From: Philippe Proulx Date: Wed, 1 Apr 2020 15:26:17 +0000 (-0400) Subject: Document LTTng 2.12 X-Git-Url: http://git.liburcu.org/?a=commitdiff_plain;h=96172a593b699513153a113f12c0e93ce249eb01;p=lttng-docs.git Document LTTng 2.12 Changes from 2.11: * "What's new?" section. * Installation section. * Document the `lttng clear` feature. * Document the process attribute tracking feature (update the existing PID tracking section). * Add the "inclusion set", "process attribute", "tracked process attribute", and "untracked process attribute" terms to the glossary. * Convert all Babeltrace 1 references/examples to Babeltrace 2 ones. * Fix English style issues. * Convert a few simple lists to description lists where appropriate. 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new file mode 100644 index 0000000..a612647 --- /dev/null +++ b/2.12/lttng-docs-2.12.txt @@ -0,0 +1,8039 @@ +The LTTng Documentation +======================= +Philippe Proulx +v2.12, 2 April 2020 + + +include::../common/copyright.txt[] + + +include::../common/welcome.txt[] + + +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 + getting 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. ++ +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 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 _Ta Meilleure_, a Northeast IPA +beer brewed by https://lagabiere.com/[Lagabière]. Translating to ``Your +best one'', this beer gives out strong aromas of passion fruit, lemon, +and peaches. Tastewise, expect a lot of fruit, a creamy texture, and a +smooth lingering hop bitterness. + +New features and changes in LTTng{nbsp}{revision}: + +Tracing control:: ++ +* Clear the contents of one or more <> + without having to destroy and reconfigure them + with the new man:lttng-clear(1) command. ++ +This is especially useful to clear a tracing session's tracing data +between attempts to reproduce a problem. ++ +See <>. + +* Before LTTng{nbsp}{revision}, the man:lttng-track(1) and + man:lttng-untrack(1) commands used to add and remove process IDs + (PIDs) to a whitelist so that LTTng would only trace processes with + specific PIDs. ++ +LTTng{nbsp}{revision} adds Unix user IDs (UIDs) and Unix group IDs +(GIDs) to the available <>. +You can specify numeric user/group IDs and user/group names to track, +for example: ++ +[role="term"] +---- +$ lttng track --userspace --vuid=http,999 --vgid=mysql,9 +---- ++ +While you can also track UIDs and GIDs with the +opt:lttng-enable-event(1):--filter option of the `enable-event` command, +this dedicated process attribute tracking approach reduces tracing +overhead and prevents the creation of <> for +the users and groups which LTTng doesn't track. ++ +In the command manual pages, the term ``whitelist'' is renamed to +``inclusion set'' to clarify the concept. + +* The <> can now maintain many files + virtually opened without using as many file descriptors (FD). It does + so by closing and reopening FDs as needed. ++ +This feature is meant as a workaround for users who can't bump the +system limit because of permission restrictions. ++ +The new opt:lttng-relayd(8):--fd-pool-size relay daemon option +sets the maximum number of simultaneously opened file descriptors +(using the soft `RLIMIT_NOFILE` resource limit of the process by +default; see man:getrlimit(2)). + +* By default, the relay daemon writes its traces under a predefined + directory hierarchy, + +$LTTNG_HOME/lttng-traces/__host__/__session__/__domain__+, with: ++ +-- ++__host__+:: + Remote hostname. + ++__session__+:: + <> name. + ++__domain__+:: + <> name (`ust` or `kernel`). +-- ++ +Change this hierarchy to group traces by tracing session name rather +than by hostname +(+$LTTNG_HOME/lttng-traces/__session__/__host__/__domain__+) with the +new relay daemon's opt:lttng-relayd(8):--group-output-by-session option. ++ +This feature is especially useful if you're tracing two or more hosts, +having different hostnames, which share the same tracing session name as +part of their configuration. In this scenario, you can use a descriptive +tracing session name (for example, `connection-hang`) across a fleet of +machines streaming to a single relay daemon. + +* The relay daemon has a new opt:lttng-relayd(8):--working-directory + option to override its working directory. + +Linux kernel tracing:: ++ +* New instrumentation hooks to trace the entry and exit tracepoints of + the network reception code paths of the Linux kernel. ++ +Use the resulting event records to identify the bounds of a network +reception and link the events that occur in the interim (for example, +wake-ups) to a specific network reception instance. You can also +analyze the network stack's latency thanks to those event records. + +* The `irqaction` structure's `thread` field, which specifies the + process to wake up when a threaded interrupt request (IRQ) occurs, is + now part of the `lttng_statedump_interrupt` event record. ++ +Use this information to discover which processes handle