
5988 for each Unix user (default), or for each instrumented process.
5990 See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
5992 |+--tracefile-size=__SIZE__+
5995 Set the maximum size of each trace file that this channel writes within
5996 a stream to +__SIZE__+ bytes instead of no maximum.
5998 See <<tracefile-rotation,Trace file count and size>>.
6000 |+--tracefile-count=__COUNT__+
6003 Limit the number of trace files that this channel creates to
6004 +__COUNT__+ channels instead of no limit.
6006 See <<tracefile-rotation,Trace file count and size>>.
6008 |+--switch-timer=__PERIODUS__+
6011 Set the <<channel-switch-timer,switch timer period>>
6012 to +__PERIODUS__+{nbsp}µs.
6014 |+--read-timer=__PERIODUS__+
6017 Set the <<channel-read-timer,read timer period>>
6018 to +__PERIODUS__+{nbsp}µs.
6020 |[[opt-blocking-timeout]]+--blocking-timeout=__TIMEOUTUS__+
6023 Set the timeout of user space applications which load LTTng-UST
6024 in blocking mode to +__TIMEOUTUS__+:
6027 Never block (non-blocking mode).
6030 Block forever until space is available in a sub-buffer to record
6033 __n__, a positive value::
6034 Wait for at most __n__ µs when trying to write into a sub-buffer.
6036 Note that, for this option to have any effect on an instrumented
6037 user space application, you need to run the application with a set
6038 env:LTTNG_UST_ALLOW_BLOCKING environment variable.
6040 |+--output=__TYPE__+ (Linux kernel tracing domain only)
6043 Set the channel's output type to +__TYPE__+, either `mmap` or `splice`.
6047 You can only create a channel in the Linux kernel and user space
6048 <<domain,tracing domains>>: other tracing domains have their own channel
6049 created on the fly when <<enabling-disabling-events,creating event
6054 Because of a current LTTng limitation, you must create all channels
6055 _before_ you <<basic-tracing-session-control,start tracing>> in a given
6056 tracing session, that is, before the first time you run
6059 Since LTTng automatically creates a default channel when you use the
6060 man:lttng-enable-event(1) command with a specific tracing domain, you
6061 cannot, for example, create a Linux kernel event rule, start tracing,
6062 and then create a user space event rule, because no user space channel
6063 exists yet and it's too late to create one.
6065 For this reason, make sure to configure your channels properly
6066 before starting the tracers for the first time!
6069 The following examples show how you can combine the previous
6070 command-line options to create simple to more complex channels.
6072 .Create a Linux kernel channel with default attributes.
6076 $ lttng enable-channel --kernel my-channel
6080 .Create a user space channel with four sub-buffers or 1{nbsp}MiB each, per CPU, per instrumented process.
6084 $ lttng enable-channel --userspace --num-subbuf=4 --subbuf-size=1M \
6085 --buffers-pid my-channel
6089 .[[blocking-timeout-example]]Create a default user space channel with an infinite blocking timeout.
6091 <<creating-destroying-tracing-sessions,Create a tracing-session>>,
6092 create the channel, <<enabling-disabling-events,create an event rule>>,
6093 and <<basic-tracing-session-control,start tracing>>:
6098 $ lttng enable-channel --userspace --blocking-timeout=inf blocking-channel
6099 $ lttng enable-event --userspace --channel=blocking-channel --all
6103 Run an application instrumented with LTTng-UST and allow it to block:
6107 $ LTTNG_UST_ALLOW_BLOCKING=1 my-app
6111 .Create a Linux kernel channel which rotates eight trace files of 4{nbsp}MiB each for each stream
6115 $ lttng enable-channel --kernel --tracefile-count=8 \
6116 --tracefile-size=4194304 my-channel
6120 .Create a user space channel in overwrite (or _flight recorder_) mode.
6124 $ lttng enable-channel --userspace --overwrite my-channel
6128 You can <<enabling-disabling-events,create>> the same event rule in
6129 two different channels:
6133 $ lttng enable-event --userspace --channel=my-channel app:tp
6134 $ lttng enable-event --userspace --channel=other-channel app:tp
6137 If both channels are enabled, when a tracepoint named `app:tp` is
6138 reached, LTTng records two events, one for each channel.
6142 === Disable a channel
6144 To disable a specific channel that you <<enabling-disabling-channels,created>>
6145 previously, use the man:lttng-disable-channel(1) command.
6147 .Disable a specific Linux kernel channel.
6151 $ lttng disable-channel --kernel my-channel
6155 The state of a channel precedes the individual states of event rules
6156 attached to it: event rules which belong to a disabled channel, even if
6157 they are enabled, are also considered disabled.
6161 === Add context fields to a channel
6163 Event record fields in trace files provide important information about
6164 events that occured previously, but sometimes some external context may
6165 help you solve a problem faster. Examples of context fields are:
6167 * The **process ID**, **thread ID**, **process name**, and
6168 **process priority** of the thread in which the event occurs.
6169 * The **hostname** of the system on which the event occurs.
6170 * The Linux kernel and user call stacks (since
6171 LTTng{nbsp}{revision}).
6172 * The current values of many possible **performance counters** using
6174 ** CPU cycles, stalled cycles, idle cycles, and the other cycle types.
6176 ** Branch instructions, misses, and loads.
6178 * Any context defined at the application level (supported for the
6179 JUL and log4j <<domain,tracing domains>>).
6181 To get the full list of available context fields, see
6182 `lttng add-context --list`. Some context fields are reserved for a
6183 specific <<domain,tracing domain>> (Linux kernel or user space).
6185 You add context fields to <<channel,channels>>. All the events
6186 that a channel with added context fields records contain those fields.
6188 To add context fields to one or all the channels of a given tracing
6191 * Use the man:lttng-add-context(1) command.
6193 .Add context fields to all the channels of the current tracing session.
6195 The following command line adds the virtual process identifier and
6196 the per-thread CPU cycles count fields to all the user space channels
6197 of the current tracing session.
6201 $ lttng add-context --userspace --type=vpid --type=perf:thread:cpu-cycles
6205 .Add performance counter context fields by raw ID
6207 See man:lttng-add-context(1) for the exact format of the context field
6208 type, which is partly compatible with the format used in
6213 $ lttng add-context --userspace --type=perf:thread:raw:r0110:test
6214 $ lttng add-context --kernel --type=perf:cpu:raw:r0013c:x86unhalted
6218 .Add context fields to a specific channel.
6220 The following command line adds the thread identifier and user call
6221 stack context fields to the Linux kernel channel named `my-channel` in
6222 the current tracing session.
