1 The LTTng Documentation
2 =======================
3 Philippe Proulx <pproulx@efficios.com>
7 include::../common/copyright.txt[]
10 include::../common/welcome.txt[]
13 include::../common/audience.txt[]
17 === What's in this documentation?
19 The LTTng Documentation is divided into the following sections:
21 * **<<nuts-and-bolts,Nuts and bolts>>** explains the
22 rudiments of software tracing and the rationale behind the
25 You can skip this section if you’re familiar with software tracing and
26 with the LTTng project.
28 * **<<installing-lttng,Installation>>** describes the steps to
29 install the LTTng packages on common Linux distributions and from
32 You can skip this section if you already properly installed LTTng on
35 * **<<getting-started,Quick start>>** is a concise guide to
36 getting started quickly with LTTng kernel and user space tracing.
38 We recommend this section if you're new to LTTng or to software tracing
41 You can skip this section if you're not new to LTTng.
43 * **<<core-concepts,Core concepts>>** explains the concepts at
46 It's a good idea to become familiar with the core concepts
47 before attempting to use the toolkit.
49 * **<<plumbing,Components of LTTng>>** describes the various components
50 of the LTTng machinery, like the daemons, the libraries, and the
51 command-line interface.
52 * **<<instrumenting,Instrumentation>>** shows different ways to
53 instrument user applications and the Linux kernel.
55 Instrumenting source code is essential to provide a meaningful
58 You can skip this section if you do not have a programming background.
60 * **<<controlling-tracing,Tracing control>>** is divided into topics
61 which demonstrate how to use the vast array of features that
62 LTTng{nbsp}{revision} offers.
63 * **<<reference,Reference>>** contains reference tables.
64 * **<<glossary,Glossary>>** is a specialized dictionary of terms related
65 to LTTng or to the field of software tracing.
68 include::../common/convention.txt[]
71 include::../common/acknowledgements.txt[]
75 == What's new in LTTng{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] (USDT) probe (DTrace-style marker). For
136 example, 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 * User applications and libraries instrumented with
189 <<lttng-ust,LTTng-UST>> can now safely unload (man:dlclose(3)) a
191 <<building-tracepoint-providers-and-user-application,tracepoint
194 * The <<lttng-relayd,relay daemon>> is more efficient and presents fewer
195 connectivity issues, especially when a large number of targets send
196 trace data to a given relay daemon.
198 * LTTng-UST uses https://github.com/numactl/numactl[libnuma]
199 when available to allocate <<def-sub-buffer,sub-buffers>>, making them
201 https://en.wikipedia.org/wiki/Non-uniform_memory_access[NUMA] node.
203 This change makes the tracer more efficient on NUMA systems.
209 What is LTTng? As its name suggests, the _Linux Trace Toolkit: next
210 generation_ is a modern toolkit for tracing Linux systems and
211 applications. So your first question might be:
218 As the history of software engineering progressed and led to what
219 we now take for granted--complex, numerous and
220 interdependent software applications running in parallel on
221 sophisticated operating systems like Linux--the authors of such
222 components, software developers, began feeling a natural
223 urge to have tools that would ensure the robustness and good performance
224 of their masterpieces.
226 One major achievement in this field is, inarguably, the
227 https://www.gnu.org/software/gdb/[GNU debugger (GDB)],
228 an essential tool for developers to find and fix bugs. But even the best
229 debugger won't help make your software run faster, and nowadays, faster
230 software means either more work done by the same hardware, or cheaper
231 hardware for the same work.
233 A _profiler_ is often the tool of choice to identify performance
234 bottlenecks. Profiling is suitable to identify _where_ performance is
235 lost in a given software. The profiler outputs a profile, a statistical
236 summary of observed events, which you may use to discover which
237 functions took the most time to execute. However, a profiler won't
238 report _why_ some identified functions are the bottleneck. Bottlenecks
239 might only occur when specific conditions are met, conditions that are
240 sometimes impossible to capture by a statistical profiler, or impossible
241 to reproduce with an application altered by the overhead of an
242 event-based profiler. For a thorough investigation of software
243 performance issues, a history of execution is essential, with the
244 recorded values of variables and context fields you choose, and
245 with as little influence as possible on the instrumented software. This
246 is where tracing comes in handy.
248 _Tracing_ is a technique used to understand what goes on in a running
249 software system. The software used for tracing is called a _tracer_,
250 which is conceptually similar to a tape recorder. When recording,
251 specific instrumentation points placed in the software source code
252 generate events that are saved on a giant tape: a _trace_ file. You
253 can trace user applications and the operating system at the same time,
254 opening the possibility of resolving a wide range of problems that would
255 otherwise be extremely challenging.
257 Tracing is often compared to _logging_. However, tracers and loggers are
258 two different tools, serving two different purposes. Tracers are
259 designed to record much lower-level events that occur much more
260 frequently than log messages, often in the range of thousands per
261 second, with very little execution overhead. Logging is more appropriate
262 for a very high-level analysis of less frequent events: user accesses,
263 exceptional conditions (errors and warnings, for example), database
264 transactions, instant messaging communications, and such. Simply put,
265 logging is one of the many use cases that can be satisfied with tracing.
267 The list of recorded events inside a trace file can be read manually
268 like a log file for the maximum level of detail, but it is generally
269 much more interesting to perform application-specific analyses to
270 produce reduced statistics and graphs that are useful to resolve a
271 given problem. Trace viewers and analyzers are specialized tools
274 In the end, this is what LTTng is: a powerful, open source set of
275 tools to trace the Linux kernel and user applications at the same time.
276 LTTng is composed of several components actively maintained and
277 developed by its link:/community/#where[community].
280 [[lttng-alternatives]]
281 === Alternatives to noch:{LTTng}
283 Excluding proprietary solutions, a few competing software tracers
286 * https://github.com/dtrace4linux/linux[dtrace4linux] is a port of
287 Sun Microsystems's DTrace to Linux. The cmd:dtrace tool interprets
288 user scripts and is responsible for loading code into the
289 Linux kernel for further execution and collecting the outputted data.
290 * https://en.wikipedia.org/wiki/Berkeley_Packet_Filter[eBPF] is a
291 subsystem in the Linux kernel in which a virtual machine can execute
292 programs passed from the user space to the kernel. You can attach
293 such programs to tracepoints and kprobes thanks to a system call, and
294 they can output data to the user space when executed thanks to
295 different mechanisms (pipe, VM register values, and eBPF maps, to name
297 * https://www.kernel.org/doc/Documentation/trace/ftrace.txt[ftrace]
298 is the de facto function tracer of the Linux kernel. Its user
299 interface is a set of special files in sysfs.
300 * https://perf.wiki.kernel.org/[perf] is
301 a performance analyzing tool for Linux which supports hardware
302 performance counters, tracepoints, as well as other counters and
303 types of probes. perf's controlling utility is the cmd:perf command
305 * http://linux.die.net/man/1/strace[strace]
306 is a command-line utility which records system calls made by a
307 user process, as well as signal deliveries and changes of process
308 state. strace makes use of https://en.wikipedia.org/wiki/Ptrace[ptrace]
309 to fulfill its function.
310 * http://www.sysdig.org/[sysdig], like SystemTap, uses scripts to
311 analyze Linux kernel events. You write scripts, or _chisels_ in
312 sysdig's jargon, in Lua and sysdig executes them while it traces the
313 system or afterwards. sysdig's interface is the cmd:sysdig
314 command-line tool as well as the curses-based cmd:csysdig tool.
315 * https://sourceware.org/systemtap/[SystemTap] is a Linux kernel and
316 user space tracer which uses custom user scripts to produce plain text
317 traces. SystemTap converts the scripts to the C language, and then
318 compiles them as Linux kernel modules which are loaded to produce
319 trace data. SystemTap's primary user interface is the cmd:stap
322 The main distinctive features of LTTng is that it produces correlated
323 kernel and user space traces, as well as doing so with the lowest
324 overhead amongst other solutions. It produces trace files in the
325 http://diamon.org/ctf[CTF] format, a file format optimized
326 for the production and analyses of multi-gigabyte data.
328 LTTng is the result of more than 10{nbsp}years of active open source
329 development by a community of passionate developers.
330 LTTng{nbsp}{revision} is currently available on major desktop and server
333 The main interface for tracing control is a single command-line tool
334 named cmd:lttng. The latter can create several tracing sessions, enable
335 and disable events on the fly, filter events efficiently with custom
336 user expressions, start and stop tracing, and much more. LTTng can
337 record the traces on the file system or send them over the network, and
338 keep them totally or partially. You can view the traces once tracing
339 becomes inactive or in real-time.
341 <<installing-lttng,Install LTTng now>> and
342 <<getting-started,start tracing>>!
348 **LTTng** is a set of software <<plumbing,components>> which interact to
349 <<instrumenting,instrument>> the Linux kernel and user applications, and
350 to <<controlling-tracing,control tracing>> (start and stop
351 tracing, enable and disable event rules, and the rest). Those
352 components are bundled into the following packages:
355 Libraries and command-line interface to control tracing.
358 Linux kernel modules to instrument and trace the kernel.
361 Libraries and Java/Python packages to instrument and trace user
364 Most distributions mark the LTTng-modules and LTTng-UST packages as
365 optional when installing LTTng-tools (which is always required). In the
366 following sections, we always provide the steps to install all three,
369 * You only need to install LTTng-modules if you intend to trace the
371 * You only need to install LTTng-UST if you intend to trace user
376 As of 22 October 2019, LTTng{nbsp}{revision} is not available
377 as distribution packages, except for <<arch-linux,Arch Linux>>.
379 You can <<building-from-source,build LTTng{nbsp}{revision} from source>>
380 to install and use it.
387 LTTng-UST{nbsp}{revision} is available in Arch Linux's _community_
388 repository, while LTTng-tools{nbsp}{revision} and
389 LTTng-modules{nbsp}{revision} are available in the
390 https://aur.archlinux.org/[AUR].
392 To install LTTng{nbsp}{revision} on Arch Linux, using
393 https://github.com/actionless/pikaur[pikaur] for the AUR packages:
395 . Install the main LTTng{nbsp}{revision} packages:
400 # pacman -Sy lttng-ust
401 $ pikaur -Sy lttng-tools
402 $ pikaur -Sy lttng-modules
406 . **If you need to instrument and trace <<python-application,Python
407 applications>>**, install the LTTng-UST Python agent:
412 # pacman -Sy python-lttngust
413 # pacman -Sy python2-lttngust
418 [[building-from-source]]
419 === Build from source
421 To build and install LTTng{nbsp}{revision} from source:
423 . Using your distribution's package manager, or from source, install
424 the following dependencies of LTTng-tools and LTTng-UST:
427 * https://sourceforge.net/projects/libuuid/[libuuid]
428 * http://directory.fsf.org/wiki/Popt[popt]
429 * http://liburcu.org/[Userspace RCU]
430 * http://www.xmlsoft.org/[libxml2]
431 * **Optional**: https://github.com/numactl/numactl[numactl]
434 . Download, build, and install the latest LTTng-modules{nbsp}{revision}:
440 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.11.tar.bz2 &&
441 tar -xf lttng-modules-latest-2.11.tar.bz2 &&
442 cd lttng-modules-2.11.* &&
444 sudo make modules_install &&
449 . Download, build, and install the latest LTTng-UST{nbsp}{revision}:
455 wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.11.tar.bz2 &&
456 tar -xf lttng-ust-latest-2.11.tar.bz2 &&
457 cd lttng-ust-2.11.* &&
465 Add `--disable-numa` to `./configure` if you don't have
466 https://github.com/numactl/numactl[numactl].
470 .Java and Python application tracing
472 If you need to instrument and trace <<java-application,Java
473 applications>>, pass the `--enable-java-agent-jul`,
474 `--enable-java-agent-log4j`, or `--enable-java-agent-all` options to the
475 `configure` script, depending on which Java logging framework you use.
477 If you need to instrument and trace <<python-application,Python
478 applications>>, pass the `--enable-python-agent` option to the
479 `configure` script. You can set the `PYTHON` environment variable to the
480 path to the Python interpreter for which to install the LTTng-UST Python
488 By default, LTTng-UST libraries are installed to
489 dir:{/usr/local/lib}, which is the de facto directory in which to
490 keep self-compiled and third-party libraries.
492 When <<building-tracepoint-providers-and-user-application,linking an
493 instrumented user application with `liblttng-ust`>>:
495 * Append `/usr/local/lib` to the env:LD_LIBRARY_PATH environment
497 * Pass the `-L/usr/local/lib` and `-Wl,-rpath,/usr/local/lib` options to
498 man:gcc(1), man:g++(1), or man:clang(1).
502 . Download, build, and install the latest LTTng-tools{nbsp}{revision}:
508 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.11.tar.bz2 &&
509 tar -xf lttng-tools-latest-2.11.tar.bz2 &&
510 cd lttng-tools-2.11.* &&
518 TIP: The https://github.com/eepp/vlttng[vlttng tool] can do all the
519 previous steps automatically for a given version of LTTng and confine
520 the installed files in a specific directory. This can be useful to test
521 LTTng without installing it on your system.
527 This is a short guide to get started quickly with LTTng kernel and user
530 Before you follow this guide, make sure to <<installing-lttng,install>>
533 This tutorial walks you through the steps to:
535 . <<tracing-the-linux-kernel,Trace the Linux kernel>>.
536 . <<tracing-your-own-user-application,Trace a user application>> written
538 . <<viewing-and-analyzing-your-traces,View and analyze the
542 [[tracing-the-linux-kernel]]
543 === Trace the Linux kernel
545 The following command lines start with the `#` prompt because you need
546 root privileges to trace the Linux kernel. You can also trace the kernel
547 as a regular user if your Unix user is a member of the
548 <<tracing-group,tracing group>>.
550 . Create a <<tracing-session,tracing session>> which writes its traces
551 to dir:{/tmp/my-kernel-trace}:
556 # lttng create my-kernel-session --output=/tmp/my-kernel-trace
560 . List the available kernel tracepoints and system calls:
565 # lttng list --kernel
566 # lttng list --kernel --syscall
570 . Create <<event,event rules>> which match the desired instrumentation
571 point names, for example the `sched_switch` and `sched_process_fork`
572 tracepoints, and the man:open(2) and man:close(2) system calls:
577 # lttng enable-event --kernel sched_switch,sched_process_fork
578 # lttng enable-event --kernel --syscall open,close
582 You can also create an event rule which matches _all_ the Linux kernel
583 tracepoints (this will generate a lot of data when tracing):
588 # lttng enable-event --kernel --all
592 . <<basic-tracing-session-control,Start tracing>>:
601 . Do some operation on your system for a few seconds. For example,
602 load a website, or list the files of a directory.
603 . <<creating-destroying-tracing-sessions,Destroy>> the current
613 The man:lttng-destroy(1) command does not destroy the trace data; it
614 only destroys the state of the tracing session.
616 The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command
617 implicitly (see <<basic-tracing-session-control,Start and stop a tracing
618 session>>). You need to stop tracing to make LTTng flush the remaining
619 trace data and make the trace readable.
621 . For the sake of this example, make the recorded trace accessible to
627 # chown -R $(whoami) /tmp/my-kernel-trace
631 See <<viewing-and-analyzing-your-traces,View and analyze the
632 recorded events>> to view the recorded events.
635 [[tracing-your-own-user-application]]
636 === Trace a user application
638 This section steps you through a simple example to trace a
639 _Hello world_ program written in C.
641 To create the traceable user application:
643 . Create the tracepoint provider header file, which defines the
644 tracepoints and the events they can generate:
650 #undef TRACEPOINT_PROVIDER
651 #define TRACEPOINT_PROVIDER hello_world
653 #undef TRACEPOINT_INCLUDE
654 #define TRACEPOINT_INCLUDE "./hello-tp.h"
656 #if !defined(_HELLO_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
659 #include <lttng/tracepoint.h>
669 ctf_string(my_string_field, my_string_arg)
670 ctf_integer(int, my_integer_field, my_integer_arg)
674 #endif /* _HELLO_TP_H */
676 #include <lttng/tracepoint-event.h>
680 . Create the tracepoint provider package source file:
686 #define TRACEPOINT_CREATE_PROBES
687 #define TRACEPOINT_DEFINE
689 #include "hello-tp.h"
693 . Build the tracepoint provider package:
698 $ gcc -c -I. hello-tp.c
702 . Create the _Hello World_ application source file:
709 #include "hello-tp.h"
711 int main(int argc, char *argv[])
715 puts("Hello, World!\nPress Enter to continue...");
718 * The following getchar() call is only placed here for the purpose
719 * of this demonstration, to pause the application in order for
720 * you to have time to list its tracepoints. It is not
726 * A tracepoint() call.
728 * Arguments, as defined in hello-tp.h:
730 * 1. Tracepoint provider name (required)
731 * 2. Tracepoint name (required)
732 * 3. my_integer_arg (first user-defined argument)
733 * 4. my_string_arg (second user-defined argument)
735 * Notice the tracepoint provider and tracepoint names are
736 * NOT strings: they are in fact parts of variables that the
737 * macros in hello-tp.h create.
739 tracepoint(hello_world, my_first_tracepoint, 23, "hi there!");
741 for (x = 0; x < argc; ++x) {
742 tracepoint(hello_world, my_first_tracepoint, x, argv[x]);
745 puts("Quitting now!");
746 tracepoint(hello_world, my_first_tracepoint, x * x, "x^2");
753 . Build the application:
762 . Link the application with the tracepoint provider package,
763 `liblttng-ust`, and `libdl`:
768 $ gcc -o hello hello.o hello-tp.o -llttng-ust -ldl
772 Here's the whole build process:
775 .User space tracing tutorial's build steps.
776 image::ust-flow.png[]
778 To trace the user application:
780 . Run the application with a few arguments:
785 $ ./hello world and beyond
794 Press Enter to continue...
798 . Start an LTTng <<lttng-sessiond,session daemon>>:
803 $ lttng-sessiond --daemonize
807 Note that a session daemon might already be running, for example as
808 a service that the distribution's service manager started.
810 . List the available user space tracepoints:
815 $ lttng list --userspace
819 You see the `hello_world:my_first_tracepoint` tracepoint listed
820 under the `./hello` process.
822 . Create a <<tracing-session,tracing session>>:
827 $ lttng create my-user-space-session
831 . Create an <<event,event rule>> which matches the
832 `hello_world:my_first_tracepoint` event name:
837 $ lttng enable-event --userspace hello_world:my_first_tracepoint
841 . <<basic-tracing-session-control,Start tracing>>:
850 . Go back to the running `hello` application and press Enter. The
851 program executes all `tracepoint()` instrumentation points and exits.
852 . <<creating-destroying-tracing-sessions,Destroy>> the current
862 The man:lttng-destroy(1) command does not destroy the trace data; it
863 only destroys the state of the tracing session.
865 The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command
866 implicitly (see <<basic-tracing-session-control,Start and stop a tracing
867 session>>). You need to stop tracing to make LTTng flush the remaining
868 trace data and make the trace readable.
870 By default, LTTng saves the traces in
871 +$LTTNG_HOME/lttng-traces/__name__-__date__-__time__+,
872 where +__name__+ is the tracing session name. The
873 env:LTTNG_HOME environment variable defaults to `$HOME` if not set.
875 See <<viewing-and-analyzing-your-traces,View and analyze the
876 recorded events>> to view the recorded events.
879 [[viewing-and-analyzing-your-traces]]
880 === View and analyze the recorded events
882 Once you have completed the <<tracing-the-linux-kernel,Trace the Linux
883 kernel>> and <<tracing-your-own-user-application,Trace a user
884 application>> tutorials, you can inspect the recorded events.
886 There are many tools you can use to read LTTng traces:
888 * **cmd:babeltrace** is a command-line utility which converts trace
889 formats; it supports the format that LTTng produces, CTF, as well as a
890 basic text output which can be ++grep++ed. The cmd:babeltrace command
891 is part of the http://diamon.org/babeltrace[Babeltrace] project.
892 * Babeltrace also includes
893 **https://www.python.org/[Python{nbsp}3] bindings** so
894 that you can easily open and read an LTTng trace with your own script,
895 benefiting from the power of Python.
896 * http://tracecompass.org/[**Trace Compass**]
897 is a graphical user interface for viewing and analyzing any type of
898 logs or traces, including LTTng's.
899 * https://github.com/lttng/lttng-analyses[**LTTng analyses**] is a
900 project which includes many high-level analyses of LTTng kernel
901 traces, like scheduling statistics, interrupt frequency distribution,
902 top CPU usage, and more.
904 NOTE: This section assumes that LTTng saved the traces it recorded
905 during the previous tutorials to their default location, in the
906 dir:{$LTTNG_HOME/lttng-traces} directory. The env:LTTNG_HOME
907 environment variable defaults to `$HOME` if not set.
910 [[viewing-and-analyzing-your-traces-bt]]
911 ==== Use the cmd:babeltrace command-line tool
913 The simplest way to list all the recorded events of a trace is to pass
914 its path to cmd:babeltrace with no options:
918 $ babeltrace ~/lttng-traces/my-user-space-session*
921 cmd:babeltrace finds all traces recursively within the given path and
922 prints all their events, merging them in chronological order.
924 You can pipe the output of cmd:babeltrace into a tool like man:grep(1) for
929 $ babeltrace /tmp/my-kernel-trace | grep _switch
932 You can pipe the output of cmd:babeltrace into a tool like man:wc(1) to
933 count the recorded events:
937 $ babeltrace /tmp/my-kernel-trace | grep _open | wc --lines
941 [[viewing-and-analyzing-your-traces-bt-python]]
942 ==== Use the Babeltrace{nbsp}1 Python bindings
944 The <<viewing-and-analyzing-your-traces-bt,text output of cmd:babeltrace>>
945 is useful to isolate events by simple matching using man:grep(1) and
946 similar utilities. However, more elaborate filters, such as keeping only
947 event records with a field value falling within a specific range, are
948 not trivial to write using a shell. Moreover, reductions and even the
949 most basic computations involving multiple event records are virtually
950 impossible to implement.
952 Fortunately, Babeltrace{nbsp}1 ships with Python{nbsp}3 bindings which
953 makes it easy to read the event records of an LTTng trace sequentially
954 and compute the desired information.
956 The following script accepts an LTTng Linux kernel trace path as its
957 first argument and prints the short names of the top five running
958 processes on CPU{nbsp}0 during the whole trace:
963 from collections import Counter
969 if len(sys.argv) != 2:
970 msg = 'Usage: python3 {} TRACEPATH'.format(sys.argv[0])
971 print(msg, file=sys.stderr)
974 # A trace collection contains one or more traces
975 col = babeltrace.TraceCollection()
977 # Add the trace provided by the user (LTTng traces always have
979 if col.add_trace(sys.argv[1], 'ctf') is None:
980 raise RuntimeError('Cannot add trace')
982 # This counter dict contains execution times:
984 # task command name -> total execution time (ns)
985 exec_times = Counter()
987 # This contains the last `sched_switch` timestamp
991 for event in col.events:
992 # Keep only `sched_switch` events
993 if event.name != 'sched_switch':
996 # Keep only events which happened on CPU 0
997 if event['cpu_id'] != 0:
1001 cur_ts = event.timestamp
1007 # Previous task command (short) name
1008 prev_comm = event['prev_comm']
1010 # Initialize entry in our dict if not yet done
1011 if prev_comm not in exec_times:
1012 exec_times[prev_comm] = 0
1014 # Compute previous command execution time
1015 diff = cur_ts - last_ts
1017 # Update execution time of this command
1018 exec_times[prev_comm] += diff
1020 # Update last timestamp
1024 for name, ns in exec_times.most_common(5):
1026 print('{:20}{} s'.format(name, s))
1031 if __name__ == '__main__':
1032 sys.exit(0 if top5proc() else 1)
1039 $ python3 top5proc.py /tmp/my-kernel-trace/kernel
1045 swapper/0 48.607245889 s
1046 chromium 7.192738188 s
1047 pavucontrol 0.709894415 s
1048 Compositor 0.660867933 s
1049 Xorg.bin 0.616753786 s
1052 Note that `swapper/0` is the "idle" process of CPU{nbsp}0 on Linux;
1053 since we weren't using the CPU that much when tracing, its first
1054 position in the list makes sense.
1058 == [[understanding-lttng]]Core concepts
1060 From a user's perspective, the LTTng system is built on a few concepts,
1061 or objects, on which the <<lttng-cli,cmd:lttng command-line tool>>
1062 operates by sending commands to the <<lttng-sessiond,session daemon>>.