the various +IRQs. You can also associate the events occurring in the context of +those processes with their respective IRQ. + +* New `lttng_statedump_cpu_topology` tracepoint to record the active + CPU/NUMA topology. ++ +Use this information to discover which CPUs are SMT siblings or part of +the same socket. As of LTTng{nbsp}{revision}, only the x86 architecture +is supported since all architectures describe their topologies +differently. ++ +The tracepoint's `architecture` field is statically defined and exists +for all architecture implementations. Analysis tools can therefore +anticipate the event record's layout. ++ +Event record example: ++ +[source,yaml] +---- +lttng_statedump_cpu_topology: + architecture: x86 + cpu_id: 0 + vendor: GenuineIntel + family: 6 + model: 142 + model_name: Intel(R) Core(TM) i7-7600U CPU @ 2.80GHz + physical_id: 0 + core_id: 0 + cores: 2 +---- + +* New product UUID metadata environment field, `product_uuid`, + which LTTng copies from the + https://en.wikipedia.org/wiki/Desktop_Management_Interface[Desktop + Management Interface] (DMI). ++ +Use this environment field to uniquely identify a machine (virtual or +physical) in order to correlate traces from multiple virtual machines. + + +[[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 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 software. This +is where tracing comes in handy. + +_Tracing_ is a technique used to understand what goes on in a running +software system. The 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 trace user applications and the operating system 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 is 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's 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. ++ +perf's controlling utility is the cmd:perf command line/text UI tool. + +http://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. + +http://www.sysdig.org/[sysdig]:: + Like SystemTap, uses scripts to analyze Linux kernel events. ++ +You write scripts, or _chisels_ in sysdig's jargon, in Lua and sysdig +executes them while it traces the system or afterwards. sysdig's +interface 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. +SystemTap's primary user interface 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 +http://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 tracing sessions, enable +and disable events on the fly, filter events efficiently with custom +user expressions, start and stop tracing, and much more. LTTng can +record the traces on the file system or send them over the network, and +keep them totally or partially. You can view the traces once tracing +becomes inactive or in real-time. + +<> and +<>! + + +[[installing-lttng]] +== Installation + +**LTTng** is a set of software <> which interact to +<> the Linux kernel and user applications, and +to <> (start and stop +tracing, enable and disable 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 trace the + Linux kernel. +* You only need to install LTTng-UST if you intend to trace user + applications. + +[IMPORTANT] +==== +As of 1 April 2020, LTTng{nbsp}{revision} isn't available +as distribution packages, except for <>. + +<> +to install and use it. +==== + + +[[arch-linux]] +=== Arch Linux + +LTTng-UST{nbsp}{revision} is available in Arch Linux's _community_ +repository, while LTTng-tools{nbsp}{revision} and +LTTng-modules{nbsp}{revision} are available in the +https://aur.archlinux.org/[AUR]. + +To install LTTng{nbsp}{revision} on Arch Linux, using +https://github.com/Jguer/yay[yay] for the AUR packages: + +. Install the main LTTng{nbsp}{revision} packages: ++ +-- +[role="term"] +---- +# pacman -Sy lttng-ust +$ yay -Sy lttng-tools +$ yay -Sy lttng-modules +---- +-- + +. **If you need to instrument and trace <>**, install the LTTng-UST Python agent: ++ +-- +[role="term"] +---- +# pacman -Sy python-lttngust +# pacman -Sy python2-lttngust +---- +-- + + +[[building-from-source]] +=== Build from source + +To build and install LTTng{nbsp}{revision} from source: + +. Using your distribution's package manager, or from source, install + the following dependencies of LTTng-tools and LTTng-UST: ++ +-- +* https://sourceforge.net/projects/libuuid/[libuuid] +* http://directory.fsf.org/wiki/Popt[popt] +* http://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 $(mktemp -d) && +wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.12.tar.bz2 && +tar -xf lttng-modules-latest-2.12.tar.bz2 && +cd lttng-modules-2.12.* && +make && +sudo make modules_install && +sudo depmod -a +---- +-- + +. Download, build, and install the latest LTTng-UST{nbsp}{revision}: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && +wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.12.tar.bz2 && +tar -xf lttng-ust-latest-2.12.tar.bz2 && +cd lttng-ust-2.12.