6226 $ lttng add-context --kernel --channel=my-channel \
6227 --type=tid --type=callstack-user
6231 .Add an application-specific context field to a specific channel.
6233 The following command line adds the `cur_msg_id` context field of the
6234 `retriever` context retriever for all the instrumented
6235 <<java-application,Java applications>> recording <<event,event records>>
6236 in the channel named `my-channel`:
6240 $ lttng add-context --kernel --channel=my-channel \
6241 --type='$app:retriever:cur_msg_id'
6244 IMPORTANT: Make sure to always quote the `$` character when you
6245 use man:lttng-add-context(1) from a shell.
6248 NOTE: You cannot remove context fields from a channel once you add it.
6253 === Track process IDs
6255 It's often useful to allow only specific process IDs (PIDs) to emit
6256 events. For example, you may wish to record all the system calls made by
6257 a given process (Ă la http://linux.die.net/man/1/strace[strace]).
6259 The man:lttng-track(1) and man:lttng-untrack(1) commands serve this
6260 purpose. Both commands operate on a whitelist of process IDs. You _add_
6261 entries to this whitelist with the man:lttng-track(1) command and remove
6262 entries with the man:lttng-untrack(1) command. Any process which has one
6263 of the PIDs in the whitelist is allowed to emit LTTng events which pass
6264 an enabled <<event,event rule>>.
6266 NOTE: The PID tracker tracks the _numeric process IDs_. Should a
6267 process with a given tracked ID exit and another process be given this
6268 ID, then the latter would also be allowed to emit events.
6270 .Track and untrack process IDs.
6272 For the sake of the following example, assume the target system has
6273 16{nbsp}possible PIDs.
6276 <<creating-destroying-tracing-sessions,create a tracing session>>,
6277 the whitelist contains all the possible PIDs:
6280 .All PIDs are tracked.
6281 image::track-all.png[]
6283 When the whitelist is full and you use the man:lttng-track(1) command to
6284 specify some PIDs to track, LTTng first clears the whitelist, then it
6285 tracks the specific PIDs. After:
6289 $ lttng track --pid=3,4,7,10,13
6295 .PIDs 3, 4, 7, 10, and 13 are tracked.
6296 image::track-3-4-7-10-13.png[]
6298 You can add more PIDs to the whitelist afterwards:
6302 $ lttng track --pid=1,15,16
6308 .PIDs 1, 15, and 16 are added to the whitelist.
6309 image::track-1-3-4-7-10-13-15-16.png[]
6311 The man:lttng-untrack(1) command removes entries from the PID tracker's
6312 whitelist. Given the previous example, the following command:
6316 $ lttng untrack --pid=3,7,10,13
6319 leads to this whitelist:
6322 .PIDs 3, 7, 10, and 13 are removed from the whitelist.
6323 image::track-1-4-15-16.png[]
6325 LTTng can track all possible PIDs again using the
6326 opt:lttng-track(1):--all option:
6330 $ lttng track --pid --all
6333 The result is, again:
6336 .All PIDs are tracked.
6337 image::track-all.png[]
6340 .Track only specific PIDs
6342 A very typical use case with PID tracking is to start with an empty
6343 whitelist, then <<basic-tracing-session-control,start the tracers>>, and
6344 then add PIDs manually while tracers are active. You can accomplish this
6345 by using the opt:lttng-untrack(1):--all option of the
6346 man:lttng-untrack(1) command to clear the whitelist after you
6347 <<creating-destroying-tracing-sessions,create a tracing session>>:
6351 $ lttng untrack --pid --all
6357 .No PIDs are tracked.
6358 image::untrack-all.png[]
6360 If you trace with this whitelist configuration, the tracer records no
6361 events for this <<domain,tracing domain>> because no processes are
6362 tracked. You can use the man:lttng-track(1) command as usual to track
6363 specific PIDs, for example:
6367 $ lttng track --pid=6,11
6373 .PIDs 6 and 11 are tracked.
6374 image::track-6-11.png[]
6379 [[saving-loading-tracing-session]]
6380 === Save and load tracing session configurations
6382 Configuring a <<tracing-session,tracing session>> can be long. Some of
6383 the tasks involved are:
6385 * <<enabling-disabling-channels,Create channels>> with
6386 specific attributes.
6387 * <<adding-context,Add context fields>> to specific channels.
6388 * <<enabling-disabling-events,Create event rules>> with specific log
6389 level and filter conditions.
6391 If you use LTTng to solve real world problems, chances are you have to
6392 record events using the same tracing session setup over and over,
6393 modifying a few variables each time in your instrumented program
6394 or environment. To avoid constant tracing session reconfiguration,
6395 the man:lttng(1) command-line tool can save and load tracing session
6396 configurations to/from XML files.
6398 To save a given tracing session configuration:
6400 * Use the man:lttng-save(1) command:
6405 $ lttng save my-session
6409 Replace `my-session` with the name of the tracing session to save.
6411 LTTng saves tracing session configurations to
6412 dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the
6413 env:LTTNG_HOME environment variable defaults to `$HOME` if not set. Use
6414 the opt:lttng-save(1):--output-path option to change this destination
6417 LTTng saves all configuration parameters, for example:
6419 * The tracing session name.
6420 * The trace data output path.
6421 * The channels with their state and all their attributes.
6422 * The context fields you added to channels.
6423 * The event rules with their state, log level and filter conditions.
6425 To load a tracing session:
6427 * Use the man:lttng-load(1) command:
6432 $ lttng load my-session
6436 Replace `my-session` with the name of the tracing session to load.
6438 When LTTng loads a configuration, it restores your saved tracing session
6439 as if you just configured it manually.
6441 See man:lttng(1) for the complete list of command-line options. You
6442 can also save and load all many sessions at a time, and decide in which
6443 directory to output the XML files.
6446 [[sending-trace-data-over-the-network]]
6447 === Send trace data over the network
6449 LTTng can send the recorded trace data to a remote system over the
6450 network instead of writing it to the local file system.
6452 To send the trace data over the network:
6454 . On the _remote_ system (which can also be the target system),
6455 start an LTTng <<lttng-relayd,relay daemon>> (man:lttng-relayd(8)):
6464 . On the _target_ system, create a tracing session configured to
6465 send trace data over the network:
6470 $ lttng create my-session --set-url=net://remote-system
6474 Replace `remote-system` by the host name or IP address of the
6475 remote system. See man:lttng-create(1) for the exact URL format.
6477 . On the target system, use the man:lttng(1) command-line tool as usual.
6478 When tracing is active, the target's consumer daemon sends sub-buffers
6479 to the relay daemon running on the remote system instead of flushing
6480 them to the local file system. The relay daemon writes the received
6481 packets to the local file system.
