From: Philippe Proulx Date: Fri, 2 Apr 2021 03:08:51 +0000 (-0400) Subject: Document LTTng 2.13 X-Git-Url: https://git.liburcu.org/?a=commitdiff_plain;h=50e95807211991759f634010c49daeedd1559918;p=lttng-docs.git Document LTTng 2.13 Signed-off-by: Philippe Proulx --- diff --git a/2.13/images/export/concepts.png b/2.13/images/export/concepts.png new file mode 100644 index 0000000..b0977be Binary files /dev/null and b/2.13/images/export/concepts.png differ diff --git a/2.13/images/export/event-rule.png b/2.13/images/export/event-rule.png new file mode 100644 index 0000000..9c05ecf Binary files /dev/null and b/2.13/images/export/event-rule.png differ diff --git a/2.13/images/export/java-app.png b/2.13/images/export/java-app.png new file mode 100644 index 0000000..a4f13ba Binary files /dev/null and b/2.13/images/export/java-app.png differ diff --git a/2.13/images/export/live.png b/2.13/images/export/live.png new file mode 100644 index 0000000..1ab2420 Binary files /dev/null and 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+include::../common/audience.txt[] + + +[[chapters]] +=== What's in this documentation? + +The LTTng Documentation is divided into the following sections: + +* ``**<>**'' explains the + rudiments of software tracing and the rationale behind the + LTTng project. ++ +Skip this section if you’re familiar with software tracing and with the +LTTng project. + +* ``**<>**'' describes the steps to + install the LTTng packages on common Linux distributions and from + their sources. ++ +Skip this section if you already properly installed LTTng on your target +system. + +* ``**<>**'' is a concise guide to + get started quickly with LTTng kernel and user space tracing. ++ +We recommend this section if you're new to LTTng or to software tracing +in general. ++ +Skip this section if you're not new to LTTng. + +* ``**<>**'' explains the concepts at + the heart of LTTng. ++ +It's a good idea to become familiar with the core concepts +before attempting to use the toolkit. + +* ``**<>**'' describes the various + components of the LTTng machinery, like the daemons, the libraries, + and the command-line interface. + +* ``**<>**'' shows different ways to + instrument user applications and the Linux kernel for LTTng tracing. ++ +Instrumenting source code is essential to provide a meaningful +source of events. ++ +Skip this section if you don't have a programming background. + +* ``**<>**'' is divided into topics + which demonstrate how to use the vast array of features that + LTTng{nbsp}{revision} offers. + +* ``**<>**'' contains API reference tables. + +* ``**<>**'' is a specialized dictionary of terms + related to LTTng or to the field of software tracing. + + +include::../common/convention.txt[] + + +include::../common/acknowledgements.txt[] + + +[[whats-new]] +== What's new in LTTng{nbsp}{revision}? + +LTTng{nbsp}{revision} bears the name _Nordicité_, the product of a +collaboration between https://champlibre.co/[Champ Libre] and +https://champlibre.co/[Boréale]. This farmhouse IPA is brewed with +https://en.wikipedia.org/wiki/Kveik[Kveik] yeast and Québec-grown +barley, oats, and juniper branches. The result is a remarkable, fruity, +hazy golden IPA that offers a balanced touch of resinous and woodsy +bitterness. + +New features and changes in LTTng{nbsp}{revision}: + +General:: ++ +* The LTTng trigger API of <> now + offers the ``__event rule matches__'' condition (an <> matches an event) as well as the following new actions: ++ +-- +* <> a recording session. +* <> of a + recording session (rotate). +* <> of a recording session. +-- ++ +As a reminder, a <> is a condition-actions pair. When +the condition of a trigger is satisfied, LTTng attempts to execute its +actions. ++ +This feature is also available with the new man:lttng-add-trigger(1), +man:lttng-remove-trigger(1), and man:lttng-list-triggers(1) +<> commands. ++ +Starting from LTTng{nbsp}{revision}, a trigger may have more than one +action. ++ +See “<>” to learn more. + +* The LTTng <> and <> + tracers offer the new namespace context field `time_ns`, which is the + inode number, in the proc file system, of the current clock namespace. ++ +See man:lttng-add-context(1), man:lttng-ust(3), and +man:time_namespaces(7). + +* The link:/man[manual pages] of LTTng-tools now have a terminology and + style which match the LTTng Documentation, many fixes, more internal + and manual page links, clearer lists and procedures, superior + consistency, and usage examples. ++ +The new man:lttng-event-rule(7) manual page explains the new, common +way to specify an event rule on the command line. ++ +The new man:lttng-concepts(7) manual page explains the core concepts of +LTTng. Its contents is essentially the ``<>'' section of this documentation, but more adapted to the +manual page style. + +User space tracing:: ++ +[IMPORTANT] +==== +The major version part of the `liblttng-ust` +https://en.wikipedia.org/wiki/Soname[soname] is bumped, which means you +**must recompile** your instrumented applications/libraries and +<> to use +LTTng-UST{nbsp}{revision}. + +This change became a necessity to clean up the library and for +`liblttng-ust` to stop exporting private symbols. + +Also, LTTng{nbsp}{revision} prepends the `lttng_ust_` and `LTTNG_UST_` +prefix to all public macro/definition/function names to offer a +consistent API namespace. The LTTng{nbsp}2.12 API is still available; +see the ``Compatibility with previous APIs'' section of +man:lttng-ust(3). +==== ++ +Other notable changes: ++ +* The `liblttng-ust` C{nbsp}API offers the new man:lttng_ust_vtracef(3) + and man:lttng_ust_vtracelog(3) macros which are to + man:lttng_ust_tracef(3) and man:lttng_ust_tracelog(3) what + man:vprintf(3) is to man:printf(3). + +* LTTng-UST now only depends on https://liburcu.org/[`liburcu`] at build + time, not at run time. + +Kernel tracing:: ++ +* The preferred display base of event record integer fields which + contain memory addresses is now hexadecimal instead of decimal. + +* The `pid` field is removed from `lttng_statedump_file_descriptor` + event records and the `file_table_address` field is added. ++ +This new field is the address of the `files_struct` structure which +contains the file descriptor. ++ +See the +``https://github.com/lttng/lttng-modules/commit/e7a0ca7205fd4be7c829d171baa8823fe4784c90[statedump: introduce `file_table_address`]'' +patch to learn more. + +* The `flags` field of `syscall_entry_clone` event records is now a + structure containing two enumerations (exit signal and options). ++ +This change makes the flag values more readable and meaningful. ++ +See the +``https://github.com/lttng/lttng-modules/commit/d775625e2ba4825b73b5897e7701ad6e2bdba115[syscalls: Make `clone()`'s `flags` field a 2 enum struct]'' +patch to learn more. + +* The memory footprint of the kernel tracer is improved: the latter only + generates metadata for the specific system call recording event rules + that you <>. + + +[[nuts-and-bolts]] +== Nuts and bolts + +What is LTTng? As its name suggests, the _Linux Trace Toolkit: next +generation_ is a modern toolkit for tracing Linux systems and +applications. So your first question might be: +**what is tracing?** + + +[[what-is-tracing]] +=== What is tracing? + +As the history of software engineering progressed and led to what +we now take for granted--complex, numerous and +interdependent software applications running in parallel on +sophisticated operating systems like Linux--the authors of such +components, software developers, began feeling a natural +urge to have tools that would ensure the robustness and good performance +of their masterpieces. + +One major achievement in this field is, inarguably, the +https://www.gnu.org/software/gdb/[GNU debugger (GDB)], +an essential tool for developers to find and fix bugs. But even the best +debugger won't help make your software run faster, and nowadays, faster +software means either more work done by the same hardware, or cheaper +hardware for the same work. + +A _profiler_ is often the tool of choice to identify performance +bottlenecks. Profiling is suitable to identify _where_ performance is +lost in a given piece of software. The profiler outputs a profile, a +statistical summary of observed events, which you may use to discover +which functions took the most time to execute. However, a profiler won't +report _why_ some identified functions are the bottleneck. Bottlenecks +might only occur when specific conditions are met, conditions that are +sometimes impossible to capture by a statistical profiler, or impossible +to reproduce with an application altered by the overhead of an +event-based profiler. For a thorough investigation of software +performance issues, a history of execution is essential, with the +recorded values of variables and context fields you choose, and with as +little influence as possible on the instrumented application. This is +where tracing comes in handy. + +_Tracing_ is a technique used to understand what goes on in a running +software system. The piece of software used for tracing is called a +_tracer_, which is conceptually similar to a tape recorder. When +recording, specific instrumentation points placed in the software source +code generate events that are saved on a giant tape: a _trace_ file. You +can record user application and operating system events at the same +time, opening the possibility of resolving a wide range of problems that +would otherwise be extremely challenging. + +Tracing is often compared to _logging_. However, tracers and loggers are +two different tools, serving two different purposes. Tracers are +designed to record much lower-level events that occur much more +frequently than log messages, often in the range of thousands per +second, with very little execution overhead. Logging is more appropriate +for a very high-level analysis of less frequent events: user accesses, +exceptional conditions (errors and warnings, for example), database +transactions, instant messaging communications, and such. Simply put, +logging is one of the many use cases that can be satisfied with tracing. + +The list of recorded events inside a trace file can be read manually +like a log file for the maximum level of detail, but it's generally +much more interesting to perform application-specific analyses to +produce reduced statistics and graphs that are useful to resolve a +given problem. Trace viewers and analyzers are specialized tools +designed to do this. + +In the end, this is what LTTng is: a powerful, open source set of +tools to trace the Linux kernel and user applications at the same time. +LTTng is composed of several components actively maintained and +developed by its link:/community/#where[community]. + + +[[lttng-alternatives]] +=== Alternatives to noch:{LTTng} + +Excluding proprietary solutions, a few competing software tracers +exist for Linux: + +https://github.com/dtrace4linux/linux[dtrace4linux]:: + A port of Sun Microsystems' DTrace to Linux. ++ +The cmd:dtrace tool interprets user scripts and is responsible for +loading code into the Linux kernel for further execution and collecting +the outputted data. + +https://en.wikipedia.org/wiki/Berkeley_Packet_Filter[eBPF]:: + A subsystem in the Linux kernel in which a virtual machine can + execute programs passed from the user space to the kernel. ++ +You can attach such programs to tracepoints and kprobes thanks to a +system call, and they can output data to the user space when executed +thanks to different mechanisms (pipe, VM register values, and eBPF maps, +to name a few). + +https://www.kernel.org/doc/Documentation/trace/ftrace.txt[ftrace]:: + The de facto function tracer of the Linux kernel. ++ +Its user interface is a set of special files in sysfs. + +https://perf.wiki.kernel.org/[perf]:: + A performance analysis tool for Linux which supports hardware + performance counters, tracepoints, as well as other counters and + types of probes. ++ +The controlling utility of perf is the cmd:perf command line/text UI +tool. + +https://linux.die.net/man/1/strace[strace]:: + A command-line utility which records system calls made by a + user process, as well as signal deliveries and changes of process + state. ++ +strace makes use of https://en.wikipedia.org/wiki/Ptrace[ptrace] to +fulfill its function. + +https://www.sysdig.org/[sysdig]:: + Like SystemTap, uses scripts to analyze Linux kernel events. ++ +You write scripts, or _chisels_ in the jargon of sysdig, in Lua and +sysdig executes them while it traces the system or afterwards. The +interface of sysdig is the cmd:sysdig command-line tool as well as the +text UI-based cmd:csysdig tool. + +https://sourceware.org/systemtap/[SystemTap]:: + A Linux kernel and user space tracer which uses custom user scripts + to produce plain text traces. ++ +SystemTap converts the scripts to the C language, and then compiles them +as Linux kernel modules which are loaded to produce trace data. The +primary user interface of SystemTap is the cmd:stap command-line tool. + +The main distinctive features of LTTng is that it produces correlated +kernel and user space traces, as well as doing so with the lowest +overhead amongst other solutions. It produces trace files in the +https://diamon.org/ctf[CTF] format, a file format optimized +for the production and analyses of multi-gigabyte data. + +LTTng is the result of more than 10{nbsp}years of active open source +development by a community of passionate developers. LTTng is currently +available on major desktop and server Linux distributions. + +The main interface for tracing control is a single command-line tool +named cmd:lttng. The latter can create several recording sessions, enable +and disable recording event rules on the fly, filter events efficiently +with custom user expressions, start and stop tracing, and much more. +LTTng can write the traces on the file system or send them over the +network, and keep them totally or partially. You can make LTTng execute +user-defined actions when LTTng emits an event. You can view the traces +once tracing becomes inactive or as LTTng records events. + +<> and +<>! + + +[[installing-lttng]] +== Installation + +**LTTng** is a set of software <> which interact to +<> the Linux kernel and user applications, and +to <> (start and stop +recording, create recording event rules, and the rest). Those +components are bundled into the following packages: + +LTTng-tools:: + Libraries and command-line interface to control tracing. + +LTTng-modules:: + Linux kernel modules to instrument and trace the kernel. + +LTTng-UST:: + Libraries and Java/Python packages to instrument and trace user + applications. + +Most distributions mark the LTTng-modules and LTTng-UST packages as +optional when installing LTTng-tools (which is always required). In the +following sections, we always provide the steps to install all three, +but note that: + +* You only need to install LTTng-modules if you intend to use + the Linux kernel LTTng tracer. + +* You only need to install LTTng-UST if you intend to use the user + space LTTng tracer. + +[NOTE] +==== +As of 10{nbsp}June{nbsp}2021, LTTng{nbsp}{revision} is not yet available +in any major non-enterprise Linux distribution. + +For https://www.redhat.com/[RHEL] and https://www.suse.com/[SLES] +packages, see https://packages.efficios.com/[EfficiOS Enterprise +Packages]. + +For other distributions, <>. +==== + + +[[building-from-source]] +=== Build from source + +To build and install LTTng{nbsp}{revision} from source: + +. Using the package manager of your distribution, or from source, + install the following dependencies of LTTng-tools and LTTng-UST: ++ +-- +* https://sourceforge.net/projects/libuuid/[libuuid] +* https://directory.fsf.org/wiki/Popt[popt] +* https://liburcu.org/[Userspace RCU] +* http://www.xmlsoft.org/[libxml2] +* **Optional**: https://github.com/numactl/numactl[numactl] +-- + +. Download, build, and install the latest LTTng-modules{nbsp}{revision}: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && + wget https://lttng.org/files/lttng-modules/lttng-modules-latest-2.13.tar.bz2 && + tar -xf lttng-modules-latest-2.13.tar.bz2 && + cd lttng-modules-2.13.* && + make && + sudo make modules_install && + sudo depmod -a +---- +-- + +. Download, build, and install the latest LTTng-UST{nbsp}{revision}: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && + wget https://lttng.org/files/lttng-ust/lttng-ust-latest-2.13.tar.bz2 && + tar -xf lttng-ust-latest-2.13.tar.bz2 && + cd lttng-ust-2.13.* && + ./configure && + make && + sudo make install && + sudo ldconfig +---- +-- ++ +Add `--disable-numa` to `./configure` if you don't have +https://github.com/numactl/numactl[numactl]. ++ +-- +[IMPORTANT] +.Java and Python application tracing +==== +If you need to instrument and have LTTng trace <>, pass the `--enable-java-agent-jul`, +`--enable-java-agent-log4j`, or `--enable-java-agent-all` options to the +`configure` script, depending on which Java logging framework you use. + +If you need to instrument and have LTTng trace +<>, pass the +`--enable-python-agent` option to the `configure` script. You can set +the env:PYTHON environment variable to the path to the Python interpreter +for which to install the LTTng-UST Python agent package. +==== +-- ++ +-- +[NOTE] +==== +By default, LTTng-UST libraries are installed to +dir:{/usr/local/lib}, which is the de facto directory in which to +keep self-compiled and third-party libraries. + +When <>: + +* Append `/usr/local/lib` to the env:LD_LIBRARY_PATH environment + variable. + +* Pass the `-L/usr/local/lib` and `-Wl,-rpath,/usr/local/lib` options to + man:gcc(1), man:g++(1), or man:clang(1). +==== +-- + +. Download, build, and install the latest LTTng-tools{nbsp}{revision}: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && + wget https://lttng.org/files/lttng-tools/lttng-tools-latest-2.13.tar.bz2 && + tar -xf lttng-tools-latest-2.13.tar.bz2 && + cd lttng-tools-2.13.* && + ./configure && + make && + sudo make install && + sudo ldconfig +---- +-- + +TIP: The https://github.com/eepp/vlttng[vlttng tool] can do all the +previous steps automatically for a given version of LTTng and confine +the installed files to a specific directory. This can be useful to try +LTTng without installing it on your system. + + +[[getting-started]] +== Quick start + +This is a short guide to get started quickly with LTTng kernel and user +space tracing. + +Before you follow this guide, make sure to <> +LTTng. + +This tutorial walks you through the steps to: + +. <>. + +. <> written in C. + +. <>. + + +[[tracing-the-linux-kernel]] +=== Record Linux kernel events + +NOTE: The following command lines start with the `#` prompt because you +need root privileges to control the Linux kernel LTTng tracer. You can +also control the kernel tracer as a regular user if your Unix user is a +member of the <>. + +. Create a <> to write LTTng traces + to dir:{/tmp/my-kernel-trace}: ++ +-- +[role="term"] +---- +# lttng create my-kernel-session --output=/tmp/my-kernel-trace +---- +-- + +. List the available kernel tracepoints and system calls: ++ +-- +[role="term"] +---- +# lttng list --kernel +# lttng list --kernel --syscall +---- +-- + +. Create <> which match events having + the desired names, for example the `sched_switch` and + `sched_process_fork` tracepoints, and the man:open(2) and man:close(2) + system calls: ++ +-- +[role="term"] +---- +# lttng enable-event --kernel sched_switch,sched_process_fork +# lttng enable-event --kernel --syscall open,close +---- +-- ++ +Create a recording event rule which matches _all_ the Linux kernel +tracepoint events with the opt:lttng-enable-event(1):--all option +(recording with such a recording event rule generates a lot of data): ++ +-- +[role="term"] +---- +# lttng enable-event --kernel --all +---- +-- + +. <>: ++ +-- +[role="term"] +---- +# lttng start +---- +-- + +. Do some operation on your system for a few seconds. For example, + load a website, or list the files of a directory. + +. <> the current + recording session: ++ +-- +[role="term"] +---- +# lttng destroy +---- +-- ++ +The man:lttng-destroy(1) command doesn't destroy the trace data; it +only destroys the state of the recording session. ++ +The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command +implicitly (see ``<>''). You need to stop recording to make LTTng flush +the remaining trace data and make the trace readable. + +. For the sake of this example, make the recorded trace accessible to + the non-root users: ++ +-- +[role="term"] +---- +# chown -R $(whoami) /tmp/my-kernel-trace +---- +-- + +See ``<>'' to view the recorded events. + + +[[tracing-your-own-user-application]] +=== Record user application events + +This section walks you through a simple example to record the events of +a _Hello world_ program written in{nbsp}C. + +To create the traceable user application: + +. Create the tracepoint provider header file, which defines the + tracepoints and the events they can generate: ++ +-- +[source,c] +.path:{hello-tp.h} +---- +#undef LTTNG_UST_TRACEPOINT_PROVIDER +#define LTTNG_UST_TRACEPOINT_PROVIDER hello_world + +#undef LTTNG_UST_TRACEPOINT_INCLUDE +#define LTTNG_UST_TRACEPOINT_INCLUDE "./hello-tp.h" + +#if !defined(_HELLO_TP_H) || defined(LTTNG_UST_TRACEPOINT_HEADER_MULTI_READ) +#define _HELLO_TP_H + +#include + +LTTNG_UST_TRACEPOINT_EVENT( + hello_world, + my_first_tracepoint, + LTTNG_UST_TP_ARGS( + int, my_integer_arg, + char *, my_string_arg + ), + LTTNG_UST_TP_FIELDS( + lttng_ust_field_string(my_string_field, my_string_arg) + lttng_ust_field_integer(int, my_integer_field, my_integer_arg) + ) +) + +#endif /* _HELLO_TP_H */ + +#include +---- +-- + +. Create the tracepoint provider package source file: ++ +-- +[source,c] +.path:{hello-tp.c} +---- +#define LTTNG_UST_TRACEPOINT_CREATE_PROBES +#define LTTNG_UST_TRACEPOINT_DEFINE + +#include "hello-tp.h" +---- +-- + +. Build the tracepoint provider package: ++ +-- +[role="term"] +---- +$ gcc -c -I. hello-tp.c +---- +-- + +. Create the _Hello World_ application source file: ++ +-- +[source,c] +.path:{hello.c} +---- +#include +#include "hello-tp.h" + +int main(int argc, char *argv[]) +{ + unsigned int i; + + puts("Hello, World!\nPress Enter to continue..."); + + /* + * The following getchar() call only exists for the purpose of this + * demonstration, to pause the application in order for you to have + * time to list its tracepoints. You don't need it otherwise. + */ + getchar(); + + /* + * An lttng_ust_tracepoint() call. + * + * Arguments, as defined in `hello-tp.h`: + * + * 1. Tracepoint provider name (required) + * 2. Tracepoint name (required) + * 3. `my_integer_arg` (first user-defined argument) + * 4. `my_string_arg` (second user-defined argument) + * + * Notice the tracepoint provider and tracepoint names are + * C identifiers, NOT strings: they're in fact parts of variables + * that the macros in `hello-tp.h` create. + */ + lttng_ust_tracepoint(hello_world, my_first_tracepoint, 23, + "hi there!"); + + for (i = 0; i < argc; i++) { + lttng_ust_tracepoint(hello_world, my_first_tracepoint, + i, argv[i]); + } + + puts("Quitting now!"); + lttng_ust_tracepoint(hello_world, my_first_tracepoint, + i * i, "i^2"); + return 0; +} +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -c hello.c +---- +-- + +. Link the application with the tracepoint provider package, + `liblttng-ust` and `libdl`: ++ +-- +[role="term"] +---- +$ gcc -o hello hello.o hello-tp.o -llttng-ust -ldl +---- +-- + +Here's the whole build process: + +[role="img-100"] +.Build steps of the user space tracing tutorial. +image::ust-flow.png[] + +To record the events of the user application: + +. Run the application with a few arguments: ++ +-- +[role="term"] +---- +$ ./hello world and beyond +---- +-- ++ +You see: ++ +-- +---- +Hello, World! +Press Enter to continue... +---- +-- + +. Start an LTTng <>: ++ +-- +[role="term"] +---- +$ lttng-sessiond --daemonize +---- +-- ++ +NOTE: A session daemon might already be running, for example as a +service that the service manager of your distribution started. + +. List the available user space tracepoints: ++ +-- +[role="term"] +---- +$ lttng list --userspace +---- +-- ++ +You see the `hello_world:my_first_tracepoint` tracepoint listed +under the `./hello` process. + +. Create a <>: ++ +-- +[role="term"] +---- +$ lttng create my-user-space-session +---- +-- + +. Create a <> which matches user space + tracepoint events named `hello_world:my_first_tracepoint`: ++ +-- +[role="term"] +---- +$ lttng enable-event --userspace hello_world:my_first_tracepoint +---- +-- + +. <>: ++ +-- +[role="term"] +---- +$ lttng start +---- +-- + +. Go back to the running `hello` application and press **Enter**. ++ +The program executes all `lttng_ust_tracepoint()` instrumentation +points, emitting events as the event rule you created in step{nbsp}5 +matches them, and +exits. + +. <> the current + recording session: ++ +-- +[role="term"] +---- +$ lttng destroy +---- +-- ++ +The man:lttng-destroy(1) command doesn't destroy the trace data; it +only destroys the state of the recording session. ++ +The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command +implicitly (see ``<>''). You need to stop recording to make LTTng flush +the remaining trace data and make the trace readable. + +By default, LTTng saves the traces to the ++$LTTNG_HOME/lttng-traces/__NAME__-__DATE__-__TIME__+ directory, where ++__NAME__+ is the recording session name. The env:LTTNG_HOME environment +variable defaults to `$HOME` if not set. + + +[[viewing-and-analyzing-your-traces]] +=== View and analyze the recorded events + +Once you have completed the <> and <> tutorials, you can inspect the recorded events. + +There are many tools you can use to read LTTng traces: + +https://babeltrace.org/[Babeltrace{nbsp}2]:: + A rich, flexible trace manipulation toolkit which includes + a versatile command-line interface + (man:babeltrace2(1)), + a https://babeltrace.org/docs/v2.0/libbabeltrace2/[C{nbsp}library], + and https://babeltrace.org/docs/v2.0/python/bt2/[Python{nbsp}3 bindings] + so that you can easily process or convert an LTTng trace with + your own script. ++ +The Babeltrace{nbsp}2 project ships with a plugin +(man:babeltrace2-plugin-ctf(7)) which supports the format of the traces +which LTTng produces, https://diamon.org/ctf/[CTF]. + +http://tracecompass.org/[Trace Compass]:: + A graphical user interface for viewing and analyzing any type of + logs or traces, including those of LTTng. + +https://github.com/lttng/lttng-analyses[LTTng analyses]:: + An experimental project which includes many high-level analyses of + LTTng kernel traces, like scheduling statistics, interrupt + frequency distribution, top CPU usage, and more. + +NOTE: This section assumes that LTTng wrote the traces it recorded +during the previous tutorials to their default location, in the +dir:{$LTTNG_HOME/lttng-traces} directory. The env:LTTNG_HOME +environment variable defaults to `$HOME` if not set. + + +[[viewing-and-analyzing-your-traces-bt]] +==== Use the cmd:babeltrace2 command-line tool + +The simplest way to list all the recorded events of an LTTng trace is to +pass its path to man:babeltrace2(1), without options: + +[role="term"] +---- +$ babeltrace2 ~/lttng-traces/my-user-space-session* +---- + +The cmd:babeltrace2 command finds all traces recursively within the +given path and prints all their events, sorting them chronologically. + +Pipe the output of cmd:babeltrace2 into a tool like man:grep(1) for +further filtering: + +[role="term"] +---- +$ babeltrace2 /tmp/my-kernel-trace | grep _switch +---- + +Pipe the output of cmd:babeltrace2 into a tool like man:wc(1) to count +the recorded events: + +[role="term"] +---- +$ babeltrace2 /tmp/my-kernel-trace | grep _open | wc --lines +---- + + +[[viewing-and-analyzing-your-traces-bt-python]] +==== Use