1063 Understanding how those objects relate to eachother is key in mastering
1066 The core concepts are:
1068 * <<tracing-session,Tracing session>>
1069 * <<domain,Tracing domain>>
1070 * <<channel,Channel and ring buffer>>
1071 * <<"event","Instrumentation point, event rule, event, and event record">>
1077 A _tracing session_ is a stateful dialogue between you and
1078 a <<lttng-sessiond,session daemon>>. You can
1079 <<creating-destroying-tracing-sessions,create a new tracing
1080 session>> with the `lttng create` command.
1082 Anything that you do when you control LTTng tracers happens within a
1083 tracing session. In particular, a tracing session:
1086 * Has its own set of trace files.
1087 * Has its own state of activity (started or stopped).
1088 * Has its own <<tracing-session-mode,mode>> (local, network streaming,
1090 * Has its own <<channel,channels>> to which are associated their own
1091 <<event,event rules>>.
1094 .A _tracing session_ contains <<channel,channels>> that are members of <<domain,tracing domains>> and contain <<event,event rules>>.
1095 image::concepts.png[]
1097 Those attributes and objects are completely isolated between different
1100 A tracing session is analogous to a cash machine session:
1101 the operations you do on the banking system through the cash machine do
1102 not alter the data of other users of the same system. In the case of
1103 the cash machine, a session lasts as long as your bank card is inside.
1104 In the case of LTTng, a tracing session lasts from the `lttng create`
1105 command to the `lttng destroy` command.
1108 .Each Unix user has its own set of tracing sessions.
1109 image::many-sessions.png[]
1112 [[tracing-session-mode]]
1113 ==== Tracing session mode
1115 LTTng can send the generated trace data to different locations. The
1116 _tracing session mode_ dictates where to send it. The following modes
1117 are available in LTTng{nbsp}{revision}:
1120 LTTng writes the traces to the file system of the machine it traces
1123 Network streaming mode::
1124 LTTng sends the traces over the network to a
1125 <<lttng-relayd,relay daemon>> running on a remote system.
1128 LTTng does not write the traces by default. Instead, you can request
1129 LTTng to <<taking-a-snapshot,take a snapshot>>, that is, a copy of the
1130 tracing session's current sub-buffers, and to write it to the
1131 target's file system or to send it over the network to a
1132 <<lttng-relayd,relay daemon>> running on a remote system.
1135 This mode is similar to the network streaming mode, but a live
1136 trace viewer can connect to the distant relay daemon to
1137 <<lttng-live,view event records as LTTng generates them>>.
1143 A _tracing domain_ is a namespace for event sources. A tracing domain
1144 has its own properties and features.
1146 There are currently five available tracing domains:
1150 * `java.util.logging` (JUL)
1154 You must specify a tracing domain when using some commands to avoid
1155 ambiguity. For example, since all the domains support named tracepoints
1156 as event sources (instrumentation points that you manually insert in the
1157 source code), you need to specify a tracing domain when
1158 <<enabling-disabling-events,creating an event rule>> because all the
1159 tracing domains could have tracepoints with the same names.
1161 Some features are reserved to specific tracing domains. Dynamic function
1162 entry and return instrumentation points, for example, are currently only
1163 supported in the Linux kernel tracing domain, but support for other
1164 tracing domains could be added in the future.
1166 You can create <<channel,channels>> in the Linux kernel and user space
1167 tracing domains. The other tracing domains have a single default
1172 === Channel and ring buffer
1174 A _channel_ is an object which is responsible for a set of ring buffers.
1175 Each ring buffer is divided into multiple sub-buffers. When an LTTng
1176 tracer emits an event, it can record it to one or more
1177 sub-buffers. The attributes of a channel determine what to do when
1178 there's no space left for a new event record because all sub-buffers
1179 are full, where to send a full sub-buffer, and other behaviours.
1181 A channel is always associated to a <<domain,tracing domain>>. The
1182 `java.util.logging` (JUL), log4j, and Python tracing domains each have
1183 a default channel which you cannot configure.
1185 A channel also owns <<event,event rules>>. When an LTTng tracer emits
1186 an event, it records it to the sub-buffers of all
1187 the enabled channels with a satisfied event rule, as long as those
1188 channels are part of active <<tracing-session,tracing sessions>>.
1191 [[channel-buffering-schemes]]
1192 ==== Per-user vs. per-process buffering schemes
1194 A channel has at least one ring buffer _per CPU_. LTTng always
1195 records an event to the ring buffer associated to the CPU on which it
1198 Two _buffering schemes_ are available when you
1199 <<enabling-disabling-channels,create a channel>> in the
1200 user space <<domain,tracing domain>>:
1202 Per-user buffering::
1203 Allocate one set of ring buffers--one per CPU--shared by all the
1204 instrumented processes of each Unix user.
1208 .Per-user buffering scheme.
1209 image::per-user-buffering.png[]
1212 Per-process buffering::
1213 Allocate one set of ring buffers--one per CPU--for each
1214 instrumented process.
1218 .Per-process buffering scheme.
1219 image::per-process-buffering.png[]
1222 The per-process buffering scheme tends to consume more memory than the
1223 per-user option because systems generally have more instrumented
1224 processes than Unix users running instrumented processes. However, the
1225 per-process buffering scheme ensures that one process having a high
1226 event throughput won't fill all the shared sub-buffers of the same
1229 The Linux kernel tracing domain has only one available buffering scheme
1230 which is to allocate a single set of ring buffers for the whole system.
1231 This scheme is similar to the per-user option, but with a single, global
1232 user "running" the kernel.
1235 [[channel-overwrite-mode-vs-discard-mode]]
1236 ==== Overwrite vs. discard event record loss modes
1238 When an event occurs, LTTng records it to a specific sub-buffer (yellow
1239 arc in the following animations) of a specific channel's ring buffer.
1240 When there's no space left in a sub-buffer, the tracer marks it as
1241 consumable (red) and another, empty sub-buffer starts receiving the
1242 following event records. A <<lttng-consumerd,consumer daemon>>
1243 eventually consumes the marked sub-buffer (returns to white).
1246 [role="docsvg-channel-subbuf-anim"]
1251 In an ideal world, sub-buffers are consumed faster than they are filled,
1252 as it is the case in the previous animation. In the real world,
1253 however, all sub-buffers can be full at some point, leaving no space to
1254 record the following events.
1256 By default, LTTng-modules and LTTng-UST are _non-blocking_ tracers: when
1257 no empty sub-buffer is available, it is acceptable to lose event records
1258 when the alternative would be to cause substantial delays in the
1259 instrumented application's execution. LTTng privileges performance over
1260 integrity; it aims at perturbing the target system as little as possible
1261 in order to make tracing of subtle race conditions and rare interrupt
1264 Since LTTng{nbsp}2.10, the LTTng user space tracer, LTTng-UST, supports
1265 a _blocking mode_. See the <<blocking-timeout-example,blocking timeout
1266 example>> to learn how to use the blocking mode.
1268 When it comes to losing event records because no empty sub-buffer is
1269 available, or because the <<opt-blocking-timeout,blocking timeout>> is
1270 reached, the channel's _event record loss mode_ determines what to do.
1271 The available event record loss modes are:
1274 Drop the newest event records until a the tracer releases a
1277 This is the only available mode when you specify a
1278 <<opt-blocking-timeout,blocking timeout>>.
1281 Clear the sub-buffer containing the oldest event records and start
1282 writing the newest event records there.
1284 This mode is sometimes called _flight recorder mode_ because it's
1286 https://en.wikipedia.org/wiki/Flight_recorder[flight recorder]:
1287 always keep a fixed amount of the latest data.
1289 Which mechanism you should choose depends on your context: prioritize
1290 the newest or the oldest event records in the ring buffer?
1292 Beware that, in overwrite mode, the tracer abandons a _whole sub-buffer_
1293 as soon as a there's no space left for a new event record, whereas in
1294 discard mode, the tracer only discards the event record that doesn't
1297 In discard mode, LTTng increments a count of lost event records when an
1298 event record is lost and saves this count to the trace. In overwrite
1299 mode, since LTTng{nbsp}2.8, LTTng increments a count of lost sub-buffers
1300 when a sub-buffer is lost and saves this count to the trace. In this
1301 mode, LTTng does not write to the trace the exact number of lost event
1302 records in those lost sub-buffers. Trace analyses can use the trace's
1303 saved discarded event record and sub-buffer counts to decide whether or
1304 not to perform the analyses even if trace data is known to be missing.
1306 There are a few ways to decrease your probability of losing event
1308 <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>> shows
1309 how you can fine-tune the sub-buffer count and size of a channel to
1310 virtually stop losing event records, though at the cost of greater
1314 [[channel-subbuf-size-vs-subbuf-count]]
1315 ==== Sub-buffer count and size
1317 When you <<enabling-disabling-channels,create a channel>>, you can
1318 set its number of sub-buffers and their size.
1320 Note that there is noticeable CPU overhead introduced when
1321 switching sub-buffers (marking a full one as consumable and switching
1322 to an empty one for the following events to be recorded). Knowing this,
1323 the following list presents a few practical situations along with how
1324 to configure the sub-buffer count and size for them:
1326 * **High event throughput**: In general, prefer bigger sub-buffers to
1327 lower the risk of losing event records.
1329 Having bigger sub-buffers also ensures a lower
1330 <<channel-switch-timer,sub-buffer switching frequency>>.
1332 The number of sub-buffers is only meaningful if you create the channel
1333 in overwrite mode: in this case, if a sub-buffer overwrite happens, the
1334 other sub-buffers are left unaltered.
1336 * **Low event throughput**: In general, prefer smaller sub-buffers
1337 since the risk of losing event records is low.
1339 Because events occur less frequently, the sub-buffer switching frequency
1340 should remain low and thus the tracer's overhead should not be a
1343 * **Low memory system**: If your target system has a low memory
1344 limit, prefer fewer first, then smaller sub-buffers.
1346 Even if the system is limited in memory, you want to keep the
1347 sub-buffers as big as possible to avoid a high sub-buffer switching
1350 Note that LTTng uses http://diamon.org/ctf/[CTF] as its trace format,
1351 which means event data is very compact. For example, the average
1352 LTTng kernel event record weights about 32{nbsp}bytes. Thus, a
1353 sub-buffer size of 1{nbsp}MiB is considered big.
1355 The previous situations highlight the major trade-off between a few big
1356 sub-buffers and more, smaller sub-buffers: sub-buffer switching
1357 frequency vs. how much data is lost in overwrite mode. Assuming a
1358 constant event throughput and using the overwrite mode, the two
1359 following configurations have the same ring buffer total size:
1362 [role="docsvg-channel-subbuf-size-vs-count-anim"]
1367 * **Two sub-buffers of 4{nbsp}MiB each**: Expect a very low sub-buffer
1368 switching frequency, but if a sub-buffer overwrite happens, half of
1369 the event records so far (4{nbsp}MiB) are definitely lost.
1370 * **Eight sub-buffers of 1{nbsp}MiB each**: Expect four times the tracer's
1371 overhead as the previous configuration, but if a sub-buffer
1372 overwrite happens, only the eighth of event records so far are
1375 In discard mode, the sub-buffers count parameter is pointless: use two
1376 sub-buffers and set their size according to the requirements of your
1380 [[channel-switch-timer]]
1381 ==== Switch timer period
1383 The _switch timer period_ is an important configurable attribute of
1384 a channel to ensure periodic sub-buffer flushing.
1386 When the _switch timer_ expires, a sub-buffer switch happens. You can
1387 set the switch timer period attribute when you
1388 <<enabling-disabling-channels,create a channel>> to ensure that LTTng
1389 consumes and commits trace data to trace files or to a distant relay
1390 daemon periodically in case of a low event throughput.
1393 [role="docsvg-channel-switch-timer"]
1398 This attribute is also convenient when you use big sub-buffers to cope
1399 with a sporadic high event throughput, even if the throughput is
1403 [[channel-read-timer]]
1404 ==== Read timer period
1406 By default, the LTTng tracers use a notification mechanism to signal a
1407 full sub-buffer so that a consumer daemon can consume it. When such
1408 notifications must be avoided, for example in real-time applications,
1409 you can use the channel's _read timer_ instead. When the read timer
1410 fires, the <<lttng-consumerd,consumer daemon>> checks for full,
1411 consumable sub-buffers.
1414 [[tracefile-rotation]]
1415 ==== Trace file count and size
1417 By default, trace files can grow as large as needed. You can set the
1418 maximum size of each trace file that a channel writes when you
1419 <<enabling-disabling-channels,create a channel>>. When the size of
1420 a trace file reaches the channel's fixed maximum size, LTTng creates
1421 another file to contain the next event records. LTTng appends a file
1422 count to each trace file name in this case.
1424 If you set the trace file size attribute when you create a channel, the
1425 maximum number of trace files that LTTng creates is _unlimited_ by
1426 default. To limit them, you can also set a maximum number of trace
1427 files. When the number of trace files reaches the channel's fixed
1428 maximum count, the oldest trace file is overwritten. This mechanism is
1429 called _trace file rotation_.
1433 Even if you don't limit the trace file count, you cannot assume that
1434 LTTng doesn't manage any trace file.
1436 In other words, there is no safe way to know if LTTng still holds a
1437 given trace file open with the trace file rotation feature.
1439 The only way to obtain an unmanaged, self-contained LTTng trace before
1440 you <<creating-destroying-tracing-sessions,destroy>> the tracing session
1441 is with the <<session-rotation,tracing session rotation>> feature
1442 (available since LTTng{nbsp}2.11).
1447 === Instrumentation point, event rule, event, and event record
1449 An _event rule_ is a set of conditions which must be **all** satisfied
1450 for LTTng to record an occuring event.
1452 You set the conditions when you <<enabling-disabling-events,create
1455 You always attach an event rule to <<channel,channel>> when you create
1458 When an event passes the conditions of an event rule, LTTng records it
1459 in one of the attached channel's sub-buffers.
1461 The available conditions, as of LTTng{nbsp}{revision}, are:
1463 * The event rule _is enabled_.
1464 * The instrumentation point's type _is{nbsp}T_.
1465 * The instrumentation point's name (sometimes called _event name_)
1466 _matches{nbsp}N_, but _is not{nbsp}E_.
1467 * The instrumentation point's log level _is as severe as{nbsp}L_, or
1468 _is exactly{nbsp}L_.
1469 * The fields of the event's payload _satisfy_ a filter
1470 expression{nbsp}__F__.
1472 As you can see, all the conditions but the dynamic filter are related to
1473 the event rule's status or to the instrumentation point, not to the
1474 occurring events. This is why, without a filter, checking if an event
1475 passes an event rule is not a dynamic task: when you create or modify an
1476 event rule, all the tracers of its tracing domain enable or disable the
1477 instrumentation points themselves once. This is possible because the
1478 attributes of an instrumentation point (type, name, and log level) are
1479 defined statically. In other words, without a dynamic filter, the tracer
1480 _does not evaluate_ the arguments of an instrumentation point unless it
1481 matches an enabled event rule.
1483 Note that, for LTTng to record an event, the <<channel,channel>> to
1484 which a matching event rule is attached must also be enabled, and the
1485 <<tracing-session,tracing session>> owning this channel must be active
1489 .Logical path from an instrumentation point to an event record.
1490 image::event-rule.png[]
1492 .Event, event record, or event rule?
1494 With so many similar terms, it's easy to get confused.
1496 An **event** is the consequence of the execution of an _instrumentation
1497 point_, like a tracepoint that you manually place in some source code,
1498 or a Linux kernel kprobe. An event is said to _occur_ at a specific
1499 time. Different actions can be taken upon the occurrence of an event,
1500 like record the event's payload to a buffer.
1502 An **event record** is the representation of an event in a sub-buffer. A
1503 tracer is responsible for capturing the payload of an event, current
1504 context variables, the event's ID, and the event's timestamp. LTTng
1505 can append this sub-buffer to a trace file.
1507 An **event rule** is a set of conditions which must _all_ be satisfied
1508 for LTTng to record an occuring event. Events still occur without
1509 satisfying event rules, but LTTng does not record them.
1514 == Components of noch:{LTTng}
1516 The second _T_ in _LTTng_ stands for _toolkit_: it would be wrong
1517 to call LTTng a simple _tool_ since it is composed of multiple
1518 interacting components. This section describes those components,
1519 explains their respective roles, and shows how they connect together to
1520 form the LTTng ecosystem.
1522 The following diagram shows how the most important components of LTTng
1523 interact with user applications, the Linux kernel, and you:
1526 .Control and trace data paths between LTTng components.
1527 image::plumbing.png[]
1529 The LTTng project incorporates:
1531 * **LTTng-tools**: Libraries and command-line interface to
1532 control tracing sessions.
1533 ** <<lttng-sessiond,Session daemon>> (man:lttng-sessiond(8)).
1534 ** <<lttng-consumerd,Consumer daemon>> (cmd:lttng-consumerd).
1535 ** <<lttng-relayd,Relay daemon>> (man:lttng-relayd(8)).
1536 ** <<liblttng-ctl-lttng,Tracing control library>> (`liblttng-ctl`).
1537 ** <<lttng-cli,Tracing control command-line tool>> (man:lttng(1)).
1538 * **LTTng-UST**: Libraries and Java/Python packages to trace user
1540 ** <<lttng-ust,User space tracing library>> (`liblttng-ust`) and its
1541 headers to instrument and trace any native user application.
1542 ** <<prebuilt-ust-helpers,Preloadable user space tracing helpers>>:
1543 *** `liblttng-ust-libc-wrapper`
1544 *** `liblttng-ust-pthread-wrapper`
1545 *** `liblttng-ust-cyg-profile`
1546 *** `liblttng-ust-cyg-profile-fast`
1547 *** `liblttng-ust-dl`
1548 ** User space tracepoint provider source files generator command-line
1549 tool (man:lttng-gen-tp(1)).
1550 ** <<lttng-ust-agents,LTTng-UST Java agent>> to instrument and trace
1551 Java applications using `java.util.logging` or
1552 Apache log4j{nbsp}1.2 logging.
1553 ** <<lttng-ust-agents,LTTng-UST Python agent>> to instrument
1554 Python applications using the standard `logging` package.
1555 * **LTTng-modules**: <<lttng-modules,Linux kernel modules>> to trace
1557 ** LTTng kernel tracer module.
1558 ** Tracing ring buffer kernel modules.
1559 ** Probe kernel modules.
1560 ** LTTng logger kernel module.
1564 === Tracing control command-line interface
1567 .The tracing control command-line interface.
1568 image::plumbing-lttng-cli.png[]
1570 The _man:lttng(1) command-line tool_ is the standard user interface to
1571 control LTTng <<tracing-session,tracing sessions>>. The cmd:lttng tool
1572 is part of LTTng-tools.
1574 The cmd:lttng tool is linked with
1575 <<liblttng-ctl-lttng,`liblttng-ctl`>> to communicate with
1576 one or more <<lttng-sessiond,session daemons>> behind the scenes.
1578 The cmd:lttng tool has a Git-like interface:
1582 $ lttng <GENERAL OPTIONS> <COMMAND> <COMMAND OPTIONS>
1585 The <<controlling-tracing,Tracing control>> section explores the
1586 available features of LTTng using the cmd:lttng tool.
1589 [[liblttng-ctl-lttng]]
1590 === Tracing control library
1593 .The tracing control library.
1594 image::plumbing-liblttng-ctl.png[]
1596 The _LTTng control library_, `liblttng-ctl`, is used to communicate
1597 with a <<lttng-sessiond,session daemon>> using a C API that hides the
1598 underlying protocol's details. `liblttng-ctl` is part of LTTng-tools.
1600 The <<lttng-cli,cmd:lttng command-line tool>>
1601 is linked with `liblttng-ctl`.
1603 You can use `liblttng-ctl` in C or $$C++$$ source code by including its
1608 #include <lttng/lttng.h>
1611 Some objects are referenced by name (C string), such as tracing
1612 sessions, but most of them require to create a handle first using
1613 `lttng_create_handle()`.
1615 The best available developer documentation for `liblttng-ctl` is, as of
1616 LTTng{nbsp}{revision}, its installed header files. Every function and
1617 structure is thoroughly documented.
1621 === User space tracing library
1624 .The user space tracing library.
1625 image::plumbing-liblttng-ust.png[]
1627 The _user space tracing library_, `liblttng-ust` (see man:lttng-ust(3)),
1628 is the LTTng user space tracer. It receives commands from a
1629 <<lttng-sessiond,session daemon>>, for example to
1630 enable and disable specific instrumentation points, and writes event
1631 records to ring buffers shared with a
1632 <<lttng-consumerd,consumer daemon>>.
1633 `liblttng-ust` is part of LTTng-UST.
1635 Public C header files are installed beside `liblttng-ust` to
1636 instrument any <<c-application,C or $$C++$$ application>>.
1638 <<lttng-ust-agents,LTTng-UST agents>>, which are regular Java and Python
1639 packages, use their own library providing tracepoints which is
1640 linked with `liblttng-ust`.
1642 An application or library does not have to initialize `liblttng-ust`
1643 manually: its constructor does the necessary tasks to properly register
1644 to a session daemon. The initialization phase also enables the
1645 instrumentation points matching the <<event,event rules>> that you
1649 [[lttng-ust-agents]]
1650 === User space tracing agents
1653 .The user space tracing agents.
1654 image::plumbing-lttng-ust-agents.png[]
1656 The _LTTng-UST Java and Python agents_ are regular Java and Python
1657 packages which add LTTng tracing capabilities to the
1658 native logging frameworks. The LTTng-UST agents are part of LTTng-UST.
1660 In the case of Java, the
1661 https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[`java.util.logging`
1662 core logging facilities] and
1663 https://logging.apache.org/log4j/1.2/[Apache log4j{nbsp}1.2] are supported.
1664 Note that Apache Log4{nbsp}2 is not supported.
1666 In the case of Python, the standard
1667 https://docs.python.org/3/library/logging.html[`logging`] package
1668 is supported. Both Python{nbsp}2 and Python{nbsp}3 modules can import the
1669 LTTng-UST Python agent package.
1671 The applications using the LTTng-UST agents are in the
1672 `java.util.logging` (JUL),
1673 log4j, and Python <<domain,tracing domains>>.
1675 Both agents use the same mechanism to trace the log statements. When an
1676 agent initializes, it creates a log handler that attaches to the root
1677 logger. The agent also registers to a <<lttng-sessiond,session daemon>>.
1678 When the application executes a log statement, the root logger passes it
1679 to the agent's log handler. The agent's log handler calls a native
1680 function in a tracepoint provider package shared library linked with
1681 <<lttng-ust,`liblttng-ust`>>, passing the formatted log message and
1682 other fields, like its logger name and its log level. This native
1683 function contains a user space instrumentation point, hence tracing the
1686 The log level condition of an
1687 <<event,event rule>> is considered when tracing
1688 a Java or a Python application, and it's compatible with the standard
1689 JUL, log4j, and Python log levels.
1693 === LTTng kernel modules
1696 .The LTTng kernel modules.
1697 image::plumbing-lttng-modules.png[]
1699 The _LTTng kernel modules_ are a set of Linux kernel modules
1700 which implement the kernel tracer of the LTTng project. The LTTng
1701 kernel modules are part of LTTng-modules.
1703 The LTTng kernel modules include:
1705 * A set of _probe_ modules.
1707 Each module attaches to a specific subsystem
1708 of the Linux kernel using its tracepoint instrument points. There are
1709 also modules to attach to the entry and return points of the Linux
1710 system call functions.
1712 * _Ring buffer_ modules.
1714 A ring buffer implementation is provided as kernel modules. The LTTng
1715 kernel tracer writes to the ring buffer; a
1716 <<lttng-consumerd,consumer daemon>> reads from the ring buffer.
1718 * The _LTTng kernel tracer_ module.
1719 * The _LTTng logger_ module.
1721 The LTTng logger module implements the special path:{/proc/lttng-logger}
1722 file so that any executable can generate LTTng events by opening and
1723 writing to this file.
1725 See <<proc-lttng-logger-abi,LTTng logger>>.
1727 Generally, you do not have to load the LTTng kernel modules manually
1728 (using man:modprobe(8), for example): a root <<lttng-sessiond,session
1729 daemon>> loads the necessary modules when starting. If you have extra
1730 probe modules, you can specify to load them to the session daemon on
1733 The LTTng kernel modules are installed in
1734 +/usr/lib/modules/__release__/extra+ by default, where +__release__+ is
1735 the kernel release (see `uname --kernel-release`).
1742 .The session daemon.
1743 image::plumbing-sessiond.png[]
1745 The _session daemon_, man:lttng-sessiond(8), is a daemon responsible for
1746 managing tracing sessions and for controlling the various components of
1747 LTTng. The session daemon is part of LTTng-tools.
1749 The session daemon sends control requests to and receives control
1752 * The <<lttng-ust,user space tracing library>>.