6483 The relay daemon writes trace files to
6484 +$LTTNG_HOME/lttng-traces/__hostname__/__session__+ by default, where
6485 +__hostname__+ is the host name of the target system and +__session__+
6486 is the tracing session name. Note that the env:LTTNG_HOME environment
6487 variable defaults to `$HOME` if not set. Use the
6488 opt:lttng-relayd(8):--output option of man:lttng-relayd(8) to write
6489 trace files to another base directory.
6494 === View events as LTTng emits them (noch:{LTTng} live)
6496 LTTng live is a network protocol implemented by the <<lttng-relayd,relay
6497 daemon>> (man:lttng-relayd(8)) to allow compatible trace viewers to
6498 display events as LTTng emits them on the target system while tracing is
6501 The relay daemon creates a _tee_: it forwards the trace data to both
6502 the local file system and to connected live viewers:
6505 .The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live viewer.
6510 . On the _target system_, create a <<tracing-session,tracing session>>
6516 $ lttng create my-session --live
6520 This spawns a local relay daemon.
6522 . Start the live viewer and configure it to connect to the relay
6523 daemon. For example, with http://diamon.org/babeltrace[Babeltrace]:
6528 $ babeltrace --input-format=lttng-live \
6529 net://localhost/host/hostname/my-session
6536 * `hostname` with the host name of the target system.
6537 * `my-session` with the name of the tracing session to view.
6540 . Configure the tracing session as usual with the man:lttng(1)
6541 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6543 You can list the available live tracing sessions with Babeltrace:
6547 $ babeltrace --input-format=lttng-live net://localhost
6550 You can start the relay daemon on another system. In this case, you need
6551 to specify the relay daemon's URL when you create the tracing session
6552 with the opt:lttng-create(1):--set-url option. You also need to replace
6553 `localhost` in the procedure above with the host name of the system on
6554 which the relay daemon is running.
6556 See man:lttng-create(1) and man:lttng-relayd(8) for the complete list of
6557 command-line options.
6561 [[taking-a-snapshot]]
6562 === Take a snapshot of the current sub-buffers of a tracing session
6564 The normal behavior of LTTng is to append full sub-buffers to growing
6565 trace data files. This is ideal to keep a full history of the events
6566 that occurred on the target system, but it can
6567 represent too much data in some situations. For example, you may wish
6568 to trace your application continuously until some critical situation
6569 happens, in which case you only need the latest few recorded
6570 events to perform the desired analysis, not multi-gigabyte trace files.
6572 With the man:lttng-snapshot(1) command, you can take a snapshot of the
6573 current sub-buffers of a given <<tracing-session,tracing session>>.
6574 LTTng can write the snapshot to the local file system or send it over
6578 .A snapshot is a copy of the current sub-buffers, which are not cleared after the operation.
6579 image::snapshot.png[]
6581 If you wish to create unmanaged, self-contained, non-overlapping
6582 trace chunk archives instead of a simple copy of the current
6583 sub-buffers, see the <<session-rotation,tracing session rotation>>
6584 feature (available since LTTng{nbsp}2.11).
6588 . Create a tracing session in _snapshot mode_:
6593 $ lttng create my-session --snapshot
6597 The <<channel-overwrite-mode-vs-discard-mode,event record loss mode>> of
6598 <<channel,channels>> created in this mode is automatically set to
6599 _overwrite_ (flight recorder mode).
6601 . Configure the tracing session as usual with the man:lttng(1)
6602 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6604 . **Optional**: When you need to take a snapshot,
6605 <<basic-tracing-session-control,stop tracing>>.
6607 You can take a snapshot when the tracers are active, but if you stop
6608 them first, you are sure that the data in the sub-buffers does not
6609 change before you actually take the snapshot.
6616 $ lttng snapshot record --name=my-first-snapshot
6620 LTTng writes the current sub-buffers of all the current tracing
6621 session's channels to trace files on the local file system. Those trace
6622 files have `my-first-snapshot` in their name.
6624 There is no difference between the format of a normal trace file and the
6625 format of a snapshot: viewers of LTTng traces also support LTTng
6628 By default, LTTng writes snapshot files to the path shown by
6629 `lttng snapshot list-output`. You can change this path or decide to send
6630 snapshots over the network using either:
6632 . An output path or URL that you specify when you
6633 <<creating-destroying-tracing-sessions,create the tracing session>>.
6634 . A snapshot output path or URL that you add using
6635 `lttng snapshot add-output`.
6636 . An output path or URL that you provide directly to the
6637 `lttng snapshot record` command.
6639 Method{nbsp}3 overrides method{nbsp}2, which overrides method 1. When
6640 you specify a URL, a relay daemon must listen on a remote system (see
6641 <<sending-trace-data-over-the-network,Send trace data over the
6646 [[session-rotation]]
6647 === Archive the current trace chunk (rotate a tracing session)
6649 The <<taking-a-snapshot,snapshot user guide>> shows how you can dump
6650 a tracing session's current sub-buffers to the file system or send them
6651 over the network. When you take a snapshot, LTTng does not clear the
6652 tracing session's ring buffers: if you take another snapshot immediately
6653 after, both snapshots could contain overlapping trace data.
6655 Inspired by https://en.wikipedia.org/wiki/Log_rotation[log rotation],
6656 _tracing session rotation_ is a feature which appends the content of the
6657 ring buffers to what's already on the file system or sent over the
6658 network since the tracing session's creation or since the last
6659 rotation, and then clears those ring buffers to avoid trace data
6662 What LTTng is about to write when performing a tracing session rotation
6663 is called the _current trace chunk_. When this current trace chunk is
6664 written to the file system or sent over the network, it is called a
6665 _trace chunk archive_. Therefore, a tracing session rotation _archives_
6666 the current trace chunk.
6669 .A tracing session rotation operation _archives_ the current trace chunk.
6670 image::rotation.png[]
6672 A trace chunk archive is a self-contained LTTng trace and is not managed
6673 anymore by LTTng: you can read it, modify it, move it, or remove it.
6675 There are two methods to perform a tracing session rotation:
6676 immediately or automatically.
6678 To perform an immediate tracing session rotation:
6680 . <<creating-destroying-tracing-sessions,Create a tracing session>>
6681 in _normal mode_ or _network streaming mode_
6682 (only those two creation modes support tracing session rotation):
6687 $ lttng create my-session
6691 . <<enabling-disabling-events,Create one or more event rules>>
6692 and <<basic-tracing-session-control,start tracing>>:
6697 $ lttng enable-event --kernel sched_'*'
6702 . When needed, immediately rotate the current tracing session:
6711 The cmd:lttng-rotate command prints the path to the created trace
6712 chunk archive. See man:lttng-rotate(1) to learn about the format
6713 of trace chunk archive directory names.