the Babeltrace{nbsp}2 Python bindings + +The <> is useful to isolate event records by simple matching +using man:grep(1) and similar utilities. However, more elaborate +filters, such as keeping only event records with a field value falling +within a specific range, are not trivial to write using a shell. +Moreover, reductions and even the most basic computations involving +multiple event records are virtually impossible to implement. + +Fortunately, Babeltrace{nbsp}2 ships with +https://babeltrace.org/docs/v2.0/python/bt2/[Python{nbsp}3 bindings] +which make it easy to read the event records of an LTTng trace +sequentially and compute the desired information. + +The following script accepts an LTTng Linux kernel trace path as its +first argument and prints the short names of the top five running +processes on CPU{nbsp}0 during the whole trace: + +[source,python] +.path:{top5proc.py} +---- +import bt2 +import sys +import collections + + +def top5proc(): + # Get the trace path from the first command-line argument + it = bt2.TraceCollectionMessageIterator(sys.argv[1]) + + # This counter dictionary will hold execution times: + # + # Task command name -> Total execution time (ns) + exec_times = collections.Counter() + + # This holds the last `sched_switch` timestamp + last_ts = None + + for msg in it: + # We only care about event messages + if type(msg) is not bt2._EventMessageConst: + continue + + # Event of the event message + event = msg.event + + # Keep only `sched_switch` events + if event.cls.name != 'sched_switch': + continue + + # Keep only records of events which LTTng emitted from CPU 0 + if event.packet.context_field['cpu_id'] != 0: + continue + + # Event timestamp (ns) + cur_ts = msg.default_clock_snapshot.ns_from_origin + + if last_ts is None: + # Start here + last_ts = cur_ts + + # (Short) name of the previous task command + prev_comm = str(event.payload_field['prev_comm']) + + # Initialize an entry in our dictionary if not done yet + if prev_comm not in exec_times: + exec_times[prev_comm] = 0 + + # Compute previous command execution time + diff = cur_ts - last_ts + + # Update execution time of this command + exec_times[prev_comm] += diff + + # Update last timestamp + last_ts = cur_ts + + # Print top 5 + for name, ns in exec_times.most_common(5): + print('{:20}{} s'.format(name, ns / 1e9)) + + +if __name__ == '__main__': + top5proc() +---- + +Run this script: + +[role="term"] +---- +$ python3 top5proc.py /tmp/my-kernel-trace/kernel +---- + +Output example: + +---- +swapper/0 48.607245889 s +chromium 7.192738188 s +pavucontrol 0.709894415 s +Compositor 0.660867933 s +Xorg.bin 0.616753786 s +---- + +Note that `swapper/0` is the ``idle'' process of CPU{nbsp}0 on Linux; +since we weren't using the CPU that much when recording, its first +position in the list makes sense. + + +[[core-concepts]] +== [[understanding-lttng]]Core concepts + +From a user's perspective, the LTTng system is built on a few concepts, +or objects, on which the <> +operates by sending commands to the <> +(through <>). + +Understanding how those objects relate to each other is key to master +the toolkit. + +The core concepts of LTTng are: + +* <<"event-rule","Instrumentation point, event rule, and event">> +* <> +* <> +* <> +* <> +* <> + +NOTE: The man:lttng-concepts(7) manual page also documents the core +concepts of LTTng, with more links to other LTTng-tools manual pages. + + +[[event-rule]] +=== Instrumentation point, event rule, and event + +An _instrumentation point_ is a point, within a piece of software, +which, when executed, creates an LTTng _event_. + +LTTng offers various <>. + +An _event rule_ is a set of conditions to match a set of events. + +When LTTng creates an event{nbsp}__E__, an event rule{nbsp}__ER__ is +said to __match__{nbsp}__E__ when{nbsp}__E__ satisfies _all_ the +conditions of{nbsp}__ER__. This concept is similar to a +https://en.wikipedia.org/wiki/Regular_expression[regular expression] +which matches a set of strings. + +When an event rule matches an event, LTTng _emits_ the event, therefore +attempting to execute one or more actions. + +[IMPORTANT] +==== +[[event-creation-emission-opti]]The event creation and emission +processes are documentation concepts to help understand the journey from +an instrumentation point to the execution of actions. + +The actual creation of an event can be costly because LTTng needs to +evaluate the arguments of the instrumentation point. + +In practice, LTTng implements various optimizations for the Linux kernel +and user space <> to avoid actually creating an +event when the tracer knows, thanks to properties which are independent +from the event payload and current context, that it would never emit +such an event. Those properties are: + +* The <>. + +* The instrumentation point name. + +* The instrumentation point log level. + +* For a <>: +** The status of the rule itself. +** The status of the <>. +** The activity of the <>. +** Whether or not the process for which LTTng would create the event is + <>. + +In other words: if, for a given instrumentation point{nbsp}__IP__, the +LTTng tracer knows that it would never emit an event, +executing{nbsp}__IP__ represents a simple boolean variable check and, +for a Linux kernel recording event rule, a few process attribute checks. +==== + +As of LTTng{nbsp}{revision}, there are two places where you can find an +event rule: + +<>:: + A specific type of event rule of which the action is to record the + matched event as an event record. ++ +See ``<>'' to learn more. + +``Event rule matches'' <> condition (since LTTng{nbsp}2.13):: + When the event rule of the trigger condition matches an event, LTTng + can execute user-defined actions such as sending an LTTng + notification, + <>, + and more. ++ +See “<>” to learn more. + +For LTTng to emit an event{nbsp}__E__,{nbsp}__E__ must satisfy _all_ the +basic conditions of an event rule{nbsp}__ER__, that is: + +* The instrumentation point from which LTTng + creates{nbsp}__E__ has a specific + <>. + +* A pattern matches the name of{nbsp}__E__ while another pattern + doesn't. + +* The log level of the instrumentation point from which LTTng + creates{nbsp}__E__ is at least as severe as some value, or is exactly + some value. + +* The fields of the payload of{nbsp}__E__ and the current context fields + satisfy a filter expression. + +A <> has additional, implicit conditions to +satisfy. + + +[[instrumentation-point-types]] +==== Instrumentation point types + +As of LTTng{nbsp}{revision}, the available instrumentation point +types are, depending on the <>: + +Linux kernel:: + LTTng tracepoint::: + A statically defined point in the source code of the kernel + image or of a kernel module using the + <> macros. + + Linux kernel system call::: + Entry, exit, or both of a Linux kernel system call. + + Linux https://www.kernel.org/doc/html/latest/trace/kprobes.html[kprobe]::: + A single probe dynamically placed in the compiled kernel code. ++ +When you create such an instrumentation point, you set its memory +address or symbol name. + + Linux user space probe::: + A single probe dynamically placed at the entry of a compiled + user space application/library function through the kernel. ++ +When you create such an instrumentation point, you set: ++ +-- +With the ELF method:: + Its application/library path and its symbol name. + +With the USDT method:: + Its application/library path, its provider name, and its probe name. ++ +``USDT'' stands for _SystemTap User-level Statically Defined Tracing_, +a http://dtrace.org/blogs/about/[DTrace]-style marker. +-- ++ +As of LTTng{nbsp}{revision}, LTTng only supports USDT probes which +are _not_ reference-counted. + + Linux https://www.kernel.org/doc/html/latest/trace/kprobes.html[kretprobe]::: + Entry, exit, or both of a Linux kernel function. ++ +When you create such an instrumentation point, you set the memory +address or symbol name of its function. + +User space:: + LTTng tracepoint::: + A statically defined point in the source code of a C/$$C++$$ + application/library using the + <> macros. + +`java.util.logging`, Apache log4j, and Python:: + Java or Python logging statement::: + A method call on a Java or Python logger attached to an + LTTng-UST handler. + +See ``<>'' to learn how to list available Linux kernel, user space, and +logging instrumentation points. + + +[[trigger]] +=== Trigger + +A _trigger_ associates a condition to one or more actions. + +When the condition of a trigger is satisfied, LTTng attempts to execute +its actions. + +As of LTTng{nbsp}{revision}, the available trigger conditions and +actions are: + +Conditions:: ++ +* The consumed buffer size of a given <> becomes greater than some value. + +* The buffer usage of a given <> becomes greater than + some value. + +* The buffer usage of a given channel becomes less than some value. + +* There's an ongoing <>. + +* A recording session rotation becomes completed. + +* An <> an event. + +Actions:: ++ +* <> to a user application. +* <> a given recording session. +* <> a given recording session. +* <> of a given + recording session (rotate). +* <> of a given recording session. + +A trigger belongs to a <>, not to a +specific recording session. For a given session daemon, each Unix user has +its own, private triggers. Note, however, that the `root` Unix user may, +for the root session daemon: + +* Add a trigger as another Unix user. + +* List all the triggers, regardless of their owner. + +* Remove a trigger which belongs to another Unix user. + +For a given session daemon and Unix user, a trigger has a unique name. + + +[[tracing-session]] +=== Recording session + +A _recording session_ (named ``tracing session'' prior to +LTTng{nbsp}2.13) is a stateful dialogue between you and a +<> for everything related to +<>. + +Everything that you do when you control LTTng tracers to record events +happens within a recording session. In particular, a recording session: + +* Has its own name, unique for a given session daemon. + +* Has its own set of trace files, if any. + +* Has its own state of activity (started or stopped). ++ +An active recording session is an implicit <> +condition. + +* Has its own <> (local, network streaming, + snapshot, or live). + +* Has its own <> to which are attached their own + recording event rules. + +* Has its own <>. + +[role="img-100"] +.A _recording session_ contains <> that are members of <> and contain <>. +image::concepts.png[] + +Those attributes and objects are completely isolated between different +recording sessions. + +A recording session is like an +https://en.wikipedia.org/wiki/Automated_teller_machine[ATM] session: the +operations you do on the banking system through the ATM don't alter the +data of other users of the same system. In the case of the ATM, a +session lasts as long as your bank card is inside. In the case of LTTng, +a recording session lasts from the man:lttng-create(1) command to the +man:lttng-destroy(1) command. + +[role="img-100"] +.Each Unix user has its own set of recording sessions. +image::many-sessions.png[] + +A recording session belongs to a <>. For a +given session daemon, each Unix user has its own, private recording +sessions. Note, however, that the `root` Unix user may operate on or +destroy another user's recording session. + + +[[tracing-session-mode]] +==== Recording session mode + +LTTng offers four recording session modes: + +[[local-mode]]Local mode:: + Write the trace data to the local file system. + +[[net-streaming-mode]]Network streaming mode:: + Send the trace data over the network to a listening + <>. + +[[snapshot-mode]]Snapshot mode:: + Only write the trace data to the local file system or send it to a + listening relay daemon when LTTng <>. ++ +LTTng forces all the <> +to be created to be configured to be snapshot-ready. ++ +LTTng takes a snapshot of such a recording session when: ++ +-- +* You run the man:lttng-snapshot(1) command. + +* LTTng executes a `snapshot-session` <> action. +-- + +[[live-mode]]Live mode:: + Send the trace data over the network to a listening relay daemon + for <>. ++ +An LTTng live reader (for example, man:babeltrace2(1)) can connect to +the same relay daemon to receive trace data while the recording session is +active. + + +[[domain]] +=== Tracing domain + +A _tracing domain_ identifies a type of LTTng tracer. + +A tracing domain has its own properties and features. + +There are currently five available tracing domains: + +* Linux kernel +* User space +* `java.util.logging` (JUL) +* log4j +* Python + +You must specify a tracing domain to target a type of LTTng tracer when +using some <> commands to avoid ambiguity. For +example, because the Linux kernel and user space tracing domains support +named tracepoints as <>, you need to +specify a tracing domain when you <> because both tracing domains could have tracepoints +sharing the same name. + +You can create <> in the Linux kernel and user space +tracing domains. The other tracing domains have a single, default +channel. + + +[[channel]] +=== Channel and ring buffer + +A _channel_ is an object which is responsible for a set of +_ring buffers_. + +Each ring buffer is divided into multiple _sub-buffers_. When a +<> +matches an event, LTTng can record it to one or more sub-buffers of one +or more channels. + +When you <>, you set its +final attributes, that is: + +* Its <>. + +* What to do <> for a new event record because all sub-buffers are full. + +* The <> a ring buffer has. + +* The <> of trace files. + +* The periods of its <>, + <>, and <> + timers. + +* For a Linux kernel channel: its output type. ++ +See the opt:lttng-enable-channel(1):--output option of the +man:lttng-enable-channel(1) command. + +* For a user space channel: the value of its + <>. + +A channel is always associated to a <>. The +`java.util.logging` (JUL), log4j, and Python tracing domains each have a +default channel which you can't configure. + +A channel owns <>. + + +[[channel-buffering-schemes]] +==== Buffering scheme + +A channel has at least one ring buffer _per CPU_. LTTng always records +an event to the ring buffer dedicated to the CPU which emits it. + +The buffering scheme of a user space channel determines what has its own +set of per-CPU ring buffers: + +Per-user buffering:: + Allocate one set of ring buffers--one per CPU--shared by all the + instrumented processes of: + If your Unix user is `root`::: + Each Unix user. ++ +-- +[role="img-100"] +.Per-user buffering scheme (recording session belongs to the `root` Unix user). +image::per-user-buffering-root.png[] +-- + + Otherwise::: + Your Unix user. ++ +-- +[role="img-100"] +.Per-user buffering scheme (recording session belongs to the `Bob` Unix user). +image::per-user-buffering.png[] +-- + +Per-process buffering:: + Allocate one set of ring buffers--one per CPU--for each + instrumented process of: + If your Unix user is `root`::: + All Unix users. ++ +-- +[role="img-100"] +.Per-process buffering scheme (recording session belongs to the `root` Unix user). +image::per-process-buffering-root.png[] +-- + + Otherwise::: + Your Unix user. ++ +-- +[role="img-100"] +.Per-process buffering scheme (recording session belongs to the `Bob` Unix user). +image::per-process-buffering.png[] +-- + +The per-process buffering scheme tends to consume more memory than the +per-user option because systems generally have more instrumented +processes than Unix users running instrumented processes. However, the +per-process buffering scheme ensures that one process having a high +event throughput won't fill all the shared sub-buffers of the same Unix +user, only its own. + +The buffering scheme of a Linux kernel channel is always to allocate a +single set of ring buffers for the whole system. This scheme is similar +to the per-user option, but with a single, global user ``running'' the +kernel. + + +[[channel-overwrite-mode-vs-discard-mode]] +==== Event record loss mode + +When LTTng emits an event, LTTng can record it to a specific, available +sub-buffer within the ring buffers of specific channels. When there's no +space left in a sub-buffer, the tracer marks it as consumable and +another, available sub-buffer starts receiving the following event +records. An LTTng <> eventually +consumes the marked sub-buffer, which returns to the available state. + +[NOTE] +[role="docsvg-channel-subbuf-anim"] +==== +{note-no-anim} +==== + +In an ideal world, sub-buffers are consumed faster than they're filled. +In the real world, however, all sub-buffers can be full at some point, +leaving no space to record the following events. + +In an ideal world, sub-buffers are consumed faster than they're filled, +as it's the case in the previous animation. In the real world, +however, all sub-buffers can be full at some point, leaving no space to +record the following events. + +By default, <> and <> +are _non-blocking_ tracers: when there's no available sub-buffer to +record an event, it's acceptable to lose event records when the +alternative would be to cause substantial delays in the execution of the +instrumented application. LTTng privileges performance over integrity; +it aims at perturbing the instrumented application as little as possible +in order to make the detection of subtle race conditions and rare +interrupt cascades possible. + +Since LTTng{nbsp}2.10, the LTTng user space tracer, LTTng-UST, supports +a _blocking mode_. See the <> to learn how to use the blocking mode. + +When it comes to losing event records because there's no available +sub-buffer, or because the blocking timeout of +the channel is reached, the _event record loss mode_ of the channel +determines what to do. The available event record loss modes are: + +[[discard-mode]]Discard mode:: + Drop the newest event records until a sub-buffer becomes available. ++ +This is the only available mode when you specify a blocking timeout. ++ +With this mode, LTTng increments a count of lost event records when an +event record is lost and saves this count to the trace. A trace reader +can use the saved discarded event record count of the trace to decide +whether or not to perform some analysis even if trace data is known to +be missing. + +[[overwrite-mode]]Overwrite mode:: + Clear the sub-buffer containing the oldest event records and start + writing the newest event records there. ++ +This mode is sometimes called _flight recorder mode_ because it's +similar to a https://en.wikipedia.org/wiki/Flight_recorder[flight +recorder]: always keep a fixed amount of the latest data. It's also +similar to the roll mode of an oscilloscope. ++ +Since LTTng{nbsp}2.8, with this mode, LTTng writes to a given sub-buffer +its sequence number within its data stream. With a <>, +<>, or <> recording +session, a trace reader can use such sequence numbers to report lost +packets. A trace reader can use the saved discarded sub-buffer (packet) +count of the trace to decide whether or not to perform some analysis +even if trace data is known to be missing. ++ +With this mode, LTTng doesn't write to the trace the exact number of +lost event records in the lost sub-buffers. + +Which mechanism you should choose depends on your context: prioritize +the newest or the oldest event records in the ring buffer? + +Beware that, in overwrite mode, the tracer abandons a _whole sub-buffer_ +as soon as a there's no space left for a new event record, whereas in +discard mode, the tracer only discards the event record that doesn't +fit. + +There are a few ways to decrease your probability of losing event +records. The ``<>'' section shows how to fine-tune the sub-buffer size and count +of a channel to virtually stop losing event records, though at the cost +of greater memory usage. + + +[[channel-subbuf-size-vs-subbuf-count]] +==== Sub-buffer size and count + +A channel has one or more ring buffer for each CPU of the target system. + +See the ``<>'' section to +learn how many ring buffers of a given channel are dedicated to each CPU +depending on its buffering scheme. + +Set the size of each sub-buffer the ring buffers of a channel contain +and how many there are +when you <>. + +Note that LTTng switching the current sub-buffer of a ring buffer +(marking a full one as consumable and switching to an available one for +LTTng to record the next events) introduces noticeable CPU overhead. +Knowing this, the following list presents a few practical situations +along with how to configure the sub-buffer size and count for them: + +High event throughput:: + In general, prefer large sub-buffers to lower the risk of losing + event records. ++ +Having larger sub-buffers also ensures a lower sub-buffer switching +frequency. ++ +The sub-buffer count is only meaningful if you create the channel in +<>: in this case, if LTTng overwrites a +sub-buffer, then the other sub-buffers are left unaltered. + +Low event throughput:: + In general, prefer smaller sub-buffers since the risk of losing + event records is low. ++ +Because LTTng emits events less frequently, the sub-buffer switching +frequency should remain low and therefore the overhead of the tracer +shouldn't be a problem. + +Low memory system:: + If your target system has a low memory limit, prefer fewer first, + then smaller sub-buffers. ++ +Even if the system is limited in memory, you want to keep the +sub-buffers as large as possible to avoid a high sub-buffer switching +frequency. + +Note that LTTng uses https://diamon.org/ctf/[CTF] as its trace format, +which means event record data is very compact. For example, the average +LTTng kernel event record weights about 32{nbsp}bytes. Therefore, a +sub-buffer size of 1{nbsp}MiB is considered large. + +The previous scenarios highlight the major trade-off between a few large +sub-buffers and more, smaller sub-buffers: sub-buffer switching +frequency vs. how many event records are lost in overwrite mode. +Assuming a constant event throughput and using the overwrite mode, the +two following configurations have the same ring buffer total size: + +[NOTE] +[role="docsvg-channel-subbuf-size-vs-count-anim"] +==== +{note-no-anim} +==== + +Two sub-buffers of 4{nbsp}MiB each:: + Expect a very low sub-buffer switching frequency, but if LTTng + ever needs to overwrite a sub-buffer, half of the event records so + far (4{nbsp}MiB) are definitely lost. + +Eight sub-buffers of 1{nbsp}MiB each:: + Expect four times the tracer overhead of the configuration above, + but if LTTng needs to overwrite a sub-buffer, only the eighth of + event records so far (1{nbsp}MiB) are definitely lost. + +In <>, the sub-buffer count parameter is +pointless: use two sub-buffers and set their size according to your +requirements. + + +[[tracefile-rotation]] +==== Maximum trace file size and count (trace file rotation) + +By default, trace files can grow as large as needed. + +Set the maximum size of each trace file that LTTng writes of a given +channel when you <>. + +When the size of a trace file reaches the fixed maximum size of the +channel, LTTng creates another file to contain the next event records. +LTTng appends a file count to each trace file name in this case. + +If you set the trace file size attribute when you create a channel, the +maximum number of trace files that LTTng creates is _unlimited_ by +default. To limit them, set a maximum number of trace files. When the +number of trace files reaches the fixed maximum count of the channel, +LTTng overwrites the oldest trace file. This mechanism is called _trace +file rotation_. + +[IMPORTANT] +==== +Even if you don't limit the trace file count, always assume that LTTng +manages all the trace files of the recording session. + +In other words, there's no safe way to know if LTTng still holds a given +trace file open with the trace file rotation feature. + +The only way to obtain an unmanaged, self-contained LTTng trace before +you <> +is with the <> feature, which +is available since LTTng{nbsp}2.11. +==== + + +[[channel-timers]] +==== Timers + +Each channel can have up to three optional timers: + +[[channel-switch-timer]]Switch timer:: + When this timer expires, a sub-buffer switch happens: for each ring + buffer of the channel, LTTng marks the current sub-buffer as + consumable and _switches_ to an available one to record the next + events. ++ +[NOTE] +[role="docsvg-channel-switch-timer"] +==== +{note-no-anim} +==== ++ +A switch timer is useful to ensure that LTTng consumes and commits trace +data to trace files or to a distant <> +periodically in case of a low event throughput. ++ +Such a timer is also convenient when you use large +<> to cope with a +sporadic high event throughput, even if the throughput is otherwise low. ++ +Set the period of the switch timer of a channel when you +<> with +the opt:lttng-enable-channel(1):--switch-timer option. + +[[channel-read-timer]]Read timer:: + When this timer expires, LTTng checks for full, consumable + sub-buffers. ++ +By default, the LTTng tracers use an asynchronous message mechanism to +signal a full sub-buffer so that a <> +can consume it. ++ +When such messages must be avoided, for example in real-time +applications, use this timer instead. ++ +Set the period of the read timer of a channel when you +<> with the +opt:lttng-enable-channel(1):--read-timer option. + +[[channel-monitor-timer]]Monitor timer:: + When this timer expires, the consumer daemon samples some channel + statistics to evaluate the following <> + conditions: ++ +-- +. The consumed buffer size of a given <> becomes greater than some value. +. The buffer usage of a given channel becomes greater than some value. +. The buffer usage of a given channel becomes less than some value. +-- ++ +If you disable the monitor timer of a channel{nbsp}__C__: ++ +-- +* The consumed buffer size value of the recording session of{nbsp}__C__ + could be wrong for trigger condition type{nbsp}1: the consumed buffer + size of{nbsp}__C__ won't be part of the grand total. + +* The buffer usage trigger conditions (types{nbsp}2 and{nbsp}3) + for{nbsp}__C__ will never be satisfied. +-- ++ +Set the period of the monitor timer of a channel when you +<> with the +opt:lttng-enable-channel(1):--monitor-timer option. + + +[[event]] +=== Recording event rule and event record + +A _recording event rule_ is a specific type of <> +of which the action is to serialize and record the matched event as an +_event record_. + +Set the explicit conditions of a recording event rule when you +<>. A recording event rule also has +the following implicit conditions: + +* The recording event rule itself is enabled. ++ +A recording event rule is enabled on creation. + +* The <> to which the recording event rule is attached + is enabled. ++ +A channel is enabled on creation. + +* The <> of the recording event rule is + <> (started). ++ +A recording session is inactive (stopped) on creation. + +* The process for which LTTng creates an event to match is + <>. ++ +All processes are allowed to record events on recording session +creation. + +You always attach a recording event rule to a channel, which belongs to +a recording session, when you create it. + +When a recording event rule{nbsp}__ER__ matches an event{nbsp}__E__, +LTTng attempts to serialize and record{nbsp}__E__ to one of the +available sub-buffers of the channel to which{nbsp}__E__ is attached. + +When multiple matching recording event rules are attached to the same +channel, LTTng attempts to serialize and record the matched event +_once_. In the following example, the second recording event rule is +redundant when both are enabled: + +[role="term"] +---- +$ lttng enable-event --userspace hello:world +$ lttng enable-event --userspace hello:world --loglevel=INFO +---- + +[role="img-100"] +.Logical path from an instrumentation point to an event record. +image::event-rule.png[] + +As of LTTng{nbsp}{revision}, you cannot remove a recording event +rule: it exists as