* && +./configure && +make && +sudo make install && +sudo ldconfig +---- +-- ++ +Add `--disable-numa` to `./configure` if you don't have +https://github.com/numactl/numactl[numactl]. ++ +-- +[IMPORTANT] +.Java and Python application tracing +==== +If you need to instrument and trace <>, pass the `--enable-java-agent-jul`, +`--enable-java-agent-log4j`, or `--enable-java-agent-all` options to the +`configure` script, depending on which Java logging framework you use. + +If you need to instrument and trace <>, pass the `--enable-python-agent` option to the +`configure` script. You can set the `PYTHON` environment variable to the +path to the Python interpreter for which to install the LTTng-UST Python +agent package. +==== +-- ++ +-- +[NOTE] +==== +By default, LTTng-UST libraries are installed to +dir:{/usr/local/lib}, which is the de facto directory in which to +keep self-compiled and third-party libraries. + +When <>: + +* Append `/usr/local/lib` to the env:LD_LIBRARY_PATH environment + variable. +* Pass the `-L/usr/local/lib` and `-Wl,-rpath,/usr/local/lib` options to + man:gcc(1), man:g++(1), or man:clang(1). +==== +-- + +. Download, build, and install the latest LTTng-tools{nbsp}{revision}: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && +wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.12.tar.bz2 && +tar -xf lttng-tools-latest-2.12.tar.bz2 && +cd lttng-tools-2.12.* && +./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 in a specific directory. This can be useful to test +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]] +=== Trace the Linux kernel + +The following command lines start with the `#` prompt because you need +root privileges to trace the Linux kernel. You can also trace the kernel +as a regular user if your Unix user is a member of the +<>. + +. Create a <> which writes its 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 the desired instrumentation + point 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 an event rule which matches _all_ the Linux kernel +tracepoints with the opt:lttng-enable-event(1):--all option +(this will generate a lot of data when tracing): ++ +-- +[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 + tracing session: ++ +-- +[role="term"] +---- +# lttng destroy +---- +-- ++ +The man:lttng-destroy(1) command doesn't destroy the trace data; it +only destroys the state of the tracing session. ++ +The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command +implicitly (see <>). You need to stop tracing 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 $(whoami) /tmp/my-kernel-trace +---- +-- + +See <> to view the recorded events. + + +[[tracing-your-own-user-application]] +=== Trace a user application + +This section steps you through a simple example to trace a +_Hello world_ program written in C. + +To create the traceable user application: + +. Create the tracepoint provider header file, which defines the + tracepoints and the events they can generate: ++ +-- +[source,c] +.path:{hello-tp.h} +---- +#undef TRACEPOINT_PROVIDER +#define TRACEPOINT_PROVIDER hello_world + +#undef TRACEPOINT_INCLUDE +#define TRACEPOINT_INCLUDE "./hello-tp.h" + +#if !defined(_HELLO_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ) +#define _HELLO_TP_H + +#include + +TRACEPOINT_EVENT( + hello_world, + my_first_tracepoint, + TP_ARGS( + int, my_integer_arg, + char*, my_string_arg + ), + TP_FIELDS( + ctf_string(my_string_field, my_string_arg) + ctf_integer(int, my_integer_field, my_integer_arg) + ) +) + +#endif /* _HELLO_TP_H */ + +#include +---- +-- + +. Create the tracepoint provider package source file: ++ +-- +[source,c] +.path:{hello-tp.c} +---- +#define TRACEPOINT_CREATE_PROBES +#define TRACEPOINT_DEFINE + +#include "hello-tp.h" +---- +-- + +. Build the tracepoint provider package: ++ +-- +[role="term"] +---- +$ 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[]) +{ + int x; + + puts("Hello, World!\nPress Enter to continue..."); + + /* + * The following getchar() call is only placed here for the purpose + * of this demonstration, to pause the application in order for + * you to have time to list its tracepoints. It's not needed + * otherwise. + */ + getchar(); + + /* + * A 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 + * NOT strings: they are in fact parts of variables that the + * macros in hello-tp.h create. + */ + tracepoint(hello_world, my_first_tracepoint, 23, "hi there!"); + + for (x = 0; x < argc; ++x) { + tracepoint(hello_world, my_first_tracepoint, x, argv[x]); + } + + puts("Quitting now!"); + tracepoint(hello_world, my_first_tracepoint, x * x, "x^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"] +.User space tracing tutorial's build steps. +image::ust-flow.png[] + +To trace 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 that a session daemon might already be running, for example as +a service that the distribution's service manager 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 an <> which matches the + `hello_world:my_first_tracepoint` event name: ++ +-- +[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 `tracepoint()` instrumentation