1754 Any instance of the user space tracing library first registers to
1755 a session daemon. Then, the session daemon can send requests to
1756 this instance, such as:
1759 ** Get the list of tracepoints.
1760 ** Share an <<event,event rule>> so that the user space tracing library
1761 can enable or disable tracepoints. Amongst the possible conditions
1762 of an event rule is a filter expression which `liblttng-ust` evalutes
1763 when an event occurs.
1764 ** Share <<channel,channel>> attributes and ring buffer locations.
1767 The session daemon and the user space tracing library use a Unix
1768 domain socket for their communication.
1770 * The <<lttng-ust-agents,user space tracing agents>>.
1772 Any instance of a user space tracing agent first registers to
1773 a session daemon. Then, the session daemon can send requests to
1774 this instance, such as:
1777 ** Get the list of loggers.
1778 ** Enable or disable a specific logger.
1781 The session daemon and the user space tracing agent use a TCP connection
1782 for their communication.
1784 * The <<lttng-modules,LTTng kernel tracer>>.
1785 * The <<lttng-consumerd,consumer daemon>>.
1787 The session daemon sends requests to the consumer daemon to instruct
1788 it where to send the trace data streams, amongst other information.
1790 * The <<lttng-relayd,relay daemon>>.
1792 The session daemon receives commands from the
1793 <<liblttng-ctl-lttng,tracing control library>>.
1795 The root session daemon loads the appropriate
1796 <<lttng-modules,LTTng kernel modules>> on startup. It also spawns
1797 a <<lttng-consumerd,consumer daemon>> as soon as you create
1798 an <<event,event rule>>.
1800 The session daemon does not send and receive trace data: this is the
1801 role of the <<lttng-consumerd,consumer daemon>> and
1802 <<lttng-relayd,relay daemon>>. It does, however, generate the
1803 http://diamon.org/ctf/[CTF] metadata stream.
1805 Each Unix user can have its own session daemon instance. The
1806 tracing sessions which different session daemons manage are completely
1809 The root user's session daemon is the only one which is
1810 allowed to control the LTTng kernel tracer, and its spawned consumer
1811 daemon is the only one which is allowed to consume trace data from the
1812 LTTng kernel tracer. Note, however, that any Unix user which is a member
1813 of the <<tracing-group,tracing group>> is allowed
1814 to create <<channel,channels>> in the
1815 Linux kernel <<domain,tracing domain>>, and thus to trace the Linux
1818 The <<lttng-cli,cmd:lttng command-line tool>> automatically starts a
1819 session daemon when using its `create` command if none is currently
1820 running. You can also start the session daemon manually.
1827 .The consumer daemon.
1828 image::plumbing-consumerd.png[]
1830 The _consumer daemon_, cmd:lttng-consumerd, is a daemon which shares
1831 ring buffers with user applications or with the LTTng kernel modules to
1832 collect trace data and send it to some location (on disk or to a
1833 <<lttng-relayd,relay daemon>> over the network). The consumer daemon
1834 is part of LTTng-tools.
1836 You do not start a consumer daemon manually: a consumer daemon is always
1837 spawned by a <<lttng-sessiond,session daemon>> as soon as you create an
1838 <<event,event rule>>, that is, before you start tracing. When you kill
1839 its owner session daemon, the consumer daemon also exits because it is
1840 the session daemon's child process. Command-line options of
1841 man:lttng-sessiond(8) target the consumer daemon process.
1843 There are up to two running consumer daemons per Unix user, whereas only
1844 one session daemon can run per user. This is because each process can be
1845 either 32-bit or 64-bit: if the target system runs a mixture of 32-bit
1846 and 64-bit processes, it is more efficient to have separate
1847 corresponding 32-bit and 64-bit consumer daemons. The root user is an
1848 exception: it can have up to _three_ running consumer daemons: 32-bit
1849 and 64-bit instances for its user applications, and one more
1850 reserved for collecting kernel trace data.
1858 image::plumbing-relayd.png[]
1860 The _relay daemon_, man:lttng-relayd(8), is a daemon acting as a bridge
1861 between remote session and consumer daemons, local trace files, and a
1862 remote live trace viewer. The relay daemon is part of LTTng-tools.
1864 The main purpose of the relay daemon is to implement a receiver of
1865 <<sending-trace-data-over-the-network,trace data over the network>>.
1866 This is useful when the target system does not have much file system
1867 space to record trace files locally.
1869 The relay daemon is also a server to which a
1870 <<lttng-live,live trace viewer>> can
1871 connect. The live trace viewer sends requests to the relay daemon to
1872 receive trace data as the target system emits events. The
1873 communication protocol is named _LTTng live_; it is used over TCP
1876 Note that you can start the relay daemon on the target system directly.
1877 This is the setup of choice when the use case is to view events as
1878 the target system emits them without the need of a remote system.
1882 == [[using-lttng]]Instrumentation
1884 There are many examples of tracing and monitoring in our everyday life:
1886 * You have access to real-time and historical weather reports and
1887 forecasts thanks to weather stations installed around the country.
1888 * You know your heart is safe thanks to an electrocardiogram.
1889 * You make sure not to drive your car too fast and to have enough fuel
1890 to reach your destination thanks to gauges visible on your dashboard.
1892 All the previous examples have something in common: they rely on
1893 **instruments**. Without the electrodes attached to the surface of your
1894 body's skin, cardiac monitoring is futile.
1896 LTTng, as a tracer, is no different from those real life examples. If
1897 you're about to trace a software system or, in other words, record its
1898 history of execution, you better have **instrumentation points** in the
1899 subject you're tracing, that is, the actual software.
1901 Various ways were developed to instrument a piece of software for LTTng
1902 tracing. The most straightforward one is to manually place
1903 instrumentation points, called _tracepoints_, in the software's source
1904 code. It is also possible to add instrumentation points dynamically in
1905 the Linux kernel <<domain,tracing domain>>.
1907 If you're only interested in tracing the Linux kernel, your
1908 instrumentation needs are probably already covered by LTTng's built-in
1909 <<lttng-modules,Linux kernel tracepoints>>. You may also wish to trace a
1910 user application which is already instrumented for LTTng tracing.
1911 In such cases, you can skip this whole section and read the topics of
1912 the <<controlling-tracing,Tracing control>> section.
1914 Many methods are available to instrument a piece of software for LTTng
1917 * <<c-application,User space instrumentation for C and $$C++$$
1919 * <<prebuilt-ust-helpers,Prebuilt user space tracing helpers>>.
1920 * <<java-application,User space Java agent>>.
1921 * <<python-application,User space Python agent>>.
1922 * <<proc-lttng-logger-abi,LTTng logger>>.
1923 * <<instrumenting-linux-kernel,LTTng kernel tracepoints>>.
1927 === [[cxx-application]]User space instrumentation for C and $$C++$$ applications
1929 The procedure to instrument a C or $$C++$$ user application with
1930 the <<lttng-ust,LTTng user space tracing library>>, `liblttng-ust`, is:
1932 . <<tracepoint-provider,Create the source files of a tracepoint provider
1934 . <<probing-the-application-source-code,Add tracepoints to
1935 the application's source code>>.
1936 . <<building-tracepoint-providers-and-user-application,Build and link
1937 a tracepoint provider package and the user application>>.
1939 If you need quick, man:printf(3)-like instrumentation, you can skip
1940 those steps and use <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>>
1943 IMPORTANT: You need to <<installing-lttng,install>> LTTng-UST to
1944 instrument a user application with `liblttng-ust`.
1947 [[tracepoint-provider]]
1948 ==== Create the source files of a tracepoint provider package
1950 A _tracepoint provider_ is a set of compiled functions which provide
1951 **tracepoints** to an application, the type of instrumentation point
1952 supported by LTTng-UST. Those functions can emit events with
1953 user-defined fields and serialize those events as event records to one
1954 or more LTTng-UST <<channel,channel>> sub-buffers. The `tracepoint()`
1955 macro, which you <<probing-the-application-source-code,insert in a user
1956 application's source code>>, calls those functions.
1958 A _tracepoint provider package_ is an object file (`.o`) or a shared
1959 library (`.so`) which contains one or more tracepoint providers.
1960 Its source files are:
1962 * One or more <<tpp-header,tracepoint provider header>> (`.h`).
1963 * A <<tpp-source,tracepoint provider package source>> (`.c`).
1965 A tracepoint provider package is dynamically linked with `liblttng-ust`,
1966 the LTTng user space tracer, at run time.
1969 .User application linked with `liblttng-ust` and containing a tracepoint provider.
1970 image::ust-app.png[]
1972 NOTE: If you need quick, man:printf(3)-like instrumentation, you can
1973 skip creating and using a tracepoint provider and use
1974 <<tracef,`tracef()`>> or <<tracelog,`tracelog()`>> instead.
1978 ===== Create a tracepoint provider header file template
1980 A _tracepoint provider header file_ contains the tracepoint
1981 definitions of a tracepoint provider.
1983 To create a tracepoint provider header file:
1985 . Start from this template:
1989 .Tracepoint provider header file template (`.h` file extension).
1991 #undef TRACEPOINT_PROVIDER
1992 #define TRACEPOINT_PROVIDER provider_name
1994 #undef TRACEPOINT_INCLUDE
1995 #define TRACEPOINT_INCLUDE "./tp.h"
1997 #if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ)
2000 #include <lttng/tracepoint.h>
2003 * Use TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(),
2004 * TRACEPOINT_EVENT_INSTANCE(), and TRACEPOINT_LOGLEVEL() here.
2009 #include <lttng/tracepoint-event.h>
2015 * `provider_name` with the name of your tracepoint provider.
2016 * `"tp.h"` with the name of your tracepoint provider header file.
2018 . Below the `#include <lttng/tracepoint.h>` line, put your
2019 <<defining-tracepoints,tracepoint definitions>>.
2021 Your tracepoint provider name must be unique amongst all the possible
2022 tracepoint provider names used on the same target system. We
2023 suggest to include the name of your project or company in the name,
2024 for example, `org_lttng_my_project_tpp`.
2026 TIP: [[lttng-gen-tp]]You can use the man:lttng-gen-tp(1) tool to create
2027 this boilerplate for you. When using cmd:lttng-gen-tp, all you need to
2028 write are the <<defining-tracepoints,tracepoint definitions>>.
2031 [[defining-tracepoints]]
2032 ===== Create a tracepoint definition
2034 A _tracepoint definition_ defines, for a given tracepoint:
2036 * Its **input arguments**. They are the macro parameters that the
2037 `tracepoint()` macro accepts for this particular tracepoint
2038 in the user application's source code.
2039 * Its **output event fields**. They are the sources of event fields
2040 that form the payload of any event that the execution of the
2041 `tracepoint()` macro emits for this particular tracepoint.
2043 You can create a tracepoint definition by using the
2044 `TRACEPOINT_EVENT()` macro below the `#include <lttng/tracepoint.h>`
2046 <<tpp-header,tracepoint provider header file template>>.
2048 The syntax of the `TRACEPOINT_EVENT()` macro is:
2051 .`TRACEPOINT_EVENT()` macro syntax.
2054 /* Tracepoint provider name */
2057 /* Tracepoint name */
2060 /* Input arguments */
2065 /* Output event fields */
2074 * `provider_name` with your tracepoint provider name.
2075 * `tracepoint_name` with your tracepoint name.
2076 * `arguments` with the <<tpp-def-input-args,input arguments>>.
2077 * `fields` with the <<tpp-def-output-fields,output event field>>
2080 This tracepoint emits events named `provider_name:tracepoint_name`.
2083 .Event name's length limitation
2085 The concatenation of the tracepoint provider name and the
2086 tracepoint name must not exceed **254{nbsp}characters**. If it does, the
2087 instrumented application compiles and runs, but LTTng throws multiple
2088 warnings and you could experience serious issues.
2091 [[tpp-def-input-args]]The syntax of the `TP_ARGS()` macro is:
2094 .`TP_ARGS()` macro syntax.
2103 * `type` with the C type of the argument.
2104 * `arg_name` with the argument name.
2106 You can repeat `type` and `arg_name` up to 10{nbsp}times to have
2107 more than one argument.
2109 .`TP_ARGS()` usage with three arguments.
2121 The `TP_ARGS()` and `TP_ARGS(void)` forms are valid to create a
2122 tracepoint definition with no input arguments.
2124 [[tpp-def-output-fields]]The `TP_FIELDS()` macro contains a list of
2125 `ctf_*()` macros. Each `ctf_*()` macro defines one event field. See
2126 man:lttng-ust(3) for a complete description of the available `ctf_*()`
2127 macros. A `ctf_*()` macro specifies the type, size, and byte order of
2130 Each `ctf_*()` macro takes an _argument expression_ parameter. This is a
2131 C expression that the tracer evalutes at the `tracepoint()` macro site
2132 in the application's source code. This expression provides a field's
2133 source of data. The argument expression can include input argument names
2134 listed in the `TP_ARGS()` macro.
2136 Each `ctf_*()` macro also takes a _field name_ parameter. Field names
2137 must be unique within a given tracepoint definition.
2139 Here's a complete tracepoint definition example:
2141 .Tracepoint definition.
2143 The following tracepoint definition defines a tracepoint which takes
2144 three input arguments and has four output event fields.
2148 #include "my-custom-structure.h"
2154 const struct my_custom_structure*, my_custom_structure,
2159 ctf_string(query_field, query)
2160 ctf_float(double, ratio_field, ratio)
2161 ctf_integer(int, recv_size, my_custom_structure->recv_size)
2162 ctf_integer(int, send_size, my_custom_structure->send_size)
2167 You can refer to this tracepoint definition with the `tracepoint()`
2168 macro in your application's source code like this:
2172 tracepoint(my_provider, my_tracepoint,
2173 my_structure, some_ratio, the_query);
2177 NOTE: The LTTng tracer only evaluates tracepoint arguments at run time
2178 if they satisfy an enabled <<event,event rule>>.
2181 [[using-tracepoint-classes]]
2182 ===== Use a tracepoint class
2184 A _tracepoint class_ is a class of tracepoints which share the same
2185 output event field definitions. A _tracepoint instance_ is one
2186 instance of such a defined tracepoint class, with its own tracepoint
2189 The <<defining-tracepoints,`TRACEPOINT_EVENT()` macro>> is actually a
2190 shorthand which defines both a tracepoint class and a tracepoint
2191 instance at the same time.
2193 When you build a tracepoint provider package, the C or $$C++$$ compiler
2194 creates one serialization function for each **tracepoint class**. A
2195 serialization function is responsible for serializing the event fields
2196 of a tracepoint to a sub-buffer when tracing.
2198 For various performance reasons, when your situation requires multiple
2199 tracepoint definitions with different names, but with the same event
2200 fields, we recommend that you manually create a tracepoint class
2201 and instantiate as many tracepoint instances as needed. One positive
2202 effect of such a design, amongst other advantages, is that all
2203 tracepoint instances of the same tracepoint class reuse the same
2204 serialization function, thus reducing
2205 https://en.wikipedia.org/wiki/Cache_pollution[cache pollution].
2207 .Use a tracepoint class and tracepoint instances.
2209 Consider the following three tracepoint definitions:
2221 ctf_integer(int, userid, userid)
2222 ctf_integer(size_t, len, len)
2234 ctf_integer(int, userid, userid)
2235 ctf_integer(size_t, len, len)
2247 ctf_integer(int, userid, userid)
2248 ctf_integer(size_t, len, len)
2253 In this case, we create three tracepoint classes, with one implicit
2254 tracepoint instance for each of them: `get_account`, `get_settings`, and
2255 `get_transaction`. However, they all share the same event field names
2256 and types. Hence three identical, yet independent serialization
2257 functions are created when you build the tracepoint provider package.
2259 A better design choice is to define a single tracepoint class and three
2260 tracepoint instances:
2264 /* The tracepoint class */
2265 TRACEPOINT_EVENT_CLASS(
2266 /* Tracepoint provider name */
2269 /* Tracepoint class name */
2272 /* Input arguments */
2278 /* Output event fields */
2280 ctf_integer(int, userid, userid)
2281 ctf_integer(size_t, len, len)
2285 /* The tracepoint instances */
2286 TRACEPOINT_EVENT_INSTANCE(
2287 /* Tracepoint provider name */
2290 /* Tracepoint class name */
2293 /* Tracepoint name */
2296 /* Input arguments */
2302 TRACEPOINT_EVENT_INSTANCE(
2311 TRACEPOINT_EVENT_INSTANCE(
2324 [[assigning-log-levels]]
2325 ===== Assign a log level to a tracepoint definition
2327 You can assign an optional _log level_ to a
2328 <<defining-tracepoints,tracepoint definition>>.
2330 Assigning different levels of severity to tracepoint definitions can
2331 be useful: when you <<enabling-disabling-events,create an event rule>>,
2332 you can target tracepoints having a log level as severe as a specific
2335 The concept of LTTng-UST log levels is similar to the levels found
2336 in typical logging frameworks:
2338 * In a logging framework, the log level is given by the function
2339 or method name you use at the log statement site: `debug()`,
2340 `info()`, `warn()`, `error()`, and so on.
2341 * In LTTng-UST, you statically assign the log level to a tracepoint
2342 definition; any `tracepoint()` macro invocation which refers to
2343 this definition has this log level.
2345 You can assign a log level to a tracepoint definition with the
2346 `TRACEPOINT_LOGLEVEL()` macro. You must use this macro _after_ the
2347 <<defining-tracepoints,`TRACEPOINT_EVENT()`>> or
2348 <<using-tracepoint-classes,`TRACEPOINT_INSTANCE()`>> macro for a given
2351 The syntax of the `TRACEPOINT_LOGLEVEL()` macro is:
2354 .`TRACEPOINT_LOGLEVEL()` macro syntax.
2356 TRACEPOINT_LOGLEVEL(provider_name, tracepoint_name, log_level)
2361 * `provider_name` with the tracepoint provider name.
2362 * `tracepoint_name` with the tracepoint name.
2363 * `log_level` with the log level to assign to the tracepoint
2364 definition named `tracepoint_name` in the `provider_name`
2365 tracepoint provider.
2367 See man:lttng-ust(3) for a list of available log level names.
2369 .Assign the `TRACE_DEBUG_UNIT` log level to a tracepoint definition.
2373 /* Tracepoint definition */
2382 ctf_integer(int, userid, userid)
2383 ctf_integer(size_t, len, len)
2387 /* Log level assignment */
2388 TRACEPOINT_LOGLEVEL(my_app, get_transaction, TRACE_DEBUG_UNIT)
2394 ===== Create a tracepoint provider package source file
2396 A _tracepoint provider package source file_ is a C source file which
2397 includes a <<tpp-header,tracepoint provider header file>> to expand its
2398 macros into event serialization and other functions.
2400 You can always use the following tracepoint provider package source
2404 .Tracepoint provider package source file template.
2406 #define TRACEPOINT_CREATE_PROBES
2411 Replace `tp.h` with the name of your <<tpp-header,tracepoint provider
2412 header file>> name. You may also include more than one tracepoint
2413 provider header file here to create a tracepoint provider package
2414 holding more than one tracepoint providers.
2417 [[probing-the-application-source-code]]
2418 ==== Add tracepoints to an application's source code
2420 Once you <<tpp-header,create a tracepoint provider header file>>, you
2421 can use the `tracepoint()` macro in your application's
2422 source code to insert the tracepoints that this header
2423 <<defining-tracepoints,defines>>.
2425 The `tracepoint()` macro takes at least two parameters: the tracepoint
2426 provider name and the tracepoint name. The corresponding tracepoint
2427 definition defines the other parameters.
2429 .`tracepoint()` usage.
2431 The following <<defining-tracepoints,tracepoint definition>> defines a
2432 tracepoint which takes two input arguments and has two output event
2436 .Tracepoint provider header file.
2438 #include "my-custom-structure.h"
2445 const char*, cmd_name
2448 ctf_string(cmd_name, cmd_name)
2449 ctf_integer(int, number_of_args, argc)
2454 You can refer to this tracepoint definition with the `tracepoint()`
2455 macro in your application's source code like this:
2458 .Application's source file.
2462 int main(int argc, char* argv[])
2464 tracepoint(my_provider, my_tracepoint, argc, argv[0]);
2470 Note how the application's source code includes
2471 the tracepoint provider header file containing the tracepoint
2472 definitions to use, path:{tp.h}.
2475 .`tracepoint()` usage with a complex tracepoint definition.
2477 Consider this complex tracepoint definition, where multiple event
2478 fields refer to the same input arguments in their argument expression
2482 .Tracepoint provider header file.
2484 /* For `struct stat` */
2485 #include <sys/types.h>
2486 #include <sys/stat.h>
2498 ctf_integer(int, my_constant_field, 23 + 17)
2499 ctf_integer(int, my_int_arg_field, my_int_arg)
2500 ctf_integer(int, my_int_arg_field2, my_int_arg * my_int_arg)
2501 ctf_integer(int, sum4_field, my_str_arg[0] + my_str_arg[1] +
2502 my_str_arg[2] + my_str_arg[3])
2503 ctf_string(my_str_arg_field, my_str_arg)
2504 ctf_integer_hex(off_t, size_field, st->st_size)
2505 ctf_float(double, size_dbl_field, (double) st->st_size)
2506 ctf_sequence_text(char, half_my_str_arg_field, my_str_arg,
2507 size_t, strlen(my_str_arg) / 2)
2512 You can refer to this tracepoint definition with the `tracepoint()`
2513 macro in your application's source code like this:
2516 .Application's source file.
2518 #define TRACEPOINT_DEFINE
2525 stat("/etc/fstab", &s);
2526 tracepoint(my_provider, my_tracepoint, 23, "Hello, World!", &s);
2532 If you look at the event record that LTTng writes when tracing this
2533 program, assuming the file size of path:{/etc/fstab} is 301{nbsp}bytes,
2534 it should look like this:
2536 .Event record fields
2538 |Field's name |Field's value
2539 |`my_constant_field` |40
2540 |`my_int_arg_field` |23
2541 |`my_int_arg_field2` |529
2543 |`my_str_arg_field` |`Hello, World!`
2544 |`size_field` |0x12d
2545 |`size_dbl_field` |301.0
2546 |`half_my_str_arg_field` |`Hello,`
2550 Sometimes, the arguments you pass to `tracepoint()` are expensive to
2551 compute--they use the call stack, for example. To avoid this
2552 computation when the tracepoint is disabled, you can use the
2553 `tracepoint_enabled()` and `do_tracepoint()` macros.
2555 The syntax of the `tracepoint_enabled()` and `do_tracepoint()` macros
2559 .`tracepoint_enabled()` and `do_tracepoint()` macros syntax.
2561 tracepoint_enabled(provider_name, tracepoint_name)
2562 do_tracepoint(provider_name, tracepoint_name, ...)
2567 * `provider_name` with the tracepoint provider name.
2568 * `tracepoint_name` with the tracepoint name.
2570 `tracepoint_enabled()` returns a non-zero value if the tracepoint named
2571 `tracepoint_name` from the provider named `provider_name` is enabled
2574 `do_tracepoint()` is like `tracepoint()`, except that it doesn't check
2575 if the tracepoint is enabled. Using `tracepoint()` with
2576 `tracepoint_enabled()` is dangerous since `tracepoint()` also contains
2577 the `tracepoint_enabled()` check, thus a race condition is
2578 possible in this situation:
2581 .Possible race condition when using `tracepoint_enabled()` with `tracepoint()`.
2583 if (tracepoint_enabled(my_provider, my_tracepoint)) {
2584 stuff = prepare_stuff();
2587 tracepoint(my_provider, my_tracepoint, stuff);
2590 If the tracepoint is enabled after the condition, then `stuff` is not
2591 prepared: the emitted event will either contain wrong data, or the whole
2592 application could crash (segmentation fault, for example).
2594 NOTE: Neither `tracepoint_enabled()` nor `do_tracepoint()` have an
2595 `STAP_PROBEV()` call. If you need it, you must emit
2599 [[building-tracepoint-providers-and-user-application]]
2600 ==== Build and link a tracepoint provider package and an application
2602 Once you have one or more <<tpp-header,tracepoint provider header
2603 files>> and a <<tpp-source,tracepoint provider package source file>>,
2604 you can create the tracepoint provider package by compiling its source
2605 file. From here, multiple build and run scenarios are possible. The
2606 following table shows common application and library configurations
2607 along with the required command lines to achieve them.
2609 In the following diagrams, we use the following file names:
2612 Executable application.
2615 Application's object file.
2618 Tracepoint provider package object file.
2621 Tracepoint provider package archive file.
2624 Tracepoint provider package shared object file.
2627 User library object file.
2630 User library shared object file.
2632 We use the following symbols in the diagrams of table below:
2635 .Symbols used in the build scenario diagrams.