6715 You can perform other immediate rotations while the tracing session is
6716 active. It is guaranteed that all the trace chunk archives do not
6717 contain overlapping trace data. You can also perform an immediate
6718 rotation once the tracing session is
6719 <<basic-tracing-session-control,stopped>>.
6721 . When you are done tracing,
6722 <<creating-destroying-tracing-sessions,destroy the current tracing
6732 The tracing session destruction operation creates one last trace
6733 chunk archive from the current trace chunk.
6735 An automatic tracing session rotation is a rotation which LTTng
6736 performs automatically based on one of the following conditions:
6738 * A timer with a configured period times out.
6739 * The total size of the flushed part of the current trace chunk
6740 becomes greater than or equal to a configured value.
6742 To configure a future, automatic tracing session rotation, you need
6743 to set a _rotation schedule_.
6745 To set a rotation schedule:
6747 . <<creating-destroying-tracing-sessions,Create a tracing session>>
6748 in _normal mode_ or _network streaming mode_
6749 (only those two creation modes support tracing session rotation):
6754 $ lttng create my-session
6758 . <<enabling-disabling-events,Create one or more event rules>>:
6763 $ lttng enable-event --kernel sched_'*'
6767 . Set a tracing session rotation schedule:
6772 $ lttng enable-rotation --timer=12s
6776 In this example, we set a rotation schedule so that LTTng performs a
6777 tracing session rotation every 12{nbsp}seconds.
6779 See man:lttng-enable-rotation(1) to learn more about other ways to set a
6782 . <<basic-tracing-session-control,Start tracing>>:
6791 LTTng performs tracing session rotations automatically while the tracing
6792 session is active thanks to the rotation schedule.
6794 . When you are done tracing,
6795 <<creating-destroying-tracing-sessions,destroy the current tracing
6805 The tracing session destruction operation creates one last trace chunk
6806 archive from the current trace chunk.
6808 You can use man:lttng-disable-rotation(1) to unset an a tracing session
6811 NOTE: man:lttng-rotate(1) and man:lttng-enable-rotation(1) list
6812 limitations regarding those two commands.
6817 === Use the machine interface
6819 With any command of the man:lttng(1) command-line tool, you can set the
6820 opt:lttng(1):--mi option to `xml` (before the command name) to get an
6821 XML machine interface output, for example:
6825 $ lttng --mi=xml enable-event --kernel --syscall open
6828 A schema definition (XSD) is
6829 https://github.com/lttng/lttng-tools/blob/stable-2.11/src/common/mi-lttng-3.0.xsd[available]
6830 to ease the integration with external tools as much as possible.
6834 [[metadata-regenerate]]
6835 === Regenerate the metadata of an LTTng trace
6837 An LTTng trace, which is a http://diamon.org/ctf[CTF] trace, has both
6838 data stream files and a metadata file. This metadata file contains,
6839 amongst other things, information about the offset of the clock sources
6840 used to timestamp <<event,event records>> when tracing.
6842 If, once a <<tracing-session,tracing session>> is
6843 <<basic-tracing-session-control,started>>, a major
6844 https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction
6845 happens, the trace's clock offset also needs to be updated. You
6846 can use the `metadata` item of the man:lttng-regenerate(1) command
6849 The main use case of this command is to allow a system to boot with
6850 an incorrect wall time and trace it with LTTng before its wall time
6851 is corrected. Once the system is known to be in a state where its
6852 wall time is correct, it can run `lttng regenerate metadata`.
6854 To regenerate the metadata of an LTTng trace:
6856 * Use the `metadata` item of the man:lttng-regenerate(1) command:
6861 $ lttng regenerate metadata
6867 `lttng regenerate metadata` has the following limitations:
6869 * Tracing session <<creating-destroying-tracing-sessions,created>>
6871 * User space <<channel,channels>>, if any, are using
6872 <<channel-buffering-schemes,per-user buffering>>.
6877 [[regenerate-statedump]]
6878 === Regenerate the state dump of a tracing session
6880 The LTTng kernel and user space tracers generate state dump
6881 <<event,event records>> when the application starts or when you
6882 <<basic-tracing-session-control,start a tracing session>>. An analysis
6883 can use the state dump event records to set an initial state before it
6884 builds the rest of the state from the following event records.
6885 http://tracecompass.org/[Trace Compass] is a notable example of an
6886 application which uses the state dump of an LTTng trace.
6888 When you <<taking-a-snapshot,take a snapshot>>, it's possible that the
6889 state dump event records are not included in the snapshot because they
6890 were recorded to a sub-buffer that has been consumed or overwritten
6893 You can use the `lttng regenerate statedump` command to emit the state
6894 dump event records again.
6896 To regenerate the state dump of the current tracing session, provided
6897 create it in snapshot mode, before you take a snapshot:
6899 . Use the `statedump` item of the man:lttng-regenerate(1) command:
6904 $ lttng regenerate statedump
6908 . <<basic-tracing-session-control,Stop the tracing session>>:
6917 . <<taking-a-snapshot,Take a snapshot>>:
6922 $ lttng snapshot record --name=my-snapshot
6926 Depending on the event throughput, you should run steps 1 and 2
6927 as closely as possible.
6929 NOTE: To record the state dump events, you need to
6930 <<enabling-disabling-events,create event rules>> which enable them.
6931 LTTng-UST state dump tracepoints start with `lttng_ust_statedump:`.
6932 LTTng-modules state dump tracepoints start with `lttng_statedump_`.
6936 [[persistent-memory-file-systems]]
6937 === Record trace data on persistent memory file systems
6939 https://en.wikipedia.org/wiki/Non-volatile_random-access_memory[Non-volatile random-access memory]
6940 (NVRAM) is random-access memory that retains its information when power
6941 is turned off (non-volatile). Systems with such memory can store data
6942 structures in RAM and retrieve them after a reboot, without flushing
6943 to typical _storage_.
6945 Linux supports NVRAM file systems thanks to either
6946 http://pramfs.sourceforge.net/[PRAMFS] or
6947 https://www.kernel.org/doc/Documentation/filesystems/dax.txt[DAX]{nbsp}+{nbsp}http://lkml.iu.edu/hypermail/linux/kernel/1504.1/03463.html[pmem]
6948 (requires Linux{nbsp}4.1+).
6950 This section does not describe how to operate such file systems;
6951 we assume that you have a working persistent memory file system.