points and exits. +. <> the current + tracing session: ++ +-- +[role="term"] +---- +$ lttng destroy +---- +-- ++ +The man:lttng-destroy(1) command doesn't destroy the trace data; it +only destroys the state of the tracing session. ++ +The man:lttng-destroy(1) command also runs the man:lttng-stop(1) command +implicitly (see <>). You need to stop tracing to make LTTng flush the remaining +trace data and make the trace readable. + +By default, LTTng saves the traces in ++$LTTNG_HOME/lttng-traces/__name__-__date__-__time__+, +where +__name__+ is the tracing session name. The +env:LTTNG_HOME environment variable defaults to `$HOME` if not set. + +See <> to view the recorded events. + + +[[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 + (https://babeltrace.org/docs/v2.0/man1/babeltrace2.1/[cmd:babeltrace2]), + a https://babeltrace.org/docs/v2.0/libbabeltrace2/[C 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 +https://babeltrace.org/docs/v2.0/man7/babeltrace2-plugin-ctf.7/[plugin] +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 LTTng's. + +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 saved 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 +https://babeltrace.org/docs/v2.0/man1/babeltrace2.1/[cmd:babeltrace2] +without options: + +[role="term"] +---- +$ babeltrace2 ~/lttng-traces/my-user-space-session* +---- + +cmd:babeltrace2 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 events 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 message's event. + event = msg.event + + # Keep only `sched_switch` events. + if event.cls.name != 'sched_switch': + continue + + # Keep only events which occurred on 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: + # We start here. + last_ts = cur_ts + + # Previous task command's (short) name. + prev_comm = str(event.payload_field['prev_comm']) + + # Initialize an entry in our dictionary if not yet done. + 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 tracing, 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 <>. +Understanding how those objects relate to eachother is key in mastering +the toolkit. + +The core concepts are: + +* <> +* <> +* <> +* <<"event","Instrumentation point, event rule, event, and event record">> + + +[[tracing-session]] +=== Tracing session + +A _tracing session_ is a stateful dialogue between you and +a <>. You can +<> with the `lttng create` command. + +Most of what you do when you control LTTng tracers happens within a +tracing session. In particular, a tracing session: + +* Has its own name. +* Has its own set of trace files. +* Has its own state of activity (started or stopped). +* Has its own <> (local, network streaming, + snapshot, or live). +* Has its own <> to which are associated their own + <>. +* Has its own <> inclusion + sets. + +[role="img-100"] +.A _tracing session_ contains <> that are members of <> and contain <>. +image::concepts.png[] + +Those attributes and objects are completely isolated between different +tracing sessions. + +A tracing session is analogous to a cash machine session: +the operations you do on the banking system through the cash machine do +not alter the data of other users of the same system. In the case of +the cash machine, a session lasts as long as your bank card is inside. +In the case of LTTng, a tracing session lasts from the `lttng create` +command to the `lttng destroy` command. + +[role="img-100"] +.Each Unix user has its own set of tracing sessions. +image::many-sessions.png[] + + +[[tracing-session-mode]] +==== Tracing session mode + +LTTng can send the generated trace data to different locations. The +_tracing session mode_ dictates where to send it. The following modes +are available in LTTng{nbsp}{revision}: + +Local mode:: + LTTng writes the traces to the file system of the machine it traces + (target system). + +Network streaming mode:: + LTTng sends the traces over the network to a + <> running on a remote system. + +Snapshot mode:: + LTTng doesn't write the traces by default. ++ +Instead, you can request LTTng to <>, +that is, a copy of the tracing session's current sub-buffers, and to +write it to the target's file system or to send it over the network to a +<> running on a remote system. + +Live mode:: + This mode is similar to the network streaming mode, but a live + trace viewer can connect to the distant relay daemon to + <>. + + +[[domain]] +=== Tracing domain + +A _tracing domain_ is a namespace for event sources. 