2636 image::ust-sit-symbols.png[]
2638 We assume that path:{.} is part of the env:LD_LIBRARY_PATH environment
2639 variable in the following instructions.
2641 [role="growable ust-scenarios",cols="asciidoc,asciidoc"]
2642 .Common tracepoint provider package scenarios.
2644 |Scenario |Instructions
2647 The instrumented application is statically linked with
2648 the tracepoint provider package object.
2650 image::ust-sit+app-linked-with-tp-o+app-instrumented.png[]
2653 include::../common/ust-sit-step-tp-o.txt[]
2655 To build the instrumented application:
2657 . In path:{app.c}, before including path:{tpp.h}, add the following line:
2662 #define TRACEPOINT_DEFINE
2666 . Compile the application source file:
2675 . Build the application:
2680 $ gcc -o app app.o tpp.o -llttng-ust -ldl
2684 To run the instrumented application:
2686 * Start the application:
2696 The instrumented application is statically linked with the
2697 tracepoint provider package archive file.
2699 image::ust-sit+app-linked-with-tp-a+app-instrumented.png[]
2702 To create the tracepoint provider package archive file:
2704 . Compile the <<tpp-source,tracepoint provider package source file>>:
2713 . Create the tracepoint provider package archive file:
2718 $ ar rcs tpp.a tpp.o
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.a -llttng-ust -ldl
2751 To run the instrumented application:
2753 * Start the application:
2763 The instrumented application is linked with the tracepoint provider
2764 package shared object.
2766 image::ust-sit+app-linked-with-tp-so+app-instrumented.png[]
2769 include::../common/ust-sit-step-tp-so.txt[]
2771 To build the instrumented application:
2773 . In path:{app.c}, before including path:{tpp.h}, add the following line:
2778 #define TRACEPOINT_DEFINE
2782 . Compile the application source file:
2791 . Build the application:
2796 $ gcc -o app app.o -ldl -L. -ltpp
2800 To run the instrumented application:
2802 * Start the application:
2812 The tracepoint provider package shared object is preloaded before the
2813 instrumented application starts.
2815 image::ust-sit+tp-so-preloaded+app-instrumented.png[]
2818 include::../common/ust-sit-step-tp-so.txt[]
2820 To build the instrumented application:
2822 . In path:{app.c}, before including path:{tpp.h}, add the
2828 #define TRACEPOINT_DEFINE
2829 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
2833 . Compile the application source file:
2842 . Build the application:
2847 $ gcc -o app app.o -ldl
2851 To run the instrumented application with tracing support:
2853 * Preload the tracepoint provider package shared object and
2854 start the application:
2859 $ LD_PRELOAD=./libtpp.so ./app
2863 To run the instrumented application without tracing support:
2865 * Start the application:
2875 The instrumented application dynamically loads the tracepoint provider
2876 package shared object.
2878 image::ust-sit+app-dlopens-tp-so+app-instrumented.png[]
2881 include::../common/ust-sit-step-tp-so.txt[]
2883 To build the instrumented application:
2885 . In path:{app.c}, before including path:{tpp.h}, add the
2891 #define TRACEPOINT_DEFINE
2892 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
2896 . Compile the application source file:
2905 . Build the application:
2910 $ gcc -o app app.o -ldl
2914 To run the instrumented application:
2916 * Start the application:
2926 The application is linked with the instrumented user library.
2928 The instrumented user library is statically linked with the tracepoint
2929 provider package object file.
2931 image::ust-sit+app-linked-with-lib+lib-linked-with-tp-o+lib-instrumented.png[]
2934 include::../common/ust-sit-step-tp-o-fpic.txt[]
2936 To build the instrumented user library:
2938 . In path:{emon.c}, before including path:{tpp.h}, add the
2944 #define TRACEPOINT_DEFINE
2948 . Compile the user library source file:
2953 $ gcc -I. -fpic -c emon.c
2957 . Build the user library shared object:
2962 $ gcc -shared -o libemon.so emon.o tpp.o -llttng-ust -ldl
2966 To build the application:
2968 . Compile the application source file:
2977 . Build the application:
2982 $ gcc -o app app.o -L. -lemon
2986 To run the application:
2988 * Start the application:
2998 The application is linked with the instrumented user library.
3000 The instrumented user library is linked with the tracepoint provider
3001 package shared object.
3003 image::ust-sit+app-linked-with-lib+lib-linked-with-tp-so+lib-instrumented.png[]
3006 include::../common/ust-sit-step-tp-so.txt[]
3008 To build the instrumented user library:
3010 . In path:{emon.c}, before including path:{tpp.h}, add the
3016 #define TRACEPOINT_DEFINE
3020 . Compile the user library source file:
3025 $ gcc -I. -fpic -c emon.c
3029 . Build the user library shared object:
3034 $ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
3038 To build the application:
3040 . Compile the application source file:
3049 . Build the application:
3054 $ gcc -o app app.o -L. -lemon
3058 To run the application:
3060 * Start the application:
3070 The tracepoint provider package shared object is preloaded before the
3073 The application is linked with the instrumented user library.
3075 image::ust-sit+tp-so-preloaded+app-linked-with-lib+lib-instrumented.png[]
3078 include::../common/ust-sit-step-tp-so.txt[]
3080 To build the instrumented user library:
3082 . In path:{emon.c}, before including path:{tpp.h}, add the
3088 #define TRACEPOINT_DEFINE
3089 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3093 . Compile the user library source file:
3098 $ gcc -I. -fpic -c emon.c
3102 . Build the user library shared object:
3107 $ gcc -shared -o libemon.so emon.o -ldl
3111 To build the application:
3113 . Compile the application source file:
3122 . Build the application:
3127 $ gcc -o app app.o -L. -lemon
3131 To run the application with tracing support:
3133 * Preload the tracepoint provider package shared object and
3134 start the application:
3139 $ LD_PRELOAD=./libtpp.so ./app
3143 To run the application without tracing support:
3145 * Start the application:
3155 The application is linked with the instrumented user library.
3157 The instrumented user library dynamically loads the tracepoint provider
3158 package shared object.
3160 image::ust-sit+app-linked-with-lib+lib-dlopens-tp-so+lib-instrumented.png[]
3163 include::../common/ust-sit-step-tp-so.txt[]
3165 To build the instrumented user library:
3167 . In path:{emon.c}, before including path:{tpp.h}, add the
3173 #define TRACEPOINT_DEFINE
3174 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3178 . Compile the user library source file:
3183 $ gcc -I. -fpic -c emon.c
3187 . Build the user library shared object:
3192 $ gcc -shared -o libemon.so emon.o -ldl
3196 To build the application:
3198 . Compile the application source file:
3207 . Build the application:
3212 $ gcc -o app app.o -L. -lemon
3216 To run the application:
3218 * Start the application:
3228 The application dynamically loads the instrumented user library.
3230 The instrumented user library is linked with the tracepoint provider
3231 package shared object.
3233 image::ust-sit+app-dlopens-lib+lib-linked-with-tp-so+lib-instrumented.png[]
3236 include::../common/ust-sit-step-tp-so.txt[]
3238 To build the instrumented user library:
3240 . In path:{emon.c}, before including path:{tpp.h}, add the
3246 #define TRACEPOINT_DEFINE
3250 . Compile the user library source file:
3255 $ gcc -I. -fpic -c emon.c
3259 . Build the user library shared object:
3264 $ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp
3268 To build the application:
3270 . Compile the application source file:
3279 . Build the application:
3284 $ gcc -o app app.o -ldl -L. -lemon
3288 To run the application:
3290 * Start the application:
3300 The application dynamically loads the instrumented user library.
3302 The instrumented user library dynamically loads the tracepoint provider
3303 package shared object.
3305 image::ust-sit+app-dlopens-lib+lib-dlopens-tp-so+lib-instrumented.png[]
3308 include::../common/ust-sit-step-tp-so.txt[]
3310 To build the instrumented user library:
3312 . In path:{emon.c}, before including path:{tpp.h}, add the
3318 #define TRACEPOINT_DEFINE
3319 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3323 . Compile the user library source file:
3328 $ gcc -I. -fpic -c emon.c
3332 . Build the user library shared object:
3337 $ gcc -shared -o libemon.so emon.o -ldl
3341 To build the application:
3343 . Compile the application source file:
3352 . Build the application:
3357 $ gcc -o app app.o -ldl -L. -lemon
3361 To run the application:
3363 * Start the application:
3373 The tracepoint provider package shared object is preloaded before the
3376 The application dynamically loads the instrumented user library.
3378 image::ust-sit+tp-so-preloaded+app-dlopens-lib+lib-instrumented.png[]
3381 include::../common/ust-sit-step-tp-so.txt[]
3383 To build the instrumented user library:
3385 . In path:{emon.c}, before including path:{tpp.h}, add the
3391 #define TRACEPOINT_DEFINE
3392 #define TRACEPOINT_PROBE_DYNAMIC_LINKAGE
3396 . Compile the user library source file:
3401 $ gcc -I. -fpic -c emon.c
3405 . Build the user library shared object:
3410 $ gcc -shared -o libemon.so emon.o -ldl
3414 To build the application:
3416 . Compile the application source file:
3425 . Build the application:
3430 $ gcc -o app app.o -L. -lemon
3434 To run the application with tracing support:
3436 * Preload the tracepoint provider package shared object and
3437 start the application:
3442 $ LD_PRELOAD=./libtpp.so ./app
3446 To run the application without tracing support:
3448 * Start the application:
3458 The application is statically linked with the tracepoint provider
3459 package object file.
3461 The application is linked with the instrumented user library.
3463 image::ust-sit+app-linked-with-tp-o+app-linked-with-lib+lib-instrumented.png[]
3466 include::../common/ust-sit-step-tp-o.txt[]
3468 To build the instrumented user library:
3470 . In path:{emon.c}, before including path:{tpp.h}, add the
3476 #define TRACEPOINT_DEFINE
3480 . Compile the user library source file:
3485 $ gcc -I. -fpic -c emon.c
3489 . Build the user library shared object:
3494 $ gcc -shared -o libemon.so emon.o
3498 To build the application:
3500 . Compile the application source file:
3509 . Build the application:
3514 $ gcc -o app app.o tpp.o -llttng-ust -ldl -L. -lemon
3518 To run the instrumented application:
3520 * Start the application:
3530 The application is statically linked with the tracepoint provider
3531 package object file.
3533 The application dynamically loads the instrumented user library.
3535 image::ust-sit+app-linked-with-tp-o+app-dlopens-lib+lib-instrumented.png[]
3538 include::../common/ust-sit-step-tp-o.txt[]
3540 To build the application:
3542 . In path:{app.c}, before including path:{tpp.h}, add the following line:
3547 #define TRACEPOINT_DEFINE
3551 . Compile the application source file:
3560 . Build the application:
3565 $ gcc -Wl,--export-dynamic -o app app.o tpp.o \
3570 The `--export-dynamic` option passed to the linker is necessary for the
3571 dynamically loaded library to ``see'' the tracepoint symbols defined in
3574 To build the instrumented user library:
3576 . Compile the user library source file:
3581 $ gcc -I. -fpic -c emon.c
3585 . Build the user library shared object:
3590 $ gcc -shared -o libemon.so emon.o
3594 To run the application:
3596 * Start the application:
3607 [[using-lttng-ust-with-daemons]]
3608 ===== Use noch:{LTTng-UST} with daemons
3610 If your instrumented application calls man:fork(2), man:clone(2),
3611 or BSD's man:rfork(2), without a following man:exec(3)-family
3612 system call, you must preload the path:{liblttng-ust-fork.so} shared
3613 object when you start the application.
3617 $ LD_PRELOAD=liblttng-ust-fork.so ./my-app
3620 If your tracepoint provider package is
3621 a shared library which you also preload, you must put both
3622 shared objects in env:LD_PRELOAD:
3626 $ LD_PRELOAD=liblttng-ust-fork.so:/path/to/tp.so ./my-app
3632 ===== Use noch:{LTTng-UST} with applications which close file descriptors that don't belong to them
3634 If your instrumented application closes one or more file descriptors
3635 which it did not open itself, you must preload the
3636 path:{liblttng-ust-fd.so} shared object when you start the application:
3640 $ LD_PRELOAD=liblttng-ust-fd.so ./my-app
3643 Typical use cases include closing all the file descriptors after
3644 man:fork(2) or man:rfork(2) and buggy applications doing
3648 [[lttng-ust-pkg-config]]
3649 ===== Use noch:{pkg-config}
3651 On some distributions, LTTng-UST ships with a
3652 https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config]
3653 metadata file. If this is your case, then you can use cmd:pkg-config to
3654 build an application on the command line:
3658 $ gcc -o my-app my-app.o tp.o $(pkg-config --cflags --libs lttng-ust)
3662 [[instrumenting-32-bit-app-on-64-bit-system]]
3663 ===== [[advanced-instrumenting-techniques]]Build a 32-bit instrumented application for a 64-bit target system
3665 In order to trace a 32-bit application running on a 64-bit system,
3666 LTTng must use a dedicated 32-bit
3667 <<lttng-consumerd,consumer daemon>>.
3669 The following steps show how to build and install a 32-bit consumer
3670 daemon, which is _not_ part of the default 64-bit LTTng build, how to
3671 build and install the 32-bit LTTng-UST libraries, and how to build and
3672 link an instrumented 32-bit application in that context.
3674 To build a 32-bit instrumented application for a 64-bit target system,
3675 assuming you have a fresh target system with no installed Userspace RCU
3678 . Download, build, and install a 32-bit version of Userspace RCU:
3683 $ cd $(mktemp -d) &&
3684 wget http://lttng.org/files/urcu/userspace-rcu-latest-0.9.tar.bz2 &&
3685 tar -xf userspace-rcu-latest-0.9.tar.bz2 &&
3686 cd userspace-rcu-0.9.* &&
3687 ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 &&
3689 sudo make install &&
3694 . Using your distribution's package manager, or from source, install
3695 the following 32-bit versions of the following dependencies of
3696 LTTng-tools and LTTng-UST:
3699 * https://sourceforge.net/projects/libuuid/[libuuid]
3700 * http://directory.fsf.org/wiki/Popt[popt]
3701 * http://www.xmlsoft.org/[libxml2]
3704 . Download, build, and install a 32-bit version of the latest
3705 LTTng-UST{nbsp}{revision}:
3710 $ cd $(mktemp -d) &&
3711 wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.11.tar.bz2 &&
3712 tar -xf lttng-ust-latest-2.11.tar.bz2 &&
3713 cd lttng-ust-2.11.* &&
3714 ./configure --libdir=/usr/local/lib32 \
3715 CFLAGS=-m32 CXXFLAGS=-m32 \
3716 LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' &&
3718 sudo make install &&
3725 Depending on your distribution,
3726 32-bit libraries could be installed at a different location than
3727 `/usr/lib32`. For example, Debian is known to install
3728 some 32-bit libraries in `/usr/lib/i386-linux-gnu`.
3730 In this case, make sure to set `LDFLAGS` to all the
3731 relevant 32-bit library paths, for example:
3735 $ LDFLAGS='-L/usr/lib/i386-linux-gnu -L/usr/lib32'
3739 . Download the latest LTTng-tools{nbsp}{revision}, build, and install
3740 the 32-bit consumer daemon:
3745 $ cd $(mktemp -d) &&
3746 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.11.tar.bz2 &&
3747 tar -xf lttng-tools-latest-2.11.tar.bz2 &&
3748 cd lttng-tools-2.11.* &&
3749 ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \
3750 LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' \
3751 --disable-bin-lttng --disable-bin-lttng-crash \
3752 --disable-bin-lttng-relayd --disable-bin-lttng-sessiond &&
3754 cd src/bin/lttng-consumerd &&
3755 sudo make install &&
3760 . From your distribution or from source,
3761 <<installing-lttng,install>> the 64-bit versions of
3762 LTTng-UST and Userspace RCU.
3763 . Download, build, and install the 64-bit version of the
3764 latest LTTng-tools{nbsp}{revision}:
3769 $ cd $(mktemp -d) &&
3770 wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.11.tar.bz2 &&
3771 tar -xf lttng-tools-latest-2.11.tar.bz2 &&
3772 cd lttng-tools-2.11.* &&
3773 ./configure --with-consumerd32-libdir=/usr/local/lib32 \
3774 --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd &&
3776 sudo make install &&
3781 . Pass the following options to man:gcc(1), man:g++(1), or man:clang(1)
3782 when linking your 32-bit application:
3785 -m32 -L/usr/lib32 -L/usr/local/lib32 \
3786 -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32
3789 For example, let's rebuild the quick start example in
3790 <<tracing-your-own-user-application,Trace a user application>> as an
3791 instrumented 32-bit application:
3796 $ gcc -m32 -c -I. hello-tp.c
3797 $ gcc -m32 -c hello.c
3798 $ gcc -m32 -o hello hello.o hello-tp.o \
3799 -L/usr/lib32 -L/usr/local/lib32 \
3800 -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 \
3805 No special action is required to execute the 32-bit application and
3806 to trace it: use the command-line man:lttng(1) tool as usual.
3813 man:tracef(3) is a small LTTng-UST API designed for quick,
3814 man:printf(3)-like instrumentation without the burden of
3815 <<tracepoint-provider,creating>> and
3816 <<building-tracepoint-providers-and-user-application,building>>
3817 a tracepoint provider package.
3819 To use `tracef()` in your application:
3821 . In the C or C++ source files where you need to use `tracef()`,
3822 include `<lttng/tracef.h>`:
3827 #include <lttng/tracef.h>
3831 . In the application's source code, use `tracef()` like you would use
3839 tracef("my message: %d (%s)", my_integer, my_string);
3845 . Link your application with `liblttng-ust`:
3850 $ gcc -o app app.c -llttng-ust
3854 To trace the events that `tracef()` calls emit:
3856 * <<enabling-disabling-events,Create an event rule>> which matches the
3857 `lttng_ust_tracef:*` event name:
3862 $ lttng enable-event --userspace 'lttng_ust_tracef:*'
3867 .Limitations of `tracef()`
3869 The `tracef()` utility function was developed to make user space tracing
3870 super simple, albeit with notable disadvantages compared to
3871 <<defining-tracepoints,user-defined tracepoints>>:
3873 * All the emitted events have the same tracepoint provider and
3874 tracepoint names, respectively `lttng_ust_tracef` and `event`.
3875 * There is no static type checking.
3876 * The only event record field you actually get, named `msg`, is a string
3877 potentially containing the values you passed to `tracef()`
3878 using your own format string. This also means that you cannot filter
3879 events with a custom expression at run time because there are no
3881 * Since `tracef()` uses the C standard library's man:vasprintf(3)
3882 function behind the scenes to format the strings at run time, its
3883 expected performance is lower than with user-defined tracepoints,
3884 which do not require a conversion to a string.
3886 Taking this into consideration, `tracef()` is useful for some quick
3887 prototyping and debugging, but you should not consider it for any
3888 permanent and serious applicative instrumentation.
3894 ==== Use `tracelog()`
3896 The man:tracelog(3) API is very similar to <<tracef,`tracef()`>>, with
3897 the difference that it accepts an additional log level parameter.
3899 The goal of `tracelog()` is to ease the migration from logging to
3902 To use `tracelog()` in your application:
3904 . In the C or C++ source files where you need to use `tracelog()`,
3905 include `<lttng/tracelog.h>`:
3910 #include <lttng/tracelog.h>
3914 . In the application's source code, use `tracelog()` like you would use
3915 man:printf(3), except for the first parameter which is the log
3923 tracelog(TRACE_WARNING, "my message: %d (%s)",
3924 my_integer, my_string);
3930 See man:lttng-ust(3) for a list of available log level names.
3932 . Link your application with `liblttng-ust`:
3937 $ gcc -o app app.c -llttng-ust
3941 To trace the events that `tracelog()` calls emit with a log level
3942 _as severe as_ a specific log level:
3944 * <<enabling-disabling-events,Create an event rule>> which matches the
3945 `lttng_ust_tracelog:*` event name and a minimum level
3951 $ lttng enable-event --userspace 'lttng_ust_tracelog:*'
3952 --loglevel=TRACE_WARNING
3956 To trace the events that `tracelog()` calls emit with a
3957 _specific log level_:
3959 * Create an event rule which matches the `lttng_ust_tracelog:*`
3960 event name and a specific log level:
3965 $ lttng enable-event --userspace 'lttng_ust_tracelog:*'
3966 --loglevel-only=TRACE_INFO
3971 [[prebuilt-ust-helpers]]
3972 === Prebuilt user space tracing helpers
3974 The LTTng-UST package provides a few helpers in the form or preloadable
3975 shared objects which automatically instrument system functions and
3978 The helper shared objects are normally found in dir:{/usr/lib}. If you
3979 built LTTng-UST <<building-from-source,from source>>, they are probably
3980 located in dir:{/usr/local/lib}.
3982 The installed user space tracing helpers in LTTng-UST{nbsp}{revision}
3985 path:{liblttng-ust-libc-wrapper.so}::
3986 path:{liblttng-ust-pthread-wrapper.so}::
3987 <<liblttng-ust-libc-pthread-wrapper,C{nbsp}standard library
3988 memory and POSIX threads function tracing>>.
3990 path:{liblttng-ust-cyg-profile.so}::
3991 path:{liblttng-ust-cyg-profile-fast.so}::
3992 <<liblttng-ust-cyg-profile,Function entry and exit tracing>>.
3994 path:{liblttng-ust-dl.so}::
3995 <<liblttng-ust-dl,Dynamic linker tracing>>.
3997 To use a user space tracing helper with any user application:
3999 * Preload the helper shared object when you start the application:
4004 $ LD_PRELOAD=liblttng-ust-libc-wrapper.so my-app
4008 You can preload more than one helper:
4013 $ LD_PRELOAD=liblttng-ust-libc-wrapper.so:liblttng-ust-dl.so my-app
4019 [[liblttng-ust-libc-pthread-wrapper]]
4020 ==== Instrument C standard library memory and POSIX threads functions
4022 The path:{liblttng-ust-libc-wrapper.so} and
4023 path:{liblttng-ust-pthread-wrapper.so} helpers
4024 add instrumentation to some C standard library and POSIX
4028 .Functions instrumented by preloading path:{liblttng-ust-libc-wrapper.so}.
4030 |TP provider name |TP name |Instrumented function
4032 .6+|`lttng_ust_libc` |`malloc` |man:malloc(3)
4033 |`calloc` |man:calloc(3)
4034 |`realloc` |man:realloc(3)
4035 |`free` |man:free(3)
4036 |`memalign` |man:memalign(3)
4037 |`posix_memalign` |man:posix_memalign(3)
4041 .Functions instrumented by preloading path:{liblttng-ust-pthread-wrapper.so}.
4043 |TP provider name |TP name |Instrumented function
4045 .4+|`lttng_ust_pthread` |`pthread_mutex_lock_req` |man:pthread_mutex_lock(3p) (request time)
4046 |`pthread_mutex_lock_acq` |man:pthread_mutex_lock(3p) (acquire time)
4047 |`pthread_mutex_trylock` |man:pthread_mutex_trylock(3p)
4048 |`pthread_mutex_unlock` |man:pthread_mutex_unlock(3p)
4051 When you preload the shared object, it replaces the functions listed
4052 in the previous tables by wrappers which contain tracepoints and call
4053 the replaced functions.
4056 [[liblttng-ust-cyg-profile]]
4057 ==== Instrument function entry and exit
4059 The path:{liblttng-ust-cyg-profile*.so} helpers can add instrumentation
4060 to the entry and exit points of functions.
4062 man:gcc(1) and man:clang(1) have an option named
4063 https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html[`-finstrument-functions`]
4064 which generates instrumentation calls for entry and exit to functions.
4065 The LTTng-UST function tracing helpers,
4066 path:{liblttng-ust-cyg-profile.so} and
4067 path:{liblttng-ust-cyg-profile-fast.so}, take advantage of this feature
4068 to add tracepoints to the two generated functions (which contain
4069 `cyg_profile` in their names, hence the helper's name).
4071 To use the LTTng-UST function tracing helper, the source files to
4072 instrument must be built using the `-finstrument-functions` compiler
4075 There are two versions of the LTTng-UST function tracing helper:
4077 * **path:{liblttng-ust-cyg-profile-fast.so}** is a lightweight variant
4078 that you should only use when it can be _guaranteed_ that the
4079 complete event stream is recorded without any lost event record.
4080 Any kind of duplicate information is left out.
4082 Assuming no event record is lost, having only the function addresses on
4083 entry is enough to create a call graph, since an event record always
4084 contains the ID of the CPU that generated it.
4086 You can use a tool like man:addr2line(1) to convert function addresses
4087 back to source file names and line numbers.
4089 * **path:{liblttng-ust-cyg-profile.so}** is a more robust variant
4090 which also works in use cases where event records might get discarded or
4091 not recorded from application startup.