6953 When you create a <<tracing-session,tracing session>>, you can specify
6954 the path of the shared memory holding the sub-buffers. If you specify a
6955 location on an NVRAM file system, then you can retrieve the latest
6956 recorded trace data when the system reboots after a crash.
6958 To record trace data on a persistent memory file system and retrieve the
6959 trace data after a system crash:
6961 . Create a tracing session with a sub-buffer shared memory path located
6962 on an NVRAM file system:
6967 $ lttng create my-session --shm-path=/path/to/shm
6971 . Configure the tracing session as usual with the man:lttng(1)
6972 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6974 . After a system crash, use the man:lttng-crash(1) command-line tool to
6975 view the trace data recorded on the NVRAM file system:
6980 $ lttng-crash /path/to/shm
6984 The binary layout of the ring buffer files is not exactly the same as
6985 the trace files layout. This is why you need to use man:lttng-crash(1)
6986 instead of your preferred trace viewer directly.
6988 To convert the ring buffer files to LTTng trace files:
6990 * Use the opt:lttng-crash(1):--extract option of man:lttng-crash(1):
6995 $ lttng-crash --extract=/path/to/trace /path/to/shm
7001 [[notif-trigger-api]]
7002 === Get notified when a channel's buffer usage is too high or too low
7004 With LTTng's $$C/C++$$ notification and trigger API, your user
7005 application can get notified when the buffer usage of one or more
7006 <<channel,channels>> becomes too low or too high. You can use this API
7007 and enable or disable <<event,event rules>> during tracing to avoid
7008 <<channel-overwrite-mode-vs-discard-mode,discarded event records>>.
7010 .Have a user application get notified when an LTTng channel's buffer usage is too high.
7012 In this example, we create and build an application which gets notified
7013 when the buffer usage of a specific LTTng channel is higher than
7014 75{nbsp}%. We only print that it is the case in the example, but we
7015 could as well use the API of <<liblttng-ctl-lttng,`liblttng-ctl`>> to
7016 disable event rules when this happens.
7018 . Create the application's C source file:
7026 #include <lttng/domain.h>
7027 #include <lttng/action/action.h>
7028 #include <lttng/action/notify.h>
7029 #include <lttng/condition/condition.h>
7030 #include <lttng/condition/buffer-usage.h>
7031 #include <lttng/condition/evaluation.h>
7032 #include <lttng/notification/channel.h>
7033 #include <lttng/notification/notification.h>
7034 #include <lttng/trigger/trigger.h>
7035 #include <lttng/endpoint.h>
7037 int main(int argc, char *argv[])
7039 int exit_status = 0;
7040 struct lttng_notification_channel *notification_channel;
7041 struct lttng_condition *condition;
7042 struct lttng_action *action;
7043 struct lttng_trigger *trigger;
7044 const char *tracing_session_name;
7045 const char *channel_name;
7048 tracing_session_name = argv[1];
7049 channel_name = argv[2];
7052 * Create a notification channel. A notification channel
7053 * connects the user application to the LTTng session daemon.
7054 * This notification channel can be used to listen to various
7055 * types of notifications.
7057 notification_channel = lttng_notification_channel_create(
7058 lttng_session_daemon_notification_endpoint);
7061 * Create a "high buffer usage" condition. In this case, the
7062 * condition is reached when the buffer usage is greater than or
7063 * equal to 75 %. We create the condition for a specific tracing
7064 * session name, channel name, and for the user space tracing
7067 * The "low buffer usage" condition type also exists.
7069 condition = lttng_condition_buffer_usage_high_create();
7070 lttng_condition_buffer_usage_set_threshold_ratio(condition, .75);
7071 lttng_condition_buffer_usage_set_session_name(
7072 condition, tracing_session_name);
7073 lttng_condition_buffer_usage_set_channel_name(condition,
7075 lttng_condition_buffer_usage_set_domain_type(condition,
7079 * Create an action (get a notification) to take when the
7080 * condition created above is reached.
7082 action = lttng_action_notify_create();
7085 * Create a trigger. A trigger associates a condition to an
7086 * action: the action is executed when the condition is reached.
7088 trigger = lttng_trigger_create(condition, action);
7090 /* Register the trigger to LTTng. */
7091 lttng_register_trigger(trigger);
7094 * Now that we have registered a trigger, a notification will be
7095 * emitted everytime its condition is met. To receive this
7096 * notification, we must subscribe to notifications that match
7097 * the same condition.
7099 lttng_notification_channel_subscribe(notification_channel,
7103 * Notification loop. You can put this in a dedicated thread to
7104 * avoid blocking the main thread.
7107 struct lttng_notification *notification;
7108 enum lttng_notification_channel_status status;
7109 const struct lttng_evaluation *notification_evaluation;
7110 const struct lttng_condition *notification_condition;
7111 double buffer_usage;
7113 /* Receive the next notification. */
7114 status = lttng_notification_channel_get_next_notification(
7115 notification_channel, ¬ification);
7118 case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK:
7120 case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED:
7122 * The session daemon can drop notifications if
7123 * a monitoring application is not consuming the
7124 * notifications fast enough.
7127 case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED:
7129 * The notification channel has been closed by the
7130 * session daemon. This is typically caused by a session
7131 * daemon shutting down.
7135 /* Unhandled conditions or errors. */
7141 * A notification provides, amongst other things:
7143 * * The condition that caused this notification to be
7145 * * The condition evaluation, which provides more
7146 * specific information on the evaluation of the
7149 * The condition evaluation provides the buffer usage
7150 * value at the moment the condition was reached.
7152 notification_condition = lttng_notification_get_condition(
7154 notification_evaluation = lttng_notification_get_evaluation(
7157 /* We're subscribed to only one condition. */
7158 assert(lttng_condition_get_type(notification_condition) ==
7159 LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH);
7162 * Get the exact sampled buffer usage from the
7163 * condition evaluation.
7165 lttng_evaluation_buffer_usage_get_usage_ratio(
7166 notification_evaluation, &buffer_usage);
7169 * At this point, instead of printing a message, we
7170 * could do something to reduce the channel's buffer
7171 * usage, like disable specific events.