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 when using some commands to avoid +ambiguity. For example, since all the domains support named tracepoints +as event sources (instrumentation points that you manually insert in the +source code), you need to specify a tracing domain when +<> because all the +tracing domains could have tracepoints with the same names. + +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 an LTTng +tracer emits an event, it can record it to one or more +sub-buffers. The attributes of a channel determine what to do when +there's no space left for a new event record because all sub-buffers +are full, where to send a full sub-buffer, and other behaviours. + +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 also owns <>. When an LTTng tracer emits +an event, it records it to the sub-buffers of all +the enabled channels with a satisfied event rule, as long as those +channels are part of active <>. + + +[[channel-buffering-schemes]] +==== Per-user vs. per-process buffering schemes + +A channel has at least one ring buffer _per CPU_. LTTng always +records an event to the ring buffer associated to the CPU on which it +occurs. + +Two _buffering schemes_ are available when you +<> in the +user space <>: + +Per-user buffering:: + Allocate one set of ring buffers--one per CPU--shared by all the + instrumented processes of each Unix user. ++ +-- +[role="img-100"] +.Per-user buffering scheme. +image::per-user-buffering.png[] +-- + +Per-process buffering:: + Allocate one set of ring buffers--one per CPU--for each + instrumented process. ++ +-- +[role="img-100"] +.Per-process buffering scheme. +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 +user, only its own. + +The Linux kernel tracing domain has only one available buffering scheme +which is 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]] +==== Overwrite vs. discard event record loss modes + +When an event occurs, LTTng records it to a specific sub-buffer (yellow +arc in the following animations) of a specific channel's ring buffer. +When there's no space left in a sub-buffer, the tracer marks it as +consumable (red) and another, empty sub-buffer starts receiving the +following event records. A <> +eventually consumes the marked sub-buffer (returns to white). + +[NOTE] +[role="docsvg-channel-subbuf-anim"] +==== +{note-no-anim} +==== + +In an ideal world, sub-buffers are consumed faster than they are filled, +as it is 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, LTTng-modules and LTTng-UST are _non-blocking_ tracers: when +no empty sub-buffer is available, it is acceptable to lose event records +when the alternative would be to cause substantial delays in the +instrumented application's execution. LTTng privileges performance over +integrity; it aims at perturbing the target system as little as possible +in order to make tracing 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 no empty sub-buffer is +available, or because the <> is +reached, the channel's _event record loss mode_ determines what to do. +The available event record loss modes are: + +Discard mode:: + Drop the newest event records until the tracer releases a sub-buffer. ++ +This is the only available mode when you specify a +<>. + +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. + +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. + +In discard mode, LTTng increments a count of lost event records when an +event record is lost and saves this count to the trace. In overwrite +mode, since LTTng{nbsp}2.8, LTTng increments a count of lost sub-buffers +when a sub-buffer is lost and saves this count to the trace. In this +mode, LTTng doesn't write to the trace the exact number of lost event +records in those lost sub-buffers. Trace analyses can use the trace's +saved discarded event record and sub-buffer counts to decide whether or +not to perform the analyses even if trace data is known to be missing. + +There are a few ways to decrease your probability of losing event +records. +<> shows +how to fine-tune the sub-buffer count and size 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 count and size + +When you <>, you can +set its number of sub-buffers and their size. + +Note that there is noticeable CPU overhead introduced when +switching sub-buffers (marking a full one as consumable and switching +to an empty one for the following events to be recorded). Knowing this, +the following list presents a few practical situations along with how +to configure the sub-buffer count and size for them: + +* **High event throughput**: In general, prefer bigger sub-buffers to + lower the risk of losing event records. ++ +Having bigger sub-buffers also ensures a lower +<>. ++ +The number of sub-buffers is only meaningful if you create the channel +in overwrite mode: in this case, if a sub-buffer overwrite happens, 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 events occur less frequently, the sub-buffer switching frequency +should remain low and thus the tracer's overhead 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 big as possible to avoid a high sub-buffer switching +frequency. + +Note that LTTng uses http://diamon.org/ctf/[CTF] as its trace format, +which means event data is very compact. For example, the average +LTTng kernel event record weights about 32{nbsp}bytes. Thus, a +sub-buffer size of 1{nbsp}MiB is considered big. + +The previous situations highlight the major trade-off between a few big +sub-buffers and