4092 In these cases, the trace analyzer needs more information to be
4093 able to reconstruct the program flow.
4095 See man:lttng-ust-cyg-profile(3) to learn more about the instrumentation
4096 points of this helper.
4098 All the tracepoints that this helper provides have the
4099 log level `TRACE_DEBUG_FUNCTION` (see man:lttng-ust(3)).
4101 TIP: It's sometimes a good idea to limit the number of source files that
4102 you compile with the `-finstrument-functions` option to prevent LTTng
4103 from writing an excessive amount of trace data at run time. When using
4104 man:gcc(1), you can use the
4105 `-finstrument-functions-exclude-function-list` option to avoid
4106 instrument entries and exits of specific function names.
4111 ==== Instrument the dynamic linker
4113 The path:{liblttng-ust-dl.so} helper adds instrumentation to the
4114 man:dlopen(3) and man:dlclose(3) function calls.
4116 See man:lttng-ust-dl(3) to learn more about the instrumentation points
4121 [[java-application]]
4122 === User space Java agent
4124 You can instrument any Java application which uses one of the following
4127 * The https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[**`java.util.logging`**]
4128 (JUL) core logging facilities.
4129 * http://logging.apache.org/log4j/1.2/[**Apache log4j{nbsp}1.2**], since
4130 LTTng{nbsp}2.6. Note that Apache Log4j{nbsp}2 is not supported.
4133 .LTTng-UST Java agent imported by a Java application.
4134 image::java-app.png[]
4136 Note that the methods described below are new in LTTng{nbsp}{revision}.
4137 Previous LTTng versions use another technique.
4139 NOTE: We use http://openjdk.java.net/[OpenJDK]{nbsp}8 for development
4140 and https://ci.lttng.org/[continuous integration], thus this version is
4141 directly supported. However, the LTTng-UST Java agent is also tested
4142 with OpenJDK{nbsp}7.
4147 ==== Use the LTTng-UST Java agent for `java.util.logging`
4149 To use the LTTng-UST Java agent in a Java application which uses
4150 `java.util.logging` (JUL):
4152 . In the Java application's source code, import the LTTng-UST
4153 log handler package for `java.util.logging`:
4158 import org.lttng.ust.agent.jul.LttngLogHandler;
4162 . Create an LTTng-UST JUL log handler:
4167 Handler lttngUstLogHandler = new LttngLogHandler();
4171 . Add this handler to the JUL loggers which should emit LTTng events:
4176 Logger myLogger = Logger.getLogger("some-logger");
4178 myLogger.addHandler(lttngUstLogHandler);
4182 . Use `java.util.logging` log statements and configuration as usual.
4183 The loggers with an attached LTTng-UST log handler can emit
4186 . Before exiting the application, remove the LTTng-UST log handler from
4187 the loggers attached to it and call its `close()` method:
4192 myLogger.removeHandler(lttngUstLogHandler);
4193 lttngUstLogHandler.close();
4197 This is not strictly necessary, but it is recommended for a clean
4198 disposal of the handler's resources.
4200 . Include the LTTng-UST Java agent's common and JUL-specific JAR files,
4201 path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-jul.jar},
4203 https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
4204 path] when you build the Java application.
4206 The JAR files are typically located in dir:{/usr/share/java}.
4208 IMPORTANT: The LTTng-UST Java agent must be
4209 <<installing-lttng,installed>> for the logging framework your
4212 .Use the LTTng-UST Java agent for `java.util.logging`.
4217 import java.io.IOException;
4218 import java.util.logging.Handler;
4219 import java.util.logging.Logger;
4220 import org.lttng.ust.agent.jul.LttngLogHandler;
4224 private static final int answer = 42;
4226 public static void main(String[] argv) throws Exception
4229 Logger logger = Logger.getLogger("jello");
4231 // Create an LTTng-UST log handler
4232 Handler lttngUstLogHandler = new LttngLogHandler();
4234 // Add the LTTng-UST log handler to our logger
4235 logger.addHandler(lttngUstLogHandler);
4238 logger.info("some info");
4239 logger.warning("some warning");
4241 logger.finer("finer information; the answer is " + answer);
4243 logger.severe("error!");
4245 // Not mandatory, but cleaner
4246 logger.removeHandler(lttngUstLogHandler);
4247 lttngUstLogHandler.close();
4256 $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
4259 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4260 <<enabling-disabling-events,create an event rule>> matching the
4261 `jello` JUL logger, and <<basic-tracing-session-control,start tracing>>:
4266 $ lttng enable-event --jul jello
4270 Run the compiled class:
4274 $ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
4277 <<basic-tracing-session-control,Stop tracing>> and inspect the
4287 In the resulting trace, an <<event,event record>> generated by a Java
4288 application using `java.util.logging` is named `lttng_jul:event` and
4289 has the following fields:
4292 Log record's message.
4298 Name of the class in which the log statement was executed.
4301 Name of the method in which the log statement was executed.
4304 Logging time (timestamp in milliseconds).
4307 Log level integer value.
4310 ID of the thread in which the log statement was executed.
4312 You can use the opt:lttng-enable-event(1):--loglevel or
4313 opt:lttng-enable-event(1):--loglevel-only option of the
4314 man:lttng-enable-event(1) command to target a range of JUL log levels
4315 or a specific JUL log level.
4320 ==== Use the LTTng-UST Java agent for Apache log4j
4322 To use the LTTng-UST Java agent in a Java application which uses
4323 Apache log4j{nbsp}1.2:
4325 . In the Java application's source code, import the LTTng-UST
4326 log appender package for Apache log4j:
4331 import org.lttng.ust.agent.log4j.LttngLogAppender;
4335 . Create an LTTng-UST log4j log appender:
4340 Appender lttngUstLogAppender = new LttngLogAppender();
4344 . Add this appender to the log4j loggers which should emit LTTng events:
4349 Logger myLogger = Logger.getLogger("some-logger");
4351 myLogger.addAppender(lttngUstLogAppender);
4355 . Use Apache log4j log statements and configuration as usual. The
4356 loggers with an attached LTTng-UST log appender can emit LTTng events.
4358 . Before exiting the application, remove the LTTng-UST log appender from
4359 the loggers attached to it and call its `close()` method:
4364 myLogger.removeAppender(lttngUstLogAppender);
4365 lttngUstLogAppender.close();
4369 This is not strictly necessary, but it is recommended for a clean
4370 disposal of the appender's resources.
4372 . Include the LTTng-UST Java agent's common and log4j-specific JAR
4373 files, path:{lttng-ust-agent-common.jar} and
4374 path:{lttng-ust-agent-log4j.jar}, in the
4375 https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class
4376 path] when you build the Java application.
4378 The JAR files are typically located in dir:{/usr/share/java}.
4380 IMPORTANT: The LTTng-UST Java agent must be
4381 <<installing-lttng,installed>> for the logging framework your
4384 .Use the LTTng-UST Java agent for Apache log4j.
4389 import org.apache.log4j.Appender;
4390 import org.apache.log4j.Logger;
4391 import org.lttng.ust.agent.log4j.LttngLogAppender;
4395 private static final int answer = 42;
4397 public static void main(String[] argv) throws Exception
4400 Logger logger = Logger.getLogger("jello");
4402 // Create an LTTng-UST log appender
4403 Appender lttngUstLogAppender = new LttngLogAppender();
4405 // Add the LTTng-UST log appender to our logger
4406 logger.addAppender(lttngUstLogAppender);
4409 logger.info("some info");
4410 logger.warn("some warning");
4412 logger.debug("debug information; the answer is " + answer);
4414 logger.fatal("error!");
4416 // Not mandatory, but cleaner
4417 logger.removeAppender(lttngUstLogAppender);
4418 lttngUstLogAppender.close();
4424 Build this example (`$LOG4JPATH` is the path to the Apache log4j JAR
4429 $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java
4432 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4433 <<enabling-disabling-events,create an event rule>> matching the
4434 `jello` log4j logger, and <<basic-tracing-session-control,start tracing>>:
4439 $ lttng enable-event --log4j jello
4443 Run the compiled class:
4447 $ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH:. Test
4450 <<basic-tracing-session-control,Stop tracing>> and inspect the
4460 In the resulting trace, an <<event,event record>> generated by a Java
4461 application using log4j is named `lttng_log4j:event` and
4462 has the following fields:
4465 Log record's message.
4471 Name of the class in which the log statement was executed.
4474 Name of the method in which the log statement was executed.
4477 Name of the file in which the executed log statement is located.
4480 Line number at which the log statement was executed.
4486 Log level integer value.
4489 Name of the Java thread in which the log statement was executed.
4491 You can use the opt:lttng-enable-event(1):--loglevel or
4492 opt:lttng-enable-event(1):--loglevel-only option of the
4493 man:lttng-enable-event(1) command to target a range of Apache log4j log levels
4494 or a specific log4j log level.
4498 [[java-application-context]]
4499 ==== Provide application-specific context fields in a Java application
4501 A Java application-specific context field is a piece of state provided
4502 by the application which <<adding-context,you can add>>, using the
4503 man:lttng-add-context(1) command, to each <<event,event record>>
4504 produced by the log statements of this application.
4506 For example, a given object might have a current request ID variable.
4507 You can create a context information retriever for this object and
4508 assign a name to this current request ID. You can then, using the
4509 man:lttng-add-context(1) command, add this context field by name to
4510 the JUL or log4j <<channel,channel>>.
4512 To provide application-specific context fields in a Java application:
4514 . In the Java application's source code, import the LTTng-UST
4515 Java agent context classes and interfaces:
4520 import org.lttng.ust.agent.context.ContextInfoManager;
4521 import org.lttng.ust.agent.context.IContextInfoRetriever;
4525 . Create a context information retriever class, that is, a class which
4526 implements the `IContextInfoRetriever` interface:
4531 class MyContextInfoRetriever implements IContextInfoRetriever
4534 public Object retrieveContextInfo(String key)
4536 if (key.equals("intCtx")) {
4538 } else if (key.equals("strContext")) {
4539 return "context value!";
4548 This `retrieveContextInfo()` method is the only member of the
4549 `IContextInfoRetriever` interface. Its role is to return the current
4550 value of a state by name to create a context field. The names of the
4551 context fields and which state variables they return depends on your
4554 All primitive types and objects are supported as context fields.
4555 When `retrieveContextInfo()` returns an object, the context field
4556 serializer calls its `toString()` method to add a string field to
4557 event records. The method can also return `null`, which means that
4558 no context field is available for the required name.
4560 . Register an instance of your context information retriever class to
4561 the context information manager singleton:
4566 IContextInfoRetriever cir = new MyContextInfoRetriever();
4567 ContextInfoManager cim = ContextInfoManager.getInstance();
4568 cim.registerContextInfoRetriever("retrieverName", cir);
4572 . Before exiting the application, remove your context information
4573 retriever from the context information manager singleton:
4578 ContextInfoManager cim = ContextInfoManager.getInstance();
4579 cim.unregisterContextInfoRetriever("retrieverName");
4583 This is not strictly necessary, but it is recommended for a clean
4584 disposal of some manager's resources.
4586 . Build your Java application with LTTng-UST Java agent support as
4587 usual, following the procedure for either the <<jul,JUL>> or
4588 <<log4j,Apache log4j>> framework.
4591 .Provide application-specific context fields in a Java application.
4596 import java.util.logging.Handler;
4597 import java.util.logging.Logger;
4598 import org.lttng.ust.agent.jul.LttngLogHandler;
4599 import org.lttng.ust.agent.context.ContextInfoManager;
4600 import org.lttng.ust.agent.context.IContextInfoRetriever;
4604 // Our context information retriever class
4605 private static class MyContextInfoRetriever
4606 implements IContextInfoRetriever
4609 public Object retrieveContextInfo(String key) {
4610 if (key.equals("intCtx")) {
4612 } else if (key.equals("strContext")) {
4613 return "context value!";
4620 private static final int answer = 42;
4622 public static void main(String args[]) throws Exception
4624 // Get the context information manager instance
4625 ContextInfoManager cim = ContextInfoManager.getInstance();
4627 // Create and register our context information retriever
4628 IContextInfoRetriever cir = new MyContextInfoRetriever();
4629 cim.registerContextInfoRetriever("myRetriever", cir);
4632 Logger logger = Logger.getLogger("jello");
4634 // Create an LTTng-UST log handler
4635 Handler lttngUstLogHandler = new LttngLogHandler();
4637 // Add the LTTng-UST log handler to our logger
4638 logger.addHandler(lttngUstLogHandler);
4641 logger.info("some info");
4642 logger.warning("some warning");
4644 logger.finer("finer information; the answer is " + answer);
4646 logger.severe("error!");
4648 // Not mandatory, but cleaner
4649 logger.removeHandler(lttngUstLogHandler);
4650 lttngUstLogHandler.close();
4651 cim.unregisterContextInfoRetriever("myRetriever");
4660 $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java
4663 <<creating-destroying-tracing-sessions,Create a tracing session>>
4664 and <<enabling-disabling-events,create an event rule>> matching the
4670 $ lttng enable-event --jul jello
4673 <<adding-context,Add the application-specific context fields>> to the
4678 $ lttng add-context --jul --type='$app.myRetriever:intCtx'
4679 $ lttng add-context --jul --type='$app.myRetriever:strContext'
4682 <<basic-tracing-session-control,Start tracing>>:
4689 Run the compiled class:
4693 $ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test
4696 <<basic-tracing-session-control,Stop tracing>> and inspect the
4708 [[python-application]]
4709 === User space Python agent
4711 You can instrument a Python{nbsp}2 or Python{nbsp}3 application which
4713 https://docs.python.org/3/library/logging.html[`logging`] package.
4715 Each log statement emits an LTTng event once the
4716 application module imports the
4717 <<lttng-ust-agents,LTTng-UST Python agent>> package.
4720 .A Python application importing the LTTng-UST Python agent.
4721 image::python-app.png[]
4723 To use the LTTng-UST Python agent:
4725 . In the Python application's source code, import the LTTng-UST Python
4735 The LTTng-UST Python agent automatically adds its logging handler to the
4736 root logger at import time.
4738 Any log statement that the application executes before this import does
4739 not emit an LTTng event.
4741 IMPORTANT: The LTTng-UST Python agent must be
4742 <<installing-lttng,installed>>.
4744 . Use log statements and logging configuration as usual.
4745 Since the LTTng-UST Python agent adds a handler to the _root_
4746 logger, you can trace any log statement from any logger.
4748 .Use the LTTng-UST Python agent.
4759 logging.basicConfig()
4760 logger = logging.getLogger('my-logger')
4763 logger.debug('debug message')
4764 logger.info('info message')
4765 logger.warn('warn message')
4766 logger.error('error message')
4767 logger.critical('critical message')
4771 if __name__ == '__main__':
4775 NOTE: `logging.basicConfig()`, which adds to the root logger a basic
4776 logging handler which prints to the standard error stream, is not
4777 strictly required for LTTng-UST tracing to work, but in versions of
4778 Python preceding{nbsp}3.2, you could see a warning message which indicates
4779 that no handler exists for the logger `my-logger`.
4781 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4782 <<enabling-disabling-events,create an event rule>> matching the
4783 `my-logger` Python logger, and <<basic-tracing-session-control,start
4789 $ lttng enable-event --python my-logger
4793 Run the Python script:
4800 <<basic-tracing-session-control,Stop tracing>> and inspect the recorded
4810 In the resulting trace, an <<event,event record>> generated by a Python
4811 application is named `lttng_python:event` and has the following fields:
4814 Logging time (string).
4817 Log record's message.
4823 Name of the function in which the log statement was executed.
4826 Line number at which the log statement was executed.
4829 Log level integer value.
4832 ID of the Python thread in which the log statement was executed.
4835 Name of the Python thread in which the log statement was executed.
4837 You can use the opt:lttng-enable-event(1):--loglevel or
4838 opt:lttng-enable-event(1):--loglevel-only option of the
4839 man:lttng-enable-event(1) command to target a range of Python log levels
4840 or a specific Python log level.
4842 When an application imports the LTTng-UST Python agent, the agent tries
4843 to register to a <<lttng-sessiond,session daemon>>. Note that you must
4844 <<start-sessiond,start the session daemon>> _before_ you run the Python
4845 application. If a session daemon is found, the agent tries to register
4846 to it during five seconds, after which the application continues
4847 without LTTng tracing support. You can override this timeout value with
4848 the env:LTTNG_UST_PYTHON_REGISTER_TIMEOUT environment variable
4851 If the session daemon stops while a Python application with an imported
4852 LTTng-UST Python agent runs, the agent retries to connect and to
4853 register to a session daemon every three seconds. You can override this
4854 delay with the env:LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment
4859 [[proc-lttng-logger-abi]]
4862 The `lttng-tracer` Linux kernel module, part of
4863 <<lttng-modules,LTTng-modules>>, creates the special LTTng logger file
4864 path:{/proc/lttng-logger} when it's loaded. Any application can write
4865 text data to this file to emit an LTTng event.
4868 .An application writes to the LTTng logger file to emit an LTTng event.
4869 image::lttng-logger.png[]
4871 The LTTng logger is the quickest method--not the most efficient,
4872 however--to add instrumentation to an application. It is designed
4873 mostly to instrument shell scripts:
4877 $ echo "Some message, some $variable" > /proc/lttng-logger
4880 Any event that the LTTng logger emits is named `lttng_logger` and
4881 belongs to the Linux kernel <<domain,tracing domain>>. However, unlike
4882 other instrumentation points in the kernel tracing domain, **any Unix
4883 user** can <<enabling-disabling-events,create an event rule>> which
4884 matches its event name, not only the root user or users in the
4885 <<tracing-group,tracing group>>.
4887 To use the LTTng logger:
4889 * From any application, write text data to the path:{/proc/lttng-logger}
4892 The `msg` field of `lttng_logger` event records contains the
4895 NOTE: The maximum message length of an LTTng logger event is
4896 1024{nbsp}bytes. Writing more than this makes the LTTng logger emit more
4897 than one event to contain the remaining data.
4899 You should not use the LTTng logger to trace a user application which
4900 can be instrumented in a more efficient way, namely:
4902 * <<c-application,C and $$C++$$ applications>>.
4903 * <<java-application,Java applications>>.
4904 * <<python-application,Python applications>>.
4906 .Use the LTTng logger.
4911 echo 'Hello, World!' > /proc/lttng-logger
4913 df --human-readable --print-type / > /proc/lttng-logger
4916 <<creating-destroying-tracing-sessions,Create a tracing session>>,
4917 <<enabling-disabling-events,create an event rule>> matching the
4918 `lttng_logger` Linux kernel tracepoint, and
4919 <<basic-tracing-session-control,start tracing>>:
4924 $ lttng enable-event --kernel lttng_logger
4928 Run the Bash script:
4935 <<basic-tracing-session-control,Stop tracing>> and inspect the recorded
4946 [[instrumenting-linux-kernel]]
4947 === LTTng kernel tracepoints
4949 NOTE: This section shows how to _add_ instrumentation points to the
4950 Linux kernel. The kernel's subsystems are already thoroughly
4951 instrumented at strategic places for LTTng when you
4952 <<installing-lttng,install>> the <<lttng-modules,LTTng-modules>>
4956 There are two methods to instrument the Linux kernel:
4958 . <<linux-add-lttng-layer,Add an LTTng layer>> over an existing ftrace
4959 tracepoint which uses the `TRACE_EVENT()` API.
4961 Choose this if you want to instrumentation a Linux kernel tree with an
4962 instrumentation point compatible with ftrace, perf, and SystemTap.
4964 . Use an <<linux-lttng-tracepoint-event,LTTng-only approach>> to
4965 instrument an out-of-tree kernel module.
4967 Choose this if you don't need ftrace, perf, or SystemTap support.
4971 [[linux-add-lttng-layer]]
4972 ==== [[instrumenting-linux-kernel-itself]][[mainline-trace-event]][[lttng-adaptation-layer]]Add an LTTng layer to an existing ftrace tracepoint
4974 This section shows how to add an LTTng layer to existing ftrace
4975 instrumentation using the `TRACE_EVENT()` API.
4977 This section does not document the `TRACE_EVENT()` macro. You can
4978 read the following articles to learn more about this API:
4980 * http://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part{nbsp}1)]
4981 * http://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part{nbsp}2)]
4982 * http://lwn.net/Articles/383362/[Using the TRACE_EVENT() macro (Part{nbsp}3)]
4984 The following procedure assumes that your ftrace tracepoints are
4985 correctly defined in their own header and that they are created in
4986 one source file using the `CREATE_TRACE_POINTS` definition.
4988 To add an LTTng layer over an existing ftrace tracepoint:
4990 . Make sure the following kernel configuration options are
4996 * `CONFIG_HIGH_RES_TIMERS`
4997 * `CONFIG_TRACEPOINTS`
5000 . Build the Linux source tree with your custom ftrace tracepoints.
5001 . Boot the resulting Linux image on your target system.
5003 Confirm that the tracepoints exist by looking for their names in the
5004 dir:{/sys/kernel/debug/tracing/events/subsys} directory, where `subsys`
5005 is your subsystem's name.
5007 . Get a copy of the latest LTTng-modules{nbsp}{revision}:
5012 $ cd $(mktemp -d) &&
5013 wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.11.tar.bz2 &&
5014 tar -xf lttng-modules-latest-2.11.tar.bz2 &&
5015 cd lttng-modules-2.11.*
5019 . In dir:{instrumentation/events/lttng-module}, relative to the root
5020 of the LTTng-modules source tree, create a header file named
5021 +__subsys__.h+ for your custom subsystem +__subsys__+ and write your
5022 LTTng-modules tracepoint definitions using the LTTng-modules
5025 Start with this template:
5029 .path:{instrumentation/events/lttng-module/my_subsys.h}
5032 #define TRACE_SYSTEM my_subsys
5034 #if !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ)
5035 #define _LTTNG_MY_SUBSYS_H
5037 #include "../../../probes/lttng-tracepoint-event.h"
5038 #include <linux/tracepoint.h>
5040 LTTNG_TRACEPOINT_EVENT(
5042 * Format is identical to TRACE_EVENT()'s version for the three
5043 * following macro parameters:
5046 TP_PROTO(int my_int, const char *my_string),
5047 TP_ARGS(my_int, my_string),
5049 /* LTTng-modules specific macros */
5051 ctf_integer(int, my_int_field, my_int)
5052 ctf_string(my_bar_field, my_bar)
5056 #endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */
5058 #include "../../../probes/define_trace.h"
5062 The entries in the `TP_FIELDS()` section are the list of fields for the
5063 LTTng tracepoint. This is similar to the `TP_STRUCT__entry()` part of
5064 ftrace's `TRACE_EVENT()` macro.
5066 See <<lttng-modules-tp-fields,Tracepoint fields macros>> for a
5067 complete description of the available `ctf_*()` macros.
5069 . Create the LTTng-modules probe's kernel module C source file,
5070 +probes/lttng-probe-__subsys__.c+, where +__subsys__+ is your
5075 .path:{probes/lttng-probe-my-subsys.c}
5077 #include <linux/module.h>
5078 #include "../lttng-tracer.h"
5081 * Build-time verification of mismatch between mainline
5082 * TRACE_EVENT() arguments and the LTTng-modules adaptation
5083 * layer LTTNG_TRACEPOINT_EVENT() arguments.
5085 #include <trace/events/my_subsys.h>
5087 /* Create LTTng tracepoint probes */
5088 #define LTTNG_PACKAGE_BUILD
5089 #define CREATE_TRACE_POINTS
5090 #define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module
5092 #include "../instrumentation/events/lttng-module/my_subsys.h"
5094 MODULE_LICENSE("GPL and additional rights");
5095 MODULE_AUTHOR("Your name <your-email>");
5096 MODULE_DESCRIPTION("LTTng my_subsys probes");
5097 MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "."
5098 __stringify(LTTNG_MODULES_MINOR_VERSION) "."
5099 __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION)
5100 LTTNG_MODULES_EXTRAVERSION);
5104 . Edit path:{probes/KBuild} and add your new kernel module object
5105 next to the existing ones:
5109 .path:{probes/KBuild}
5113 obj-m += lttng-probe-module.o
5114 obj-m += lttng-probe-power.o
5116 obj-m += lttng-probe-my-subsys.o
5122 . Build and install the LTTng kernel modules:
5127 $ make KERNELDIR=/path/to/linux
5128 # make modules_install && depmod -a
5132 Replace `/path/to/linux` with the path to the Linux source tree where
5133 you defined and used tracepoints with ftrace's `TRACE_EVENT()` macro.