long as its recording session exists. + + +[[plumbing]] +== Components of noch:{LTTng} + +The second _T_ in _LTTng_ stands for _toolkit_: it would be wrong +to call LTTng a simple _tool_ since it's composed of multiple +interacting components. + +This section describes those components, explains their respective +roles, and shows how they connect together to form the LTTng ecosystem. + +The following diagram shows how the most important components of LTTng +interact with user applications, the Linux kernel, and you: + +[role="img-100"] +.Control and trace data paths between LTTng components. +image::plumbing.png[] + +The LTTng project integrates: + +LTTng-tools:: + Libraries and command-line interface to control recording sessions: ++ +* <> (man:lttng-sessiond(8)). +* <> (cmd:lttng-consumerd). +* <> (man:lttng-relayd(8)). +* <> (`liblttng-ctl`). +* <> (man:lttng(1)). +* <> + (man:lttng-crash(1)). + +LTTng-UST:: + Libraries and Java/Python packages to instrument and trace user + applications: ++ +* <> (`liblttng-ust`) and its + headers to instrument and trace any native user application. +* <>: +** `liblttng-ust-libc-wrapper` +** `liblttng-ust-pthread-wrapper` +** `liblttng-ust-cyg-profile` +** `liblttng-ust-cyg-profile-fast` +** `liblttng-ust-dl` +* <> to instrument and trace + Java applications using `java.util.logging` or + Apache log4j{nbsp}1.2 logging. +* <> to instrument + Python applications using the standard `logging` package. + +LTTng-modules:: + <> to instrument and trace the + kernel: ++ +* LTTng kernel tracer module. +* Recording ring buffer kernel modules. +* Probe kernel modules. +* LTTng logger kernel module. + + +[[lttng-cli]] +=== Tracing control command-line interface + +The _man:lttng(1) command-line tool_ is the standard user interface to +control LTTng <>. + +The cmd:lttng tool is part of LTTng-tools. + +The cmd:lttng tool is linked with +<> to communicate with +one or more <> behind the scenes. + +The cmd:lttng tool has a Git-like interface: + +[role="term"] +---- +$ lttng [GENERAL OPTIONS] [COMMAND OPTIONS] +---- + +The ``<>'' section explores the +available features of LTTng through its cmd:lttng tool. + + +[[liblttng-ctl-lttng]] +=== Tracing control library + +[role="img-100"] +.The tracing control library. +image::plumbing-liblttng-ctl.png[] + +The _LTTng control library_, `liblttng-ctl`, is used to communicate with +a <> using a C{nbsp}API that hides the +underlying details of the protocol. + +`liblttng-ctl` is part of LTTng-tools. + +The <> is linked with +`liblttng-ctl`. + +Use `liblttng-ctl` in C or $$C++$$ source code by including its +``master'' header: + +[source,c] +---- +#include +---- + +As of LTTng{nbsp}{revision}, the best available developer documentation +for `liblttng-ctl` is its installed header files. Functions and +structures are documented with header comments. + + +[[lttng-ust]] +=== User space tracing library + +[role="img-100"] +.The user space tracing library. +image::plumbing-liblttng-ust.png[] + +The _user space tracing library_, `liblttng-ust` (see man:lttng-ust(3)), +is the LTTng user space tracer. + +`liblttng-ust` receives commands from a <>, for example to allow specific instrumentation points to emit +LTTng <>, and writes event records to <> shared with a <>. + +`liblttng-ust` is part of LTTng-UST. + +`liblttng-ust` can also send asynchronous messages to the session daemon +when it emits an event. This supports the ``event rule matches'' +<> condition feature (see +“<>”). + +Public C{nbsp}header files are installed beside `liblttng-ust` to +instrument any <>. + +<>, which are regular Java and Python +packages, use their own <> which is linked with `liblttng-ust`. + +An application or library doesn't have to initialize `liblttng-ust` +manually: its constructor does the necessary tasks to register the +application to a session daemon. The initialization phase also +configures instrumentation points depending on the <> that you already created. + + +[[lttng-ust-agents]] +=== User space tracing agents + +[role="img-100"] +.The user space tracing agents. +image::plumbing-lttng-ust-agents.png[] + +The _LTTng-UST Java and Python agents_ are regular Java and Python +packages which add LTTng tracing capabilities to the +native logging frameworks. + +The LTTng-UST agents are part of LTTng-UST. + +In the case of Java, the +https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[`java.util.logging` +core logging facilities] and +https://logging.apache.org/log4j/1.2/[Apache log4j{nbsp}1.2] are supported. +Note that Apache Log4j{nbsp}2 isn't supported. + +In the case of Python, the standard +https://docs.python.org/3/library/logging.html[`logging`] package +is supported. Both Python{nbsp}2 and Python{nbsp}3 modules can import the +LTTng-UST Python agent package. + +The applications using the LTTng-UST agents are in the +`java.util.logging` (JUL), log4j, and Python <>. + +Both agents use the same mechanism to convert log statements to LTTng +events. When an agent initializes, it creates a log handler that +attaches to the root logger. The agent also registers to a +<>. When the user application executes a +log statement, the root logger passes it to the log handler of the +agent. The custom log handler of the agent calls a native function in a +tracepoint provider package shared library linked with +<>, passing the formatted log message and +other fields, like its logger name and its log level. This native +function contains a user space instrumentation point, therefore tracing +the log statement. + +The log level condition of a <> is +considered when tracing a Java or a Python application, and it's +compatible with the standard `java.util.logging`, log4j, and Python log +levels. + + +[[lttng-modules]] +=== LTTng kernel modules + +[role="img-100"] +.The LTTng kernel modules. +image::plumbing-lttng-modules.png[] + +The _LTTng kernel modules_ are a set of Linux kernel modules +which implement the kernel tracer of the LTTng project. + +The LTTng kernel modules are part of LTTng-modules. + +The LTTng kernel modules include: + +* A set of _probe_ modules. ++ +Each module attaches to a specific subsystem +of the Linux kernel using its tracepoint instrument points. ++ +There are also modules to attach to the entry and return points of the +Linux system call functions. + +* _Ring buffer_ modules. ++ +A ring buffer implementation is provided as kernel modules. The LTTng +kernel tracer writes to ring buffers; a +<> reads from ring buffers. + +* The _LTTng kernel tracer_ module. +* The <> module. ++ +The LTTng logger module implements the special path:{/proc/lttng-logger} +(and path:{/dev/lttng-logger}, since LTTng{nbsp}2.11) files so that any +executable can generate LTTng events by opening those files and +writing to them. + +The LTTng kernel tracer can also send asynchronous messages to the +<> when it emits an event. +This supports the ``event rule matches'' +<> condition feature (see +“<>”). + +Generally, you don't have to load the LTTng kernel modules manually +(using man:modprobe(8), for example): a root session daemon loads the +necessary modules when starting. If you have extra probe modules, you +can specify to load them to the session daemon on the command line +(see the opt:lttng-sessiond(8):--extra-kmod-probes option). + +The LTTng kernel modules are installed in ++/usr/lib/modules/__release__/extra+ by default, where +__release__+ is +the kernel release (output of `uname --kernel-release`). + + +[[lttng-sessiond]] +=== Session daemon + +[role="img-100"] +.The session daemon. +image::plumbing-sessiond.png[] + +The _session daemon_, man:lttng-sessiond(8), is a +https://en.wikipedia.org/wiki/Daemon_(computing)[daemon] which: + +* Manages <>. + +* Controls the various components (like tracers and + <>) of LTTng. + +* Sends <> to user + applications. + +The session daemon is part of LTTng-tools. + +The session daemon sends control requests to and receives control +responses from: + +* The <>. ++ +Any instance of the user space tracing library first registers to +a session daemon. Then, the session daemon can send requests to +this instance, such as: ++ +-- +** Get the list of tracepoints. +** Share a <> so that the user space tracing + library can decide whether or not a given tracepoint can emit events. + Amongst the possible conditions of a recording event rule is a filter + expression which `liblttng-ust` evaluates before it emits an event. +** Share <> attributes and ring buffer locations. +-- ++ +The session daemon and the user space tracing library use a Unix +domain socket to communicate. + +* The <>. ++ +Any instance of a user space tracing agent first registers to +a session daemon. Then, the session daemon can send requests to +this instance, such as: ++ +-- +** Get the list of loggers. +** Enable or disable a specific logger. +-- ++ +The session daemon and the user space tracing agent use a TCP connection +to communicate. + +* The <>. +* The <>. ++ +The session daemon sends requests to the consumer daemon to instruct +it where to send the trace data streams, amongst other information. + +* The <>. + +The session daemon receives commands from the +<>. + +The session daemon can receive asynchronous messages from the +<> and <> tracers +when they emit events. This supports the ``event rule matches'' +<> condition feature (see +“<>”). + +The root session daemon loads the appropriate +<> on startup. It also spawns +one or more <> as soon as you create +a <>. + +The session daemon doesn't send and receive trace data: this is the +role of the <> and +<>. It does, however, generate the +https://diamon.org/ctf/[CTF] metadata stream. + +Each Unix user can have its own session daemon instance. The +recording sessions which different session daemons manage are completely +independent. + +The root user's session daemon is the only one which is +allowed to control the LTTng kernel tracer, and its spawned consumer +daemon is the only one which is allowed to consume trace data from the +LTTng kernel tracer. Note, however, that any Unix user which is a member +of the <> is allowed +to create <> in the +Linux kernel <>, and therefore to use the Linux +kernel LTTng tracer. + +The <> automatically starts a +session daemon when using its `create` command if none is currently +running. You can also start the session daemon manually. + + +[[lttng-consumerd]] +=== Consumer daemon + +[role="img-100"] +.The consumer daemon. +image::plumbing-consumerd.png[] + +The _consumer daemon_, cmd:lttng-consumerd, is a +https://en.wikipedia.org/wiki/Daemon_(computing)[daemon] which shares +ring buffers with user applications or with the LTTng kernel modules to +collect trace data and send it to some location (file system or to a +<> over the network). + +The consumer daemon is part of LTTng-tools. + +You don't start a consumer daemon manually: a consumer daemon is always +spawned by a <> as soon as you create a +<>, that is, before you start recording. When +you kill its owner session daemon, the consumer daemon also exits +because it's the child process of the session daemon. Command-line +options of man:lttng-sessiond(8) target the consumer daemon process. + +There are up to two running consumer daemons per Unix user, whereas only +one session daemon can run per user. This is because each process can be +either 32-bit or 64-bit: if the target system runs a mixture of 32-bit +and 64-bit processes, it's more efficient to have separate +corresponding 32-bit and 64-bit consumer daemons. The root user is an +exception: it can have up to _three_ running consumer daemons: 32-bit +and 64-bit instances for its user applications, and one more +reserved for collecting kernel trace data. + + +[[lttng-relayd]] +=== Relay daemon + +[role="img-100"] +.The relay daemon. +image::plumbing-relayd.png[] + +The _relay daemon_, man:lttng-relayd(8), is a +https://en.wikipedia.org/wiki/Daemon_(computing)[daemon] acting as a bridge +between remote session and consumer daemons, local trace files, and a +remote live trace reader. + +The relay daemon is part of LTTng-tools. + +The main purpose of the relay daemon is to implement a receiver of +<>. +This is useful when the target system doesn't have much file system +space to write trace files locally. + +The relay daemon is also a server to which a +<> can +connect. The live trace reader sends requests to the relay daemon to +receive trace data as the target system records events. The +communication protocol is named _LTTng live_; it's used over TCP +connections. + +Note that you can start the relay daemon on the target system directly. +This is the setup of choice when the use case is to view/analyze events +as the target system records them without the need of a remote system. + + +[[instrumenting]] +== [[using-lttng]]Instrumentation + +There are many examples of tracing and monitoring in our everyday life: + +* You have access to real-time and historical weather reports and + forecasts thanks to weather stations installed around the country. +* You know your heart is safe thanks to an electrocardiogram. +* You make sure not to drive your car too fast and to have enough fuel + to reach your destination thanks to gauges visible on your dashboard. + +All the previous examples have something in common: they rely on +**instruments**. Without the electrodes attached to the surface of your +body skin, cardiac monitoring is futile. + +LTTng, as a tracer, is no different from those real life examples. If +you're about to trace a software system or, in other words, record its +history of execution, you better have **instrumentation points** in the +subject you're tracing, that is, the actual software system. + +<> were developed to +instrument a piece of software for LTTng tracing. The most +straightforward one is to manually place static instrumentation points, +called _tracepoints_, in the source code of the application. The Linux +kernel <> also makes it possible to dynamically +add instrumentation points. + +If you're only interested in tracing the Linux kernel, your +instrumentation needs are probably already covered by the built-in +<> of LTTng. You may +also wish to have LTTng trace a user application which is already +instrumented for LTTng tracing. In such cases, skip this whole section +and read the topics of the ``<>'' +section. + +Many methods are available to instrument a piece of software for LTTng +tracing: + +* <>. +* <>. +* <>. +* <>. +* <>. +* <>. + + +[[c-application]] +=== [[cxx-application]]Instrument a C/$$C++$$ user application + +The high level procedure to instrument a C or $$C++$$ user application +with the <>, `liblttng-ust`, +is: + +. <>. + +. <>. + +. <>. + +If you need quick, man:printf(3)-like instrumentation, skip those steps +and use <> or +<> instead. + +IMPORTANT: You need to <> LTTng-UST to +instrument a user application with `liblttng-ust`. + + +[[tracepoint-provider]] +==== Create the source files of a tracepoint provider package + +A _tracepoint provider_ is a set of compiled functions which provide +**tracepoints** to an application, the type of instrumentation point +which LTTng-UST provides. + +Those functions can make LTTng emit events with user-defined fields and +serialize those events as event records to one or more LTTng-UST +<> sub-buffers. The `lttng_ust_tracepoint()` macro, +which you <>, calls those functions. + +A _tracepoint provider package_ is an object file (`.o`) or a shared +library (`.so`) which contains one or more tracepoint providers. Its +source files are: + +* One or more <> (`.h`). +* A <> (`.c`). + +A tracepoint provider package is dynamically linked with `liblttng-ust`, +the LTTng user space tracer, at run time. + +[role="img-100"] +.User application linked with `liblttng-ust` and containing a tracepoint provider. +image::ust-app.png[] + +NOTE: If you need quick, man:printf(3)-like instrumentation, skip +creating and using a tracepoint provider and use +<> or <> +instead. + + +[[tpp-header]] +===== Create a tracepoint provider header file template + +A _tracepoint provider header file_ contains the tracepoint definitions +of a tracepoint provider. + +To create a tracepoint provider header file: + +. Start from this template: ++ +-- +[source,c] +.Tracepoint provider header file template (`.h` file extension). +---- +#undef LTTNG_UST_TRACEPOINT_PROVIDER +#define LTTNG_UST_TRACEPOINT_PROVIDER provider_name + +#undef LTTNG_UST_TRACEPOINT_INCLUDE +#define LTTNG_UST_TRACEPOINT_INCLUDE "./tp.h" + +#if !defined(_TP_H) || defined(LTTNG_UST_TRACEPOINT_HEADER_MULTI_READ) +#define _TP_H + +#include + +/* + * Use LTTNG_UST_TRACEPOINT_EVENT(), LTTNG_UST_TRACEPOINT_EVENT_CLASS(), + * LTTNG_UST_TRACEPOINT_EVENT_INSTANCE(), and + * LTTNG_UST_TRACEPOINT_LOGLEVEL() here. + */ + +#endif /* _TP_H */ + +#include +---- +-- ++ +Replace: ++ +* +__provider_name__+ with the name of your tracepoint provider. +* `"tp.h"` with the name of your tracepoint provider header file. + +. Below the `#include ` line, put your + <>. + +Your tracepoint provider name must be unique amongst all the possible +tracepoint provider names used on the same target system. We suggest to +include the name of your project or company in the name, for example, +`org_lttng_my_project_tpp`. + + +[[defining-tracepoints]] +===== Create a tracepoint definition + +A _tracepoint definition_ defines, for a given tracepoint: + +* Its **input arguments**. ++ +They're the macro parameters that the `lttng_ust_tracepoint()` macro +accepts for this particular tracepoint in the source code of the user +application. + +* Its **output event fields**. ++ +They're the sources of event fields that form the payload of any event +that the execution of the `lttng_ust_tracepoint()` macro emits for this +particular tracepoint. + +Create a tracepoint definition with the +`LTTNG_UST_TRACEPOINT_EVENT()` macro below the `#include ` +line in the +<>. + +The syntax of the `LTTNG_UST_TRACEPOINT_EVENT()` macro is: + +[source,c] +.`LTTNG_UST_TRACEPOINT_EVENT()` macro syntax. +---- +LTTNG_UST_TRACEPOINT_EVENT( + /* Tracepoint provider name */ + provider_name, + + /* Tracepoint name */ + tracepoint_name, + + /* Input arguments */ + LTTNG_UST_TP_ARGS( + arguments + ), + + /* Output event fields */ + LTTNG_UST_TP_FIELDS( + fields + ) +) +---- + +Replace: + +* +__provider_name__+ with your tracepoint provider name. +* +__tracepoint_name__+ with your tracepoint name. +* +__arguments__+ with the <>. +* +__fields__+ with the <> + definitions. + +The full name of this tracepoint is `provider_name:tracepoint_name`. + +[IMPORTANT] +.Event name length limitation +==== +The concatenation of the tracepoint provider name and the tracepoint +name must not exceed **254{nbsp}characters**. If it does, the +instrumented application compiles and runs, but LTTng throws multiple +warnings and you could experience serious issues. +==== + +[[tpp-def-input-args]]The syntax of the `LTTNG_UST_TP_ARGS()` macro is: + +[source,c] +.`LTTNG_UST_TP_ARGS()` macro syntax. +---- +LTTNG_UST_TP_ARGS( + type, arg_name +) +---- + +Replace: + +* +__type__+ with the C{nbsp}type of the argument. +* +__arg_name__+ with the argument name. + +You can repeat +__type__+ and +__arg_name__+ up to 10{nbsp}times to have +more than one argument. + +.`LTTNG_UST_TP_ARGS()` usage with three arguments. +==== +[source,c] +---- +LTTNG_UST_TP_ARGS( + int, count, + float, ratio, + const char*, query +) +---- +==== + +The `LTTNG_UST_TP_ARGS()` and `LTTNG_UST_TP_ARGS(void)` forms are valid +to create a tracepoint definition with no input arguments. + +[[tpp-def-output-fields]]The `LTTNG_UST_TP_FIELDS()` macro contains a +list of `lttng_ust_field_*()` macros. Each `lttng_ust_field_*()` macro +defines one event field. See man:lttng-ust(3) for a complete description +of the available `lttng_ust_field_*()` macros. A `lttng_ust_field_*()` +macro specifies the type, size, and byte order of one event field. + +Each `lttng_ust_field_*()` macro takes an _argument expression_ +parameter. This is a C{nbsp}expression that the tracer evaluates at the +`lttng_ust_tracepoint()` macro site in the source code of the +application. This expression provides the source of data of a field. The +argument expression can include input argument names listed in the +`LTTNG_UST_TP_ARGS()` macro. + +Each `lttng_ust_field_*()` macro also takes a _field name_ parameter. +Field names must be unique within a given tracepoint definition. + +Here's a complete tracepoint definition example: + +.Tracepoint definition. +==== +The following tracepoint definition defines a tracepoint which takes +three input arguments and has four output event fields. + +[source,c] +---- +#include "my-custom-structure.h" + +LTTNG_UST_TRACEPOINT_EVENT( + my_provider, + my_tracepoint, + LTTNG_UST_TP_ARGS( + const struct my_custom_structure *, my_custom_structure, + float, ratio, + const char *, query + ), + LTTNG_UST_TP_FIELDS( + lttng_ust_field_string(query_field, query) + lttng_ust_field_float(double, ratio_field, ratio) + lttng_ust_field_integer(int, recv_size, + my_custom_structure->recv_size) + lttng_ust_field_integer(int, send_size, + my_custom_structure->send_size) + ) +) +---- + +Refer to this tracepoint definition with the `lttng_ust_tracepoint()` +macro in the source code of your application like this: + +[source,c] +---- +lttng_ust_tracepoint(my_provider, my_tracepoint, + my_structure, some_ratio, the_query); +---- +==== + +NOTE: The LTTng-UST tracer only evaluates the arguments of a tracepoint +at run time when such a tracepoint _could_ emit an event. See +<> to learn more. + + +[[using-tracepoint-classes]] +===== Use a tracepoint class + +A _tracepoint class_ is a class of tracepoints which share the same +output event field definitions. A _tracepoint instance_ is one +instance of such a defined tracepoint class, with its own tracepoint +name. + +The <> is +actually a shorthand which defines both a tracepoint class and a +tracepoint instance at the same time. + +When you build a tracepoint provider package, the C or $$C++$$ compiler +creates one serialization function for each **tracepoint class**. A +serialization function is responsible for serializing the event fields +of a tracepoint to a sub-buffer when recording. + +For various performance reasons, when your situation requires multiple +tracepoint definitions with different names, but with the same event +fields, we recommend that you manually create a tracepoint class and +instantiate as many tracepoint instances as needed. One positive effect +of such a design, amongst other advantages, is that all tracepoint +instances of the same tracepoint class reuse the same serialization +function, thus reducing +https://en.wikipedia.org/wiki/Cache_pollution[cache pollution]. + +.Use a tracepoint class and tracepoint instances. +==== +Consider the following three tracepoint definitions: + +[source,c] +---- +LTTNG_UST_TRACEPOINT_EVENT( + my_app, + get_account, + LTTNG_UST_TP_ARGS( + int, userid, + size_t, len + ), + LTTNG_UST_TP_FIELDS( + lttng_ust_field_integer(int, userid, userid) + lttng_ust_field_integer(size_t, len, len) + ) +) + +LTTNG_UST_TRACEPOINT_EVENT( + my_app, + get_settings, + LTTNG_UST_TP_ARGS( + int, userid, + size_t, len + ), + LTTNG_UST_TP_FIELDS( + lttng_ust_field_integer(int, userid, userid) + lttng_ust_field_integer(size_t, len, len) + ) +) + +LTTNG_UST_TRACEPOINT_EVENT( + my_app, + get_transaction, + LTTNG_UST_TP_ARGS( + int, userid, + size_t, len + ), + LTTNG_UST_TP_FIELDS( + lttng_ust_field_integer(int, userid, userid) + lttng_ust_field_integer(size_t, len, len) + ) +) +---- + +In this case, we create three tracepoint classes, with one implicit +tracepoint instance for each of them: `get_account`, `get_settings`, and +`get_transaction`. However, they all share the same event field names +and types. Hence three identical, yet independent serialization +functions are created when you build the tracepoint provider package. + +A better design choice is to define a single tracepoint class and three +tracepoint instances: + +[source,c] +---- +/* The tracepoint class */ +LTTNG_UST_TRACEPOINT_EVENT_CLASS( + /* Tracepoint class provider name */ + my_app, + + /* Tracepoint class name */ + my_class, + + /* Input arguments */ + LTTNG_UST_TP_ARGS( + int, userid, + size_t, len + ), + + /* Output event fields */ + LTTNG_UST_TP_FIELDS( + lttng_ust_field_integer(int, userid, userid) + lttng_ust_field_integer(size_t, len, len) + ) +) + +/* The tracepoint instances */ +LTTNG_UST_TRACEPOINT_EVENT_INSTANCE( + /* Tracepoint class provider name */ + my_app, + + /* Tracepoint class name */ + my_class, + + /* Instance provider name */ + my_app, + + /* Tracepoint name */ + get_account, + + /* Input arguments */ + LTTNG_UST_TP_ARGS( + int, userid, + size_t, len + ) +) +LTTNG_UST_TRACEPOINT_EVENT_INSTANCE( + my_app, + my_class, + get_settings, + LTTNG_UST_TP_ARGS( + int, userid, + size_t, len + ) +) +LTTNG_UST_TRACEPOINT_EVENT_INSTANCE( + my_app, + my_class, + get_transaction, + LTTNG_UST_TP_ARGS( + int, userid, + size_t, len + ) +) +---- +==== + +The tracepoint class and instance provider names must be the same if the +`LTTNG_UST_TRACEPOINT_EVENT_CLASS()` and +`LTTNG_UST_TRACEPOINT_EVENT_INSTANCE()` expansions are part of the same +translation unit. See man:lttng-ust(3) to learn more. + + +[[assigning-log-levels]] +===== Assign a log level to a tracepoint definition + +Assign a _log level_ to a <> +with the `LTTNG_UST_TRACEPOINT_LOGLEVEL()` macro. + +Assigning different levels of severity to tracepoint definitions can be +useful: when you <>, you can target tracepoints having a log level at least as severe +as a specific value. + +The concept of LTTng-UST log levels is similar to the levels found +in typical logging frameworks: + +* In a logging framework, the log level is given by the function + or method name you use at the log statement site: `debug()`, + `info()`, `warn()`, `error()`, and so on. + +* In LTTng-UST, you statically assign the log level to a tracepoint + definition; any `lttng_ust_tracepoint()` macro invocation which refers + to this definition has this log level. + +You must use `LTTNG_UST_TRACEPOINT_LOGLEVEL()` _after_ the +<> or +<> macro for +a given tracepoint. + +The syntax of the `LTTNG_UST_TRACEPOINT_LOGLEVEL()` macro is: + +[source,c] +.`LTTNG_UST_TRACEPOINT_LOGLEVEL()` macro syntax. +---- +LTTNG_UST_TRACEPOINT_LOGLEVEL(provider_name, tracepoint_name, log_level) +---- + +Replace: + +* +__provider_name__+ with the tracepoint provider name. +* +__tracepoint_name__+ with the tracepoint name. +* +__log_level__+ with the log level to assign to the tracepoint + definition named +__tracepoint_name__+ in the +__provider_name__+ + tracepoint provider. ++ +See man:lttng-ust(3) for a list of available log level names. + +.Assign the `LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_UNIT` log level to a +tracepoint definition. +==== +[source,c] +---- +/* Tracepoint definition */ +LTTNG_UST_TRACEPOINT_EVENT( + my_app, + get_transaction, + LTTNG_UST_TP_ARGS( + int, userid, + size_t, len + ), + LTTNG_UST_TP_FIELDS( + lttng_ust_field_integer(int, userid, userid) + lttng_ust_field_integer(size_t, len, len) + ) +) + +/* Log level assignment */ +LTTNG_UST_TRACEPOINT_LOGLEVEL(my_app, get_transaction, + LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_UNIT) +---- +==== + + +[[tpp-source]] +===== Create a tracepoint provider package source file + +A _tracepoint provider package source file_ is a C source file which +includes a <> to expand its +macros into event serialization and other functions. + +Use the following tracepoint provider package source file template: + +[source,c] +.Tracepoint provider package source file template. +---- +#define LTTNG_UST_TRACEPOINT_CREATE_PROBES + +#include "tp.h" +---- + +Replace `tp.h` with the name of your <> name. You may also include more than one tracepoint +provider header file here to create a tracepoint provider package +holding more than one tracepoint providers. + + +[[probing-the-application-source-code]] +==== Add tracepoints to the source code of an application + +Once you <>, use +the `lttng_ust_tracepoint()` macro in the source code of your +application to insert the tracepoints that this header +<>. + +The `lttng_ust_tracepoint()` macro takes at least two parameters: the +tracepoint provider name and the tracepoint name. The corresponding +tracepoint definition defines the other parameters. + +.