more, smaller sub-buffers: sub-buffer switching +frequency vs. how much data is 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 a sub-buffer overwrite happens, 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's + overhead as the previous configuration, but if a sub-buffer + overwrite happens, only the eighth of event records so far are + definitely lost. + +In discard mode, the sub-buffers count parameter is pointless: use two +sub-buffers and set their size according to the requirements of your +situation. + + +[[channel-switch-timer]] +==== Switch timer period + +The _switch timer period_ is an important configurable attribute of +a channel to ensure periodic sub-buffer flushing. + +When the _switch timer_ expires, a sub-buffer switch happens. Set +the switch timer period attribute when you +<> to ensure that LTTng +consumes and commits trace data to trace files or to a distant relay +daemon periodically in case of a low event throughput. + +[NOTE] +[role="docsvg-channel-switch-timer"] +==== +{note-no-anim} +==== + +This attribute is also convenient when you use big sub-buffers to cope +with a sporadic high event throughput, even if the throughput is +normally low. + + +[[channel-read-timer]] +==== Read timer period + +By default, the LTTng tracers use a notification mechanism to signal a +full sub-buffer so that a consumer daemon can consume it. When such +notifications must be avoided, for example in real-time applications, +use the channel's _read timer_ instead. When the read timer fires, the +<> checks for full, consumable +sub-buffers. + + +[[tracefile-rotation]] +==== Trace file count and size + +By default, trace files can grow as large as needed. Set the +maximum size of each trace file that a channel writes when you +<>. When the size of +a trace file reaches the channel's fixed maximum size, 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 channel's fixed maximum count, the +oldest trace file is overwritten. This mechanism is called _trace file +rotation_. + +[IMPORTANT] +==== +Even if you don't limit the trace file count, you can't assume that +LTTng doesn't manage any trace file. + +In other words, there is 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 <> the tracing session +is with the <> feature +(available since LTTng{nbsp}2.11). +==== + + +[[event]] +=== Instrumentation point, event rule, event, and event record + +An _event rule_ is a set of conditions which must be **all** satisfied +for LTTng to record an occuring event. + +You set the conditions when you <>. + +You always attach an event rule to a <> when you create +it. + +When an event passes the conditions of an event rule, LTTng records it +in one of the attached channel's sub-buffers. + +The available conditions, as of LTTng{nbsp}{revision}, are: + +* The event rule _is enabled_. +* The instrumentation point's type _is{nbsp}T_. +* The instrumentation point's name (sometimes called _event name_) + _matches{nbsp}N_, but _isn't{nbsp}E_. +* The instrumentation point's log level _is as severe as{nbsp}L_, or + _is exactly{nbsp}L_. +* The fields of the event's payload _satisfy_ a filter + expression{nbsp}__F__. + +As you can see, all the conditions but the dynamic filter are related to +the event rule's status or to the instrumentation point, not to the +occurring events. This is why, without a filter, checking if an event +passes an event rule isn't a dynamic task: when you create or modify an +event rule, all the tracers of its tracing domain enable or disable the +instrumentation points themselves once. This is possible because the +attributes of an instrumentation point (type, name, and log level) are +defined statically. In other words, without a dynamic filter, the tracer +_doesn't evaluate_ the arguments of an instrumentation point unless it +matches an enabled event rule. + +Note that, for LTTng to record an event, the <> to +which a matching event rule is attached must also be enabled, and the +<> owning this channel must be active +(started). + +[role="img-100"] +.Logical path from an instrumentation point to an event record. +image::event-rule.png[] + +.Event, event record, or event rule? +**** +With so many similar terms, it's easy to get confused. + +An **event** is the consequence of the execution of an _instrumentation +point_, like a tracepoint that you manually place in some source code, +or a Linux kernel kprobe. An event is said to _occur_ at a specific +time. Different actions can be taken upon the occurrence of an event, +like record the event's payload to a buffer. + +An **event record** is the representation of an event in a sub-buffer. A +tracer is responsible for capturing the payload of an event, current +context variables, the event's ID, and the event's timestamp. LTTng +can append this sub-buffer to a trace file. + +An **event rule** is a set of conditions which must _all_ be satisfied +for LTTng to record an occuring event. Events still occur without +satisfying event rules, but