7173 printf("Buffer usage is %f %% in tracing session \"%s\", "
7174 "user space channel \"%s\".\n", buffer_usage * 100,
7175 tracing_session_name, channel_name);
7176 lttng_notification_destroy(notification);
7180 lttng_action_destroy(action);
7181 lttng_condition_destroy(condition);
7182 lttng_trigger_destroy(trigger);
7183 lttng_notification_channel_destroy(notification_channel);
7189 . Build the `notif-app` application, linking it to `liblttng-ctl`:
7194 $ gcc -o notif-app notif-app.c -llttng-ctl
7198 . <<creating-destroying-tracing-sessions,Create a tracing session>>,
7199 <<enabling-disabling-events,create an event rule>> matching all the
7200 user space tracepoints, and
7201 <<basic-tracing-session-control,start tracing>>:
7206 $ lttng create my-session
7207 $ lttng enable-event --userspace --all
7212 If you create the channel manually with the man:lttng-enable-channel(1)
7213 command, you can control how frequently are the current values of the
7214 channel's properties sampled to evaluate user conditions with the
7215 opt:lttng-enable-channel(1):--monitor-timer option.
7217 . Run the `notif-app` application. This program accepts the
7218 <<tracing-session,tracing session>> name and the user space channel
7219 name as its two first arguments. The channel which LTTng automatically
7220 creates with the man:lttng-enable-event(1) command above is named
7226 $ ./notif-app my-session channel0
7230 . In another terminal, run an application with a very high event
7231 throughput so that the 75{nbsp}% buffer usage condition is reached.
7233 In the first terminal, the application should print lines like this:
7236 Buffer usage is 81.45197 % in tracing session "my-session", user space
7240 If you don't see anything, try modifying the condition in
7241 path:{notif-app.c} to a lower value (0.1, for example), rebuilding it
7242 (step{nbsp}2) and running it again (step{nbsp}4).
7249 [[lttng-modules-ref]]
7250 === noch:{LTTng-modules}
7254 [[lttng-tracepoint-enum]]
7255 ==== `LTTNG_TRACEPOINT_ENUM()` usage
7257 Use the `LTTNG_TRACEPOINT_ENUM()` macro to define an enumeration:
7261 LTTNG_TRACEPOINT_ENUM(name, TP_ENUM_VALUES(entries))
7266 * `name` with the name of the enumeration (C identifier, unique
7267 amongst all the defined enumerations).
7268 * `entries` with a list of enumeration entries.
7270 The available enumeration entry macros are:
7272 +ctf_enum_value(__name__, __value__)+::
7273 Entry named +__name__+ mapped to the integral value +__value__+.
7275 +ctf_enum_range(__name__, __begin__, __end__)+::
7276 Entry named +__name__+ mapped to the range of integral values between
7277 +__begin__+ (included) and +__end__+ (included).
7279 +ctf_enum_auto(__name__)+::
7280 Entry named +__name__+ mapped to the integral value following the
7281 last mapping's value.
7283 The last value of a `ctf_enum_value()` entry is its +__value__+
7286 The last value of a `ctf_enum_range()` entry is its +__end__+ parameter.
7288 If `ctf_enum_auto()` is the first entry in the list, its integral
7291 Use the `ctf_enum()` <<lttng-modules-tp-fields,field definition macro>>
7292 to use a defined enumeration as a tracepoint field.
7294 .Define an enumeration with `LTTNG_TRACEPOINT_ENUM()`.
7298 LTTNG_TRACEPOINT_ENUM(
7301 ctf_enum_auto("AUTO: EXPECT 0")
7302 ctf_enum_value("VALUE: 23", 23)
7303 ctf_enum_value("VALUE: 27", 27)
7304 ctf_enum_auto("AUTO: EXPECT 28")
7305 ctf_enum_range("RANGE: 101 TO 303", 101, 303)
7306 ctf_enum_auto("AUTO: EXPECT 304")
7314 [[lttng-modules-tp-fields]]
7315 ==== Tracepoint fields macros (for `TP_FIELDS()`)
7317 [[tp-fast-assign]][[tp-struct-entry]]The available macros to define
7318 tracepoint fields, which must be listed within `TP_FIELDS()` in
7319 `LTTNG_TRACEPOINT_EVENT()`, are:
7321 [role="func-desc growable",cols="asciidoc,asciidoc"]
7322 .Available macros to define LTTng-modules tracepoint fields
7324 |Macro |Description and parameters
7327 +ctf_integer(__t__, __n__, __e__)+
7329 +ctf_integer_nowrite(__t__, __n__, __e__)+
7331 +ctf_user_integer(__t__, __n__, __e__)+
7333 +ctf_user_integer_nowrite(__t__, __n__, __e__)+
7335 Standard integer, displayed in base{nbsp}10.
7338 Integer C type (`int`, `long`, `size_t`, ...).
7344 Argument expression.
7347 +ctf_integer_hex(__t__, __n__, __e__)+
7349 +ctf_user_integer_hex(__t__, __n__, __e__)+
7351 Standard integer, displayed in base{nbsp}16.
7360 Argument expression.
7362 |+ctf_integer_oct(__t__, __n__, __e__)+
7364 Standard integer, displayed in base{nbsp}8.
7373 Argument expression.
7376 +ctf_integer_network(__t__, __n__, __e__)+
7378 +ctf_user_integer_network(__t__, __n__, __e__)+
7380 Integer in network byte order (big-endian), displayed in base{nbsp}10.
7389 Argument expression.
7392 +ctf_integer_network_hex(__t__, __n__, __e__)+
7394 +ctf_user_integer_network_hex(__t__, __n__, __e__)+
7396 Integer in network byte order, displayed in base{nbsp}16.
7405 Argument expression.
7408 +ctf_enum(__N__, __t__, __n__, __e__)+
7410 +ctf_enum_nowrite(__N__, __t__, __n__, __e__)+
7412 +ctf_user_enum(__N__, __t__, __n__, __e__)+
7414 +ctf_user_enum_nowrite(__N__, __t__, __n__, __e__)+
7419 Name of a <<lttng-tracepoint-enum,previously defined enumeration>>.
7422 Integer C type (`int`, `long`, `size_t`, ...).
7428 Argument expression.
7431 +ctf_string(__n__, __e__)+
7433 +ctf_string_nowrite(__n__, __e__)+
7435 +ctf_user_string(__n__, __e__)+
7437 +ctf_user_string_nowrite(__n__, __e__)+
7439 Null-terminated string; undefined behavior if +__e__+ is `NULL`.
7445 Argument expression.
7448 +ctf_array(__t__, __n__, __e__, __s__)+
7450 +ctf_array_nowrite(__t__, __n__, __e__, __s__)+
7452 +ctf_user_array(__t__, __n__, __e__, __s__)+
7454 +ctf_user_array_nowrite(__t__, __n__, __e__, __s__)+
7456 Statically-sized array of integers.
7459 Array element C type.
7465 Argument expression.
7471 +ctf_array_bitfield(__t__, __n__, __e__, __s__)+
7473 +ctf_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
7475 +ctf_user_array_bitfield(__t__, __n__, __e__, __s__)+
7477 +ctf_user_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
7479 Statically-sized array of bits.
7481 The type of +__e__+ must be an integer type. +__s__+ is the number
7482 of elements of such type in +__e__+, not the number of bits.