`lttng_ust_tracepoint()` usage. +==== +The following <> defines a +tracepoint which takes two input arguments and has two output event +fields. + +[source,c] +.Tracepoint provider header file. +---- +#include "my-custom-structure.h" + +LTTNG_UST_TRACEPOINT_EVENT( + my_provider, + my_tracepoint, + LTTNG_UST_TP_ARGS( + int, argc, + const char *, cmd_name + ), + LTTNG_UST_TP_FIELDS( + lttng_ust_field_string(cmd_name, cmd_name) + lttng_ust_field_integer(int, number_of_args, argc) + ) +) +---- + +Refer to this tracepoint definition with the `lttng_ust_tracepoint()` +macro in the source code of your application like this: + +[source,c] +.Application source file. +---- +#include "tp.h" + +int main(int argc, char* argv[]) +{ + lttng_ust_tracepoint(my_provider, my_tracepoint, argc, argv[0]); + return 0; +} +---- + +Note how the source code of the application includes +the tracepoint provider header file containing the tracepoint +definitions to use, path:{tp.h}. +==== + +.`lttng_ust_tracepoint()` usage with a complex tracepoint definition. +==== +Consider this complex tracepoint definition, where multiple event +fields refer to the same input arguments in their argument expression +parameter: + +[source,c] +.Tracepoint provider header file. +---- +/* For `struct stat` */ +#include +#include +#include + +LTTNG_UST_TRACEPOINT_EVENT( + my_provider, + my_tracepoint, + LTTNG_UST_TP_ARGS( + int, my_int_arg, + char *, my_str_arg, + struct stat *, st + ), + LTTNG_UST_TP_FIELDS( + lttng_ust_field_integer(int, my_constant_field, 23 + 17) + lttng_ust_field_integer(int, my_int_arg_field, my_int_arg) + lttng_ust_field_integer(int, my_int_arg_field2, + my_int_arg * my_int_arg) + lttng_ust_field_integer(int, sum4_field, + my_str_arg[0] + my_str_arg[1] + + my_str_arg[2] + my_str_arg[3]) + lttng_ust_field_string(my_str_arg_field, my_str_arg) + lttng_ust_field_integer_hex(off_t, size_field, st->st_size) + lttng_ust_field_float(double, size_dbl_field, (double) st->st_size) + lttng_ust_field_sequence_text(char, half_my_str_arg_field, + my_str_arg, size_t, + strlen(my_str_arg) / 2) + ) +) +---- + +Refer to this tracepoint definition with the `lttng_ust_tracepoint()` +macro in the source code of your application like this: + +[source,c] +.Application source file. +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +#include "tp.h" + +int main(void) +{ + struct stat s; + + stat("/etc/fstab", &s); + lttng_ust_tracepoint(my_provider, my_tracepoint, 23, + "Hello, World!", &s); + + return 0; +} +---- + +If you look at the event record that LTTng writes when recording this +program, assuming the file size of path:{/etc/fstab} is 301{nbsp}bytes, +it should look like this: + +.Event record fields +|==== +|Field name |Field value +|`my_constant_field` |40 +|`my_int_arg_field` |23 +|`my_int_arg_field2` |529 +|`sum4_field` |389 +|`my_str_arg_field` |`Hello, World!` +|`size_field` |0x12d +|`size_dbl_field` |301.0 +|`half_my_str_arg_field` |`Hello,` +|==== +==== + +Sometimes, the arguments you pass to `lttng_ust_tracepoint()` are +expensive to evaluate--they use the call stack, for example. To avoid +this computation when LTTng wouldn't emit any event anyway, use the +`lttng_ust_tracepoint_enabled()` and `lttng_ust_do_tracepoint()` macros. + +The syntax of the `lttng_ust_tracepoint_enabled()` and +`lttng_ust_do_tracepoint()` macros is: + +[source,c] +.`lttng_ust_tracepoint_enabled()` and `lttng_ust_do_tracepoint()` macros syntax. +---- +lttng_ust_tracepoint_enabled(provider_name, tracepoint_name) + +lttng_ust_do_tracepoint(provider_name, tracepoint_name, ...) +---- + +Replace: + +* +__provider_name__+ with the tracepoint provider name. +* +__tracepoint_name__+ with the tracepoint name. + +`lttng_ust_tracepoint_enabled()` returns a non-zero value if executing +the tracepoint named `tracepoint_name` from the provider named +`provider_name` _could_ make LTTng emit an event, depending on the +payload of said event. + +`lttng_ust_do_tracepoint()` is like `lttng_ust_tracepoint()`, except +that it doesn't check what `lttng_ust_tracepoint_enabled()` checks. +Using `lttng_ust_tracepoint()` with `lttng_ust_tracepoint_enabled()` is +dangerous because `lttng_ust_tracepoint()` also contains the +`lttng_ust_tracepoint_enabled()` check; therefore, a race condition is +possible in this situation: + +[source,c] +.Possible race condition when using `lttng_ust_tracepoint_enabled()` with `lttng_ust_tracepoint()`. +---- +if (lttng_ust_tracepoint_enabled(my_provider, my_tracepoint)) { + stuff = prepare_stuff(); +} + +lttng_ust_tracepoint(my_provider, my_tracepoint, stuff); +---- + +If `lttng_ust_tracepoint_enabled()` is false, but would be true after +the conditional block, then `stuff` isn't prepared: the emitted event +will either contain wrong data, or the whole application could crash +(with a segmentation fault, for example). + +NOTE: Neither `lttng_ust_tracepoint_enabled()` nor +`lttng_ust_do_tracepoint()` have an `STAP_PROBEV()` call. If you need +it, you must emit this call yourself. + + +[[building-tracepoint-providers-and-user-application]] +==== Build and link a tracepoint provider package and an application + +Once you have one or more <> and a <>, +create the tracepoint provider package by compiling its source +file. From here, multiple build and run scenarios are possible. The +following table shows common application and library configurations +along with the required command lines to achieve them. + +In the following diagrams, we use the following file names: + +`app`:: + Executable application. + +`app.o`:: + Application object file. + +`tpp.o`:: + Tracepoint provider package object file. + +`tpp.a`:: + Tracepoint provider package archive file. + +`libtpp.so`:: + Tracepoint provider package shared object file. + +`emon.o`:: + User library object file. + +`libemon.so`:: + User library shared object file. + +We use the following symbols in the diagrams of table below: + +[role="img-100"] +.Symbols used in the build scenario diagrams. +image::ust-sit-symbols.png[] + +We assume that path:{.} is part of the env:LD_LIBRARY_PATH environment +variable in the following instructions. + +[role="growable ust-scenarios",cols="asciidoc,asciidoc"] +.Common tracepoint provider package scenarios. +|==== +|Scenario |Instructions + +| +The instrumented application is statically linked with +the tracepoint provider package object. + +image::ust-sit+app-linked-with-tp-o+app-instrumented.png[] + +| +include::../common/ust-sit-step-tp-o.txt[] + +To build the instrumented application: + +. In path:{app.c}, before including path:{tpp.h}, add the following line: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +---- +-- + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o tpp.o -llttng-ust -ldl +---- +-- + +To run the instrumented application: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The instrumented application is statically linked with the +tracepoint provider package archive file. + +image::ust-sit+app-linked-with-tp-a+app-instrumented.png[] + +| +To create the tracepoint provider package archive file: + +. Compile the <>: ++ +-- +[role="term"] +---- +$ gcc -I. -c tpp.c +---- +-- + +. Create the tracepoint provider package archive file: ++ +-- +[role="term"] +---- +$ ar rcs tpp.a tpp.o +---- +-- + +To build the instrumented application: + +. In path:{app.c}, before including path:{tpp.h}, add the following line: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +---- +-- + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o tpp.a -llttng-ust -ldl +---- +-- + +To run the instrumented application: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The instrumented application is linked with the tracepoint provider +package shared object. + +image::ust-sit+app-linked-with-tp-so+app-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented application: + +. In path:{app.c}, before including path:{tpp.h}, add the following line: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +---- +-- + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o -ldl -L. -ltpp +---- +-- + +To run the instrumented application: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The tracepoint provider package shared object is preloaded before the +instrumented application starts. + +image::ust-sit+tp-so-preloaded+app-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented application: + +. In path:{app.c}, before including path:{tpp.h}, add the + following lines: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE +---- +-- + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o -ldl +---- +-- + +To run the instrumented application with tracing support: + +* Preload the tracepoint provider package shared object and + start the application: ++ +-- +[role="term"] +---- +$ LD_PRELOAD=./libtpp.so ./app +---- +-- + +To run the instrumented application without tracing support: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The instrumented application dynamically loads the tracepoint provider +package shared object. + +image::ust-sit+app-dlopens-tp-so+app-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented application: + +. In path:{app.c}, before including path:{tpp.h}, add the + following lines: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE +---- +-- + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o -ldl +---- +-- + +To run the instrumented application: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The application is linked with the instrumented user library. + +The instrumented user library is statically linked with the tracepoint +provider package object file. + +image::ust-sit+app-linked-with-lib+lib-linked-with-tp-o+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-o-fpic.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following line: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +$ gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +$ gcc -shared -o libemon.so emon.o tpp.o -llttng-ust -ldl +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o -L. -lemon +---- +-- + +To run the application: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The application is linked with the instrumented user library. + +The instrumented user library is linked with the tracepoint provider +package shared object. + +image::ust-sit+app-linked-with-lib+lib-linked-with-tp-so+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following line: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +$ gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +$ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o -L. -lemon +---- +-- + +To run the application: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The tracepoint provider package shared object is preloaded before the +application starts. + +The application is linked with the instrumented user library. + +image::ust-sit+tp-so-preloaded+app-linked-with-lib+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following lines: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +$ gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +$ gcc -shared -o libemon.so emon.o -ldl +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o -L. -lemon +---- +-- + +To run the application with tracing support: + +* Preload the tracepoint provider package shared object and + start the application: ++ +-- +[role="term"] +---- +$ LD_PRELOAD=./libtpp.so ./app +---- +-- + +To run the application without tracing support: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The application is linked with the instrumented user library. + +The instrumented user library dynamically loads the tracepoint provider +package shared object. + +image::ust-sit+app-linked-with-lib+lib-dlopens-tp-so+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following lines: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +$ gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +$ gcc -shared -o libemon.so emon.o -ldl +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o -L. -lemon +---- +-- + +To run the application: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The application dynamically loads the instrumented user library. + +The instrumented user library is linked with the tracepoint provider +package shared object. + +image::ust-sit+app-dlopens-lib+lib-linked-with-tp-so+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following line: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +$ gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +$ gcc -shared -o libemon.so emon.o -ldl -L. -ltpp +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o -ldl -L. -lemon +---- +-- + +To run the application: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The application dynamically loads the instrumented user library. + +The instrumented user library dynamically loads the tracepoint provider +package shared object. + +image::ust-sit+app-dlopens-lib+lib-dlopens-tp-so+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following lines: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +$ gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +$ gcc -shared -o libemon.so emon.o -ldl +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o -ldl -L. -lemon +---- +-- + +To run the application: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The tracepoint provider package shared object is preloaded before the +application starts. + +The application dynamically loads the instrumented user library. + +image::ust-sit+tp-so-preloaded+app-dlopens-lib+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-so.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following lines: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +#define LTTNG_UST_TRACEPOINT_PROBE_DYNAMIC_LINKAGE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +$ gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +$ gcc -shared -o libemon.so emon.o -ldl +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o -L. -lemon +---- +-- + +To run the application with tracing support: + +* Preload the tracepoint provider package shared object and + start the application: ++ +-- +[role="term"] +---- +$ LD_PRELOAD=./libtpp.so ./app +---- +-- + +To run the application without tracing support: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The application is statically linked with the tracepoint provider +package object file. + +The application is linked with the instrumented user library. + +image::ust-sit+app-linked-with-tp-o+app-linked-with-lib+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-o.txt[] + +To build the instrumented user library: + +. In path:{emon.c}, before including path:{tpp.h}, add the + following line: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +---- +-- + +. Compile the user library source file: ++ +-- +[role="term"] +---- +$ gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +$ gcc -shared -o libemon.so emon.o +---- +-- + +To build the application: + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -o app app.o tpp.o -llttng-ust -ldl -L. -lemon +---- +-- + +To run the instrumented application: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- + +| +The application is statically linked with the tracepoint provider +package object file. + +The application dynamically loads the instrumented user library. + +image::ust-sit+app-linked-with-tp-o+app-dlopens-lib+lib-instrumented.png[] + +| +include::../common/ust-sit-step-tp-o.txt[] + +To build the application: + +. In path:{app.c}, before including path:{tpp.h}, add the following line: ++ +-- +[source,c] +---- +#define LTTNG_UST_TRACEPOINT_DEFINE +---- +-- + +. Compile the application source file: ++ +-- +[role="term"] +---- +$ gcc -c app.c +---- +-- + +. Build the application: ++ +-- +[role="term"] +---- +$ gcc -Wl,--export-dynamic -o app app.o tpp.o \ + -llttng-ust -ldl +---- +-- ++ +The `--export-dynamic` option passed to the linker is necessary for the +dynamically loaded library to ``see'' the tracepoint symbols defined in +the application. + +To build the instrumented user library: + +. Compile the user library source file: ++ +-- +[role="term"] +---- +$ gcc -I. -fpic -c emon.c +---- +-- + +. Build the user library shared object: ++ +-- +[role="term"] +---- +$ gcc -shared -o libemon.so emon.o +---- +-- + +To run the application: + +* Start the application: ++ +-- +[role="term"] +---- +$ ./app +---- +-- +|==== + + +[[using-lttng-ust-with-daemons]] +===== Use noch:{LTTng-UST} with daemons + +If your instrumented application calls man:fork(2), man:clone(2), +or BSD's man:rfork(2), without a following man:exec(3)-family +system call, you must preload the path:{liblttng-ust-fork.so} shared +object when you start the application. + +[role="term"] +---- +$ LD_PRELOAD=liblttng-ust-fork.so ./my-app +---- + +If your tracepoint provider package is +a shared library which you also preload, you must put both +shared objects in env:LD_PRELOAD: + +[role="term"] +---- +$ LD_PRELOAD=liblttng-ust-fork.so:/path/to/tp.so ./my-app +---- + + +[role="since-2.9"] +[[liblttng-ust-fd]] +===== Use noch:{LTTng-UST} with applications which close file descriptors that don't belong to them + +If your instrumented application closes one or more file descriptors +which it did not open itself, you must preload the +path:{liblttng-ust-fd.so} shared object when you start the application: + +[role="term"] +---- +$ LD_PRELOAD=liblttng-ust-fd.so ./my-app +---- + +Typical use cases include closing all the file descriptors after +man:fork(2) or man:rfork(2) and buggy applications doing +``double closes''. + + +[[lttng-ust-pkg-config]] +===== Use noch:{pkg-config} + +On some distributions, LTTng-UST ships with a +https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config] +metadata file. If this is your case, then use cmd:pkg-config to +build an application on the command line: + +[role="term"] +---- +$ gcc -o my-app my-app.o tp.o $(pkg-config --cflags --libs lttng-ust) +---- + + +[[instrumenting-32-bit-app-on-64-bit-system]] +===== [[advanced-instrumenting-techniques]]Build a 32-bit instrumented application for a 64-bit target system + +In order to trace a 32-bit application running on a 64-bit system, +LTTng must use a dedicated 32-bit +<>. + +The following steps show how to build and install a 32-bit consumer +daemon, which is _not_ part of the default 64-bit LTTng build, how to +build and install the 32-bit LTTng-UST libraries, and how to build and +link an instrumented 32-bit application in that context. + +To build a 32-bit instrumented application for a 64-bit target system, +assuming you have a fresh target system with no installed Userspace RCU +or LTTng packages: + +. Download, build, and install a 32-bit version of Userspace RCU: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && + wget https://lttng.org/files/urcu/userspace-rcu-latest-0.13.tar.bz2 && + tar -xf userspace-rcu-latest-0.13.tar.bz2 && + cd userspace-rcu-0.13.* && + ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 && + make && + sudo make install && + sudo ldconfig +---- +-- + +. Using the package manager of your distribution, or from source, + install the 32-bit versions of the following dependencies of + LTTng-tools and LTTng-UST: ++ +-- +* https://sourceforge.net/projects/libuuid/[libuuid] +* https://directory.fsf.org/wiki/Popt[popt] +* https://www.xmlsoft.org/[libxml2] +* **Optional**: https://github.com/numactl/numactl[numactl] +-- + +. Download, build, and install a 32-bit version of the latest + LTTng-UST{nbsp}{revision}: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && + wget https://lttng.org/files/lttng-ust/lttng-ust-latest-2.13.tar.bz2 && + tar -xf lttng-ust-latest-2.13.tar.bz2 && + cd lttng-ust-2.13.* && + ./configure --libdir=/usr/local/lib32 \ + CFLAGS=-m32 CXXFLAGS=-m32 \ + LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' && + make && + sudo make install && + sudo ldconfig +---- +-- ++ +Add `--disable-numa` to `./configure` if you don't have +https://github.com/numactl/numactl[numactl]. ++ +[NOTE] +==== +Depending on your distribution, 32-bit libraries could be installed at a +different location than `/usr/lib32`. For example, Debian is known to +install some 32-bit libraries in `/usr/lib/i386-linux-gnu`. + +In this case, make sure to set `LDFLAGS` to all the +relevant 32-bit library paths, for example: + +[role="term"] +---- +$ LDFLAGS='-L/usr/lib/i386-linux-gnu -L/usr/lib32' +---- +==== + +. Download the latest LTTng-tools{nbsp}{revision}, build, and install + the 32-bit consumer daemon: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && + wget https://lttng.org/files/lttng-tools/lttng-tools-latest-2.13.tar.bz2 && + tar -xf lttng-tools-latest-2.13.tar.bz2 && + cd lttng-tools-2.13.* && + ./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \ + LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' \ + --disable-bin-lttng --disable-bin-lttng-crash \ + --disable-bin-lttng-relayd --disable-bin-lttng-sessiond && + make && + cd src/bin/lttng-consumerd && + sudo make install && + sudo ldconfig +---- +-- + +. From your distribution or from source, <> + the 64-bit versions of LTTng-UST and Userspace RCU. + +. Download, build, and install the 64-bit version of the + latest LTTng-tools{nbsp}{revision}: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && + wget https://lttng.org/files/lttng-tools/lttng-tools-latest-2.13.tar.bz2 && + tar -xf lttng-tools-latest-2.13.tar.bz2 && + cd lttng-tools-2.13.* && + ./configure --with-consumerd32-libdir=/usr/local/lib32 \ + --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd && + make && + sudo make install && + sudo ldconfig +---- +-- + +. Pass the following options to man:gcc(1), man:g++(1), or man:clang(1) + when linking your 32-bit application: ++ +---- +-m32 -L/usr/lib32 -L/usr/local/lib32 \ +-Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 +---- ++ +For example, let's rebuild the quick start example in +``<>'' +as an instrumented 32-bit application: ++ +-- +[role="term"] +---- +$ gcc -m32 -c -I. hello-tp.c +$ gcc -m32 -c hello.c +$ gcc -m32 -o hello hello.o hello-tp.o \ + -L/usr/lib32 -L/usr/local/lib32 \ + -Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 \ + -llttng-ust -ldl +---- +-- + +No special action is required to execute the 32-bit application and +for LTTng to trace it: use the command-line man:lttng(1) tool as usual. + + +[role="since-2.5"] +[[tracef]] +==== Use `lttng_ust_tracef()` + +man:lttng_ust_tracef(3) is a small LTTng-UST API designed for quick, +man:printf(3)-like instrumentation without the burden of +<> and +<> +a tracepoint provider package. + +To use `lttng_ust_tracef()` in your application: + +. In the C or $$C++$$ source files where you need to use + `lttng_ust_tracef()`, include ``: ++ +-- +[source,c] +---- +#include +---- +-- + +. In the source code of the application, use `lttng_ust_tracef()` like + you would use man:printf(3): ++ +-- +[source,c] +---- + /* ... */ + + lttng_ust_tracef("my message: %d (%s)", my_integer, my_string); + + /* ... */ +---- +-- + +. Link your application with `liblttng-ust`: ++ +-- +[role="term"] +---- +$ gcc -o app app.c -llttng-ust +---- +-- + +To record the events that `lttng_ust_tracef()` calls emit: + +* <> which + matches user space events named `lttng_ust_tracef:*`: ++ +-- +[role="term"] +---- +$ lttng enable-event --userspace 'lttng_ust_tracef:*' +---- +-- + +[IMPORTANT] +.Limitations of `lttng_ust_tracef()` +==== +The `lttng_ust_tracef()` utility function was developed to make user +space tracing super simple, albeit with notable disadvantages compared +to <>: + +* All the created events have the same tracepoint provider and + tracepoint names, respectively `lttng_ust_tracef` and `event`. +* There's no static type checking. +* The only event record field you actually get, named `msg`, is a string + potentially containing the values you passed to `lttng_ust_tracef()` + using your own format string. This also means that you can't filter + events with a custom expression at run time because there are no + isolated fields. +* Since `lttng_ust_tracef()` uses the man:vasprintf(3) function of the + C{nbsp}standard library behind the scenes to format the strings at run + time, its expected performance is lower than with user-defined + tracepoints, which don't require a conversion to a string. + +Taking this into consideration, `lttng_ust_tracef()` is useful for some +quick prototyping and debugging, but you shouldn't consider it for any +permanent and serious applicative instrumentation. +==== + + +[role="since-2.7"] +[[tracelog]] +==== Use `lttng_ust_tracelog()` + +The man:tracelog(3) API is very similar to +<>, with the difference that it accepts an +additional log level parameter. + +The goal of `lttng_ust_tracelog()` is to ease the migration from logging +to tracing. + +To use `lttng_ust_tracelog()` in your application: + +. In the C or $$C++$$ source files where you need to use `tracelog()`, + include ``: ++ +-- +[source,c] +---- +#include +---- +-- + +. In the source code of the application, use `lttng_ust_tracelog()` like + you would use man:printf(3), except for the first parameter which is + the log level: ++ +-- +[source,c] +---- + /* ... */ + + tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_WARNING, + "my message: %d (%s)", my_integer, my_string); + + /* ... */ +---- +-- ++ +See man:lttng-ust(3) for a list of available log level names. + +. Link your application with `liblttng-ust`: ++ +-- +[role="term"] +---- +$ gcc -o app app.c -llttng-ust +---- +-- + +To record the events that `lttng_ust_tracelog()` calls emit with a log +level _at least as severe as_ a specific log level: + +* <> which + matches user space tracepoint events named `lttng_ust_tracelog:*` and + with some minimum level of severity: ++ +-- +[role="term"] +---- +$ lttng enable-event --userspace 'lttng_ust_tracelog:*' \ + --loglevel=WARNING +---- +-- + +To record the events that `lttng_ust_tracelog()` calls emit with a +_specific log level_: + +* Create a recording event rule which matches tracepoint events named + `lttng_ust_tracelog:*` and with a specific log level: ++ +-- +[role="term"] +---- +$ lttng enable-event --userspace 'lttng_ust_tracelog:*' \ + --loglevel-only=INFO +---- +-- + + +[[prebuilt-ust-helpers]] +=== Load a prebuilt user space tracing helper + +The LTTng-UST package provides a few helpers in the form of preloadable +shared objects which automatically instrument system functions and +calls. + +The helper shared objects are normally found in dir:{/usr/lib}. If you +built LTTng-UST <>, they're probably +located in dir:{/usr/local/lib}. + +The installed user space tracing helpers in LTTng-UST{nbsp}{revision} +are: + +path:{liblttng-ust-libc-wrapper.so}:: +path:{liblttng-ust-pthread-wrapper.so}:: + <>. + +path:{liblttng-ust-cyg-profile.so}:: +path:{liblttng-ust-cyg-profile-fast.so}:: + <>. + +path:{liblttng-ust-dl.so}:: + <>. + +To use a user space tracing helper with any user application: + +* Preload the helper shared object when you start the application: ++ +-- +[role="term"] +---- +$ LD_PRELOAD=liblttng-ust-libc-wrapper.so my-app +---- +-- ++ +You can preload more than one helper: ++ +-- +[role="term"] +---- +$ LD_PRELOAD=liblttng-ust-libc-wrapper.so:liblttng-ust-dl.so my-app +---- +-- + + +[role="since-2.3"] +[[liblttng-ust-libc-pthread-wrapper]] +==== Instrument C standard library memory and POSIX threads functions + +The