5135 Note that you can also use the
5136 <<lttng-tracepoint-event-code,`LTTNG_TRACEPOINT_EVENT_CODE()` macro>>
5137 instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and
5138 C code that need to be executed before the event fields are recorded.
5140 The best way to learn how to use the previous LTTng-modules macros is to
5141 inspect the existing LTTng-modules tracepoint definitions in the
5142 dir:{instrumentation/events/lttng-module} header files. Compare them
5143 with the Linux kernel mainline versions in the
5144 dir:{include/trace/events} directory of the Linux source tree.
5148 [[lttng-tracepoint-event-code]]
5149 ===== Use custom C code to access the data for tracepoint fields
5151 Although we recommended to always use the
5152 <<lttng-adaptation-layer,`LTTNG_TRACEPOINT_EVENT()`>> macro to describe
5153 the arguments and fields of an LTTng-modules tracepoint when possible,
5154 sometimes you need a more complex process to access the data that the
5155 tracer records as event record fields. In other words, you need local
5156 variables and multiple C{nbsp}statements instead of simple
5157 argument-based expressions that you pass to the
5158 <<lttng-modules-tp-fields,`ctf_*()` macros of `TP_FIELDS()`>>.
5160 You can use the `LTTNG_TRACEPOINT_EVENT_CODE()` macro instead of
5161 `LTTNG_TRACEPOINT_EVENT()` to declare custom local variables and define
5162 a block of C{nbsp}code to be executed before LTTng records the fields.
5163 The structure of this macro is:
5166 .`LTTNG_TRACEPOINT_EVENT_CODE()` macro syntax.
5168 LTTNG_TRACEPOINT_EVENT_CODE(
5170 * Format identical to the LTTNG_TRACEPOINT_EVENT()
5171 * version for the following three macro parameters:
5174 TP_PROTO(int my_int, const char *my_string),
5175 TP_ARGS(my_int, my_string),
5177 /* Declarations of custom local variables */
5180 unsigned long b = 0;
5181 const char *name = "(undefined)";
5182 struct my_struct *my_struct;
5186 * Custom code which uses both tracepoint arguments
5187 * (in TP_ARGS()) and local variables (in TP_locvar()).
5189 * Local variables are actually members of a structure pointed
5190 * to by the special variable tp_locvar.
5194 tp_locvar->a = my_int + 17;
5195 tp_locvar->my_struct = get_my_struct_at(tp_locvar->a);
5196 tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct);
5197 tp_locvar->name = my_struct_get_name(tp_locvar->my_struct);
5198 put_my_struct(tp_locvar->my_struct);
5207 * Format identical to the LTTNG_TRACEPOINT_EVENT()
5208 * version for this, except that tp_locvar members can be
5209 * used in the argument expression parameters of
5210 * the ctf_*() macros.
5213 ctf_integer(unsigned long, my_struct_b, tp_locvar->b)
5214 ctf_integer(int, my_struct_a, tp_locvar->a)
5215 ctf_string(my_string_field, my_string)
5216 ctf_string(my_struct_name, tp_locvar->name)
5221 IMPORTANT: The C code defined in `TP_code()` must not have any side
5222 effects when executed. In particular, the code must not allocate
5223 memory or get resources without deallocating this memory or putting
5224 those resources afterwards.
5227 [[instrumenting-linux-kernel-tracing]]
5228 ==== Load and unload a custom probe kernel module
5230 You must load a <<lttng-adaptation-layer,created LTTng-modules probe
5231 kernel module>> in the kernel before it can emit LTTng events.
5233 To load the default probe kernel modules and a custom probe kernel
5236 * Use the opt:lttng-sessiond(8):--extra-kmod-probes option to give extra
5237 probe modules to load when starting a root <<lttng-sessiond,session
5241 .Load the `my_subsys`, `usb`, and the default probe modules.
5245 # lttng-sessiond --extra-kmod-probes=my_subsys,usb
5250 You only need to pass the subsystem name, not the whole kernel module
5253 To load _only_ a given custom probe kernel module:
5255 * Use the opt:lttng-sessiond(8):--kmod-probes option to give the probe
5256 modules to load when starting a root session daemon:
5259 .Load only the `my_subsys` and `usb` probe modules.
5263 # lttng-sessiond --kmod-probes=my_subsys,usb
5268 To confirm that a probe module is loaded:
5275 $ lsmod | grep lttng_probe_usb
5279 To unload the loaded probe modules:
5281 * Kill the session daemon with `SIGTERM`:
5286 # pkill lttng-sessiond
5290 You can also use man:modprobe(8)'s `--remove` option if the session
5291 daemon terminates abnormally.
5294 [[controlling-tracing]]
5297 Once an application or a Linux kernel is
5298 <<instrumenting,instrumented>> for LTTng tracing,
5301 This section is divided in topics on how to use the various
5302 <<plumbing,components of LTTng>>, in particular the <<lttng-cli,cmd:lttng
5303 command-line tool>>, to _control_ the LTTng daemons and tracers.
5305 NOTE: In the following subsections, we refer to an man:lttng(1) command
5306 using its man page name. For example, instead of _Run the `create`
5307 command to..._, we use _Run the man:lttng-create(1) command to..._.
5311 === Start a session daemon
5313 In some situations, you need to run a <<lttng-sessiond,session daemon>>
5314 (man:lttng-sessiond(8)) _before_ you can use the man:lttng(1)
5317 You will see the following error when you run a command while no session
5321 Error: No session daemon is available
5324 The only command that automatically runs a session daemon is
5325 man:lttng-create(1), which you use to
5326 <<creating-destroying-tracing-sessions,create a tracing session>>. While
5327 this is most of the time the first operation that you do, sometimes it's
5328 not. Some examples are:
5330 * <<list-instrumentation-points,List the available instrumentation points>>.
5331 * <<saving-loading-tracing-session,Load a tracing session configuration>>.
5333 [[tracing-group]] Each Unix user must have its own running session
5334 daemon to trace user applications. The session daemon that the root user
5335 starts is the only one allowed to control the LTTng kernel tracer. Users
5336 that are part of the _tracing group_ can control the root session
5337 daemon. The default tracing group name is `tracing`; you can set it to
5338 something else with the opt:lttng-sessiond(8):--group option when you
5339 start the root session daemon.
5341 To start a user session daemon:
5343 * Run man:lttng-sessiond(8):
5348 $ lttng-sessiond --daemonize
5352 To start the root session daemon:
5354 * Run man:lttng-sessiond(8) as the root user:
5359 # lttng-sessiond --daemonize
5363 In both cases, remove the opt:lttng-sessiond(8):--daemonize option to
5364 start the session daemon in foreground.
5366 To stop a session daemon, use man:kill(1) on its process ID (standard
5369 Note that some Linux distributions could manage the LTTng session daemon
5370 as a service. In this case, you should use the service manager to
5371 start, restart, and stop session daemons.
5374 [[creating-destroying-tracing-sessions]]
5375 === Create and destroy a tracing session
5377 Almost all the LTTng control operations happen in the scope of
5378 a <<tracing-session,tracing session>>, which is the dialogue between the
5379 <<lttng-sessiond,session daemon>> and you.
5381 To create a tracing session with a generated name:
5383 * Use the man:lttng-create(1) command:
5392 The created tracing session's name is `auto` followed by the
5395 To create a tracing session with a specific name:
5397 * Use the optional argument of the man:lttng-create(1) command:
5402 $ lttng create my-session
5406 Replace `my-session` with the specific tracing session name.
5408 LTTng appends the creation date to the created tracing session's name.
5410 LTTng writes the traces of a tracing session in
5411 +$LTTNG_HOME/lttng-trace/__name__+ by default, where +__name__+ is the
5412 name of the tracing session. Note that the env:LTTNG_HOME environment
5413 variable defaults to `$HOME` if not set.
5415 To output LTTng traces to a non-default location:
5417 * Use the opt:lttng-create(1):--output option of the man:lttng-create(1) command:
5422 $ lttng create my-session --output=/tmp/some-directory
5426 You may create as many tracing sessions as you wish.
5428 To list all the existing tracing sessions for your Unix user:
5430 * Use the man:lttng-list(1) command:
5439 When you create a tracing session, it is set as the _current tracing
5440 session_. The following man:lttng(1) commands operate on the current
5441 tracing session when you don't specify one:
5443 [role="list-3-cols"]
5444 * man:lttng-add-context(1)
5445 * man:lttng-destroy(1)
5446 * man:lttng-disable-channel(1)
5447 * man:lttng-disable-event(1)
5448 * man:lttng-disable-rotation(1)
5449 * man:lttng-enable-channel(1)
5450 * man:lttng-enable-event(1)
5451 * man:lttng-enable-rotation(1)
5453 * man:lttng-regenerate(1)
5454 * man:lttng-rotate(1)
5456 * man:lttng-snapshot(1)
5457 * man:lttng-start(1)
5458 * man:lttng-status(1)
5460 * man:lttng-track(1)
5461 * man:lttng-untrack(1)
5464 To change the current tracing session:
5466 * Use the man:lttng-set-session(1) command:
5471 $ lttng set-session new-session
5475 Replace `new-session` by the name of the new current tracing session.
5477 When you are done tracing in a given tracing session, you can destroy
5478 it. This operation frees the resources taken by the tracing session
5479 to destroy; it does not destroy the trace data that LTTng wrote for
5480 this tracing session.
5482 To destroy the current tracing session:
5484 * Use the man:lttng-destroy(1) command:
5493 The man:lttng-destroy(1) command also runs the man:lttng-stop(1)
5494 command implicitly (see <<basic-tracing-session-control,Start and stop a
5495 tracing session>>). You need to stop tracing to make LTTng flush the
5496 remaining trace data and make the trace readable.
5499 [[list-instrumentation-points]]
5500 === List the available instrumentation points
5502 The <<lttng-sessiond,session daemon>> can query the running instrumented
5503 user applications and the Linux kernel to get a list of available
5504 instrumentation points. For the Linux kernel <<domain,tracing domain>>,
5505 they are tracepoints and system calls. For the user space tracing
5506 domain, they are tracepoints. For the other tracing domains, they are
5509 To list the available instrumentation points:
5511 * Use the man:lttng-list(1) command with the requested tracing domain's
5515 * opt:lttng-list(1):--kernel: Linux kernel tracepoints (your Unix user
5516 must be a root user, or it must be a member of the
5517 <<tracing-group,tracing group>>).
5518 * opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall: Linux
5519 kernel system calls (your Unix user must be a root user, or it must be
5520 a member of the tracing group).
5521 * opt:lttng-list(1):--userspace: user space tracepoints.
5522 * opt:lttng-list(1):--jul: `java.util.logging` loggers.
5523 * opt:lttng-list(1):--log4j: Apache log4j loggers.
5524 * opt:lttng-list(1):--python: Python loggers.
5527 .List the available user space tracepoints.
5531 $ lttng list --userspace
5535 .List the available Linux kernel system call tracepoints.
5539 $ lttng list --kernel --syscall
5544 [[enabling-disabling-events]]
5545 === Create and enable an event rule
5547 Once you <<creating-destroying-tracing-sessions,create a tracing
5548 session>>, you can create <<event,event rules>> with the
5549 man:lttng-enable-event(1) command.
5551 You specify each condition with a command-line option. The available
5552 condition arguments are shown in the following table.
5554 [role="growable",cols="asciidoc,asciidoc,default"]
5555 .Condition command-line arguments for the man:lttng-enable-event(1) command.
5557 |Argument |Description |Applicable tracing domains
5563 . +--probe=__ADDR__+
5564 . +--function=__ADDR__+
5565 . +--userspace-probe=__PATH__:__SYMBOL__+
5566 . +--userspace-probe=sdt:__PATH__:__PROVIDER__:__NAME__+
5569 Instead of using the default _tracepoint_ instrumentation type, use:
5571 . A Linux system call (entry and exit).
5572 . A Linux https://lwn.net/Articles/132196/[kprobe] (symbol or address).
5573 . The entry and return points of a Linux function (symbol or address).
5574 . The entry point of a user application or library function (path to
5575 application/library and symbol).
5576 . A https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[SystemTap
5577 Statically Defined Tracing] (USDT) probe (path to application/library,
5578 provider and probe names).
5582 |First positional argument.
5585 Tracepoint or system call name.
5587 With the opt:lttng-enable-event(1):--probe,
5588 opt:lttng-enable-event(1):--function, and
5589 opt:lttng-enable-event(1):--userspace-probe options, this is a custom
5590 name given to the event rule. With the JUL, log4j, and Python domains,
5591 this is a logger name.
5593 With a tracepoint, logger, or system call name, you can use the special
5594 `*` globbing character to match anything (for example, `sched_*`,
5602 . +--loglevel=__LEVEL__+
5603 . +--loglevel-only=__LEVEL__+
5606 . Match only tracepoints or log statements with a logging level at
5607 least as severe as +__LEVEL__+.
5608 . Match only tracepoints or log statements with a logging level
5609 equal to +__LEVEL__+.
5611 See man:lttng-enable-event(1) for the list of available logging level
5614 |User space, JUL, log4j, and Python.
5616 |+--exclude=__EXCLUSIONS__+
5619 When you use a `*` character at the end of the tracepoint or logger
5620 name (first positional argument), exclude the specific names in the
5621 comma-delimited list +__EXCLUSIONS__+.
5624 User space, JUL, log4j, and Python.
5626 |+--filter=__EXPR__+
5629 Match only events which satisfy the expression +__EXPR__+.
5631 See man:lttng-enable-event(1) to learn more about the syntax of a
5638 You attach an event rule to a <<channel,channel>> on creation. If you do
5639 not specify the channel with the opt:lttng-enable-event(1):--channel
5640 option, and if the event rule to create is the first in its
5641 <<domain,tracing domain>> for a given tracing session, then LTTng
5642 creates a _default channel_ for you. This default channel is reused in
5643 subsequent invocations of the man:lttng-enable-event(1) command for the
5644 same tracing domain.
5646 An event rule is always enabled at creation time.
5648 The following examples show how you can combine the previous
5649 command-line options to create simple to more complex event rules.
5651 .Create an event rule targetting a Linux kernel tracepoint (default channel).
5655 $ lttng enable-event --kernel sched_switch
5659 .Create an event rule matching four Linux kernel system calls (default channel).
5663 $ lttng enable-event --kernel --syscall open,write,read,close
5667 .Create event rules matching tracepoints with filter expressions (default channel).
5671 $ lttng enable-event --kernel sched_switch --filter='prev_comm == "bash"'
5676 $ lttng enable-event --kernel --all \
5677 --filter='$ctx.tid == 1988 || $ctx.tid == 1534'
5682 $ lttng enable-event --jul my_logger \
5683 --filter='$app.retriever:cur_msg_id > 3'
5686 IMPORTANT: Make sure to always quote the filter string when you
5687 use man:lttng(1) from a shell.
5690 .Create an event rule matching any user space tracepoint of a given tracepoint provider with a log level range (default channel).
5694 $ lttng enable-event --userspace my_app:'*' --loglevel=TRACE_INFO
5697 IMPORTANT: Make sure to always quote the wildcard character when you
5698 use man:lttng(1) from a shell.
5701 .Create an event rule matching multiple Python loggers with a wildcard and with exclusions (default channel).
5705 $ lttng enable-event --python my-app.'*' \
5706 --exclude='my-app.module,my-app.hello'
5710 .Create an event rule matching any Apache log4j logger with a specific log level (default channel).
5714 $ lttng enable-event --log4j --all --loglevel-only=LOG4J_WARN
5718 .Create an event rule attached to a specific channel matching a specific user space tracepoint provider and tracepoint.
5722 $ lttng enable-event --userspace my_app:my_tracepoint --channel=my-channel
5726 .Create an event rule matching the `malloc` function entry in path:{/usr/lib/libc.so.6}:
5730 $ lttng enable-event --kernel --userspace-probe=/usr/lib/libc.so.6:malloc \
5735 .Create an event rule matching the `server`/`accept_request` https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[USDT probe] in path:{/usr/bin/serv}:
5739 $ lttng enable-event --kernel --userspace-probe=sdt:serv:server:accept_request \
5740 server_accept_request
5744 The event rules of a given channel form a whitelist: as soon as an
5745 emitted event passes one of them, LTTng can record the event. For
5746 example, an event named `my_app:my_tracepoint` emitted from a user space
5747 tracepoint with a `TRACE_ERROR` log level passes both of the following
5752 $ lttng enable-event --userspace my_app:my_tracepoint
5753 $ lttng enable-event --userspace my_app:my_tracepoint \
5754 --loglevel=TRACE_INFO
5757 The second event rule is redundant: the first one includes
5761 [[disable-event-rule]]
5762 === Disable an event rule
5764 To disable an event rule that you <<enabling-disabling-events,created>>
5765 previously, use the man:lttng-disable-event(1) command. This command
5766 disables _all_ the event rules (of a given tracing domain and channel)
5767 which match an instrumentation point. The other conditions are not
5768 supported as of LTTng{nbsp}{revision}.
5770 The LTTng tracer does not record an emitted event which passes
5771 a _disabled_ event rule.
5773 .Disable an event rule matching a Python logger (default channel).
5777 $ lttng disable-event --python my-logger
5781 .Disable an event rule matching all `java.util.logging` loggers (default channel).
5785 $ lttng disable-event --jul '*'
5789 .Disable _all_ the event rules of the default channel.
5791 The opt:lttng-disable-event(1):--all-events option is not, like the
5792 opt:lttng-enable-event(1):--all option of man:lttng-enable-event(1), the
5793 equivalent of the event name `*` (wildcard): it disables _all_ the event
5794 rules of a given channel.
5798 $ lttng disable-event --jul --all-events
5802 NOTE: You cannot delete an event rule once you create it.
5806 === Get the status of a tracing session
5808 To get the status of the current tracing session, that is, its
5809 parameters, its channels, event rules, and their attributes:
5811 * Use the man:lttng-status(1) command:
5821 To get the status of any tracing session:
5823 * Use the man:lttng-list(1) command with the tracing session's name:
5828 $ lttng list my-session
5832 Replace `my-session` with the desired tracing session's name.
5835 [[basic-tracing-session-control]]
5836 === Start and stop a tracing session
5838 Once you <<creating-destroying-tracing-sessions,create a tracing
5840 <<enabling-disabling-events,create one or more event rules>>,
5841 you can start and stop the tracers for this tracing session.
5843 To start tracing in the current tracing session:
5845 * Use the man:lttng-start(1) command:
5854 LTTng is very flexible: you can launch user applications before
5855 or after the you start the tracers. The tracers only record the events
5856 if they pass enabled event rules and if they occur while the tracers are
5859 To stop tracing in the current tracing session:
5861 * Use the man:lttng-stop(1) command:
5870 If there were <<channel-overwrite-mode-vs-discard-mode,lost event
5871 records>> or lost sub-buffers since the last time you ran
5872 man:lttng-start(1), warnings are printed when you run the
5873 man:lttng-stop(1) command.
5875 IMPORTANT: You need to stop tracing to make LTTng flush the remaining
5876 trace data and make the trace readable. Note that the
5877 man:lttng-destroy(1) command (see
5878 <<creating-destroying-tracing-sessions,Create and destroy a tracing
5879 session>>) also runs the man:lttng-stop(1) command implicitly.
5882 [[enabling-disabling-channels]]
5883 === Create a channel
5885 Once you create a tracing session, you can create a <<channel,channel>>
5886 with the man:lttng-enable-channel(1) command.
5888 Note that LTTng automatically creates a default channel when, for a
5889 given <<domain,tracing domain>>, no channels exist and you
5890 <<enabling-disabling-events,create>> the first event rule. This default
5891 channel is named `channel0` and its attributes are set to reasonable
5892 values. Therefore, you only need to create a channel when you need
5893 non-default attributes.
5895 You specify each non-default channel attribute with a command-line
5896 option when you use the man:lttng-enable-channel(1) command. The
5897 available command-line options are:
5899 [role="growable",cols="asciidoc,asciidoc"]
5900 .Command-line options for the man:lttng-enable-channel(1) command.
5902 |Option |Description
5908 <<channel-overwrite-mode-vs-discard-mode,event record loss mode>> instead
5909 of the default _discard_ mode.
5911 |`--buffers-pid` (user space tracing domain only)
5914 Use the per-process <<channel-buffering-schemes,buffering scheme>>
5915 instead of the default per-user buffering scheme.
5917 |+--subbuf-size=__SIZE__+
5920 Allocate sub-buffers of +__SIZE__+ bytes (power of two), for each CPU,
5921 either for each Unix user (default), or for each instrumented process.
5923 See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
5925 |+--num-subbuf=__COUNT__+
5928 Allocate +__COUNT__+ sub-buffers (power of two), for each CPU, either
5929 for each Unix user (default), or for each instrumented process.
5931 See <<channel-subbuf-size-vs-subbuf-count,Sub-buffer count and size>>.
5933 |+--tracefile-size=__SIZE__+
5936 Set the maximum size of each trace file that this channel writes within
5937 a stream to +__SIZE__+ bytes instead of no maximum.
5939 See <<tracefile-rotation,Trace file count and size>>.
5941 |+--tracefile-count=__COUNT__+
5944 Limit the number of trace files that this channel creates to
5945 +__COUNT__+ channels instead of no limit.
5947 See <<tracefile-rotation,Trace file count and size>>.
5949 |+--switch-timer=__PERIODUS__+
5952 Set the <<channel-switch-timer,switch timer period>>
5953 to +__PERIODUS__+{nbsp}µs.
5955 |+--read-timer=__PERIODUS__+
5958 Set the <<channel-read-timer,read timer period>>
5959 to +__PERIODUS__+{nbsp}µs.
5961 |[[opt-blocking-timeout]]+--blocking-timeout=__TIMEOUTUS__+
5964 Set the timeout of user space applications which load LTTng-UST
5965 in blocking mode to +__TIMEOUTUS__+:
5968 Never block (non-blocking mode).
5971 Block forever until space is available in a sub-buffer to record
5974 __n__, a positive value::
5975 Wait for at most __n__ µs when trying to write into a sub-buffer.
5977 Note that, for this option to have any effect on an instrumented
5978 user space application, you need to run the application with a set
5979 env:LTTNG_UST_ALLOW_BLOCKING environment variable.
5981 |+--output=__TYPE__+ (Linux kernel tracing domain only)
5984 Set the channel's output type to +__TYPE__+, either `mmap` or `splice`.
5988 You can only create a channel in the Linux kernel and user space
5989 <<domain,tracing domains>>: other tracing domains have their own channel
5990 created on the fly when <<enabling-disabling-events,creating event
5995 Because of a current LTTng limitation, you must create all channels
5996 _before_ you <<basic-tracing-session-control,start tracing>> in a given
5997 tracing session, that is, before the first time you run
6000 Since LTTng automatically creates a default channel when you use the
6001 man:lttng-enable-event(1) command with a specific tracing domain, you
6002 cannot, for example, create a Linux kernel event rule, start tracing,
6003 and then create a user space event rule, because no user space channel
6004 exists yet and it's too late to create one.
6006 For this reason, make sure to configure your channels properly
6007 before starting the tracers for the first time!
6010 The following examples show how you can combine the previous
6011 command-line options to create simple to more complex channels.
6013 .Create a Linux kernel channel with default attributes.
6017 $ lttng enable-channel --kernel my-channel
6021 .Create a user space channel with four sub-buffers or 1{nbsp}MiB each, per CPU, per instrumented process.
6025 $ lttng enable-channel --userspace --num-subbuf=4 --subbuf-size=1M \
6026 --buffers-pid my-channel
6030 .[[blocking-timeout-example]]Create a default user space channel with an infinite blocking timeout.
6032 <<creating-destroying-tracing-sessions,Create a tracing-session>>,
6033 create the channel, <<enabling-disabling-events,create an event rule>>,
6034 and <<basic-tracing-session-control,start tracing>>:
6039 $ lttng enable-channel --userspace --blocking-timeout=inf blocking-channel
6040 $ lttng enable-event --userspace --channel=blocking-channel --all
6044 Run an application instrumented with LTTng-UST and allow it to block:
6048 $ LTTNG_UST_ALLOW_BLOCKING=1 my-app
6052 .Create a Linux kernel channel which rotates eight trace files of 4{nbsp}MiB each for each stream
6056 $ lttng enable-channel --kernel --tracefile-count=8 \
6057 --tracefile-size=4194304 my-channel
6061 .Create a user space channel in overwrite (or _flight recorder_) mode.
6065 $ lttng enable-channel --userspace --overwrite my-channel
6069 You can <<enabling-disabling-events,create>> the same event rule in
6070 two different channels:
6074 $ lttng enable-event --userspace --channel=my-channel app:tp
6075 $ lttng enable-event --userspace --channel=other-channel app:tp
6078 If both channels are enabled, when a tracepoint named `app:tp` is
6079 reached, LTTng records two events, one for each channel.