7485 Array element C type.
7491 Argument expression.
7497 +ctf_array_text(__t__, __n__, __e__, __s__)+
7499 +ctf_array_text_nowrite(__t__, __n__, __e__, __s__)+
7501 +ctf_user_array_text(__t__, __n__, __e__, __s__)+
7503 +ctf_user_array_text_nowrite(__t__, __n__, __e__, __s__)+
7505 Statically-sized array, printed as text.
7507 The string does not need to be null-terminated.
7510 Array element C type (always `char`).
7516 Argument expression.
7522 +ctf_sequence(__t__, __n__, __e__, __T__, __E__)+
7524 +ctf_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
7526 +ctf_user_sequence(__t__, __n__, __e__, __T__, __E__)+
7528 +ctf_user_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
7530 Dynamically-sized array of integers.
7532 The type of +__E__+ must be unsigned.
7535 Array element C type.
7541 Argument expression.
7544 Length expression C type.
7550 +ctf_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
7552 +ctf_user_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
7554 Dynamically-sized array of integers, displayed in base{nbsp}16.
7556 The type of +__E__+ must be unsigned.
7559 Array element C type.
7565 Argument expression.
7568 Length expression C type.
7573 |+ctf_sequence_network(__t__, __n__, __e__, __T__, __E__)+
7575 Dynamically-sized array of integers in network byte order (big-endian),
7576 displayed in base{nbsp}10.
7578 The type of +__E__+ must be unsigned.
7581 Array element C type.
7587 Argument expression.
7590 Length expression C type.
7596 +ctf_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
7598 +ctf_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
7600 +ctf_user_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
7602 +ctf_user_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
7604 Dynamically-sized array of bits.
7606 The type of +__e__+ must be an integer type. +__s__+ is the number
7607 of elements of such type in +__e__+, not the number of bits.
7609 The type of +__E__+ must be unsigned.
7612 Array element C type.
7618 Argument expression.
7621 Length expression C type.
7627 +ctf_sequence_text(__t__, __n__, __e__, __T__, __E__)+
7629 +ctf_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
7631 +ctf_user_sequence_text(__t__, __n__, __e__, __T__, __E__)+
7633 +ctf_user_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
7635 Dynamically-sized array, displayed as text.
7637 The string does not need to be null-terminated.
7639 The type of +__E__+ must be unsigned.
7641 The behaviour is undefined if +__e__+ is `NULL`.
7644 Sequence element C type (always `char`).
7650 Argument expression.
7653 Length expression C type.
7659 Use the `_user` versions when the argument expression, `e`, is
7660 a user space address. In the cases of `ctf_user_integer*()` and
7661 `ctf_user_float*()`, `&e` must be a user space address, thus `e` must
7664 The `_nowrite` versions omit themselves from the session trace, but are
7665 otherwise identical. This means the `_nowrite` fields won't be written
7666 in the recorded trace. Their primary purpose is to make some
7667 of the event context available to the
7668 <<enabling-disabling-events,event filters>> without having to
7669 commit the data to sub-buffers.
7675 Terms related to LTTng and to tracing in general:
7678 The http://diamon.org/babeltrace[Babeltrace] project, which includes
7679 the cmd:babeltrace command, libraries, and Python bindings.
7681 [[def-buffering-scheme]]<<channel-buffering-schemes,buffering scheme>>::
7682 A layout of <<def-sub-buffer,sub-buffers>> applied to a given channel.
7684 <<channel,channel>>::
7685 An entity which is responsible for a set of
7686 <<def-ring-buffer,ring buffers>>.
7688 <<event,Event rules>> are always attached to a specific channel.
7691 A source of time for a <<def-tracer,tracer>>.
7693 <<lttng-consumerd,consumer daemon>>::
7694 A process which is responsible for consuming the full
7695 <<def-sub-buffer,sub-buffers>> and write them to a file system or
7696 send them over the network.
7698 [[def-current-trace-chunk]]current trace chunk::
7699 A <<def-trace-chunk,trace chunk>> which includes the current content
7700 of all the <<tracing-session,tracing session>>'s
7701 <<def-sub-buffer,sub-buffers>> and the stream files produced since the
7702 latest event amongst:
7704 * The creation of the tracing session.
7705 * The last <<session-rotation,tracing session rotation>>, if any.
7707 <<channel-overwrite-mode-vs-discard-mode,discard mode>>:: The event
7708 record loss mode in which the <<def-tracer,tracer>> _discards_ new
7709 event records when there's no
7710 <<def-sub-buffer,sub-buffer>> space left to store them.
7712 [[def-event]]event::
7713 The consequence of the execution of an
7714 <<def-instrumentation-point,instrumentation point>>, like a
7715 <<def-tracepoint,tracepoint>> that you manually place in some source
7716 code, or a Linux kernel kprobe.
7718 An event is said to _occur_ at a specific time. Different actions can
7719 be taken upon the occurrence of an event, like record the event's payload
7720 to a <<def-sub-buffer,sub-buffer>>.
7722 [[def-event-name]]event name::
7723 The name of an event, which is also the name of the event record.
7724 This is also called the _instrumentation point name_.
7726 [[def-event-record]]event record::
7727 A record, in a <<def-trace,trace>>, of the payload of an event
7730 [[def-event-record-loss-mode]]<<channel-overwrite-mode-vs-discard-mode,event record loss mode>>::
7731 The mechanism by which event records of a given <<channel,channel>>
7732 are lost (not recorded) when there is no <<def-sub-buffer,sub-buffer>>
7733 space left to store them.
7735 <<event,event rule>>::
7736 Set of conditions which must be satisfied for one or more occuring
7737 events to be recorded.
7739 `java.util.logging`::
7741 https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities].
7743 <<instrumenting,instrumentation>>::
7744 The use of LTTng probes to make a piece of software traceable.
7746 [[def-instrumentation-point]]instrumentation point::
7747 A point in the execution path of a piece of software that, when
7748 reached by this execution, can emit an <<def-event,event>>.
7750 instrumentation point name::
7751 See _<<def-event-name,event name>>_.
7754 A http://logging.apache.org/log4j/1.2/[logging library] for Java
7755 developed by the Apache Software Foundation.
7758 Level of severity of a log statement or user space
7759 instrumentation point.