LTTng doesn't record them. +**** + + +[[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 is 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 incorporates: + +* **LTTng-tools**: Libraries and command-line interface to + control tracing sessions. +** <> (man:lttng-sessiond(8)). +** <> (cmd:lttng-consumerd). +** <> (man:lttng-relayd(8)). +** <> (`liblttng-ctl`). +** <> (man:lttng(1)). +* **LTTng-UST**: Libraries and Java/Python packages to 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` +** User space tracepoint provider source files generator command-line + tool (man:lttng-gen-tp(1)). +** <> 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 trace + the kernel. +** LTTng kernel tracer module. +** Tracing ring buffer kernel modules. +** Probe kernel modules. +** LTTng logger kernel module. + + +[[lttng-cli]] +=== Tracing control command-line interface + +[role="img-100"] +.The tracing control command-line interface. +image::plumbing-lttng-cli.png[] + +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 +---- + +The <> section explores the +available features of LTTng using the 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 API that hides the +underlying protocol's details. `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 +---- + +Some objects are referenced by name (C string), such as tracing +sessions, but most of them require to create a handle first using +`lttng_create_handle()`. + +As of LTTng{nbsp}{revision}, the best available developer documentation for +`liblttng-ctl` is its installed header files. Every function and structure is +thoroughly documented. + + +[[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. It receives commands from a +<>, for example to +enable and disable specific instrumentation points, and writes event +records to ring buffers shared with a +<>. +`liblttng-ust` is part of LTTng-UST. + +Public C header files are installed beside `liblttng-ust` to +instrument any <>. + +<>, which are regular Java and Python +packages, use their own library providing tracepoints 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 properly register +to a session daemon. The initialization phase also enables the +instrumentation points matching 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 Log4{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 trace the log statements. When an +agent initializes, it creates a log handler that attaches to the root +logger. The agent also registers to a <>. +When the application executes a log statement, the root logger passes it +to the agent's log handler. The agent's log handler 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, hence tracing the +log statement. + +The log level condition of an +<> is considered when tracing +a Java or a Python application, and it's compatible with the standard +JUL, 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 the ring buffer; a +<> reads from the ring buffer. + +* The _LTTng kernel tracer_ module. +* The _LTTng logger_ 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 and writing to those +files. ++ +See <>. + +Generally, you don't have to load the LTTng kernel modules manually +(using man:modprobe(8), for example): a root <> 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. + +The LTTng kernel modules are installed in ++/usr/lib/modules/__release__/extra+ by default, where +__release__+ is +the kernel release (see `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 daemon responsible for +managing tracing sessions and for controlling the various components of +LTTng. 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 an <> so that the user space tracing library + can enable or disable tracepoints. Amongst the possible conditions + of an event rule is a filter expression which `liblttng-ust` evalutes + when an event occurs. +** Share <> attributes and ring buffer locations. +-- ++ +The session daemon and the user space tracing library use a Unix +domain socket for their communication. + +* 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 +for their communication. + +* 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 root session daemon loads the appropriate +<> on startup. It also spawns +a <> as soon as you create +an <>. + +The session daemon doesn't send and receive trace data: this is the +role of the <> and +<>. It does, however, generate the +http://diamon.org/ctf/[CTF] metadata stream. + +Each Unix user can have its own session daemon instance. The +tracing 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 thus to trace the Linux +kernel. + +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 daemon which shares +ring buffers with user applications or with the LTTng kernel modules to +collect trace data and send it to some location (on disk 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 an +<>, that is, before you start tracing. When you kill +its owner session daemon, the consumer daemon also exits because it is +the session daemon's child process. 