6083 === Disable a channel
6085 To disable a specific channel that you <<enabling-disabling-channels,created>>
6086 previously, use the man:lttng-disable-channel(1) command.
6088 .Disable a specific Linux kernel channel.
6092 $ lttng disable-channel --kernel my-channel
6096 The state of a channel precedes the individual states of event rules
6097 attached to it: event rules which belong to a disabled channel, even if
6098 they are enabled, are also considered disabled.
6102 === Add context fields to a channel
6104 Event record fields in trace files provide important information about
6105 events that occured previously, but sometimes some external context may
6106 help you solve a problem faster. Examples of context fields are:
6108 * The **process ID**, **thread ID**, **process name**, and
6109 **process priority** of the thread in which the event occurs.
6110 * The **hostname** of the system on which the event occurs.
6111 * The Linux kernel and user call stacks (since
6112 LTTng{nbsp}{revision}).
6113 * The current values of many possible **performance counters** using
6115 ** CPU cycles, stalled cycles, idle cycles, and the other cycle types.
6117 ** Branch instructions, misses, and loads.
6119 * Any context defined at the application level (supported for the
6120 JUL and log4j <<domain,tracing domains>>).
6122 To get the full list of available context fields, see
6123 `lttng add-context --list`. Some context fields are reserved for a
6124 specific <<domain,tracing domain>> (Linux kernel or user space).
6126 You add context fields to <<channel,channels>>. All the events
6127 that a channel with added context fields records contain those fields.
6129 To add context fields to one or all the channels of a given tracing
6132 * Use the man:lttng-add-context(1) command.
6134 .Add context fields to all the channels of the current tracing session.
6136 The following command line adds the virtual process identifier and
6137 the per-thread CPU cycles count fields to all the user space channels
6138 of the current tracing session.
6142 $ lttng add-context --userspace --type=vpid --type=perf:thread:cpu-cycles
6146 .Add performance counter context fields by raw ID
6148 See man:lttng-add-context(1) for the exact format of the context field
6149 type, which is partly compatible with the format used in
6154 $ lttng add-context --userspace --type=perf:thread:raw:r0110:test
6155 $ lttng add-context --kernel --type=perf:cpu:raw:r0013c:x86unhalted
6159 .Add context fields to a specific channel.
6161 The following command line adds the thread identifier and user call
6162 stack context fields to the Linux kernel channel named `my-channel` in
6163 the current tracing session.
6167 $ lttng add-context --kernel --channel=my-channel \
6168 --type=tid --type=callstack-user
6172 .Add an application-specific context field to a specific channel.
6174 The following command line adds the `cur_msg_id` context field of the
6175 `retriever` context retriever for all the instrumented
6176 <<java-application,Java applications>> recording <<event,event records>>
6177 in the channel named `my-channel`:
6181 $ lttng add-context --kernel --channel=my-channel \
6182 --type='$app:retriever:cur_msg_id'
6185 IMPORTANT: Make sure to always quote the `$` character when you
6186 use man:lttng-add-context(1) from a shell.
6189 NOTE: You cannot remove context fields from a channel once you add it.
6194 === Track process IDs
6196 It's often useful to allow only specific process IDs (PIDs) to emit
6197 events. For example, you may wish to record all the system calls made by
6198 a given process (Ă la http://linux.die.net/man/1/strace[strace]).
6200 The man:lttng-track(1) and man:lttng-untrack(1) commands serve this
6201 purpose. Both commands operate on a whitelist of process IDs. You _add_
6202 entries to this whitelist with the man:lttng-track(1) command and remove
6203 entries with the man:lttng-untrack(1) command. Any process which has one
6204 of the PIDs in the whitelist is allowed to emit LTTng events which pass
6205 an enabled <<event,event rule>>.
6207 NOTE: The PID tracker tracks the _numeric process IDs_. Should a
6208 process with a given tracked ID exit and another process be given this
6209 ID, then the latter would also be allowed to emit events.
6211 .Track and untrack process IDs.
6213 For the sake of the following example, assume the target system has
6214 16{nbsp}possible PIDs.
6217 <<creating-destroying-tracing-sessions,create a tracing session>>,
6218 the whitelist contains all the possible PIDs:
6221 .All PIDs are tracked.
6222 image::track-all.png[]
6224 When the whitelist is full and you use the man:lttng-track(1) command to
6225 specify some PIDs to track, LTTng first clears the whitelist, then it
6226 tracks the specific PIDs. After:
6230 $ lttng track --pid=3,4,7,10,13
6236 .PIDs 3, 4, 7, 10, and 13 are tracked.
6237 image::track-3-4-7-10-13.png[]
6239 You can add more PIDs to the whitelist afterwards:
6243 $ lttng track --pid=1,15,16
6249 .PIDs 1, 15, and 16 are added to the whitelist.
6250 image::track-1-3-4-7-10-13-15-16.png[]
6252 The man:lttng-untrack(1) command removes entries from the PID tracker's
6253 whitelist. Given the previous example, the following command:
6257 $ lttng untrack --pid=3,7,10,13
6260 leads to this whitelist:
6263 .PIDs 3, 7, 10, and 13 are removed from the whitelist.
6264 image::track-1-4-15-16.png[]
6266 LTTng can track all possible PIDs again using the
6267 opt:lttng-track(1):--all option:
6271 $ lttng track --pid --all
6274 The result is, again:
6277 .All PIDs are tracked.
6278 image::track-all.png[]
6281 .Track only specific PIDs
6283 A very typical use case with PID tracking is to start with an empty
6284 whitelist, then <<basic-tracing-session-control,start the tracers>>, and
6285 then add PIDs manually while tracers are active. You can accomplish this
6286 by using the opt:lttng-untrack(1):--all option of the
6287 man:lttng-untrack(1) command to clear the whitelist after you
6288 <<creating-destroying-tracing-sessions,create a tracing session>>:
6292 $ lttng untrack --pid --all
6298 .No PIDs are tracked.
6299 image::untrack-all.png[]
6301 If you trace with this whitelist configuration, the tracer records no
6302 events for this <<domain,tracing domain>> because no processes are
6303 tracked. You can use the man:lttng-track(1) command as usual to track
6304 specific PIDs, for example:
6308 $ lttng track --pid=6,11
6314 .PIDs 6 and 11 are tracked.
6315 image::track-6-11.png[]
6320 [[saving-loading-tracing-session]]
6321 === Save and load tracing session configurations
6323 Configuring a <<tracing-session,tracing session>> can be long. Some of
6324 the tasks involved are:
6326 * <<enabling-disabling-channels,Create channels>> with
6327 specific attributes.
6328 * <<adding-context,Add context fields>> to specific channels.
6329 * <<enabling-disabling-events,Create event rules>> with specific log
6330 level and filter conditions.
6332 If you use LTTng to solve real world problems, chances are you have to
6333 record events using the same tracing session setup over and over,
6334 modifying a few variables each time in your instrumented program
6335 or environment. To avoid constant tracing session reconfiguration,
6336 the man:lttng(1) command-line tool can save and load tracing session
6337 configurations to/from XML files.
6339 To save a given tracing session configuration:
6341 * Use the man:lttng-save(1) command:
6346 $ lttng save my-session
6350 Replace `my-session` with the name of the tracing session to save.
6352 LTTng saves tracing session configurations to
6353 dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the
6354 env:LTTNG_HOME environment variable defaults to `$HOME` if not set. Use
6355 the opt:lttng-save(1):--output-path option to change this destination
6358 LTTng saves all configuration parameters, for example:
6360 * The tracing session name.
6361 * The trace data output path.
6362 * The channels with their state and all their attributes.
6363 * The context fields you added to channels.
6364 * The event rules with their state, log level and filter conditions.
6366 To load a tracing session:
6368 * Use the man:lttng-load(1) command:
6373 $ lttng load my-session
6377 Replace `my-session` with the name of the tracing session to load.
6379 When LTTng loads a configuration, it restores your saved tracing session
6380 as if you just configured it manually.
6382 See man:lttng(1) for the complete list of command-line options. You
6383 can also save and load all many sessions at a time, and decide in which
6384 directory to output the XML files.
6387 [[sending-trace-data-over-the-network]]
6388 === Send trace data over the network
6390 LTTng can send the recorded trace data to a remote system over the
6391 network instead of writing it to the local file system.
6393 To send the trace data over the network:
6395 . On the _remote_ system (which can also be the target system),
6396 start an LTTng <<lttng-relayd,relay daemon>> (man:lttng-relayd(8)):
6405 . On the _target_ system, create a tracing session configured to
6406 send trace data over the network:
6411 $ lttng create my-session --set-url=net://remote-system
6415 Replace `remote-system` by the host name or IP address of the
6416 remote system. See man:lttng-create(1) for the exact URL format.
6418 . On the target system, use the man:lttng(1) command-line tool as usual.
6419 When tracing is active, the target's consumer daemon sends sub-buffers
6420 to the relay daemon running on the remote system instead of flushing
6421 them to the local file system. The relay daemon writes the received
6422 packets to the local file system.
6424 The relay daemon writes trace files to
6425 +$LTTNG_HOME/lttng-traces/__hostname__/__session__+ by default, where
6426 +__hostname__+ is the host name of the target system and +__session__+
6427 is the tracing session name. Note that the env:LTTNG_HOME environment
6428 variable defaults to `$HOME` if not set. Use the
6429 opt:lttng-relayd(8):--output option of man:lttng-relayd(8) to write
6430 trace files to another base directory.
6435 === View events as LTTng emits them (noch:{LTTng} live)
6437 LTTng live is a network protocol implemented by the <<lttng-relayd,relay
6438 daemon>> (man:lttng-relayd(8)) to allow compatible trace viewers to
6439 display events as LTTng emits them on the target system while tracing is
6442 The relay daemon creates a _tee_: it forwards the trace data to both
6443 the local file system and to connected live viewers:
6446 .The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live viewer.
6451 . On the _target system_, create a <<tracing-session,tracing session>>
6457 $ lttng create my-session --live
6461 This spawns a local relay daemon.
6463 . Start the live viewer and configure it to connect to the relay
6464 daemon. For example, with http://diamon.org/babeltrace[Babeltrace]:
6469 $ babeltrace --input-format=lttng-live \
6470 net://localhost/host/hostname/my-session
6477 * `hostname` with the host name of the target system.
6478 * `my-session` with the name of the tracing session to view.
6481 . Configure the tracing session as usual with the man:lttng(1)
6482 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6484 You can list the available live tracing sessions with Babeltrace:
6488 $ babeltrace --input-format=lttng-live net://localhost
6491 You can start the relay daemon on another system. In this case, you need
6492 to specify the relay daemon's URL when you create the tracing session
6493 with the opt:lttng-create(1):--set-url option. You also need to replace
6494 `localhost` in the procedure above with the host name of the system on
6495 which the relay daemon is running.
6497 See man:lttng-create(1) and man:lttng-relayd(8) for the complete list of
6498 command-line options.
6502 [[taking-a-snapshot]]
6503 === Take a snapshot of the current sub-buffers of a tracing session
6505 The normal behavior of LTTng is to append full sub-buffers to growing
6506 trace data files. This is ideal to keep a full history of the events
6507 that occurred on the target system, but it can
6508 represent too much data in some situations. For example, you may wish
6509 to trace your application continuously until some critical situation
6510 happens, in which case you only need the latest few recorded
6511 events to perform the desired analysis, not multi-gigabyte trace files.
6513 With the man:lttng-snapshot(1) command, you can take a snapshot of the
6514 current sub-buffers of a given <<tracing-session,tracing session>>.
6515 LTTng can write the snapshot to the local file system or send it over
6519 .A snapshot is a copy of the current sub-buffers, which are not cleared after the operation.
6520 image::snapshot.png[]
6522 If you wish to create unmanaged, self-contained, non-overlapping
6523 trace chunk archives instead of a simple copy of the current
6524 sub-buffers, see the <<session-rotation,tracing session rotation>>
6525 feature (available since LTTng{nbsp}2.11).
6529 . Create a tracing session in _snapshot mode_:
6534 $ lttng create my-session --snapshot
6538 The <<channel-overwrite-mode-vs-discard-mode,event record loss mode>> of
6539 <<channel,channels>> created in this mode is automatically set to
6540 _overwrite_ (flight recorder mode).
6542 . Configure the tracing session as usual with the man:lttng(1)
6543 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6545 . **Optional**: When you need to take a snapshot,
6546 <<basic-tracing-session-control,stop tracing>>.
6548 You can take a snapshot when the tracers are active, but if you stop
6549 them first, you are sure that the data in the sub-buffers does not
6550 change before you actually take the snapshot.
6557 $ lttng snapshot record --name=my-first-snapshot
6561 LTTng writes the current sub-buffers of all the current tracing
6562 session's channels to trace files on the local file system. Those trace
6563 files have `my-first-snapshot` in their name.
6565 There is no difference between the format of a normal trace file and the
6566 format of a snapshot: viewers of LTTng traces also support LTTng
6569 By default, LTTng writes snapshot files to the path shown by
6570 `lttng snapshot list-output`. You can change this path or decide to send
6571 snapshots over the network using either:
6573 . An output path or URL that you specify when you
6574 <<creating-destroying-tracing-sessions,create the tracing session>>.
6575 . A snapshot output path or URL that you add using
6576 `lttng snapshot add-output`.
6577 . An output path or URL that you provide directly to the
6578 `lttng snapshot record` command.
6580 Method{nbsp}3 overrides method{nbsp}2, which overrides method 1. When
6581 you specify a URL, a relay daemon must listen on a remote system (see
6582 <<sending-trace-data-over-the-network,Send trace data over the
6587 [[session-rotation]]
6588 === Archive the current trace chunk (rotate a tracing session)
6590 The <<taking-a-snapshot,snapshot user guide>> shows how you can dump
6591 a tracing session's current sub-buffers to the file system or send them
6592 over the network. When you take a snapshot, LTTng does not clear the
6593 tracing session's ring buffers: if you take another snapshot immediately
6594 after, both snapshots could contain overlapping trace data.
6596 Inspired by https://en.wikipedia.org/wiki/Log_rotation[log rotation],
6597 _tracing session rotation_ is a feature which appends the content of the
6598 ring buffers to what's already on the file system or sent over the
6599 network since the tracing session's creation or since the last
6600 rotation, and then clears those ring buffers to avoid trace data
6603 What LTTng is about to write when performing a tracing session rotation
6604 is called the _current trace chunk_. When this current trace chunk is
6605 written to the file system or sent over the network, it becomes a _trace
6606 chunk archive_. Therefore, a tracing session rotation _archives_ the
6607 current trace chunk.
6610 .A tracing session rotation operation _archives_ the current trace chunk.
6611 image::rotation.png[]
6613 A trace chunk archive is a self-contained LTTng trace which LTTng
6614 doesn't manage anymore: you can read it, modify it, move it, or remove
6617 There are two methods to perform a tracing session rotation: immediately
6618 or with a rotation schedule.
6620 To perform an immediate tracing session rotation:
6622 . <<creating-destroying-tracing-sessions,Create a tracing session>>
6623 in _normal mode_ or _network streaming mode_
6624 (only those two creation modes support tracing session rotation):
6629 $ lttng create my-session
6633 . <<enabling-disabling-events,Create one or more event rules>>
6634 and <<basic-tracing-session-control,start tracing>>:
6639 $ lttng enable-event --kernel sched_'*'
6644 . When needed, immediately rotate the current tracing session:
6653 The cmd:lttng-rotate command prints the path to the created trace
6654 chunk archive. See man:lttng-rotate(1) to learn about the format
6655 of trace chunk archive directory names.
6657 You can perform other immediate rotations while the tracing session is
6658 active. It is guaranteed that all the trace chunk archives do not
6659 contain overlapping trace data. You can also perform an immediate
6660 rotation once you have <<basic-tracing-session-control,stopped>> the
6663 . When you are done tracing,
6664 <<creating-destroying-tracing-sessions,destroy the current tracing
6674 The tracing session destruction operation creates one last trace
6675 chunk archive from the current trace chunk.
6677 A tracing session rotation schedule is a planned rotation which LTTng
6678 performs automatically based on one of the following conditions:
6680 * A timer with a configured period times out.
6682 * The total size of the flushed part of the current trace chunk
6683 becomes greater than or equal to a configured value.
6685 To schedule a tracing session rotation, set a _rotation schedule_:
6687 . <<creating-destroying-tracing-sessions,Create a tracing session>>
6688 in _normal mode_ or _network streaming mode_
6689 (only those two creation modes support tracing session rotation):
6694 $ lttng create my-session
6698 . <<enabling-disabling-events,Create one or more event rules>>:
6703 $ lttng enable-event --kernel sched_'*'
6707 . Set a tracing session rotation schedule:
6712 $ lttng enable-rotation --timer=10s
6716 In this example, we set a rotation schedule so that LTTng performs a
6717 tracing session rotation every ten seconds.
6719 See man:lttng-enable-rotation(1) to learn more about other ways to set a
6722 . <<basic-tracing-session-control,Start tracing>>:
6731 LTTng performs tracing session rotations automatically while the tracing
6732 session is active thanks to the rotation schedule.
6734 . When you are done tracing,
6735 <<creating-destroying-tracing-sessions,destroy the current tracing
6745 The tracing session destruction operation creates one last trace chunk
6746 archive from the current trace chunk.
6748 You can use man:lttng-disable-rotation(1) to unset a tracing session
6751 NOTE: man:lttng-rotate(1) and man:lttng-enable-rotation(1) list
6752 limitations regarding those two commands.
6757 === Use the machine interface
6759 With any command of the man:lttng(1) command-line tool, you can set the
6760 opt:lttng(1):--mi option to `xml` (before the command name) to get an
6761 XML machine interface output, for example:
6765 $ lttng --mi=xml enable-event --kernel --syscall open
6768 A schema definition (XSD) is
6769 https://github.com/lttng/lttng-tools/blob/stable-2.11/src/common/mi-lttng-3.0.xsd[available]
6770 to ease the integration with external tools as much as possible.
6774 [[metadata-regenerate]]
6775 === Regenerate the metadata of an LTTng trace
6777 An LTTng trace, which is a http://diamon.org/ctf[CTF] trace, has both
6778 data stream files and a metadata file. This metadata file contains,
6779 amongst other things, information about the offset of the clock sources
6780 used to timestamp <<event,event records>> when tracing.
6782 If, once a <<tracing-session,tracing session>> is
6783 <<basic-tracing-session-control,started>>, a major
6784 https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction
6785 happens, the trace's clock offset also needs to be updated. You
6786 can use the `metadata` item of the man:lttng-regenerate(1) command
6789 The main use case of this command is to allow a system to boot with
6790 an incorrect wall time and trace it with LTTng before its wall time
6791 is corrected. Once the system is known to be in a state where its
6792 wall time is correct, it can run `lttng regenerate metadata`.
6794 To regenerate the metadata of an LTTng trace:
6796 * Use the `metadata` item of the man:lttng-regenerate(1) command:
6801 $ lttng regenerate metadata
6807 `lttng regenerate metadata` has the following limitations:
6809 * Tracing session <<creating-destroying-tracing-sessions,created>>
6811 * User space <<channel,channels>>, if any, are using
6812 <<channel-buffering-schemes,per-user buffering>>.
6817 [[regenerate-statedump]]
6818 === Regenerate the state dump of a tracing session
6820 The LTTng kernel and user space tracers generate state dump
6821 <<event,event records>> when the application starts or when you
6822 <<basic-tracing-session-control,start a tracing session>>. An analysis
6823 can use the state dump event records to set an initial state before it
6824 builds the rest of the state from the following event records.
6825 http://tracecompass.org/[Trace Compass] is a notable example of an
6826 application which uses the state dump of an LTTng trace.
6828 When you <<taking-a-snapshot,take a snapshot>>, it's possible that the
6829 state dump event records are not included in the snapshot because they
6830 were recorded to a sub-buffer that has been consumed or overwritten
6833 You can use the `lttng regenerate statedump` command to emit the state
6834 dump event records again.
6836 To regenerate the state dump of the current tracing session, provided
6837 create it in snapshot mode, before you take a snapshot:
6839 . Use the `statedump` item of the man:lttng-regenerate(1) command:
6844 $ lttng regenerate statedump
6848 . <<basic-tracing-session-control,Stop the tracing session>>:
6857 . <<taking-a-snapshot,Take a snapshot>>:
6862 $ lttng snapshot record --name=my-snapshot
6866 Depending on the event throughput, you should run steps 1 and 2
6867 as closely as possible.
6869 NOTE: To record the state dump events, you need to
6870 <<enabling-disabling-events,create event rules>> which enable them.
6871 LTTng-UST state dump tracepoints start with `lttng_ust_statedump:`.
6872 LTTng-modules state dump tracepoints start with `lttng_statedump_`.
6876 [[persistent-memory-file-systems]]
6877 === Record trace data on persistent memory file systems
6879 https://en.wikipedia.org/wiki/Non-volatile_random-access_memory[Non-volatile random-access memory]
6880 (NVRAM) is random-access memory that retains its information when power
6881 is turned off (non-volatile). Systems with such memory can store data
6882 structures in RAM and retrieve them after a reboot, without flushing
6883 to typical _storage_.
6885 Linux supports NVRAM file systems thanks to either
6886 http://pramfs.sourceforge.net/[PRAMFS] or
6887 https://www.kernel.org/doc/Documentation/filesystems/dax.txt[DAX]{nbsp}+{nbsp}http://lkml.iu.edu/hypermail/linux/kernel/1504.1/03463.html[pmem]
6888 (requires Linux{nbsp}4.1+).
6890 This section does not describe how to operate such file systems;
6891 we assume that you have a working persistent memory file system.
6893 When you create a <<tracing-session,tracing session>>, you can specify
6894 the path of the shared memory holding the sub-buffers. If you specify a
6895 location on an NVRAM file system, then you can retrieve the latest
6896 recorded trace data when the system reboots after a crash.
6898 To record trace data on a persistent memory file system and retrieve the
6899 trace data after a system crash:
6901 . Create a tracing session with a sub-buffer shared memory path located
6902 on an NVRAM file system:
6907 $ lttng create my-session --shm-path=/path/to/shm
6911 . Configure the tracing session as usual with the man:lttng(1)
6912 command-line tool, and <<basic-tracing-session-control,start tracing>>.
6914 . After a system crash, use the man:lttng-crash(1) command-line tool to
6915 view the trace data recorded on the NVRAM file system:
6920 $ lttng-crash /path/to/shm
6924 The binary layout of the ring buffer files is not exactly the same as
6925 the trace files layout. This is why you need to use man:lttng-crash(1)
6926 instead of your preferred trace viewer directly.
6928 To convert the ring buffer files to LTTng trace files:
6930 * Use the opt:lttng-crash(1):--extract option of man:lttng-crash(1):
6935 $ lttng-crash --extract=/path/to/trace /path/to/shm
6941 [[notif-trigger-api]]
6942 === Get notified when a channel's buffer usage is too high or too low
6944 With LTTng's $$C/C++$$ notification and trigger API, your user
6945 application can get notified when the buffer usage of one or more
6946 <<channel,channels>> becomes too low or too high. You can use this API
6947 and enable or disable <<event,event rules>> during tracing to avoid
6948 <<channel-overwrite-mode-vs-discard-mode,discarded event records>>.
6950 .Have a user application get notified when an LTTng channel's buffer usage is too high.
6952 In this example, we create and build an application which gets notified
6953 when the buffer usage of a specific LTTng channel is higher than
6954 75{nbsp}%. We only print that it is the case in the example, but we
6955 could as well use the API of <<liblttng-ctl-lttng,`liblttng-ctl`>> to
6956 disable event rules when this happens.
6958 . Create the application's C source file:
6966 #include <lttng/domain.h>
6967 #include <lttng/action/action.h>
6968 #include <lttng/action/notify.h>
6969 #include <lttng/condition/condition.h>
6970 #include <lttng/condition/buffer-usage.h>
6971 #include <lttng/condition/evaluation.h>
6972 #include <lttng/notification/channel.h>
6973 #include <lttng/notification/notification.h>
6974 #include <lttng/trigger/trigger.h>
6975 #include <lttng/endpoint.h>
6977 int main(int argc, char *argv[])
6979 int exit_status = 0;
6980 struct lttng_notification_channel *notification_channel;
6981 struct lttng_condition *condition;
6982 struct lttng_action *action;
6983 struct lttng_trigger *trigger;
6984 const char *tracing_session_name;
6985 const char *channel_name;
6988 tracing_session_name = argv[1];
6989 channel_name = argv[2];
6992 * Create a notification channel. A notification channel
6993 * connects the user application to the LTTng session daemon.
6994 * This notification channel can be used to listen to various
6995 * types of notifications.