path:{liblttng-ust-libc-wrapper.so} and +path:{liblttng-ust-pthread-wrapper.so} helpers +add instrumentation to some C standard library and POSIX +threads functions. + +[role="growable"] +.Functions instrumented by preloading path:{liblttng-ust-libc-wrapper.so}. +|==== +|TP provider name |TP name |Instrumented function + +.6+|`lttng_ust_libc` |`malloc` |man:malloc(3) + |`calloc` |man:calloc(3) + |`realloc` |man:realloc(3) + |`free` |man:free(3) + |`memalign` |man:memalign(3) + |`posix_memalign` |man:posix_memalign(3) +|==== + +[role="growable"] +.Functions instrumented by preloading path:{liblttng-ust-pthread-wrapper.so}. +|==== +|TP provider name |TP name |Instrumented function + +.4+|`lttng_ust_pthread` |`pthread_mutex_lock_req` |man:pthread_mutex_lock(3p) (request time) + |`pthread_mutex_lock_acq` |man:pthread_mutex_lock(3p) (acquire time) + |`pthread_mutex_trylock` |man:pthread_mutex_trylock(3p) + |`pthread_mutex_unlock` |man:pthread_mutex_unlock(3p) +|==== + +When you preload the shared object, it replaces the functions listed +in the previous tables by wrappers which contain tracepoints and call +the replaced functions. + + +[[liblttng-ust-cyg-profile]] +==== Instrument function entry and exit + +The path:{liblttng-ust-cyg-profile*.so} helpers can add instrumentation +to the entry and exit points of functions. + +man:gcc(1) and man:clang(1) have an option named +https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html[`-finstrument-functions`] +which generates instrumentation calls for entry and exit to functions. +The LTTng-UST function tracing helpers, +path:{liblttng-ust-cyg-profile.so} and +path:{liblttng-ust-cyg-profile-fast.so}, take advantage of this feature +to add tracepoints to the two generated functions (which contain +`cyg_profile` in their names, hence the name of the helper). + +To use the LTTng-UST function tracing helper, the source files to +instrument must be built using the `-finstrument-functions` compiler +flag. + +There are two versions of the LTTng-UST function tracing helper: + +* **path:{liblttng-ust-cyg-profile-fast.so}** is a lightweight variant + that you should only use when it can be _guaranteed_ that the + complete event stream is recorded without any lost event record. + Any kind of duplicate information is left out. ++ +Assuming no event record is lost, having only the function addresses on +entry is enough to create a call graph, since an event record always +contains the ID of the CPU that generated it. ++ +Use a tool like man:addr2line(1) to convert function addresses back to +source file names and line numbers. + +* **path:{liblttng-ust-cyg-profile.so}** is a more robust variant +which also works in use cases where event records might get discarded or +not recorded from application startup. +In these cases, the trace analyzer needs more information to be +able to reconstruct the program flow. + +See man:lttng-ust-cyg-profile(3) to learn more about the instrumentation +points of this helper. + +All the tracepoints that this helper provides have the log level +`LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_FUNCTION` (see man:lttng-ust(3)). + +TIP: It's sometimes a good idea to limit the number of source files that +you compile with the `-finstrument-functions` option to prevent LTTng +from writing an excessive amount of trace data at run time. When using +man:gcc(1), use the +`-finstrument-functions-exclude-function-list` option to avoid +instrument entries and exits of specific function names. + + +[role="since-2.4"] +[[liblttng-ust-dl]] +==== Instrument the dynamic linker + +The path:{liblttng-ust-dl.so} helper adds instrumentation to the +man:dlopen(3) and man:dlclose(3) function calls. + +See man:lttng-ust-dl(3) to learn more about the instrumentation points +of this helper. + + +[role="since-2.4"] +[[java-application]] +=== Instrument a Java application + +You can instrument any Java application which uses one of the following +logging frameworks: + +* The https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[**`java.util.logging`**] + (JUL) core logging facilities. + +* https://logging.apache.org/log4j/1.2/[**Apache log4j{nbsp}1.2**], since + LTTng{nbsp}2.6. Note that Apache Log4j{nbsp}2 isn't supported. + +[role="img-100"] +.LTTng-UST Java agent imported by a Java application. +image::java-app.png[] + +Note that the methods described below are new in LTTng{nbsp}2.8. +Previous LTTng versions use another technique. + +NOTE: We use https://openjdk.java.net/[OpenJDK]{nbsp}8 for development +and https://ci.lttng.org/[continuous integration], thus this version is +directly supported. However, the LTTng-UST Java agent is also tested +with OpenJDK{nbsp}7. + + +[role="since-2.8"] +[[jul]] +==== Use the LTTng-UST Java agent for `java.util.logging` + +To use the LTTng-UST Java agent in a Java application which uses +`java.util.logging` (JUL): + +. In the source code of the Java application, import the LTTng-UST log + handler package for `java.util.logging`: ++ +-- +[source,java] +---- +import org.lttng.ust.agent.jul.LttngLogHandler; +---- +-- + +. Create an LTTng-UST `java.util.logging` log handler: ++ +-- +[source,java] +---- +Handler lttngUstLogHandler = new LttngLogHandler(); +---- +-- + +. Add this handler to the `java.util.logging` loggers which should emit + LTTng events: ++ +-- +[source,java] +---- +Logger myLogger = Logger.getLogger("some-logger"); + +myLogger.addHandler(lttngUstLogHandler); +---- +-- + +. Use `java.util.logging` log statements and configuration as usual. + The loggers with an attached LTTng-UST log handler can emit + LTTng events. + +. Before exiting the application, remove the LTTng-UST log handler from + the loggers attached to it and call its `close()` method: ++ +-- +[source,java] +---- +myLogger.removeHandler(lttngUstLogHandler); +lttngUstLogHandler.close(); +---- +-- ++ +This isn't strictly necessary, but it's recommended for a clean +disposal of the resources of the handler. + +. Include the common and JUL-specific JAR files of the LTTng-UST Java agent, + path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-jul.jar}, + in the + https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class + path] when you build the Java application. ++ +The JAR files are typically located in dir:{/usr/share/java}. ++ +IMPORTANT: The LTTng-UST Java agent must be +<> for the logging framework your +application uses. + +.Use the LTTng-UST Java agent for `java.util.logging`. +==== +[source,java] +.path:{Test.java} +---- +import java.io.IOException; +import java.util.logging.Handler; +import java.util.logging.Logger; +import org.lttng.ust.agent.jul.LttngLogHandler; + +public class Test +{ + private static final int answer = 42; + + public static void main(String[] argv) throws Exception + { + // Create a logger + Logger logger = Logger.getLogger("jello"); + + // Create an LTTng-UST log handler + Handler lttngUstLogHandler = new LttngLogHandler(); + + // Add the LTTng-UST log handler to our logger + logger.addHandler(lttngUstLogHandler); + + // Log at will! + logger.info("some info"); + logger.warning("some warning"); + Thread.sleep(500); + logger.finer("finer information; the answer is " + answer); + Thread.sleep(123); + logger.severe("error!"); + + // Not mandatory, but cleaner + logger.removeHandler(lttngUstLogHandler); + lttngUstLogHandler.close(); + } +} +---- + +Build this example: + +[role="term"] +---- +$ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java +---- + +<>, +<> matching JUL +events named `jello`, and <>: + +[role="term"] +---- +$ lttng create +$ lttng enable-event --jul jello +$ lttng start +---- + +Run the compiled class: + +[role="term"] +---- +$ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test +---- + +<> and inspect the +recorded events: + +[role="term"] +---- +$ lttng stop +$ lttng view +---- +==== + +In the resulting trace, an <> which a Java +application using `java.util.logging` generated is named +`lttng_jul:event` and has the following fields: + +`msg`:: + Log record message. + +`logger_name`:: + Logger name. + +`class_name`:: + Name of the class in which the log statement was executed. + +`method_name`:: + Name of the method in which the log statement was executed. + +`long_millis`:: + Logging time (timestamp in milliseconds). + +`int_loglevel`:: + Log level integer value. + +`int_threadid`:: + ID of the thread in which the log statement was executed. + +Use the opt:lttng-enable-event(1):--loglevel or +opt:lttng-enable-event(1):--loglevel-only option of the +man:lttng-enable-event(1) command to target a range of +`java.util.logging` log levels or a specific `java.util.logging` log +level. + + +[role="since-2.8"] +[[log4j]] +==== Use the LTTng-UST Java agent for Apache log4j + +To use the LTTng-UST Java agent in a Java application which uses +Apache log4j{nbsp}1.2: + +. In the source code of the Java application, import the LTTng-UST log + appender package for Apache log4j: ++ +-- +[source,java] +---- +import org.lttng.ust.agent.log4j.LttngLogAppender; +---- +-- + +. Create an LTTng-UST log4j log appender: ++ +-- +[source,java] +---- +Appender lttngUstLogAppender = new LttngLogAppender(); +---- +-- + +. Add this appender to the log4j loggers which should emit LTTng events: ++ +-- +[source,java] +---- +Logger myLogger = Logger.getLogger("some-logger"); + +myLogger.addAppender(lttngUstLogAppender); +---- +-- + +. Use Apache log4j log statements and configuration as usual. The + loggers with an attached LTTng-UST log appender can emit LTTng events. + +. Before exiting the application, remove the LTTng-UST log appender from + the loggers attached to it and call its `close()` method: ++ +-- +[source,java] +---- +myLogger.removeAppender(lttngUstLogAppender); +lttngUstLogAppender.close(); +---- +-- ++ +This isn't strictly necessary, but it's recommended for a clean +disposal of the resources of the appender. + +. Include the common and log4j-specific JAR + files of the LTTng-UST Java agent, path:{lttng-ust-agent-common.jar} and + path:{lttng-ust-agent-log4j.jar}, in the + https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class + path] when you build the Java application. ++ +The JAR files are typically located in dir:{/usr/share/java}. ++ +IMPORTANT: The LTTng-UST Java agent must be +<> for the logging framework your +application uses. + +.Use the LTTng-UST Java agent for Apache log4j. +==== +[source,java] +.path:{Test.java} +---- +import org.apache.log4j.Appender; +import org.apache.log4j.Logger; +import org.lttng.ust.agent.log4j.LttngLogAppender; + +public class Test +{ + private static final int answer = 42; + + public static void main(String[] argv) throws Exception + { + // Create a logger + Logger logger = Logger.getLogger("jello"); + + // Create an LTTng-UST log appender + Appender lttngUstLogAppender = new LttngLogAppender(); + + // Add the LTTng-UST log appender to our logger + logger.addAppender(lttngUstLogAppender); + + // Log at will! + logger.info("some info"); + logger.warn("some warning"); + Thread.sleep(500); + logger.debug("debug information; the answer is " + answer); + Thread.sleep(123); + logger.fatal("error!"); + + // Not mandatory, but cleaner + logger.removeAppender(lttngUstLogAppender); + lttngUstLogAppender.close(); + } +} + +---- + +Build this example (`$LOG4JPATH` is the path to the Apache log4j JAR +file): + +[role="term"] +---- +$ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java +---- + +<>, +<> matching +log4j events named `jello`, and <>: + +[role="term"] +---- +$ lttng create +$ lttng enable-event --log4j jello +$ lttng start +---- + +Run the compiled class: + +[role="term"] +---- +$ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH:. Test +---- + +<> and inspect the +recorded events: + +[role="term"] +---- +$ lttng stop +$ lttng view +---- +==== + +In the resulting trace, an <> which a Java +application using log4j generated is named `lttng_log4j:event` and +has the following fields: + +`msg`:: + Log record message. + +`logger_name`:: + Logger name. + +`class_name`:: + Name of the class in which the log statement was executed. + +`method_name`:: + Name of the method in which the log statement was executed. + +`filename`:: + Name of the file in which the executed log statement is located. + +`line_number`:: + Line number at which the log statement was executed. + +`timestamp`:: + Logging timestamp. + +`int_loglevel`:: + Log level integer value. + +`thread_name`:: + Name of the Java thread in which the log statement was executed. + +Use the opt:lttng-enable-event(1):--loglevel or +opt:lttng-enable-event(1):--loglevel-only option of the +man:lttng-enable-event(1) command to target a range of Apache log4j +log levels or a specific log4j log level. + + +[role="since-2.8"] +[[java-application-context]] +==== Provide application-specific context fields in a Java application + +A Java application-specific context field is a piece of state which +the Java application provides. You can <> such +a context field to be recorded, using the +man:lttng-add-context(1) command, to each <> +which the log statements of this application produce. + +For example, a given object might have a current request ID variable. +You can create a context information retriever for this object and +assign a name to this current request ID. You can then, using the +man:lttng-add-context(1) command, add this context field by name so that +LTTng writes it to the event records of a given `java.util.logging` or +log4j <>. + +To provide application-specific context fields in a Java application: + +. In the source code of the Java application, import the LTTng-UST + Java agent context classes and interfaces: ++ +-- +[source,java] +---- +import org.lttng.ust.agent.context.ContextInfoManager; +import org.lttng.ust.agent.context.IContextInfoRetriever; +---- +-- + +. Create a context information retriever class, that is, a class which + implements the `IContextInfoRetriever` interface: ++ +-- +[source,java] +---- +class MyContextInfoRetriever implements IContextInfoRetriever +{ + @Override + public Object retrieveContextInfo(String key) + { + if (key.equals("intCtx")) { + return (short) 17; + } else if (key.equals("strContext")) { + return "context value!"; + } else { + return null; + } + } +} +---- +-- ++ +This `retrieveContextInfo()` method is the only member of the +`IContextInfoRetriever` interface. Its role is to return the current +value of a state by name to create a context field. The names of the +context fields and which state variables they return depends on your +specific scenario. ++ +All primitive types and objects are supported as context fields. +When `retrieveContextInfo()` returns an object, the context field +serializer calls its `toString()` method to add a string field to +event records. The method can also return `null`, which means that +no context field is available for the required name. + +. Register an instance of your context information retriever class to + the context information manager singleton: ++ +-- +[source,java] +---- +IContextInfoRetriever cir = new MyContextInfoRetriever(); +ContextInfoManager cim = ContextInfoManager.getInstance(); +cim.registerContextInfoRetriever("retrieverName", cir); +---- +-- + +. Before exiting the application, remove your context information + retriever from the context information manager singleton: ++ +-- +[source,java] +---- +ContextInfoManager cim = ContextInfoManager.getInstance(); +cim.unregisterContextInfoRetriever("retrieverName"); +---- +-- ++ +This isn't strictly necessary, but it's recommended for a clean +disposal of some resources of the manager. + +. Build your Java application with LTTng-UST Java agent support as + usual, following the procedure for either the + <> or <> framework. + +.Provide application-specific context fields in a Java application. +==== +[source,java] +.path:{Test.java} +---- +import java.util.logging.Handler; +import java.util.logging.Logger; +import org.lttng.ust.agent.jul.LttngLogHandler; +import org.lttng.ust.agent.context.ContextInfoManager; +import org.lttng.ust.agent.context.IContextInfoRetriever; + +public class Test +{ + // Our context information retriever class + private static class MyContextInfoRetriever + implements IContextInfoRetriever + { + @Override + public Object retrieveContextInfo(String key) { + if (key.equals("intCtx")) { + return (short) 17; + } else if (key.equals("strContext")) { + return "context value!"; + } else { + return null; + } + } + } + + private static final int answer = 42; + + public static void main(String args[]) throws Exception + { + // Get the context information manager instance + ContextInfoManager cim = ContextInfoManager.getInstance(); + + // Create and register our context information retriever + IContextInfoRetriever cir = new MyContextInfoRetriever(); + cim.registerContextInfoRetriever("myRetriever", cir); + + // Create a logger + Logger logger = Logger.getLogger("jello"); + + // Create an LTTng-UST log handler + Handler lttngUstLogHandler = new LttngLogHandler(); + + // Add the LTTng-UST log handler to our logger + logger.addHandler(lttngUstLogHandler); + + // Log at will! + logger.info("some info"); + logger.warning("some warning"); + Thread.sleep(500); + logger.finer("finer information; the answer is " + answer); + Thread.sleep(123); + logger.severe("error!"); + + // Not mandatory, but cleaner + logger.removeHandler(lttngUstLogHandler); + lttngUstLogHandler.close(); + cim.unregisterContextInfoRetriever("myRetriever"); + } +} +---- + +Build this example: + +[role="term"] +---- +$ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java +---- + +<> and +<> matching +`java.util.logging` events named `jello`: + +[role="term"] +---- +$ lttng create +$ lttng enable-event --jul jello +---- + +<> to be +recorded to the event records of the `java.util.logging` channel: + +[role="term"] +---- +$ lttng add-context --jul --type='$app.myRetriever:intCtx' +$ lttng add-context --jul --type='$app.myRetriever:strContext' +---- + +<>: + +[role="term"] +---- +$ lttng start +---- + +Run the compiled class: + +[role="term"] +---- +$ java -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar:. Test +---- + +<> and inspect the +recorded events: + +[role="term"] +---- +$ lttng stop +$ lttng view +---- +==== + + +[role="since-2.7"] +[[python-application]] +=== Instrument a Python application + +You can instrument a Python{nbsp}2 or Python{nbsp}3 application which +uses the standard +https://docs.python.org/3/library/logging.html[`logging`] package. + +Each log statement creates an LTTng event once the application module +imports the <> package. + +[role="img-100"] +.A Python application importing the LTTng-UST Python agent. +image::python-app.png[] + +To use the LTTng-UST Python agent: + +. In the source code of the Python application, import the LTTng-UST + Python agent: ++ +-- +[source,python] +---- +import lttngust +---- +-- ++ +The LTTng-UST Python agent automatically adds its logging handler to the +root logger at import time. ++ +A log statement that the application executes before this import doesn't +create an LTTng event. ++ +IMPORTANT: The LTTng-UST Python agent must be +<>. + +. Use log statements and logging configuration as usual. + Since the LTTng-UST Python agent adds a handler to the _root_ + logger, any log statement from any logger can emit an LTTng event. + +.Use the LTTng-UST Python agent. +==== +[source,python] +.path:{test.py} +---- +import lttngust +import logging +import time + + +def example(): + logging.basicConfig() + logger = logging.getLogger('my-logger') + + while True: + logger.debug('debug message') + logger.info('info message') + logger.warn('warn message') + logger.error('error message') + logger.critical('critical message') + time.sleep(1) + + +if __name__ == '__main__': + example() +---- + +NOTE: `logging.basicConfig()`, which adds to the root logger a basic +logging handler which prints to the standard error stream, isn't +strictly required for LTTng-UST tracing to work, but in versions of +Python preceding{nbsp}3.2, you could see a warning message which +indicates that no handler exists for the logger `my-logger`. + +<>, +<> matching +Python logging events named `my-logger`, and +<>: + +[role="term"] +---- +$ lttng create +$ lttng enable-event --python my-logger +$ lttng start +---- + +Run the Python script: + +[role="term"] +---- +$ python test.py +---- + +<> and inspect the +recorded events: + +[role="term"] +---- +$ lttng stop +$ lttng view +---- +==== + +In the resulting trace, an <> which a Python +application generated is named `lttng_python:event` and has the +following fields: + +`asctime`:: + Logging time (string). + +`msg`:: + Log record message. + +`logger_name`:: + Logger name. + +`funcName`:: + Name of the function in which the log statement was executed. + +`lineno`:: + Line number at which the log statement was executed. + +`int_loglevel`:: + Log level integer value. + +`thread`:: + ID of the Python thread in which the log statement was executed. + +`threadName`:: + Name of the Python thread in which the log statement was executed. + +Use the opt:lttng-enable-event(1):--loglevel or +opt:lttng-enable-event(1):--loglevel-only option of the +man:lttng-enable-event(1) command to target a range of Python log levels +or a specific Python log level. + +When an application imports the LTTng-UST Python agent, the agent tries +to register to a <>. Note that you must +<> _before_ you run the Python +application. If a session daemon is found, the agent tries to register +to it during five seconds, after which the application continues +without LTTng tracing support. Override this timeout value with +the env:LTTNG_UST_PYTHON_REGISTER_TIMEOUT environment variable +(milliseconds). + +If the session daemon stops while a Python application with an imported +LTTng-UST Python agent runs, the agent retries to connect and to +register to a session daemon every three seconds. Override this +delay with the env:LTTNG_UST_PYTHON_REGISTER_RETRY_DELAY environment +variable. + + +[role="since-2.5"] +[[proc-lttng-logger-abi]] +=== Use the LTTng logger + +The `lttng-tracer` Linux kernel module, part of +<>, creates the special LTTng logger files +path:{/proc/lttng-logger} and path:{/dev/lttng-logger} (since +LTTng{nbsp}2.11) when it's loaded. Any application can write text data +to any of those files to create one or more LTTng events. + +[role="img-100"] +.An application writes to the LTTng logger file to create one or more LTTng events. +image::lttng-logger.png[] + +The LTTng logger is the quickest method--not the most efficient, +however--to add instrumentation to an application. It's designed +mostly to instrument shell scripts: + +[role="term"] +---- +$ echo "Some message, some $variable" > /dev/lttng-logger +---- + +Any event that the LTTng logger creates is named `lttng_logger` and +belongs to the Linux kernel <>. However, unlike +other instrumentation points in the kernel tracing domain, **any Unix +user** can <> +which matches events named `lttng_logger`, not only the root user or +users in the <>. + +To use the LTTng logger: + +* From any application, write text data to the path:{/dev/lttng-logger} + file. + +The `msg` field of `lttng_logger` event records contains the +recorded message. + +NOTE: The maximum message length of an LTTng logger event is +1024{nbsp}bytes. Writing more than this makes the LTTng logger emit more +than one event to contain the remaining data. + +You shouldn't use the LTTng logger to trace a user application which you +can instrument in a more efficient way, namely: + +* <>. +* <>. +* <>. + +.Use the LTTng logger. +==== +[source,bash] +.path:{test.bash} +---- +echo 'Hello, World!' > /dev/lttng-logger +sleep 2 +df --human-readable --print-type / > /dev/lttng-logger +---- + +<>, +<> matching +Linux kernel tracepoint events named `lttng_logger`, and +<>: + +[role="term"] +---- +$ lttng create +$ lttng enable-event --kernel lttng_logger +$ lttng start +---- + +Run the Bash script: + +[role="term"] +---- +$ bash test.bash +---- + +<> and inspect the recorded +events: + +[role="term"] +---- +$ lttng stop +$ lttng view +---- +==== + + +[[instrumenting-linux-kernel]] +=== Instrument a Linux kernel image or module + +NOTE: This section shows how to _add_ instrumentation points to the +Linux kernel. The subsystems of the kernel are already thoroughly +instrumented at strategic points for LTTng when you +<> the <> +package. + + +[[linux-add-lttng-layer]] +==== [[instrumenting-linux-kernel-itself]][[mainline-trace-event]][[lttng-adaptation-layer]]Add an LTTng layer to an existing ftrace tracepoint + +This section shows how to add an LTTng layer to existing ftrace +instrumentation using the `TRACE_EVENT()` API. + +This section doesn't document the `TRACE_EVENT()` macro. Read the +following articles to learn more about this API: + +* https://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part{nbsp}1)] +* https://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part{nbsp}2)] +* https://lwn.net/Articles/383362/[Using the TRACE_EVENT() macro (Part{nbsp}3)] + +The following procedure assumes that your ftrace tracepoints are +correctly defined in their own header and that they're created in +one source file using the `CREATE_TRACE_POINTS` definition. + +To add an LTTng layer over an existing ftrace tracepoint: + +. Make sure the following kernel configuration options are + enabled: ++ +-- +* `CONFIG_MODULES` +* `CONFIG_KALLSYMS` +* `CONFIG_HIGH_RES_TIMERS` +* `CONFIG_TRACEPOINTS` +-- + +. Build the Linux source tree with your custom ftrace tracepoints. +. Boot the resulting Linux image on your target system. ++ +Confirm that the tracepoints exist by looking for their names in the +dir:{/sys/kernel/debug/tracing/events/subsys} directory, where `subsys` +is your subsystem name. + +. Get a copy of the latest LTTng-modules{nbsp}{revision}: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && + wget https://lttng.org/files/lttng-modules/lttng-modules-latest-2.13.tar.bz2 && + tar -xf lttng-modules-latest-2.13.tar.bz2 && + cd lttng-modules-2.13.* +---- +-- + +. In dir:{instrumentation/events/lttng-module}, relative to the root + of the LTTng-modules source tree, create a header file named + +__subsys__.h+ for your custom subsystem +__subsys__+ and write your + LTTng-modules tracepoint definitions using the LTTng-modules + macros in it. ++ +Start with this template: ++ +-- +[source,c] +.path:{instrumentation/events/lttng-module/my_subsys.h} +---- +#undef TRACE_SYSTEM +#define TRACE_SYSTEM my_subsys + +#if !