7762 The _Linux Trace Toolkit: next generation_ project.
7764 <<lttng-cli,cmd:lttng>>::
7765 A command-line tool provided by the LTTng-tools project which you
7766 can use to send and receive control messages to and from a
7770 The https://github.com/lttng/lttng-analyses[LTTng analyses] project,
7771 which is a set of analyzing programs that are used to obtain a
7772 higher level view of an LTTng <<def-trace,trace>>.
7774 cmd:lttng-consumerd::
7775 The name of the consumer daemon program.
7778 A utility provided by the LTTng-tools project which can convert
7779 <<def-ring-buffer,ring buffer>> files (usually
7780 <<persistent-memory-file-systems,saved on a persistent memory file system>>)
7781 to <<def-trace,trace>> files.
7783 LTTng Documentation::
7786 <<lttng-live,LTTng live>>::
7787 A communication protocol between the <<lttng-relayd,relay daemon>> and
7788 live viewers which makes it possible to see <<def-event-record,event
7789 records>> "live", as they are received by the relay daemon.
7791 <<lttng-modules,LTTng-modules>>::
7792 The https://github.com/lttng/lttng-modules[LTTng-modules] project,
7793 which contains the Linux kernel modules to make the Linux kernel
7794 <<def-instrumentation-point,instrumentation points>> available for
7798 The name of the <<lttng-relayd,relay daemon>> program.
7800 cmd:lttng-sessiond::
7801 The name of the <<lttng-sessiond,session daemon>> program.
7804 The https://github.com/lttng/lttng-tools[LTTng-tools] project, which
7805 contains the various programs and libraries used to
7806 <<controlling-tracing,control tracing>>.
7808 <<lttng-ust,LTTng-UST>>::
7809 The https://github.com/lttng/lttng-ust[LTTng-UST] project, which
7810 contains libraries to instrument
7811 <<def-user-application,user applications>>.
7813 <<lttng-ust-agents,LTTng-UST Java agent>>::
7814 A Java package provided by the LTTng-UST project to allow the
7815 LTTng instrumentation of `java.util.logging` and Apache log4j{nbsp}1.2
7818 <<lttng-ust-agents,LTTng-UST Python agent>>::
7819 A Python package provided by the LTTng-UST project to allow the
7820 LTTng instrumentation of Python logging statements.
7822 <<channel-overwrite-mode-vs-discard-mode,overwrite mode>>::
7823 The <<def-event-record-loss-mode,event record loss mode>> in which new
7824 <<def-event-record,event records>> _overwrite_ older event records
7825 when there's no <<def-sub-buffer,sub-buffer>> space left to store
7828 <<channel-buffering-schemes,per-process buffering>>::
7829 A <<def-buffering-scheme,buffering scheme>> in which each instrumented
7830 process has its own <<def-sub-buffer,sub-buffers>> for a given user
7831 space <<channel,channel>>.
7833 <<channel-buffering-schemes,per-user buffering>>::
7834 A <<def-buffering-scheme,buffering scheme>> in which all the processes
7835 of a Unix user share the same <<def-sub-buffer,sub-buffers>> for a
7836 given user space <<channel,channel>>.
7838 <<lttng-relayd,relay daemon>>::
7839 A process which is responsible for receiving the <<def-trace,trace>>
7840 data sent by a distant <<lttng-consumerd,consumer daemon>>.
7842 [[def-ring-buffer]]ring buffer::
7843 A set of <<def-sub-buffer,sub-buffers>>.
7846 See _<<def-tracing-session-rotation,tracing session rotation>>_.
7848 <<lttng-sessiond,session daemon>>::
7849 A process which receives control commands from you and orchestrates
7850 the <<def-tracer,tracers>> and various LTTng daemons.
7852 <<taking-a-snapshot,snapshot>>::
7853 A copy of the current data of all the <<def-sub-buffer,sub-buffers>>
7854 of a given <<tracing-session,tracing session>>, saved as
7855 <<def-trace,trace>> files.
7857 [[def-sub-buffer]]sub-buffer::
7858 One part of an LTTng <<def-ring-buffer,ring buffer>> which contains
7859 <<def-event-record,event records>>.
7862 The time information attached to an
7863 <<def-event,event>> when it is emitted.
7865 [[def-trace]]trace (_noun_)::
7868 * One http://diamon.org/ctf/[CTF] metadata stream file.
7869 * One or more CTF data stream files which are the concatenations of one
7870 or more flushed <<def-sub-buffer,sub-buffers>>.
7873 The action of recording the <<def-event,events>> emitted by an
7874 application or by a system, or to initiate such recording by
7875 controlling a tracer.
7877 [[def-trace-chunk]]trace chunk::
7878 A self-contained trace which is part of a <<tracing-session,tracing
7879 session>>. Each <<session-rotation, tracing session rotation>>
7880 produces a trace chunk archive.
7882 trace chunk archive::
7883 The result of a <<session-rotation, tracing session rotation>>. A
7884 trace chunk archive is not managed by LTTng, even if its containing
7885 <<tracing-session,tracing session>> is still active: you are free to
7886 read it, modify it, move it, or remove it.
7889 The http://tracecompass.org[Trace Compass] project and application.
7891 [[def-tracepoint]]tracepoint::
7892 An instrumentation point using the tracepoint mechanism of the Linux
7893 kernel or of LTTng-UST.
7895 tracepoint definition::
7896 The definition of a single tracepoint.
7899 The name of a tracepoint.
7901 tracepoint provider::
7902 A set of functions providing tracepoints to an instrumented user
7905 Not to be confused with a _tracepoint provider package_: many tracepoint
7906 providers can exist within a tracepoint provider package.
7908 tracepoint provider package::
7909 One or more tracepoint providers compiled as an
7910 https://en.wikipedia.org/wiki/Object_file[object file] or as
7911 a link:https://en.wikipedia.org/wiki/Library_(computing)#Shared_libraries[shared library].
7913 [[def-tracer]]tracer::
7914 A software which records emitted <<def-event,events>>.
7916 <<domain,tracing domain>>::
7917 A namespace for <<def-event,event>> sources.
7919 <<tracing-group,tracing group>>::
7920 The Unix group in which a Unix user can be to be allowed to trace the
7923 [[def-tracing-session-rotation]]<<tracing-session,tracing session>>::
7924 A stateful dialogue between you and a <<lttng-sessiond,session
7927 <<session-rotation,tracing session rotation>>::
7928 The action of archiving the
7929 <<def-current-trace-chunk,current trace chunk>> of a
7930 <<tracing-session,tracing session>>.
7932 [[def-user-application]]user application::
7933 An application running in user space, as opposed to a Linux kernel
7934 module, for example.