6997 notification_channel = lttng_notification_channel_create(
6998 lttng_session_daemon_notification_endpoint);
7001 * Create a "high buffer usage" condition. In this case, the
7002 * condition is reached when the buffer usage is greater than or
7003 * equal to 75 %. We create the condition for a specific tracing
7004 * session name, channel name, and for the user space tracing
7007 * The "low buffer usage" condition type also exists.
7009 condition = lttng_condition_buffer_usage_high_create();
7010 lttng_condition_buffer_usage_set_threshold_ratio(condition, .75);
7011 lttng_condition_buffer_usage_set_session_name(
7012 condition, tracing_session_name);
7013 lttng_condition_buffer_usage_set_channel_name(condition,
7015 lttng_condition_buffer_usage_set_domain_type(condition,
7019 * Create an action (get a notification) to take when the
7020 * condition created above is reached.
7022 action = lttng_action_notify_create();
7025 * Create a trigger. A trigger associates a condition to an
7026 * action: the action is executed when the condition is reached.
7028 trigger = lttng_trigger_create(condition, action);
7030 /* Register the trigger to LTTng. */
7031 lttng_register_trigger(trigger);
7034 * Now that we have registered a trigger, a notification will be
7035 * emitted everytime its condition is met. To receive this
7036 * notification, we must subscribe to notifications that match
7037 * the same condition.
7039 lttng_notification_channel_subscribe(notification_channel,
7043 * Notification loop. You can put this in a dedicated thread to
7044 * avoid blocking the main thread.
7047 struct lttng_notification *notification;
7048 enum lttng_notification_channel_status status;
7049 const struct lttng_evaluation *notification_evaluation;
7050 const struct lttng_condition *notification_condition;
7051 double buffer_usage;
7053 /* Receive the next notification. */
7054 status = lttng_notification_channel_get_next_notification(
7055 notification_channel, ¬ification);
7058 case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK:
7060 case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED:
7062 * The session daemon can drop notifications if
7063 * a monitoring application is not consuming the
7064 * notifications fast enough.
7067 case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED:
7069 * The notification channel has been closed by the
7070 * session daemon. This is typically caused by a session
7071 * daemon shutting down.
7075 /* Unhandled conditions or errors. */
7081 * A notification provides, amongst other things:
7083 * * The condition that caused this notification to be
7085 * * The condition evaluation, which provides more
7086 * specific information on the evaluation of the
7089 * The condition evaluation provides the buffer usage
7090 * value at the moment the condition was reached.
7092 notification_condition = lttng_notification_get_condition(
7094 notification_evaluation = lttng_notification_get_evaluation(
7097 /* We're subscribed to only one condition. */
7098 assert(lttng_condition_get_type(notification_condition) ==
7099 LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH);
7102 * Get the exact sampled buffer usage from the
7103 * condition evaluation.
7105 lttng_evaluation_buffer_usage_get_usage_ratio(
7106 notification_evaluation, &buffer_usage);
7109 * At this point, instead of printing a message, we
7110 * could do something to reduce the channel's buffer
7111 * usage, like disable specific events.
7113 printf("Buffer usage is %f %% in tracing session \"%s\", "
7114 "user space channel \"%s\".\n", buffer_usage * 100,
7115 tracing_session_name, channel_name);
7116 lttng_notification_destroy(notification);
7120 lttng_action_destroy(action);
7121 lttng_condition_destroy(condition);
7122 lttng_trigger_destroy(trigger);
7123 lttng_notification_channel_destroy(notification_channel);
7129 . Build the `notif-app` application, linking it to `liblttng-ctl`:
7134 $ gcc -o notif-app notif-app.c -llttng-ctl
7138 . <<creating-destroying-tracing-sessions,Create a tracing session>>,
7139 <<enabling-disabling-events,create an event rule>> matching all the
7140 user space tracepoints, and
7141 <<basic-tracing-session-control,start tracing>>:
7146 $ lttng create my-session
7147 $ lttng enable-event --userspace --all
7152 If you create the channel manually with the man:lttng-enable-channel(1)
7153 command, you can control how frequently are the current values of the
7154 channel's properties sampled to evaluate user conditions with the
7155 opt:lttng-enable-channel(1):--monitor-timer option.
7157 . Run the `notif-app` application. This program accepts the
7158 <<tracing-session,tracing session>> name and the user space channel
7159 name as its two first arguments. The channel which LTTng automatically
7160 creates with the man:lttng-enable-event(1) command above is named
7166 $ ./notif-app my-session channel0
7170 . In another terminal, run an application with a very high event
7171 throughput so that the 75{nbsp}% buffer usage condition is reached.
7173 In the first terminal, the application should print lines like this:
7176 Buffer usage is 81.45197 % in tracing session "my-session", user space
7180 If you don't see anything, try modifying the condition in
7181 path:{notif-app.c} to a lower value (0.1, for example), rebuilding it
7182 (step{nbsp}2) and running it again (step{nbsp}4).
7189 [[lttng-modules-ref]]
7190 === noch:{LTTng-modules}
7194 [[lttng-tracepoint-enum]]
7195 ==== `LTTNG_TRACEPOINT_ENUM()` usage
7197 Use the `LTTNG_TRACEPOINT_ENUM()` macro to define an enumeration:
7201 LTTNG_TRACEPOINT_ENUM(name, TP_ENUM_VALUES(entries))
7206 * `name` with the name of the enumeration (C identifier, unique
7207 amongst all the defined enumerations).
7208 * `entries` with a list of enumeration entries.
7210 The available enumeration entry macros are:
7212 +ctf_enum_value(__name__, __value__)+::
7213 Entry named +__name__+ mapped to the integral value +__value__+.
7215 +ctf_enum_range(__name__, __begin__, __end__)+::
7216 Entry named +__name__+ mapped to the range of integral values between
7217 +__begin__+ (included) and +__end__+ (included).
7219 +ctf_enum_auto(__name__)+::
7220 Entry named +__name__+ mapped to the integral value following the
7221 last mapping's value.
7223 The last value of a `ctf_enum_value()` entry is its +__value__+
7226 The last value of a `ctf_enum_range()` entry is its +__end__+ parameter.
7228 If `ctf_enum_auto()` is the first entry in the list, its integral
7231 Use the `ctf_enum()` <<lttng-modules-tp-fields,field definition macro>>
7232 to use a defined enumeration as a tracepoint field.
7234 .Define an enumeration with `LTTNG_TRACEPOINT_ENUM()`.
7238 LTTNG_TRACEPOINT_ENUM(
7241 ctf_enum_auto("AUTO: EXPECT 0")
7242 ctf_enum_value("VALUE: 23", 23)
7243 ctf_enum_value("VALUE: 27", 27)
7244 ctf_enum_auto("AUTO: EXPECT 28")
7245 ctf_enum_range("RANGE: 101 TO 303", 101, 303)
7246 ctf_enum_auto("AUTO: EXPECT 304")
7254 [[lttng-modules-tp-fields]]
7255 ==== Tracepoint fields macros (for `TP_FIELDS()`)
7257 [[tp-fast-assign]][[tp-struct-entry]]The available macros to define
7258 tracepoint fields, which must be listed within `TP_FIELDS()` in
7259 `LTTNG_TRACEPOINT_EVENT()`, are:
7261 [role="func-desc growable",cols="asciidoc,asciidoc"]
7262 .Available macros to define LTTng-modules tracepoint fields
7264 |Macro |Description and parameters
7267 +ctf_integer(__t__, __n__, __e__)+
7269 +ctf_integer_nowrite(__t__, __n__, __e__)+
7271 +ctf_user_integer(__t__, __n__, __e__)+
7273 +ctf_user_integer_nowrite(__t__, __n__, __e__)+
7275 Standard integer, displayed in base{nbsp}10.
7278 Integer C type (`int`, `long`, `size_t`, ...).
7284 Argument expression.
7287 +ctf_integer_hex(__t__, __n__, __e__)+
7289 +ctf_user_integer_hex(__t__, __n__, __e__)+
7291 Standard integer, displayed in base{nbsp}16.
7300 Argument expression.
7302 |+ctf_integer_oct(__t__, __n__, __e__)+
7304 Standard integer, displayed in base{nbsp}8.
7313 Argument expression.
7316 +ctf_integer_network(__t__, __n__, __e__)+
7318 +ctf_user_integer_network(__t__, __n__, __e__)+
7320 Integer in network byte order (big-endian), displayed in base{nbsp}10.
7329 Argument expression.
7332 +ctf_integer_network_hex(__t__, __n__, __e__)+
7334 +ctf_user_integer_network_hex(__t__, __n__, __e__)+
7336 Integer in network byte order, displayed in base{nbsp}16.
7345 Argument expression.
7348 +ctf_enum(__N__, __t__, __n__, __e__)+
7350 +ctf_enum_nowrite(__N__, __t__, __n__, __e__)+
7352 +ctf_user_enum(__N__, __t__, __n__, __e__)+
7354 +ctf_user_enum_nowrite(__N__, __t__, __n__, __e__)+
7359 Name of a <<lttng-tracepoint-enum,previously defined enumeration>>.
7362 Integer C type (`int`, `long`, `size_t`, ...).
7368 Argument expression.
7371 +ctf_string(__n__, __e__)+
7373 +ctf_string_nowrite(__n__, __e__)+
7375 +ctf_user_string(__n__, __e__)+
7377 +ctf_user_string_nowrite(__n__, __e__)+
7379 Null-terminated string; undefined behavior if +__e__+ is `NULL`.
7385 Argument expression.
7388 +ctf_array(__t__, __n__, __e__, __s__)+
7390 +ctf_array_nowrite(__t__, __n__, __e__, __s__)+
7392 +ctf_user_array(__t__, __n__, __e__, __s__)+
7394 +ctf_user_array_nowrite(__t__, __n__, __e__, __s__)+
7396 Statically-sized array of integers.
7399 Array element C type.
7405 Argument expression.
7411 +ctf_array_bitfield(__t__, __n__, __e__, __s__)+
7413 +ctf_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
7415 +ctf_user_array_bitfield(__t__, __n__, __e__, __s__)+
7417 +ctf_user_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+
7419 Statically-sized array of bits.
7421 The type of +__e__+ must be an integer type. +__s__+ is the number
7422 of elements of such type in +__e__+, not the number of bits.
7425 Array element C type.
7431 Argument expression.
7437 +ctf_array_text(__t__, __n__, __e__, __s__)+
7439 +ctf_array_text_nowrite(__t__, __n__, __e__, __s__)+
7441 +ctf_user_array_text(__t__, __n__, __e__, __s__)+
7443 +ctf_user_array_text_nowrite(__t__, __n__, __e__, __s__)+
7445 Statically-sized array, printed as text.
7447 The string does not need to be null-terminated.
7450 Array element C type (always `char`).
7456 Argument expression.
7462 +ctf_sequence(__t__, __n__, __e__, __T__, __E__)+
7464 +ctf_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
7466 +ctf_user_sequence(__t__, __n__, __e__, __T__, __E__)+
7468 +ctf_user_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+
7470 Dynamically-sized array of integers.
7472 The type of +__E__+ must be unsigned.
7475 Array element C type.
7481 Argument expression.
7484 Length expression C type.
7490 +ctf_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
7492 +ctf_user_sequence_hex(__t__, __n__, __e__, __T__, __E__)+
7494 Dynamically-sized array of integers, displayed in base{nbsp}16.
7496 The type of +__E__+ must be unsigned.
7499 Array element C type.
7505 Argument expression.
7508 Length expression C type.
7513 |+ctf_sequence_network(__t__, __n__, __e__, __T__, __E__)+
7515 Dynamically-sized array of integers in network byte order (big-endian),
7516 displayed in base{nbsp}10.
7518 The type of +__E__+ must be unsigned.
7521 Array element C type.
7527 Argument expression.
7530 Length expression C type.
7536 +ctf_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
7538 +ctf_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
7540 +ctf_user_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+
7542 +ctf_user_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+
7544 Dynamically-sized array of bits.
7546 The type of +__e__+ must be an integer type. +__s__+ is the number
7547 of elements of such type in +__e__+, not the number of bits.
7549 The type of +__E__+ must be unsigned.
7552 Array element C type.
7558 Argument expression.
7561 Length expression C type.
7567 +ctf_sequence_text(__t__, __n__, __e__, __T__, __E__)+
7569 +ctf_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
7571 +ctf_user_sequence_text(__t__, __n__, __e__, __T__, __E__)+
7573 +ctf_user_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+
7575 Dynamically-sized array, displayed as text.
7577 The string does not need to be null-terminated.
7579 The type of +__E__+ must be unsigned.
7581 The behaviour is undefined if +__e__+ is `NULL`.
7584 Sequence element C type (always `char`).
7590 Argument expression.
7593 Length expression C type.
7599 Use the `_user` versions when the argument expression, `e`, is
7600 a user space address. In the cases of `ctf_user_integer*()` and
7601 `ctf_user_float*()`, `&e` must be a user space address, thus `e` must
7604 The `_nowrite` versions omit themselves from the session trace, but are
7605 otherwise identical. This means the `_nowrite` fields won't be written
7606 in the recorded trace. Their primary purpose is to make some
7607 of the event context available to the
7608 <<enabling-disabling-events,event filters>> without having to
7609 commit the data to sub-buffers.
7615 Terms related to LTTng and to tracing in general:
7618 The http://diamon.org/babeltrace[Babeltrace] project, which includes:
7620 * The cmd:babeltrace (Babeltrace{nbsp}1) or cmd:babeltrace2
7621 (Babeltrace{nbsp}2) command.
7622 * Libraries with a C{nbsp}API.
7623 * Python{nbsp}3 bindings.
7624 * Plugins (Babeltrace{nbsp}2).
7626 [[def-buffering-scheme]]<<channel-buffering-schemes,buffering scheme>>::
7627 A layout of <<def-sub-buffer,sub-buffers>> applied to a given channel.
7629 [[def-channel]]<<channel,channel>>::
7630 An entity which is responsible for a set of
7631 <<def-ring-buffer,ring buffers>>.
7633 <<def-event-rule,Event rules>> are always attached to a specific
7637 A source of time for a <<def-tracer,tracer>>.
7639 [[def-consumer-daemon]]<<lttng-consumerd,consumer daemon>>::
7640 A process which is responsible for consuming the full
7641 <<def-sub-buffer,sub-buffers>> and write them to a file system or
7642 send them over the network.
7644 [[def-current-trace-chunk]]current trace chunk::
7645 A <<def-trace-chunk,trace chunk>> which includes the current content
7646 of all the <<def-tracing-session-rotation,tracing session>>'s
7647 <<def-sub-buffer,sub-buffers>> and the stream files produced since the
7648 latest event amongst:
7650 * The creation of the <<def-tracing-session,tracing session>>.
7651 * The last tracing session rotation, if any.
7653 <<channel-overwrite-mode-vs-discard-mode,discard mode>>::
7654 The <<def-event-record-loss-mode,event record loss mode>> in which
7655 the <<def-tracer,tracer>> _discards_ new event records when there's no
7656 <<def-sub-buffer,sub-buffer>> space left to store them.
7658 [[def-event]]event::
7659 The consequence of the execution of an
7660 <<def-instrumentation-point,instrumentation point>>, like a
7661 <<def-tracepoint,tracepoint>> that you manually place in some source
7662 code, or a Linux kernel kprobe.
7664 An event is said to _occur_ at a specific time. <<def-lttng,LTTng>> can
7665 take various actions upon the occurrence of an event, like record the
7666 event's payload to a <<def-sub-buffer,sub-buffer>>.
7668 [[def-event-name]]event name::
7669 The name of an <<def-event,event>>, which is also the name of the
7670 <<def-event-record,event record>>.
7672 This is also called the _instrumentation point name_.
7674 [[def-event-record]]event record::
7675 A record, in a <<def-trace,trace>>, of the payload of an
7676 <<def-event,event>> which occured.
7678 [[def-event-record-loss-mode]]<<channel-overwrite-mode-vs-discard-mode,event record loss mode>>::
7679 The mechanism by which event records of a given
7680 <<def-channel,channel>> are lost (not recorded) when there is no
7681 <<def-sub-buffer,sub-buffer>> space left to store them.
7683 [[def-event-rule]]<<event,event rule>>::
7684 Set of conditions which must be satisfied for one or more occuring
7685 <<def-event,events>> to be recorded.
7687 `java.util.logging`::
7689 https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities].
7691 <<instrumenting,instrumentation>>::
7692 The use of <<def-lttng,LTTng>> probes to make a piece of software
7695 [[def-instrumentation-point]]instrumentation point::
7696 A point in the execution path of a piece of software that, when
7697 reached by this execution, can emit an <<def-event,event>>.
7699 instrumentation point name::
7700 See _<<def-event-name,event name>>_.
7703 A http://logging.apache.org/log4j/1.2/[logging library] for Java
7704 developed by the Apache Software Foundation.
7707 Level of severity of a log statement or user space
7708 <<def-instrumentation-point,instrumentation point>>.
7710 [[def-lttng]]LTTng::
7711 The _Linux Trace Toolkit: next generation_ project.
7713 <<lttng-cli,cmd:lttng>>::
7714 A command-line tool provided by the <<def-lttng-tools,LTTng-tools>>
7715 project which you can use to send and receive control messages to and
7716 from a <<def-session-daemon,session daemon>>.
7719 The https://github.com/lttng/lttng-analyses[LTTng analyses] project,
7720 which is a set of analyzing programs that you can use to obtain a
7721 higher level view of an <<def-lttng,LTTng>> <<def-trace,trace>>.
7723 cmd:lttng-consumerd::
7724 The name of the <<def-consumer-daemon,consumer daemon>> program.
7727 A utility provided by the <<def-lttng-tools,LTTng-tools>> project
7728 which can convert <<def-ring-buffer,ring buffer>> files (usually
7729 <<persistent-memory-file-systems,saved on a persistent memory file
7730 system>>) to <<def-trace,trace>> files.
7732 See man:lttng-crash(1).
7734 LTTng Documentation::
7737 <<lttng-live,LTTng live>>::
7738 A communication protocol between the <<lttng-relayd,relay daemon>> and
7739 live viewers which makes it possible to see <<def-event-record,event
7740 records>> "live", as they are received by the
7741 <<def-relay-daemon,relay daemon>>.
7743 <<lttng-modules,LTTng-modules>>::
7744 The https://github.com/lttng/lttng-modules[LTTng-modules] project,
7745 which contains the Linux kernel modules to make the Linux kernel
7746 <<def-instrumentation-point,instrumentation points>> available for
7747 <<def-lttng,LTTng>> tracing.
7750 The name of the <<def-relay-daemon,relay daemon>> program.
7752 cmd:lttng-sessiond::
7753 The name of the <<def-session-daemon,session daemon>> program.
7755 [[def-lttng-tools]]LTTng-tools::
7756 The https://github.com/lttng/lttng-tools[LTTng-tools] project, which
7757 contains the various programs and libraries used to
7758 <<controlling-tracing,control tracing>>.
7760 [[def-lttng-ust]]<<lttng-ust,LTTng-UST>>::
7761 The https://github.com/lttng/lttng-ust[LTTng-UST] project, which
7762 contains libraries to instrument
7763 <<def-user-application,user applications>>.
7765 <<lttng-ust-agents,LTTng-UST Java agent>>::
7766 A Java package provided by the <<def-lttng-ust,LTTng-UST>> project to
7767 allow the LTTng instrumentation of `java.util.logging` and Apache
7768 log4j{nbsp}1.2 logging statements.
7770 <<lttng-ust-agents,LTTng-UST Python agent>>::
7771 A Python package provided by the <<def-lttng-ust,LTTng-UST>> project
7772 to allow the <<def-lttng,LTTng>> instrumentation of Python logging
7775 <<channel-overwrite-mode-vs-discard-mode,overwrite mode>>::
7776 The <<def-event-record-loss-mode,event record loss mode>> in which new
7777 <<def-event-record,event records>> _overwrite_ older event records
7778 when there's no <<def-sub-buffer,sub-buffer>> space left to store
7781 <<channel-buffering-schemes,per-process buffering>>::
7782 A <<def-buffering-scheme,buffering scheme>> in which each instrumented
7783 process has its own <<def-sub-buffer,sub-buffers>> for a given user
7784 space <<def-channel,channel>>.
7786 <<channel-buffering-schemes,per-user buffering>>::
7787 A <<def-buffering-scheme,buffering scheme>> in which all the processes
7788 of a Unix user share the same <<def-sub-buffer,sub-buffers>> for a
7789 given user space <<def-channel,channel>>.
7791 [[def-relay-daemon]]<<lttng-relayd,relay daemon>>::
7792 A process which is responsible for receiving the <<def-trace,trace>>
7793 data which a distant <<def-consumer-daemon,consumer daemon>> sends.
7795 [[def-ring-buffer]]ring buffer::
7796 A set of <<def-sub-buffer,sub-buffers>>.
7799 See _<<def-tracing-session-rotation,tracing session rotation>>_.
7801 [[def-session-daemon]]<<lttng-sessiond,session daemon>>::
7802 A process which receives control commands from you and orchestrates
7803 the <<def-tracer,tracers>> and various <<def-lttng,LTTng>> daemons.
7805 <<taking-a-snapshot,snapshot>>::
7806 A copy of the current data of all the <<def-sub-buffer,sub-buffers>>
7807 of a given <<def-tracing-session,tracing session>>, saved as
7808 <<def-trace,trace>> files.
7810 [[def-sub-buffer]]sub-buffer::
7811 One part of an <<def-lttng,LTTng>> <<def-ring-buffer,ring buffer>>
7812 which contains <<def-event-record,event records>>.
7815 The time information attached to an <<def-event,event>> when it is
7818 [[def-trace]]trace (_noun_)::
7821 * One http://diamon.org/ctf/[CTF] metadata stream file.
7822 * One or more CTF data stream files which are the concatenations of one
7823 or more flushed <<def-sub-buffer,sub-buffers>>.
7825 [[def-trace-verb]]trace (_verb_)::
7826 The action of recording the <<def-event,events>> emitted by an
7827 application or by a system, or to initiate such recording by
7828 controlling a <<def-tracer,tracer>>.
7830 [[def-trace-chunk]]trace chunk::
7831 A self-contained <<def-trace,trace>> which is part of a
7832 <<def-tracing-session,tracing session>>. Each
7833 <<def-tracing-session-rotation, tracing session rotation>> produces a
7834 <<def-trace-chunk-archive,trace chunk archive>>.
7836 [[def-trace-chunk-archive]]trace chunk archive::
7837 The result of a <<def-tracing-session-rotation, tracing session rotation>>.
7839 <<def-lttng,LTTng>> does not manage any trace chunk archive, even if its
7840 containing <<def-tracing-session,tracing session>> is still active: you
7841 are free to read it, modify it, move it, or remove it.
7844 The http://tracecompass.org[Trace Compass] project and application.
7846 [[def-tracepoint]]tracepoint::
7847 An instrumentation point using the tracepoint mechanism of the Linux
7848 kernel or of <<def-lttng-ust,LTTng-UST>>.
7850 tracepoint definition::
7851 The definition of a single <<def-tracepoint,tracepoint>>.
7854 The name of a <<def-tracepoint,tracepoint>>.
7856 [[def-tracepoint-provider]]tracepoint provider::
7857 A set of functions providing <<def-tracepoint,tracepoints>> to an
7858 instrumented <<def-user-application,user application>>.
7860 Not to be confused with a <<def-tracepoint-provider-package,tracepoint
7861 provider package>>: many tracepoint providers can exist within a
7862 tracepoint provider package.
7864 [[def-tracepoint-provider-package]]tracepoint provider package::
7865 One or more <<def-tracepoint-provider,tracepoint providers>> compiled
7866 as an https://en.wikipedia.org/wiki/Object_file[object file] or as a
7867 link:https://en.wikipedia.org/wiki/Library_(computing)#Shared_libraries[shared
7870 [[def-tracer]]tracer::
7871 A software which records emitted <<def-event,events>>.
7873 <<domain,tracing domain>>::
7874 A namespace for <<def-event,event>> sources.
7876 <<tracing-group,tracing group>>::
7877 The Unix group in which a Unix user can be to be allowed to
7878 <<def-trace-verb,trace>> the Linux kernel.
7880 [[def-tracing-session]]<<tracing-session,tracing session>>::
7881 A stateful dialogue between you and a <<lttng-sessiond,session daemon>>.
7883 [[def-tracing-session-rotation]]<<session-rotation,tracing session rotation>>::
7884 The action of archiving the
7885 <<def-current-trace-chunk,current trace chunk>> of a
7886 <<def-tracing-session,tracing session>>.
7888 [[def-user-application]]user application::
7889 An application running in user space, as opposed to a Linux kernel
7890 module, for example.