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) +#define _LTTNG_MY_SUBSYS_H + +#include "../../../probes/lttng-tracepoint-event.h" +#include + +LTTNG_TRACEPOINT_EVENT( + /* + * Format is identical to the TRACE_EVENT() version for the three + * following macro parameters: + */ + my_subsys_my_event, + TP_PROTO(int my_int, const char *my_string), + TP_ARGS(my_int, my_string), + + /* LTTng-modules specific macros */ + TP_FIELDS( + ctf_integer(int, my_int_field, my_int) + ctf_string(my_bar_field, my_bar) + ) +) + +#endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */ + +#include "../../../probes/define_trace.h" +---- +-- ++ +The entries in the `TP_FIELDS()` section are the list of fields for the +LTTng tracepoint. This is similar to the `TP_STRUCT__entry()` part of +the `TRACE_EVENT()` ftrace macro. ++ +See ``<>'' for a +complete description of the available `ctf_*()` macros. + +. Create the kernel module C{nbsp}source file of the LTTng-modules + probe, +probes/lttng-probe-__subsys__.c+, where +__subsys__+ is your + subsystem name: ++ +-- +[source,c] +.path:{probes/lttng-probe-my-subsys.c} +---- +#include +#include "../lttng-tracer.h" + +/* + * Build-time verification of mismatch between mainline + * TRACE_EVENT() arguments and the LTTng-modules adaptation + * layer LTTNG_TRACEPOINT_EVENT() arguments. + */ +#include + +/* Create LTTng tracepoint probes */ +#define LTTNG_PACKAGE_BUILD +#define CREATE_TRACE_POINTS +#define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module + +#include "../instrumentation/events/lttng-module/my_subsys.h" + +MODULE_LICENSE("GPL and additional rights"); +MODULE_AUTHOR("Your name "); +MODULE_DESCRIPTION("LTTng my_subsys probes"); +MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "." + __stringify(LTTNG_MODULES_MINOR_VERSION) "." + __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION) + LTTNG_MODULES_EXTRAVERSION); +---- +-- + +. Edit path:{probes/KBuild} and add your new kernel module object + next to the existing ones: ++ +-- +[source,make] +.path:{probes/KBuild} +---- +# ... + +obj-m += lttng-probe-module.o +obj-m += lttng-probe-power.o + +obj-m += lttng-probe-my-subsys.o + +# ... +---- +-- + +. Build and install the LTTng kernel modules: ++ +-- +[role="term"] +---- +$ make KERNELDIR=/path/to/linux +# make modules_install && depmod -a +---- +-- ++ +Replace `/path/to/linux` with the path to the Linux source tree where +you defined and used tracepoints with the `TRACE_EVENT()` ftrace macro. + +Note that you can also use the +<> +instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and +C{nbsp}code that need to be executed before LTTng records the event +fields. + +The best way to learn how to use the previous LTTng-modules macros is to +inspect the existing LTTng-modules tracepoint definitions in the +dir:{instrumentation/events/lttng-module} header files. Compare them +with the Linux kernel mainline versions in the +dir:{include/trace/events} directory of the Linux source tree. + + +[role="since-2.7"] +[[lttng-tracepoint-event-code]] +===== Use custom C code to access the data for tracepoint fields + +Although we recommended to always use the +<> macro to describe +the arguments and fields of an LTTng-modules tracepoint when possible, +sometimes you need a more complex process to access the data that the +tracer records as event record fields. In other words, you need local +variables and multiple C{nbsp}statements instead of simple +argument-based expressions that you pass to the +<>. + +Use the `LTTNG_TRACEPOINT_EVENT_CODE()` macro instead of +`LTTNG_TRACEPOINT_EVENT()` to declare custom local variables and define +a block of C{nbsp}code to be executed before LTTng records the fields. +The structure of this macro is: + +[source,c] +.`LTTNG_TRACEPOINT_EVENT_CODE()` macro syntax. +---- +LTTNG_TRACEPOINT_EVENT_CODE( + /* + * Format identical to the LTTNG_TRACEPOINT_EVENT() + * version for the following three macro parameters: + */ + my_subsys_my_event, + TP_PROTO(int my_int, const char *my_string), + TP_ARGS(my_int, my_string), + + /* Declarations of custom local variables */ + TP_locvar( + int a = 0; + unsigned long b = 0; + const char *name = "(undefined)"; + struct my_struct *my_struct; + ), + + /* + * Custom code which uses both tracepoint arguments + * (in TP_ARGS()) and local variables (in TP_locvar()). + * + * Local variables are actually members of a structure pointed + * to by the special variable tp_locvar. + */ + TP_code( + if (my_int) { + tp_locvar->a = my_int + 17; + tp_locvar->my_struct = get_my_struct_at(tp_locvar->a); + tp_locvar->b = my_struct_compute_b(tp_locvar->my_struct); + tp_locvar->name = my_struct_get_name(tp_locvar->my_struct); + put_my_struct(tp_locvar->my_struct); + + if (tp_locvar->b) { + tp_locvar->a = 1; + } + } + ), + + /* + * Format identical to the LTTNG_TRACEPOINT_EVENT() + * version for this, except that tp_locvar members can be + * used in the argument expression parameters of + * the ctf_*() macros. + */ + TP_FIELDS( + ctf_integer(unsigned long, my_struct_b, tp_locvar->b) + ctf_integer(int, my_struct_a, tp_locvar->a) + ctf_string(my_string_field, my_string) + ctf_string(my_struct_name, tp_locvar->name) + ) +) +---- + +IMPORTANT: The C code defined in `TP_code()` must not have any side +effects when executed. In particular, the code must not allocate +memory or get resources without deallocating this memory or putting +those resources afterwards. + + +[[instrumenting-linux-kernel-tracing]] +==== Load and unload a custom probe kernel module + +You must load a <> in the kernel before it can emit LTTng events. + +To load the default probe kernel modules and a custom probe kernel +module: + +* Use the opt:lttng-sessiond(8):--extra-kmod-probes option to give extra + probe modules to load when starting a root <>: ++ +-- +.Load the `my_subsys`, `usb`, and the default probe modules. +==== +[role="term"] +---- +# lttng-sessiond --extra-kmod-probes=my_subsys,usb +---- +==== +-- ++ +You only need to pass the subsystem name, not the whole kernel module +name. + +To load _only_ a given custom probe kernel module: + +* Use the opt:lttng-sessiond(8):--kmod-probes option to give the probe + modules to load when starting a root session daemon: ++ +-- +.Load only the `my_subsys` and `usb` probe modules. +==== +[role="term"] +---- +# lttng-sessiond --kmod-probes=my_subsys,usb +---- +==== +-- + +To confirm that a probe module is loaded: + +* Use man:lsmod(8): ++ +-- +[role="term"] +---- +$ lsmod | grep lttng_probe_usb +---- +-- + +To unload the loaded probe modules: + +* Kill the session daemon with `SIGTERM`: ++ +-- +[role="term"] +---- +# pkill lttng-sessiond +---- +-- ++ +You can also use the `--remove` option of man:modprobe(8) if the session +daemon terminates abnormally. + + +[[controlling-tracing]] +== Tracing control + +Once an application or a Linux kernel is <> +for LTTng tracing, you can _trace_ it. + +In the LTTng context, _tracing_ means making sure that LTTng attempts to +execute some action(s) when a CPU executes an instrumentation point. + +This section is divided in topics on how to use the various +<>, in particular the +<>, to _control_ the LTTng +daemons and tracers. + +NOTE: In the following subsections, we refer to an man:lttng(1) command +using its man page name. For example, instead of ``Run the `create` +command to'', we write ``Run the man:lttng-create(1) command to''. + + +[[start-sessiond]] +=== Start a session daemon + +In some situations, you need to run a <> +(man:lttng-sessiond(8)) _before_ you can use the man:lttng(1) +command-line tool. + +You will see the following error when you run a command while no session +daemon is running: + +---- +Error: No session daemon is available +---- + +The only command that automatically runs a session daemon is +man:lttng-create(1), which you use to +<>. While +this could be your most used first operation, sometimes it's not. Some +examples are: + +* <>. +* <>. +* <>. + +All the examples above don't require a recording session to operate on. + +[[tracing-group]] Each Unix user can have its own running session daemon +to use the user space LTTng tracer. The session daemon that the `root` +user starts is the only one allowed to control the LTTng kernel tracer. +Members of the Unix _tracing group_ may connect to and control the root +session daemon, even for user space tracing. See the ``Session daemon +connection'' section of man:lttng(1) to learn more about the Unix +tracing group. + +To start a user session daemon: + +* Run man:lttng-sessiond(8): ++ +-- +[role="term"] +---- +$ lttng-sessiond --daemonize +---- +-- + +To start the root session daemon: + +* Run man:lttng-sessiond(8) as the `root` user: ++ +-- +[role="term"] +---- +# lttng-sessiond --daemonize +---- +-- + +In both cases, remove the opt:lttng-sessiond(8):--daemonize option to +start the session daemon in foreground. + +To stop a session daemon, kill its process (see man:kill(1)) with the +standard `TERM` signal. + +Note that some Linux distributions could manage the LTTng session daemon +as a service. In this case, we suggest that you use the service manager +to start, restart, and stop session daemons. + + +[[creating-destroying-tracing-sessions]] +=== Create and destroy a recording session + +Many LTTng control operations happen in the scope of a +<>, which is the dialogue between the +<> and you for everything related to +<>. + +To create a recording session with a generated name: + +* Use the man:lttng-create(1) command: ++ +-- +[role="term"] +---- +$ lttng create +---- +-- + +The name of the created recording session is `auto` followed by the +creation date. + +To create a recording session with a specific name: + +* Use the optional argument of the man:lttng-create(1) command: ++ +-- +[role="term"] +---- +$ lttng create SESSION +---- +-- ++ +Replace +__SESSION__+ with your specific recording session name. + +In <>, LTTng writes the traces of a recording +session to the +$LTTNG_HOME/lttng-traces/__NAME__-__DATE__-__TIME__+ +directory by default, where +__NAME__+ is the name of the recording +session. Note that the env:LTTNG_HOME environment variable defaults to +`$HOME` if not set. + +To output LTTng traces to a non-default location: + +* Use the opt:lttng-create(1):--output option of the man:lttng-create(1) + command: ++ +-- +[role="term"] +---- +$ lttng create my-session --output=/tmp/some-directory +---- +-- + +You may create as many recording sessions as you wish. + +To list all the existing recording sessions for your Unix user, or for +all users if your Unix user is `root`: + +* Use the man:lttng-list(1) command: ++ +-- +[role="term"] +---- +$ lttng list +---- +-- + +[[cur-tracing-session]]When you create a recording session, the +man:lttng-create(1) command sets it as the _current recording session_. +The following man:lttng(1) commands operate on the current recording +session when you don't specify one: + +[role="list-3-cols"] +* man:lttng-add-context(1) +* man:lttng-clear(1) +* man:lttng-destroy(1) +* man:lttng-disable-channel(1) +* man:lttng-disable-event(1) +* man:lttng-disable-rotation(1) +* man:lttng-enable-channel(1) +* man:lttng-enable-event(1) +* man:lttng-enable-rotation(1) +* man:lttng-load(1) +* man:lttng-regenerate(1) +* man:lttng-rotate(1) +* man:lttng-save(1) +* man:lttng-snapshot(1) +* man:lttng-start(1) +* man:lttng-status(1) +* man:lttng-stop(1) +* man:lttng-track(1) +* man:lttng-untrack(1) +* man:lttng-view(1) + +To change the current recording session: + +* Use the man:lttng-set-session(1) command: ++ +-- +[role="term"] +---- +$ lttng set-session SESSION +---- +-- ++ +Replace +__SESSION__+ with the name of the new current recording session. + +When you're done recording in a given recording session, destroy it. +This operation frees the resources taken by the recording session to +destroy; it doesn't destroy the trace data that LTTng wrote for this +recording session (see ``<>'' for one +way to do this). + +To destroy the current recording session: + +* Use the man:lttng-destroy(1) command: ++ +-- +[role="term"] +---- +$ lttng destroy +---- +-- + +The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command +implicitly (see ``<>''). You need to stop recording to make LTTng flush the +remaining trace data and make the trace readable. + + +[[list-instrumentation-points]] +=== List the available instrumentation points + +The <> can query the running instrumented +user applications and the Linux kernel to get a list of available +instrumentation points: + +* LTTng tracepoints and system calls for the Linux kernel + <>. + +* LTTng tracepoints for the user space tracing domain. + +To list the available instrumentation points: + +. <> there's a running + <> to which your Unix user can + connect. + +. Use the man:lttng-list(1) command with the option of the requested + tracing domain amongst: ++ +-- +opt:lttng-list(1):--kernel:: + Linux kernel tracepoints. ++ +Your Unix user must be `root`, or it must be a member of the Unix +<>. + +opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall:: + Linux kernel system calls. ++ +Your Unix user must be `root`, or it must be a member of the Unix +<>. + +opt:lttng-list(1):--userspace:: + User space tracepoints. + +opt:lttng-list(1):--jul:: + `java.util.logging` loggers. + +opt:lttng-list(1):--log4j:: + Apache log4j loggers. + +opt:lttng-list(1):--python:: + Python loggers. +-- + +.List the available user space tracepoints. +==== +[role="term"] +---- +$ lttng list --userspace +---- +==== + +.List the available Linux kernel system calls. +==== +[role="term"] +---- +$ lttng list --kernel --syscall +---- +==== + + +[[enabling-disabling-events]] +=== Create and enable a recording event rule + +Once you <>, you can create <> with the +man:lttng-enable-event(1) command. + +The man:lttng-enable-event(1) command always attaches an event rule to a +<> on creation. The command can create a _default +channel_, named `channel0`, for you. The man:lttng-enable-event(1) +command reuses the default channel each time you run it for the same +tracing domain and session. + +A recording event rule is always enabled at creation time. + +The following examples show how to combine the command-line arguments of +the man:lttng-enable-event(1) command to create simple to more complex +recording event rules within the <>. + +.Create a recording event rule matching specific Linux kernel tracepoint events (default channel). +==== +[role="term"] +---- +# lttng enable-event --kernel sched_switch +---- +==== + +.Create a recording event rule matching Linux kernel system call events with four specific names (default channel). +==== +[role="term"] +---- +# lttng enable-event --kernel --syscall open,write,read,close +---- +==== + +.Create recording event rules matching tracepoint events which satisfy a filter expressions (default channel). +==== +[role="term"] +---- +# lttng enable-event --kernel sched_switch --filter='prev_comm == "bash"' +---- + +[role="term"] +---- +# lttng enable-event --kernel --all \ + --filter='$ctx.tid == 1988 || $ctx.tid == 1534' +---- + +[role="term"] +---- +$ lttng enable-event --jul my_logger \ + --filter='$app.retriever:cur_msg_id > 3' +---- + +IMPORTANT: Make sure to always single-quote the filter string when you +run man:lttng(1) from a shell. + +See also ``<>'' +which offers another, more efficient filtering mechanism for process ID, +user ID, and group ID attributes. +==== + +.Create a recording event rule matching any user space event from the `my_app` tracepoint provider and with a log level range (default channel). +==== +[role="term"] +---- +$ lttng enable-event --userspace my_app:'*' --loglevel=INFO +---- + +IMPORTANT: Make sure to always single-quote the wildcard character when +you run man:lttng(1) from a shell. +==== + +.Create a recording event rule matching user space events named specifically, but with name exclusions (default channel). +==== +[role="term"] +---- +$ lttng enable-event --userspace my_app:'*' \ + --exclude=my_app:set_user,my_app:handle_sig +---- +==== + +.Create a recording event rule matching any Apache log4j event with a specific log level (default channel). +==== +[role="term"] +---- +$ lttng enable-event --log4j --all --loglevel-only=WARN +---- +==== + +.Create a recording event rule, attached to a specific channel, and matching user space tracepoint events named `my_app:my_tracepoint`. +==== +[role="term"] +---- +$ lttng enable-event --userspace my_app:my_tracepoint \ + --channel=my-channel +---- +==== + +.Create a recording event rule matching user space probe events for the `malloc` function entry in path:{/usr/lib/libc.so.6}: +==== +[role="term"] +---- +# lttng enable-event --kernel \ + --userspace-probe=/usr/lib/libc.so.6:malloc \ + libc_malloc +---- +==== + +.Create a recording event rule matching user space probe events for the `server`/`accept_request` https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[USDT probe] in path:{/usr/bin/serv}: +==== +[role="term"] +---- +# lttng enable-event --kernel \ + --userspace-probe=sdt:serv:server:accept_request \ + server_accept_request +---- +==== + +The recording event rules of a given channel form a whitelist: as soon +as an event rule matches an event, LTTng emits it _once_ and therefore +<> record it. For example, +the following rules both match user space tracepoint events named +`my_app:my_tracepoint` with an `INFO` log level: + +[role="term"] +---- +$ lttng enable-event --userspace my_app:my_tracepoint +$ lttng enable-event --userspace my_app:my_tracepoint \ + --loglevel=INFO +---- + +The second recording event rule is redundant: the first one includes the +second one. + + +[[disable-event-rule]] +=== Disable a recording event rule + +To disable a <> that you +<> previously, use the +man:lttng-disable-event(1) command. + +man:lttng-disable-event(1) can only find recording event rules to +disable by their <> and event name conditions. Therefore, you cannot disable +recording event rules having a specific instrumentation point log level +condition, for example. + +LTTng doesn't emit (and, therefore, won't record) an event which only +_disabled_ recording event rules match. + +.Disable event rules matching Python logging events from the `my-logger` logger (default <>, <>). +==== +[role="term"] +---- +$ lttng disable-event --python my-logger +---- +==== + +.Disable event rules matching all `java.util.logging` events (default channel, recording session `my-session`). +==== +[role="term"] +---- +$ lttng disable-event --jul --session=my-session '*' +---- +==== + +.Disable _all_ the Linux kernel recording event rules (channel `my-chan`, current recording session). +==== +The opt:lttng-disable-event(1):--all-events option isn't, like the +opt:lttng-enable-event(1):--all option of the man:lttng-enable-event(1) +command, an alias for the event name globbing pattern `*`: it disables +_all_ the recording event rules of a given channel. + +[role="term"] +---- +# lttng disable-event --kernel --channel=my-chan --all-events +---- +==== + +NOTE: You can't _remove_ a recording event rule once you create it. + + +[[status]] +=== Get the status of a recording session + +To get the status of the <>, that is, its parameters, its channels, recording event rules, +and their attributes: + +* Use the man:lttng-status(1) command: ++ +-- +[role="term"] +---- +$ lttng status +---- +-- + +To get the status of any recording session: + +* Use the man:lttng-list(1) command with the name of the recording + session: ++ +-- +[role="term"] +---- +$ lttng list SESSION +---- +-- ++ +Replace +__SESSION__+ with the recording session name. + + +[[basic-tracing-session-control]] +=== Start and stop a recording session + +Once you <> and <>, you can start and stop the tracers for this recording +session. + +To start the <>: + +* Use the man:lttng-start(1) command: ++ +-- +[role="term"] +---- +$ lttng start +---- +-- + +LTTng is flexible: you can launch user applications before or after you +start the tracers. An LTTng tracer only <> if a +recording event rule matches it, which means the tracer is active. + +The `start-session` <> action can also start a recording +session. + +To stop the current recording session: + +* Use the man:lttng-stop(1) command: ++ +-- +[role="term"] +---- +$ lttng stop +---- +-- ++ +If there were <> or lost sub-buffers since the last time you ran +man:lttng-start(1), the man:lttng-stop(1) command prints corresponding +warnings. + +IMPORTANT: You need to stop recording to make LTTng flush the remaining +trace data and make the trace readable. Note that the +man:lttng-destroy(1) command (see +``<>'') also runs the man:lttng-stop(1) command implicitly. + +The `stop-session` <> action can also stop a recording +session. + +[role="since-2.12"] +[[clear]] +=== Clear a recording session + +You might need to remove all the current tracing data of one or more +<> between multiple attempts to +reproduce a problem without interrupting the LTTng recording activity. + +To clear the tracing data of the +<>: + +* Use the man:lttng-clear(1) command: ++ +-- +[role="term"] +---- +$ lttng clear +---- +-- + +To clear the tracing data of all the recording sessions: + +* Use the `lttng clear` command with its opt:lttng-clear(1):--all + option: ++ +-- +[role="term"] +---- +$ lttng clear --all +---- +-- + + +[[enabling-disabling-channels]] +=== Create a channel + +Once you <>, you can create a <> with the +man:lttng-enable-channel(1) command. + +Note that LTTng can automatically create a default channel when you +<>. +Therefore, you only need to create a channel when you need non-default +attributes. + +Specify each non-default channel attribute with a command-line +option when you run the man:lttng-enable-channel(1) command. + +You can only create a custom channel in the Linux kernel and user space +<>: the Java/Python logging tracing domains have +their own default channel which LTTng automatically creates when you +<>. + +[IMPORTANT] +==== +As of LTTng{nbsp}{revision}, you may _not_ perform the +following operations with the man:lttng-enable-channel(1) command: + +* Change an attribute of an existing channel. + +* Enable a disabled channel once its recording session has been + <> at least once. + +* Create a channel once its recording session has been active at + least once. + +* Create a user space channel with a given + <> and create a second + user space channel with a different buffering scheme in the same + recording session. +==== + +The following examples show how to combine the command-line options of +the man:lttng-enable-channel(1) command to create simple to more complex +channels within the <>. + +.Create a Linux kernel channel with default attributes. +==== +[role="term"] +---- +# lttng enable-channel --kernel my-channel +---- +==== + +.Create a user space channel with four sub-buffers or 1{nbsp}MiB each, per CPU, per instrumented process. +==== +[role="term"] +---- +$ lttng enable-channel --userspace --num-subbuf=4 --subbuf-size=1M \ + --buffers-pid my-channel +---- +==== + +.[[blocking-timeout-example]]Create a default user space channel with an infinite blocking timeout. +==== +<>, +create the channel, <>, and <>: + +[role="term"] +---- +$ lttng create +$ lttng enable-channel --userspace --blocking-timeout=inf blocking-chan +$ lttng enable-event --userspace --channel=blocking-chan --all +$ lttng start +---- + +Run an application instrumented with LTTng-UST tracepoints and allow it +to block: + +[role="term"] +---- +$ LTTNG_UST_ALLOW_BLOCKING=1 my-app +---- +==== + +.Create a Linux kernel channel which rotates eight trace files of 4{nbsp}MiB each for each stream. +==== +[role="term"] +---- +# lttng enable-channel --kernel --tracefile-count=8 \ + --tracefile-size=4194304 my-channel +---- +==== + +.Create a user space channel in <> (or ``flight recorder'') mode. +==== +[role="term"] +---- +$ lttng enable-channel --userspace --overwrite my-channel +---- +==== + +.<> the same <> attached to two different channels. +==== +[role="term"] +---- +$ lttng enable-event --userspace --channel=my-channel app:tp +$ lttng enable-event --userspace --channel=other-channel app:tp +---- + +When a CPU executes the `app:tp` <>, the two recording event rules above match the created +event, making LTTng emit the event. Because the recording event rules +are not attached to the same channel, LTTng records the event twice. +==== + + +[[disable-channel]] +=== Disable a channel + +To disable a specific channel that you +<> previously, use the +man:lttng-disable-channel(1) command. + +.Disable a specific Linux kernel channel (<>). +==== +[role="term"] +---- +# lttng disable-channel --kernel my-channel +---- +==== + +An enabled channel is an implicit <> +condition. + +NOTE: As of LTTng{nbsp}{revision}, you may _not_ enable a disabled +channel once its recording session has been +<> at least once. + + +[[adding-context]] +=== Add context fields to be recorded to the event records of a channel + +<> fields in trace files provide important +information about previously emitted events, but sometimes some external +context may help you solve a problem faster. + +Examples of context fields are: + +* The **process ID**, **thread ID**, **process name**, and + **process priority** of the thread from which LTTng emits the event. + +* The **hostname** of the system on which LTTng emits the event. + +* The Linux kernel and user call stacks (since LTTng{nbsp}2.11). + +* The current values of many possible **performance counters** using + perf, for example: + +** CPU cycles, stalled cycles, idle cycles, and the other cycle types. +** Cache misses. +** Branch instructions, misses, and loads. +** CPU faults. + +* Any state defined at the application level (supported for the + `java.util.logging` and Apache log4j <>). + +To get the full list of available context fields: + +* Use the opt:lttng-add-context(1):--list option of the + man:lttng-add-context(1) command: ++ +[role="term"] +---- +$ lttng add-context --list +---- + +.Add context fields to be recorded to the event records of all the <> of the <>. +==== +The following command line adds the virtual process identifier and the +per-thread CPU cycles count fields to all the user space channels of the +current recording session. + +[role="term"] +---- +$ lttng add-context --userspace --type=vpid --type=perf:thread:cpu-cycles +---- +==== + +.Add performance counter context fields by raw ID +==== +See man:lttng-add-context(1) for the exact format of the context field +type, which is partly compatible with the format used in +man:perf-record(1). + +[role="term"] +---- +# lttng add-context --userspace --type=perf:thread:raw:r0110:test +# lttng add-context --kernel --type=perf:cpu:raw:r0013c:x86unhalted +---- +==== + +.Add context fields to be recorded to the event records of a specific channel. +==== +The following command line adds the thread identifier and user call +stack context fields to the Linux kernel channel named `my-channel` of +the <>. + +[role="term"] +---- +# lttng add-context --kernel --channel=my-channel \ + --type=tid --type=callstack-user +---- +==== + +.Add an <> to be recorded to the event records of a specific channel. +==== +The following command line makes sure LTTng writes the `cur_msg_id` +context field of the `retriever` context retriever to all the Java +logging <> of the channel named `my-channel`: + +[role="term"] +---- +# lttng add-context --kernel --channel=my-channel \ + --type='$app:retriever:cur_msg_id' +---- + +IMPORTANT: Make sure to always single-quote the `$` character when you +run man:lttng-add-context(1) from a shell. +==== + +NOTE: You can't undo what the man:lttng-add-context(1) command does. + + +[role="since-2.7"] +[[pid-tracking]] +=== Allow specific processes to record events + +It's often useful to only allow processes with specific attributes to +record events. For example, you may wish to record all the system calls +which a given process makes (à la man:strace(1)). + +The man:lttng-track(1) and man:lttng-untrack(1) commands serve this +purpose. Both commands operate on _inclusion sets_ of process +attributes. The available process attribute types are: + +Linux kernel <>:: ++ +* Process ID (PID). + +* Virtual process ID (VPID). ++ +This is the PID as seen by the application. + +* Unix user ID (UID). + +* Virtual Unix user ID (VUID). ++ +This is the UID as seen by the application. + +* Unix group ID (GID). + +* Virtual Unix group ID (VGID). ++ +This is the GID as seen by the application. + +User space tracing domain:: ++ +* VPID +* VUID +* VGID + +A <> has nine process +attribute inclusion sets: six for the Linux kernel <> +and three for the user space tracing domain. + +For a given recording session, a process{nbsp}__P__ is allowed to record +LTTng events for a given <>{nbsp}__D__ if _all_ +the attributes of{nbsp}__P__ are part of the inclusion sets +of{nbsp}__D__. + +Whether a process is allowed or not to record LTTng events is an +implicit condition of all <>. Therefore, if +LTTng creates an event{nbsp}__E__ for a given process, but this process +may not record events, then no recording event rule matches{nbsp}__E__, +which means LTTng won't emit and record{nbsp}__E__. + +When you <>, all its process attribute inclusion sets contain all the +possible values. In other words, all processes are allowed to record +events. + +Add values to an inclusion set with the man:lttng-track(1) command and +remove values with the man:lttng-untrack(1) command. + +[NOTE] +==== +The process attribute values are _numeric_. + +Should a process with a given ID (part of an inclusion set), for +example, exit, and then a new process be given this same ID, then the +latter would also be allowed to record events. + +With the man:lttng-track(1) command, you can add Unix user and group +_names_ to the user and group inclusion sets: the +<> finds the corresponding UID, VUID, +GID, or VGID once on _addition_ to the inclusion set. This means that if +you rename the user or group after you run the man:lttng-track(1) +command, its user/group ID remains part of the inclusion sets. +==== + +.Allow processes to record events based on their virtual process ID (VPID). +==== +For the sake of the following example, assume the target system has +16{nbsp}possible VPIDs. + +When you +<>, +the user space VPID inclusion set contains _all_ the possible VPIDs: + +[role="img-100"] +.The VPID inclusion set is full. +image::track-all.png[] + +When the inclusion set is full and you run the man:lttng-track(1) +command to specify some VPIDs, LTTng: + +. Clears the inclusion set. +. Adds the specific VPIDs to the inclusion set. + +After: + +[role="term"] +---- +$ lttng track --userspace --vpid=3,4,7,10,13 +---- + +the VPID inclusion set is: + +[role="img-100"] +.The VPID inclusion set contains the VPIDs 3, 4, 7, 10, and 13. +image::track-3-4-7-10-13.png[] + +Add more VPIDs to the inclusion set afterwards: + +[role="term"] +---- +$ lttng track --userspace --vpid=1,15,16 +---- + +The result is: + +[role="img-100"] +.VPIDs 1, 15, and 16 are added to the inclusion set. +image::track-1-3-4-7-10-13-15-16.png[] + +The man:lttng-untrack(1) command removes entries from process attribute +inclusion sets. Given the previous example, the following command: + +[role="term"] +---- +$ lttng untrack --userspace --vpid=3,7,10,13 +---- + +leads to this VPID inclusion set: + +[role="img-100"] +.VPIDs 3, 7, 10, and 13 are removed from the inclusion set. +image::track-1-4-15-16.png[] + +You can make the VPID inclusion set full again with the +opt:lttng-track(1):--all option: + +[role="term"] +---- +$ lttng track --userspace --vpid --all +---- + +The result is, again: + +[role="img-100"] +.The VPID inclusion set is full. +image::track-all.png[] +==== + +.Allow specific processes to record events based on their user ID (UID). +==== +A typical use case with process attribute inclusion sets is to start +with an empty inclusion set, then <>, and finally add values manually while the tracers are +active. + +Use the opt:lttng-untrack(1):--all option of the +man:lttng-untrack(1) command to clear the inclusion set after you +<>, for +example (with UIDs): + +[role="term"] +---- +# lttng untrack --kernel --uid --all +---- + +gives: + +[role="img-100"] +.The UID inclusion set is empty. +image::untrack-all.png[] + +If the LTTng tracer runs with this inclusion set configuration, it +records no events within the <> because no processes is allowed to do so. Use the +man:lttng-track(1) command as usual to add specific values to the UID +inclusion set when you need to, for example: + +[role="term"] +---- +# lttng track --kernel --uid=http,11 +---- + +Result: + +[role="img-100"] +.UIDs 6 (`http`) and 11 are part of the UID inclusion set. +image::track-6-11.png[] +==== + + +[role="since-2.5"] +[[saving-loading-tracing-session]] +=== Save and load recording session configurations + +Configuring a <> can be long. Some of +the tasks involved are: + +* <> with + specific attributes. + +* <> to be recorded to the + <> of specific channels. + +* <> with + specific log level, filter, and other conditions. + +If you use LTTng to solve real world problems, chances are you have to +record events using the same recording session setup over and over, +modifying a few variables each time in your instrumented program or +environment. + +To avoid constant recording session reconfiguration, the man:lttng(1) +command-line tool can save and load recording session configurations +to/from XML files. + +To save a given recording session configuration: + +* Use the man:lttng-save(1) command: ++ +-- +[role="term"] +---- +$ lttng save SESSION +---- +-- ++ +Replace +__SESSION__+ with the name of the recording session to save. + +LTTng saves recording session configurations to +dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the +env:LTTNG_HOME environment variable defaults to `$HOME` if not set. See +man:lttng-save(1) to learn more about the recording session configuration +output path. + +LTTng saves all configuration parameters, for example: + +* The recording session name. +* The trace data output path. +* The <>, with their state and all their attributes. +* The context fields you added to channels. +* The <> with their state and conditions. + +To load a recording session: + +* Use the man:lttng-load(1) command: ++ +-- +[role="term"] +---- +$ lttng load SESSION +---- +-- ++ +Replace +__SESSION__+ with the name of the recording session to load. + +When LTTng loads a configuration, it restores your saved recording session +as if you just configured it manually. + +You can also save and load many sessions at a time; see +man:lttng-save(1) and man:lttng-load(1) to learn more. + + +[[sending-trace-data-over-the-network]] +=== Send trace data over the network + +LTTng can send the recorded trace data of a <> to a remote system over the network instead of writing it to +the local file system. + +To send the trace data over the network: + +. On the _remote_ system (which can also be the target system), + start an LTTng <> (man:lttng-relayd(8)): ++ +-- +[role="term"] +---- +$ lttng-relayd +---- +-- + +. On the _target_ system, create a recording session + <> to send trace data over the network: ++ +-- +[role="term"] +---- +$ lttng create my-session --set-url=net://remote-system +---- +-- ++ +Replace +__remote-system__+ with the host name or IP address of the +remote system. See man:lttng-create(1) for the exact URL format. + +. On the target system, use the man:lttng(1) command-line tool as usual. ++ +When recording is <>, the +<> of the target sends the contents of +<> to the remote relay daemon instead of flushing +them to the local file system. The relay daemon writes the received +packets to its local file system. + +See the ``Output directory'' section of man:lttng-relayd(8) to learn +where a relay daemon writes its received trace data. + + +[role="since-2.4"] +[[lttng-live]] +=== View events as LTTng records them (noch:{LTTng} live) + +_LTTng live_ is a network protocol implemented by the +<> (man:lttng-relayd(8)) to allow compatible +trace readers to display or analyze <> as LTTng +records events on the target system while recording is +<>. + +The relay daemon creates a _tee_: it forwards the trace data to both the +local file system and to connected live readers: + +[role="img-90"] +.The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live reader. +image::live.png[] + +To use LTTng live: + +. On the _target system_, create a <> + in _live mode_: ++ +-- +[role="term"] +---- +$ lttng create my-session --live +---- +-- ++ +This operation spawns a local relay daemon. + +. Start the live reader and configure it to connect to the relay daemon. ++ +For example, with man:babeltrace2(1): ++ +-- +[role="term"] +---- +$ babeltrace2 net://localhost/host/HOSTNAME/my-session +---- +-- ++ +Replace +__HOSTNAME__+ with the host name of the target system. + +. Configure the recording session as usual with the man:lttng(1) + command-line tool, and <>. + +List the available live recording sessions with man:babeltrace2(1): + +[role="term"] +---- +$ babeltrace2 net://localhost +---- + +You can start the relay daemon on another system. In this case, you need +to specify the URL of the relay daemon when you +<> with +the opt:lttng-create(1):--set-url option of the man:lttng-create(1) +command. You also need to replace +__localhost__+ in the procedure above +with the host name of the system on which the relay daemon runs. + + +[role="since-2.3"] +[[taking-a-snapshot]] +=== Take a snapshot of the current sub-buffers of a recording session + +The normal behavior of LTTng is to append full sub-buffers to growing +trace data files. This is ideal to keep a full history of the events +which the target system emitted, but it can represent too much data in +some situations. + +For example, you may wish to have LTTng record your application +continuously until some critical situation happens, in which case you +only need the latest few recorded events to perform the desired +analysis, not multi-gigabyte trace files. + +With the man:lttng-snapshot(1) command, you can take a _snapshot_ of the +current <> of a given <>. LTTng can write the snapshot to the local file system or send +it over the network. + +[role="img-100"] +.A snapshot is a copy of the current sub-buffers, which LTTng does _not_ clear after the operation. +image::snapshot.png[] + +The snapshot feature of LTTng is similar to how a +https://en.wikipedia.org/wiki/Flight_recorder[flight recorder] or the +``roll'' mode of an oscilloscope work. + +TIP: If you wish to create unmanaged, self-contained, non-overlapping +trace chunk archives instead of a simple copy of the current +sub-buffers, see the <> +feature (available since LTTng{nbsp}2.11). + +To take a snapshot of the <>: + +. Create a recording session in <>: ++ +-- +[role="term"] +---- +$ lttng create my-session --snapshot +---- +-- ++ +The <> of +<> created in this mode is automatically set to +<>. + +. Configure the recording session as usual with the man:lttng(1) + command-line tool, and <>. + +. **Optional**: When you need to take a snapshot, + <>. ++ +You can take a snapshot when the tracers are active, but if you stop +them first, you're guaranteed that the trace data in the sub-buffers +doesn't change before you actually take the snapshot. + +. Take a snapshot: ++ +-- +[role="term"] +---- +$ lttng snapshot record --name=my-first-snapshot +---- +-- ++ +LTTng writes the current sub-buffers of all the channels of the +<> to +trace files on the local file system. Those trace files have +`my-first-snapshot` in their name. + +There's no difference between the format of a normal trace file and the +format of a snapshot: LTTng trace readers also support LTTng snapshots. + +By default, LTTng writes snapshot files to the path shown by + +[role="term"] +---- +$ lttng snapshot list-output +---- + +You can change this path or decide to send snapshots over the network +using either: + +. An output path or URL that you specify when you + <>. + +. A snapshot output path or URL that you add using the + `add-output` action of the man:lttng-snapshot(1) command. + +. An output path or URL that you provide directly to the + `record` action of the man:lttng-snapshot(1) command. + +Method{nbsp}3 overrides method{nbsp}2, which overrides method 1. When +you specify a URL, a <> must listen on a +remote system (see ``<>''). + +The `snapshot-session` <> action can also take +a recording session snapshot. + + +[role="since-2.11"] +[[session-rotation]] +=== Archive the current trace chunk (rotate a recording session) + +The <> shows how to dump the +current sub-buffers of a recording session to the file system or send them +over the network. When you take a snapshot, LTTng doesn't clear the ring +buffers of the recording session: if you take another snapshot immediately +after, both snapshots could contain overlapping trace data. + +Inspired by https://en.wikipedia.org/wiki/Log_rotation[log rotation], +_recording session rotation_ is a feature which appends the content of the +ring buffers to what's already on the file system or sent over the +network since the creation of the recording session or since the last +rotation, and then clears those ring buffers to avoid trace data +overlaps. + +What LTTng is about to write when performing a recording session rotation +is called the _current trace chunk_. When LTTng writes or sends over the +network this current trace chunk, it becomes a _trace chunk archive_. +Therefore, a recording session rotation operation _archives_ the current +trace chunk. + +[role="img-100"] +.A recording session rotation operation _archives_ the current trace chunk. +image::rotation.png[] + +A trace chunk archive is a self-contained LTTng trace which LTTng +doesn't manage anymore: you can read it, modify it, move it, or remove +it. + +As of LTTng{nbsp}{revision}, there are three methods to perform a +recording session rotation: + +* <>. + +* With a <>. + +* Through the execution of a `rotate-session` <> + action. + +[[immediate-rotation]]To perform an immediate rotation of the +<>: + +. <> in + <> or <> (only those two recording session modes support recording session + rotation): ++ +-- +[role="term"] +---- +# lttng create my-session +---- +-- + +. <> + and <>: ++ +-- +[role="term"] +---- +# lttng enable-event --kernel sched_'*' +# lttng start +---- +-- + +. When needed, immediately rotate the current recording session: ++ +-- +[role="term"] +---- +# lttng rotate +---- +-- ++ +The man:lttng-rotate(1) command prints the path to the created trace +chunk archive. See its manual page to learn about the format of trace +chunk archive directory names. ++ +Perform other immediate rotations while the recording session is active. +It's guaranteed that all the trace chunk archives don't contain +overlapping trace data. You can also perform an immediate rotation once +you have <> the recording session. + +. When you're done recording, + <>: ++ +-- +[role="term"] +---- +# lttng destroy +---- +-- ++ +The recording session destruction operation creates one last trace chunk +archive from the current trace chunk. + +[[rotation-schedule]]A recording session rotation schedule is a planned +rotation which LTTng performs automatically based on one of the +following conditions: + +* A timer with a configured period expires. + +* The total size of the _flushed_ part of the current trace chunk + becomes greater than or equal to a configured value. + +To schedule a rotation of the <>, set a _rotation schedule_: + +. <> in + <> or <> (only those two creation modes support recording session + rotation): ++ +-- +[role="term"] +---- +# lttng create my-session +---- +-- + +. <>: ++ +-- +[role="term"] +---- +# lttng enable-event --kernel sched_'*' +---- +-- + +. Set a recording session rotation schedule: ++ +-- +[role="term"] +---- +# lttng enable-rotation --timer=10s +---- +-- ++ +In this example, we set a rotation schedule so that LTTng performs a +recording session rotation every ten seconds. ++ +See man:lttng-enable-rotation(1) to learn more about other ways to set a +rotation schedule. + +. <>: ++ +-- +[role="term"] +---- +# lttng start +---- +-- ++ +LTTng performs recording session rotations automatically while the +recording session is active thanks to the rotation schedule. + +. When you're done recording, + <>: ++ +-- +[role="term"] +---- +# lttng destroy +---- +-- ++ +The recording session destruction operation creates one last trace chunk +archive from the current trace chunk. + +Unset a recording session rotation schedule with the +man:lttng-disable-rotation(1) command. + + +[role="since-2.13"] +[[add-event-rule-matches-trigger]] +=== Add an ``event rule matches'' trigger to a session daemon + +With the man:lttng-add-trigger(1) command, you can add a +<> to a <>. + +A trigger associates an LTTng tracing condition to one or more actions: +when the condition is satisfied, LTTng attempts to execute the actions. + +A trigger doesn't need any <> to exist: +it belongs to a session daemon. + +As of LTTng{nbsp}{revision}, many condition types are available through +the <> C{nbsp}API, but the +man:lttng-add-trigger(1) command only accepts the ``event rule matches'' +condition. + +An ``event rule matches'' condition is satisfied when its event rule +matches an event. + +Unlike a <>, the event rule of an +``event rule matches'' trigger condition has no implicit conditions, +that is: + +* It has no enabled/disabled state. +* It has no attached <>. +* It doesn't belong to a <>. + +Both the man:lttng-add-trigger(1) and man:lttng-enable-event(1) commands +accept command-line arguments to specify an <>. +That being said, the former is a more recent command and therefore +follows the common event rule specification format (see +man:lttng-event-rule(7)). + +.Start a <> when an event rule matches. +==== +This example shows how to add the following trigger to the root +<>: + +Condition:: + An event rule matches a Linux kernel system call event of which the + name starts with `exec` and `*/ls` matches the `filename` payload + field. ++ +With such an event rule, LTTng emits an event when the cmd:ls program +starts. + +Action:: + <> + named `pitou`. + +To add such a trigger to the root session daemon: + +. **If there's no currently running LTTng root session daemon**, start + one: ++ +[role="term"] +---- +# lttng-sessiond --daemonize +---- + +. <> + named `pitou` and + <> matching + all the system call events: ++ +[role="term"] +---- +# lttng create pitou +# lttng enable-event --kernel --syscall --all +---- + +. Add the trigger to the root session daemon: ++ +[role="term"] +---- +# lttng add-trigger --condition=event-rule-matches \ + --type=syscall --name='exec*' \ + --filter='filename == "*/ls"' \ + --action=start-session pitou +---- ++ +Confirm that the trigger exists with the man:lttng-list-triggers(1) +command: ++ +[role="term"] +---- +# lttng list-triggers +---- + +. Make sure the `pitou` recording session is still inactive (stopped): ++ +[role="term"] +---- +# lttng list pitou +---- ++ +The first line should be something like: ++ +---- +Recording session pitou: [inactive] +---- + +Run the cmd:ls program to fire the LTTng trigger above: + +[role="term"] +---- +$ ls ~ +---- + +At this point, the `pitou` recording session should be active +(started). Confirm this with the man:lttng-list(1) command again: + +[role="term"] +---- +# lttng list pitou +---- + +The first line should now look like: + +---- +Recording session pitou: [active] +---- + +This line confirms that the LTTng trigger you added fired, therefore +starting the `pitou` recording session. +==== + +.[[trigger-event-notif]]Send a notification to a user application when an event rule matches. +==== +This example shows how to add the following trigger to the root +<>: + +Condition:: + An event rule matches a Linux kernel tracepoint event named + `sched_switch` and of which the value of the `next_comm` payload + field is `bash`. ++ +With such an event rule, LTTng emits an event when Linux gives access to +the processor to a process named `bash`. + +Action:: + Send an LTTng notification to a user application. + +Moreover, we'll specify a _capture descriptor_ with the +`event-rule-matches` trigger condition so that the user application can +get the value of a specific `sched_switch` event payload field. + +First, write and build the user application: + +. Create the C{nbsp}source file of the application: ++ +-- +[source,c] +.path:{notif-app.c} +---- +#include +#include +#include +#include +#include +#include + +/* + * Subscribes to notifications, through the notification channel + * `notification_channel`, which match the condition of the trigger + * named `trigger_name`. + * + * Returns `true` on success. + */ +static bool subscribe(struct lttng_notification_channel *notification_channel, + const char *trigger_name) +{ + const struct lttng_condition *condition = NULL; + struct lttng_triggers *triggers = NULL; + unsigned int trigger_count; + unsigned int i; + enum lttng_error_code error_code; + enum lttng_trigger_status trigger_status; + bool ret = false; + + /* Get all LTTng triggers */ + error_code = lttng_list_triggers(&triggers); + assert(error_code == LTTNG_OK); + + /* Get the number of triggers */ + trigger_status = lttng_triggers_get_count(triggers, &trigger_count); + assert(trigger_status == LTTNG_TRIGGER_STATUS_OK); + + /* Find the trigger named `trigger_name` */ + for (i = 0; i < trigger_count; i++) { + const struct lttng_trigger *trigger; + const char *this_trigger_name; + + trigger = lttng_triggers_get_at_index(triggers, i); + trigger_status = lttng_trigger_get_name(trigger, &this_trigger_name); + assert(trigger_status == LTTNG_TRIGGER_STATUS_OK); + + if (strcmp(this_trigger_name, trigger_name) == 0) { + /* Trigger found: subscribe with its condition */ + enum lttng_notification_channel_status notification_channel_status; + + notification_channel_status = lttng_notification_channel_subscribe( + notification_channel, + lttng_trigger_get_const_condition(trigger)); + assert(notification_channel_status == + LTTNG_NOTIFICATION_CHANNEL_STATUS_OK); + ret = true; + break; + } + } + + lttng_triggers_destroy(triggers); + return ret; +} + +/* + * Handles the evaluation `evaluation` of a single notification. + */ +static void handle_evaluation(const struct lttng_evaluation *evaluation) +{ + enum lttng_evaluation_status evaluation_status; + const struct lttng_event_field_value *array_field_value; + const struct lttng_event_field_value *string_field_value; + enum lttng_event_field_value_status event_field_value_status; + const char *string_field_string_value; + + /* Get the value of the first captured (string) field */ + evaluation_status = lttng_evaluation_event_rule_matches_get_captured_values( + evaluation, &array_field_value); + assert(evaluation_status == LTTNG_EVALUATION_STATUS_OK); + event_field_value_status = + lttng_event_field_value_array_get_element_at_index( + array_field_value, 0, &string_field_value); + assert(event_field_value_status == LTTNG_EVENT_FIELD_VALUE_STATUS_OK); + assert(lttng_event_field_value_get_type(string_field_value) == + LTTNG_EVENT_FIELD_VALUE_TYPE_STRING); + event_field_value_status = lttng_event_field_value_string_get_value( + string_field_value, &string_field_string_value); + assert(event_field_value_status == LTTNG_EVENT_FIELD_VALUE_STATUS_OK); + + /* Print the string value of the field */ + puts(string_field_string_value); +} + +int main(int argc, char *argv[]) +{ + int exit_status = EXIT_SUCCESS; + struct lttng_notification_channel *notification_channel; + enum lttng_notification_channel_status notification_channel_status; + const struct lttng_condition *condition; + const char *trigger_name; + bool subscribe_res; + + assert(argc >= 2); + trigger_name = argv[1]; + + /* + * Create a notification channel. + * + * A notification channel connects the user application to the LTTng + * session daemon. + * + * You can use this notification channel to listen to various types + * of notifications. + */ + notification_channel = lttng_notification_channel_create( + lttng_session_daemon_notification_endpoint); + assert(notification_channel); + + /* + * Subscribe to notifications which match the condition of the + * trigger named `trigger_name`. + */ + if (!subscribe(notification_channel, trigger_name)) { + fprintf(stderr, + "Error: Failed to subscribe to notifications (trigger `%s`).\n", + trigger_name); + exit_status = EXIT_FAILURE; + goto end; + } + + /* + * Notification loop. + * + * Put this in a dedicated thread to avoid blocking the main thread. + */ + while (true) { + struct lttng_notification *notification; + enum lttng_notification_channel_status status; + const struct lttng_evaluation *notification_evaluation; + + /* Receive the next notification */ + status = lttng_notification_channel_get_next_notification( + notification_channel, ¬ification); + + switch (status) { + case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK: + break; + case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED: + /* + * The session daemon can drop notifications if a receiving + * application doesn't consume the notifications fast + * enough. + */ + continue; + case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED: + /* + * The session daemon closed the notification channel. + * + * This is typically caused by a session daemon shutting + * down. + */ + goto end; + default: + /* Unhandled conditions or errors */ + exit_status = EXIT_FAILURE; + goto end; + } + + /* + * Handle the condition evaluation. + * + * A notification provides, amongst other things: + * + * * The condition that caused LTTng to send this notification. + * + * * The condition evaluation, which provides more specific + * information on the evaluation of the condition. + */ + handle_evaluation(lttng_notification_get_evaluation(notification)); + + /* Destroy the notification object */ + lttng_notification_destroy(notification); + } + +end: + lttng_notification_channel_destroy(notification_channel); + return exit_status; +} +---- +-- ++ +This application prints the first captured string field value of the +condition evaluation of each LTTng notification it receives. + +. Build the `notif-app` application, + using https://www.freedesktop.org/wiki/Software/pkg-config/[pkg-config] + to provide the right compiler and linker flags: ++ +-- +[role="term"] +---- +$ gcc -o notif-app notif-app.c $(pkg-config --cflags --libs lttng-ctl) +---- +-- + +Now, to add the trigger to the root session daemon: + +[start=3] +. **If there's no currently running LTTng root session daemon**, start + one: ++ +[role="term"] +---- +# lttng-sessiond --daemonize +---- + +. Add the trigger, naming it `sched-switch-notif`, to the root + session daemon: ++ +[role="term"] +---- +# lttng add-trigger --name=sched-switch-notif \ + --condition=event-rule-matches \ + --type=kernel --name=sched_switch \ + --filter='next_comm == "bash"' --capture=prev_comm \ + --action=notify +---- ++ +Confirm that the `sched-switch-notif` trigger exists with the +man:lttng-list-triggers(1) command: ++ +[role="term"] +---- +# lttng list-triggers +---- + +Run the cmd:notif-app application, passing the name of the trigger +of which to watch the notifications: + +[role="term"] +---- +# ./notif-app sched-switch-notif +---- + +Now, in an interactive Bash, type a few keys to fire the +`sched-switch-notif` trigger. Watch the `notif-app` application print +the previous process names. +==== + +[role="since-2.6"] +[[mi]] +=== Use the machine interface + +With any command of the man:lttng(1) command-line tool, set the +opt:lttng(1):--mi option to `xml` (before the command name) to get an +XML machine interface output, for example: + +[role="term"] +---- +$ lttng --mi=xml list my-session +---- + +A schema definition (XSD) is +https://github.com/lttng/lttng-tools/blob/stable-{revision}/src/common/mi-lttng-4.0.xsd[available] +to ease the integration with external tools as much as possible. + + +[role="since-2.8"] +[[metadata-regenerate]] +=== Regenerate the metadata of an LTTng trace + +An LTTng trace, which is a https://diamon.org/ctf[CTF] trace, has both +data stream files and a metadata stream file. This metadata file +contains, amongst other things, information about the offset of the +clock sources which LTTng uses to assign timestamps to <> when recording. + +If, once a <> is +<>, a major +https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction +happens, the clock offset of the trace also needs to be updated. Use +the `metadata` item of the man:lttng-regenerate(1) command to do so. + +The main use case of this command is to allow a system to boot with +an incorrect wall time and have LTTng trace it before its wall time +is corrected. Once the system is known to be in a state where its +wall time is correct, you can run `lttng regenerate metadata`. + +To regenerate the metadata stream files of the +<>: + +* Use the `metadata` item of the man:lttng-regenerate(1) command: ++ +-- +[role="term"] +---- +$ lttng regenerate metadata +---- +-- + + +[role="since-2.9"] +[[regenerate-statedump]] +=== Regenerate the state dump event records of a recording session + +The LTTng kernel and user space tracers generate state dump +<> when the application starts or when you +<>. + +An analysis can use the state dump event records to set an initial state +before it builds the rest of the state from the subsequent event +records. http://tracecompass.org/[Trace Compass] and +https://github.com/lttng/lttng-analyses[LTTng analyses] are notable +examples of applications which use the state dump of an LTTng trace. + +When you <>, it's possible that the +state dump event records aren't included in the snapshot trace files +because they were recorded to a <> that has been +consumed or <> already. + +Use the `statedump` item of the man:lttng-regenerate(1) command to emit +and record the state dump events again. + +To regenerate the state dump of the <>, provided you created it in <>, before you take a snapshot: + +. Use the `statedump` item of the man:lttng-regenerate(1) command: ++ +-- +[role="term"] +---- +$ lttng regenerate statedump +---- +-- + +. <>: ++ +-- +[role="term"] +---- +$ lttng stop +---- +-- + +. <>: ++ +-- +[role="term"] +---- +$ lttng snapshot record --name=my-snapshot +---- +-- + +Depending on the event throughput, you should run steps{nbsp}1 +and{nbsp}2 as closely as possible. + +[NOTE] +==== +To record the state dump events, you need to +<> which enable +them: + +* The names of LTTng-UST state dump tracepoints start with + `lttng_ust_statedump:`. + +* The names of LTTng-modules state dump tracepoints start with + `lttng_statedump_`. +==== + + +[role="since-2.7"] +[[persistent-memory-file-systems]] +=== Record trace data on persistent memory file systems + +https://en.wikipedia.org/wiki/Non-volatile_random-access_memory[Non-volatile +random-access memory] (NVRAM) is random-access memory that retains its +information when power is turned off (non-volatile). Systems with such +memory can store data structures in RAM and retrieve them after a +reboot, without flushing to typical _storage_. + +Linux supports NVRAM file systems thanks to either +http://pramfs.sourceforge.net/[PRAMFS] or +https://www.kernel.org/doc/Documentation/filesystems/dax.txt[DAX]{nbsp}+{nbsp}http://lkml.iu.edu/hypermail/linux/kernel/1504.1/03463.html[pmem] +(requires Linux{nbsp}4.1+). + +This section doesn't describe how to operate such file systems; we +assume that you have a working persistent memory file system. + +When you <>, you can specify the path of the shared memory holding the +sub-buffers. If you specify a location on an NVRAM file system, then you +can retrieve the latest recorded trace data when the system reboots +after a crash. + +To record trace data on a persistent memory file system and retrieve the +trace data after a system crash: + +. Create a recording session with a <> shared memory + path located on an NVRAM file system: ++ +-- +[role="term"] +---- +$ lttng create my-session --shm-path=/path/to/shm/on/nvram +---- +-- + +. Configure the recording session as usual with the man:lttng(1) + command-line tool, and <>. + +. After a system crash, use the man:lttng-crash(1) command-line tool to + read the trace data recorded on the NVRAM file system: ++ +-- +[role="term"] +---- +$ lttng-crash /path/to/shm/on/nvram +---- +-- + +The binary layout of the ring buffer files isn't exactly the same as the +trace files layout. This is why you need to use man:lttng-crash(1) +instead of some standard LTTng trace reader. + +To convert the ring buffer files to LTTng trace files: + +* Use the opt:lttng-crash(1):--extract option of man:lttng-crash(1): ++ +-- +[role="term"] +---- +$ lttng-crash --extract=/path/to/trace /path/to/shm/on/nvram +---- +-- + + +[role="since-2.10"] +[[notif-trigger-api]] +=== Get notified when the buffer usage of a channel is too high or too low + +With the notification and <> C{nbsp}API of +<>, LTTng can notify your user +application when the buffer usage of one or more <> +becomes too low or too high. + +Use this API and enable or disable <> while +a recording session <> to avoid +<>, for +example. + +.Send a notification to a user application when the buffer usage of an LTTng channel is too high. +==== +In this example, we create and build an application which gets notified +when the buffer usage of a specific LTTng channel is higher than +75{nbsp}%. + +We only print that it's the case in this example, but we could as well +use the `liblttng-ctl` C{nbsp}API to <> when this happens, for example. + +. Create the C{nbsp}source file of the application: ++ +-- +[source,c] +.path:{notif-app.c} +---- +#include +#include +#include +#include + +int main(int argc, char *argv[]) +{ + int exit_status = EXIT_SUCCESS; + struct lttng_notification_channel *notification_channel; + struct lttng_condition *condition; + struct lttng_action *action; + struct lttng_trigger *trigger; + const char *recording_session_name; + const char *channel_name; + + assert(argc >= 3); + recording_session_name = argv[1]; + channel_name = argv[2]; + + /* + * Create a notification channel. + * + * A notification channel connects the user application to the LTTng + * session daemon. + * + * You can use this notification channel to listen to various types + * of notifications. + */ + notification_channel = lttng_notification_channel_create( + lttng_session_daemon_notification_endpoint); + + /* + * Create a "buffer usage becomes greater than" condition. + * + * In this case, the condition is satisfied when the buffer usage + * becomes greater than or equal to 75 %. + * + * We create the condition for a specific recording session name, + * channel name, and for the user space tracing domain. + * + * The following condition types also exist: + * + * * The buffer usage of a channel becomes less than a given value. + * + * * The consumed data size of a recording session becomes greater + * than a given value. + * + * * A recording session rotation becomes ongoing. + * + * * A recording session rotation becomes completed. + * + * * A given event rule matches an event. + */ + condition = lttng_condition_buffer_usage_high_create(); + lttng_condition_buffer_usage_set_threshold_ratio(condition, .75); + lttng_condition_buffer_usage_set_session_name(condition, + recording_session_name); + lttng_condition_buffer_usage_set_channel_name(condition, + channel_name); + lttng_condition_buffer_usage_set_domain_type(condition, + LTTNG_DOMAIN_UST); + + /* + * Create an action (receive a notification) to execute when the + * condition created above is satisfied. + */ + action = lttng_action_notify_create(); + + /* + * Create a trigger. + * + * A trigger associates a condition to an action: LTTng executes + * the action when the condition is satisfied. + */ + trigger = lttng_trigger_create(condition, action); + + /* Register the trigger to the LTTng session daemon. */ + lttng_register_trigger(trigger); + + /* + * Now that we have registered a trigger, LTTng will send a + * notification every time its condition is met through a + * notification channel. + * + * To receive this notification, we must subscribe to notifications + * which match the same condition. + */ + lttng_notification_channel_subscribe(notification_channel, + condition); + + /* + * Notification loop. + * + * Put this in a dedicated thread to avoid blocking the main thread. + */ + for (;;) { + struct lttng_notification *notification; + enum lttng_notification_channel_status status; + const struct lttng_evaluation *notification_evaluation; + const struct lttng_condition *notification_condition; + double buffer_usage; + + /* Receive the next notification. */ + status = lttng_notification_channel_get_next_notification( + notification_channel, ¬ification); + + switch (status) { + case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK: + break; + case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED: + /* + * The session daemon can drop notifications if a monitoring + * application isn't consuming the notifications fast + * enough. + */ + continue; + case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED: + /* + * The session daemon closed the notification channel. + * + * This is typically caused by a session daemon shutting + * down. + */ + goto end; + default: + /* Unhandled conditions or errors. */ + exit_status = EXIT_FAILURE; + goto end; + } + + /* + * A notification provides, amongst other things: + * + * * The condition that caused LTTng to send this notification. + * + * * The condition evaluation, which provides more specific + * information on the evaluation of the condition. + * + * The condition evaluation provides the buffer usage + * value at the moment the condition was satisfied. + */ + notification_condition = lttng_notification_get_condition( + notification); + notification_evaluation = lttng_notification_get_evaluation( + notification); + + /* We're subscribed to only one condition. */ + assert(lttng_condition_get_type(notification_condition) == + LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH); + + /* + * Get the exact sampled buffer usage from the condition + * evaluation. + */ + lttng_evaluation_buffer_usage_get_usage_ratio( + notification_evaluation, &buffer_usage); + + /* + * At this point, instead of printing a message, we could do + * something to reduce the buffer usage of the channel, like + * disable specific events, for example. + */ + printf("Buffer usage is %f %% in recording session \"%s\", " + "user space channel \"%s\".\n", buffer_usage * 100, + recording_session_name, channel_name); + + /* Destroy the notification object. */ + lttng_notification_destroy(notification); + } + +end: + lttng_action_destroy(action); + lttng_condition_destroy(condition); + lttng_trigger_destroy(trigger); + lttng_notification_channel_destroy(notification_channel); + return exit_status; +} +---- +-- + +. Build the `notif-app` application, linking it with `liblttng-ctl`: ++ +-- +[role="term"] +---- +$ gcc -o notif-app notif-app.c $(pkg-config --cflags --libs lttng-ctl) +---- +-- + +. <>, + <> matching + all the user space tracepoint events, and + <>: ++ +-- +[role="term"] +---- +$ lttng create my-session +$ lttng enable-event --userspace --all +$ lttng start +---- +-- ++ +If you create the channel manually with the man:lttng-enable-channel(1) +command, you can set its <> to +control how frequently LTTng samples the current values of the channel +properties to evaluate user conditions. + +. Run the `notif-app` application. ++ +This program accepts the <> and +user space channel names as its two first arguments. The channel +which LTTng automatically creates with the man:lttng-enable-event(1) +command above is named `channel0`: ++ +-- +[role="term"] +---- +$ ./notif-app my-session channel0 +---- +-- + +. In another terminal, run an application with a very high event + throughput so that the 75{nbsp}% buffer usage condition is reached. ++ +In the first terminal, the application should print lines like this: ++ +---- +Buffer usage is 81.45197 % in recording session "my-session", user space +channel "channel0". +---- ++ +If you don't see anything, try to make the threshold of the condition in +path:{notif-app.c} lower (0.1{nbsp}%, for example), and then rebuild the +`notif-app` application (step{nbsp}2) and run it again (step{nbsp}4). +==== + + +[[reference]] +== Reference + +[[lttng-modules-ref]] +=== noch:{LTTng-modules} + + +[role="since-2.9"] +[[lttng-tracepoint-enum]] +==== `LTTNG_TRACEPOINT_ENUM()` usage + +Use the `LTTNG_TRACEPOINT_ENUM()` macro to define an enumeration: + +[source,c] +---- +LTTNG_TRACEPOINT_ENUM(name, TP_ENUM_VALUES(entries)) +---- + +Replace: + +* `name` with the name of the enumeration (C identifier, unique + amongst all the defined enumerations). +* `entries` with a list of enumeration entries. + +The available enumeration entry macros are: + ++ctf_enum_value(__name__, __value__)+:: + Entry named +__name__+ mapped to the integral value +__value__+. + ++ctf_enum_range(__name__, __begin__, __end__)+:: + Entry named +__name__+ mapped to the range of integral values between + +__begin__+ (included) and +__end__+ (included). + ++ctf_enum_auto(__name__)+:: + Entry named +__name__+ mapped to the integral value following the + last mapping value. ++ +The last value of a `ctf_enum_value()` entry is its +__value__+ +parameter. ++ +The last value of a `ctf_enum_range()` entry is its +__end__+ parameter. ++ +If `ctf_enum_auto()` is the first entry in the list, its integral +value is 0. + +Use the `ctf_enum()` <> +to use a defined enumeration as a tracepoint field. + +.Define an enumeration with `LTTNG_TRACEPOINT_ENUM()`. +==== +[source,c] +---- +LTTNG_TRACEPOINT_ENUM( + my_enum, + TP_ENUM_VALUES( + ctf_enum_auto("AUTO: EXPECT 0") + ctf_enum_value("VALUE: 23", 23) + ctf_enum_value("VALUE: 27", 27) + ctf_enum_auto("AUTO: EXPECT 28") + ctf_enum_range("RANGE: 101 TO 303", 101, 303) + ctf_enum_auto("AUTO: EXPECT 304") + ) +) +---- +==== + + +[role="since-2.7"] +[[lttng-modules-tp-fields]] +==== Tracepoint fields macros (for `TP_FIELDS()`) + +[[tp-fast-assign]][[tp-struct-entry]]The available macros to define +tracepoint fields, which must be listed within `TP_FIELDS()` in +`LTTNG_TRACEPOINT_EVENT()`, are: + +[role="func-desc growable",cols="asciidoc,asciidoc"] +.Available macros to define LTTng-modules tracepoint fields +|==== +|Macro |Description and parameters + +| ++ctf_integer(__t__, __n__, __e__)+ + ++ctf_integer_nowrite(__t__, __n__, __e__)+ + ++ctf_user_integer(__t__, __n__, __e__)+ + ++ctf_user_integer_nowrite(__t__, __n__, __e__)+ +| +Standard integer, displayed in base{nbsp}10. + ++__t__+:: + Integer C type (`int`, `long`, `size_t`, ...). + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +| ++ctf_integer_hex(__t__, __n__, __e__)+ + ++ctf_user_integer_hex(__t__, __n__, __e__)+ +| +Standard integer, displayed in base{nbsp}16. + ++__t__+:: + Integer C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +|+ctf_integer_oct(__t__, __n__, __e__)+ +| +Standard integer, displayed in base{nbsp}8. + ++__t__+:: + Integer C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +| ++ctf_integer_network(__t__, __n__, __e__)+ + ++ctf_user_integer_network(__t__, __n__, __e__)+ +| +Integer in network byte order (big-endian), displayed in base{nbsp}10. + ++__t__+:: + Integer C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +| ++ctf_integer_network_hex(__t__, __n__, __e__)+ + ++ctf_user_integer_network_hex(__t__, __n__, __e__)+ +| +Integer in network byte order, displayed in base{nbsp}16. + ++__t__+:: + Integer C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +| ++ctf_enum(__N__, __t__, __n__, __e__)+ + ++ctf_enum_nowrite(__N__, __t__, __n__, __e__)+ + ++ctf_user_enum(__N__, __t__, __n__, __e__)+ + ++ctf_user_enum_nowrite(__N__, __t__, __n__, __e__)+ +| +Enumeration. + ++__N__+:: + Name of a <>. + ++__t__+:: + Integer C type (`int`, `long`, `size_t`, ...). + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +| ++ctf_string(__n__, __e__)+ + ++ctf_string_nowrite(__n__, __e__)+ + ++ctf_user_string(__n__, __e__)+ + ++ctf_user_string_nowrite(__n__, __e__)+ +| +Null-terminated string; undefined behavior if +__e__+ is `NULL`. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + +| ++ctf_array(__t__, __n__, __e__, __s__)+ + ++ctf_array_nowrite(__t__, __n__, __e__, __s__)+ + ++ctf_user_array(__t__, __n__, __e__, __s__)+ + ++ctf_user_array_nowrite(__t__, __n__, __e__, __s__)+ +| +Statically-sized array of integers. + ++__t__+:: + Array element C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__s__+:: + Number of elements. + +| ++ctf_array_bitfield(__t__, __n__, __e__, __s__)+ + ++ctf_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+ + ++ctf_user_array_bitfield(__t__, __n__, __e__, __s__)+ + ++ctf_user_array_bitfield_nowrite(__t__, __n__, __e__, __s__)+ +| +Statically-sized array of bits. + +The type of +__e__+ must be an integer type. +__s__+ is the number +of elements of such type in +__e__+, not the number of bits. + ++__t__+:: + Array element C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__s__+:: + Number of elements. + +| ++ctf_array_text(__t__, __n__, __e__, __s__)+ + ++ctf_array_text_nowrite(__t__, __n__, __e__, __s__)+ + ++ctf_user_array_text(__t__, __n__, __e__, __s__)+ + ++ctf_user_array_text_nowrite(__t__, __n__, __e__, __s__)+ +| +Statically-sized array, printed as text. + +The string doesn't need to be null-terminated. + ++__t__+:: + Array element C type (always `char`). + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__s__+:: + Number of elements. + +| ++ctf_sequence(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence_nowrite(__t__, __n__, __e__, __T__, __E__)+ +| +Dynamically-sized array of integers. + +The type of +__E__+ must be unsigned. + ++__t__+:: + Array element C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__T__+:: + Length expression C type. + ++__E__+:: + Length expression. + +| ++ctf_sequence_hex(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence_hex(__t__, __n__, __e__, __T__, __E__)+ +| +Dynamically-sized array of integers, displayed in base{nbsp}16. + +The type of +__E__+ must be unsigned. + ++__t__+:: + Array element C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__T__+:: + Length expression C type. + ++__E__+:: + Length expression. + +|+ctf_sequence_network(__t__, __n__, __e__, __T__, __E__)+ +| +Dynamically-sized array of integers in network byte order (big-endian), +displayed in base{nbsp}10. + +The type of +__E__+ must be unsigned. + ++__t__+:: + Array element C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__T__+:: + Length expression C type. + ++__E__+:: + Length expression. + +| ++ctf_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence_bitfield(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence_bitfield_nowrite(__t__, __n__, __e__, __T__, __E__)+ +| +Dynamically-sized array of bits. + +The type of +__e__+ must be an integer type. +__s__+ is the number +of elements of such type in +__e__+, not the number of bits. + +The type of +__E__+ must be unsigned. + ++__t__+:: + Array element C type. + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__T__+:: + Length expression C type. + ++__E__+:: + Length expression. + +| ++ctf_sequence_text(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence_text(__t__, __n__, __e__, __T__, __E__)+ + ++ctf_user_sequence_text_nowrite(__t__, __n__, __e__, __T__, __E__)+ +| +Dynamically-sized array, displayed as text. + +The string doesn't need to be null-terminated. + +The type of +__E__+ must be unsigned. + +The behaviour is undefined if +__e__+ is `NULL`. + ++__t__+:: + Sequence element C type (always `char`). + ++__n__+:: + Field name. + ++__e__+:: + Argument expression. + ++__T__+:: + Length expression C type. + ++__E__+:: + Length expression. +|==== + +Use the `_user` versions when the argument expression, `e`, is +a user space address. In the cases of `ctf_user_integer*()` and +`ctf_user_float*()`, `&e` must be a user space address, thus `e` must +be addressable. + +The `_nowrite` versions omit themselves from the trace data, but are +otherwise identical. This means LTTng won't write the `_nowrite` fields +to the recorded trace. Their primary purpose is to make some of the +event context available to the <> without having to commit the data to +<>. + + +[[glossary]] +== Glossary + +Terms related to LTTng and to tracing in general: + +[[def-action]]action:: + The part of a <> which LTTng executes when the + trigger <> is satisfied. + +Babeltrace:: + The https://diamon.org/babeltrace[Babeltrace] project, which includes: ++ +* The + https://babeltrace.org/docs/v2.0/man1/babeltrace2.1/[cmd:babeltrace2] + command-line interface. +* The libbabeltrace2 library which offers a + https://babeltrace.org/docs/v2.0/libbabeltrace2/[C API]. +* https://babeltrace.org/docs/v2.0/python/bt2/[Python{nbsp}3 bindings]. +* Plugins. + +[[def-buffering-scheme]]<>:: + A layout of <> applied to a given channel. + +[[def-channel]]<>:: + An entity which is responsible for a set of + <>. ++ +<> are always attached +to a specific channel. + +clock:: + A source of time for a <>. + +[[def-condition]]condition:: + The part of a <> which must be satisfied for + LTTng to attempt to execute the trigger <>. + +[[def-consumer-daemon]]<>:: + A program which is responsible for consuming the full + <> and write them to a file system or + send them over the network. + +[[def-current-trace-chunk]]current trace chunk:: + A <> which includes the current content + of all the <> of the + <> and the stream files + produced since the latest event amongst: ++ +* The creation of the recording session. +* The last <>, if + any. + +<>:: + The <> in which + the <> _discards_ new <> when there's no <> space left to + store them. + +[[def-event]]event:: + The execution of an <>, like a <> that you manually place + in some source code, or a Linux kprobe. ++ +When an instrumentation point is executed, LTTng creates an event. ++ +When an <> matches the event, +<> executes some action, for example: ++ +* Record its payload to a <> as an + <>. +* Attempt to execute the user-defined actions of a + <> with an + <> condition. + +[[def-event-name]]event name:: + The name of an <>, which is also the name of the + <>. ++ +This is also called the _instrumentation point name_. + +[[def-event-record]]event record:: + A record (binary serialization), in a <>, of the + payload of an <>. ++ +The payload of an event record has zero or more _fields_. + +[[def-event-record-loss-mode]]<>:: + The mechanism by which event records of a given + <> are lost (not recorded) when there's no + <> space left to store them. + +[[def-event-rule]]<>:: + Set of conditions which an <> must satisfy + for LTTng to execute some action. ++ +An event rule is said to _match_ events, like a +https://en.wikipedia.org/wiki/Regular_expression[regular expression] +matches strings. ++ +A <> is a specific type +of event rule of which the action is to <> the event +to a <>. + +[[def-incl-set]]inclusion set:: + In the <> context: a + set of <> of a given type. + +<>:: + The use of <> probes to make a kernel or + <> traceable. + +[[def-instrumentation-point]]instrumentation point:: + A point in the execution path of a kernel or + <> which, when executed, + create an <>. + +instrumentation point name:: + See _<>_. + +`java.util.logging`:: + The + https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities] + of the Java platform. + +log4j:: + A https://logging.apache.org/log4j/1.2/[logging library] for Java + developed by the Apache Software Foundation. + +log level:: + Level of severity of a log statement or user space + <>. + +[[def-lttng]]LTTng:: + The _Linux Trace Toolkit: next generation_ project. + +<>:: + A command-line tool provided by the <> + project which you can use to send and receive control messages to and + from a <>. + +LTTng analyses:: + The https://github.com/lttng/lttng-analyses[LTTng analyses] project, + which is a set of analyzing programs that you can use to obtain a + higher level view of an <> <>. + +cmd:lttng-consumerd:: + The name of the <> program. + +cmd:lttng-crash:: + A utility provided by the <> project + which can convert <> files (usually + <>) to <> files. ++ +See man:lttng-crash(1). + +LTTng Documentation:: + This document. + +<>:: + A communication protocol between the <> and + live readers which makes it possible to show or analyze + <> ``live'', as they're received by + the <>. + +<>:: + The https://github.com/lttng/lttng-modules[LTTng-modules] project, + which contains the Linux kernel modules to make the Linux kernel + <> available for + <> tracing. + +cmd:lttng-relayd:: + The name of the <> program. + +cmd:lttng-sessiond:: + The name of the <> program. + +[[def-lttng-tools]]LTTng-tools:: + The https://github.com/lttng/lttng-tools[LTTng-tools] project, which + contains the various programs and libraries used to + <>. + +[[def-lttng-ust]]<>:: + The https://github.com/lttng/lttng-ust[LTTng-UST] project, which + contains libraries to instrument + <>. + +<>:: + A Java package provided by the <> project to + allow the LTTng instrumentation of `java.util.logging` and Apache + log4j{nbsp}1.2 logging statements. + +<>:: + A Python package provided by the <> project + to allow the <> instrumentation of Python logging + statements. + +<>:: + The <> in which new + <> _overwrite_ older event records + when there's no <> space left to store + them. + +<>:: + A <> in which each instrumented + process has its own <> for a given user + space <>. + +<>:: + A <> in which all the processes + of a Unix user share the same <> for a + given user space <>. + +[[def-proc-attr]]process attribute:: + In the <> context: ++ +* A process ID. +* A virtual process ID. +* A Unix user ID. +* A virtual Unix user ID. +* A Unix group ID. +* A virtual Unix group ID. + +record (_noun_):: + See <>. + +[[def-record]]record (_verb_):: + Serialize the binary payload of an <> to a + <>. + +[[def-recording-event-rule]]<>:: + Specific type of <> of which the action is + to <> the matched event to a + <>. + +[[def-tracing-session]][[def-recording-session]]<>:: + A stateful dialogue between you and a <>. + +[[def-tracing-session-rotation]]<>:: + The action of archiving the + <> of a + <>. + +[[def-relay-daemon]]<>:: + A process which is responsible for receiving the <> + data which a distant <> sends. + +[[def-ring-buffer]]ring buffer:: + A set of <>. + +rotation:: + See _<>_. + +[[def-session-daemon]]<>:: + A process which receives control commands from you and orchestrates + the <> and various <> daemons. + +<>:: + A copy of the current data of all the <> + of a given <>, saved as + <> files. + +[[def-sub-buffer]]sub-buffer:: + One part of an <> <> + which contains <>. + +timestamp:: + The time information attached to an <> when LTTng + creates it. + +[[def-trace]]trace (_noun_):: + A set of: ++ +* One https://diamon.org/ctf/[CTF] metadata stream file. +* One or more CTF data stream files which are the concatenations of one + or more flushed <>. + +[[def-trace-verb]]trace (_verb_):: + From the perspective of a <>: attempt to execute + one or more actions when emitting an <> in an + application or in a system. + +[[def-trace-chunk]]trace chunk:: + A self-contained <> which is part of a + <>. Each + <> produces a + <>. + +[[def-trace-chunk-archive]]trace chunk archive:: + The result of a <>. ++ +<> doesn't manage any trace chunk archive, even if its +containing <> is still active: you +are free to read it, modify it, move it, or remove it. + +Trace Compass:: + The http://tracecompass.org[Trace Compass] project and application. + +[[def-tracepoint]]tracepoint:: + An instrumentation point using the tracepoint mechanism of the Linux + kernel or of <>. + +tracepoint definition:: + The definition of a single <>. + +tracepoint name:: + The name of a <>. + +[[def-tracepoint-provider]]tracepoint provider:: + A set of functions providing <> to an + instrumented <>. ++ +Not to be confused with a <>: many tracepoint providers can exist within a +tracepoint provider package. + +[[def-tracepoint-provider-package]]tracepoint provider package:: + One or more <> compiled + as an https://en.wikipedia.org/wiki/Object_file[object file] or as a + link:https://en.wikipedia.org/wiki/Library_(computing)#Shared_libraries[shared + library]. + +[[def-tracer]]tracer:: + A piece of software which executes some action when it emits + an <>, like <> it to some + buffer. + +<>:: + A type of LTTng <>. + +<>:: + The Unix group which a Unix user can be part of to be allowed to + control the Linux kernel LTTng <>. + +[[def-trigger]]<>:: + A <>-<> pair; when the + condition of a trigger is satisfied, LTTng attempts to execute its + actions. + +[[def-user-application]]user application:: + An application (program or library) running in user space, as opposed + to a Linux kernel module, for example.