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 is 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 daemon acting as a bridge +between remote session and consumer daemons, local trace files, and a +remote live trace viewer. 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 record trace files locally. + +The relay daemon is also a server to which a +<> can +connect. The live trace viewer sends requests to the relay daemon to +receive trace data as the target system emits events. The +communication protocol is named _LTTng live_; it is 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 events as +the target system emits 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's 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. + +Various ways were developed to instrument a piece of software for LTTng +tracing. The most straightforward one is to manually place +instrumentation points, called _tracepoints_, in the software's source +code. It is also possible to add instrumentation points dynamically in +the Linux kernel <>. + +If you're only interested in tracing the Linux kernel, your +instrumentation needs are probably already covered by LTTng's built-in +<>. You may also wish to 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. They are: + +* <>. +* <>. +* <>. +* <>. +* <>. +* <>. + + +[[c-application]] +=== [[cxx-application]]User space instrumentation for C and $$C++$$ applications + +The 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 +supported by LTTng-UST. Those functions can emit events with +user-defined fields and serialize those events as event records to one +or more LTTng-UST <> sub-buffers. The `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 TRACEPOINT_PROVIDER +#define TRACEPOINT_PROVIDER provider_name + +#undef TRACEPOINT_INCLUDE +#define TRACEPOINT_INCLUDE "./tp.h" + +#if !defined(_TP_H) || defined(TRACEPOINT_HEADER_MULTI_READ) +#define _TP_H + +#include + +/* + * Use TRACEPOINT_EVENT(), TRACEPOINT_EVENT_CLASS(), + * TRACEPOINT_EVENT_INSTANCE(), and 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`. + +TIP: [[lttng-gen-tp]]Use the man:lttng-gen-tp(1) tool to create +this boilerplate for you. When using cmd:lttng-gen-tp, all you need to +write are the <>. + + +[[defining-tracepoints]] +===== Create a tracepoint definition + +A _tracepoint definition_ defines, for a given tracepoint: + +* Its **input arguments**. They are the macro parameters that the + `tracepoint()` macro accepts for this particular tracepoint + in the user application's source code. +* Its **output event fields**. They are the sources of event fields + that form the payload of any event that the execution of the + `tracepoint()` macro emits for this particular tracepoint. + +Create a tracepoint definition by using the +`TRACEPOINT_EVENT()` macro below the `#include ` +line in the +<>. + +The syntax of the `TRACEPOINT_EVENT()` macro is: + +[source,c] +.`TRACEPOINT_EVENT()` macro syntax. +---- +TRACEPOINT_EVENT( + /* Tracepoint provider name */ + provider_name, + + /* Tracepoint name */ + tracepoint_name, + + /* Input arguments */ + TP_ARGS( + arguments + ), + + /* Output event fields */ + TP_FIELDS( + fields + ) +) +---- + +Replace: + +* `provider_name` with your tracepoint provider name. +* `tracepoint_name` with your tracepoint name. +* `arguments` with the <>. +* `fields` with the <> + definitions. + +This tracepoint emits events named `provider_name:tracepoint_name`. + +[IMPORTANT] +.Event name's 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 `TP_ARGS()` macro is: + +[source,c] +.`TP_ARGS()` macro syntax. +---- +TP_ARGS( + type, arg_name +) +---- + +Replace: + +* `type` with the C 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. + +.`TP_ARGS()` usage with three arguments. +==== +[source,c] +---- +TP_ARGS( + int, count, + float, ratio, + const char*, query +) +---- +==== + +The `TP_ARGS()` and `TP_ARGS(void)` forms are valid to create a +tracepoint definition with no input arguments. + +[[tpp-def-output-fields]]The `TP_FIELDS()` macro contains a list of +`ctf_*()` macros. Each `ctf_*()` macro defines one event field. See +man:lttng-ust(3) for a complete description of the available `ctf_*()` +macros. A `ctf_*()` macro specifies the type, size, and byte order of +one event field. + +Each `ctf_*()` macro takes an _argument expression_ parameter. This is a +C expression that the tracer evalutes at the `tracepoint()` macro site +in the application's source code. This expression provides a field's +source of data. The argument expression can include input argument names +listed in the `TP_ARGS()` macro. + +Each `ctf_*()` 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" + +TRACEPOINT_EVENT( + my_provider, + my_tracepoint, + TP_ARGS( + const struct my_custom_structure*, my_custom_structure, + float, ratio, + const char*, query + ), + TP_FIELDS( + ctf_string(query_field, query) + ctf_float(double, ratio_field, ratio) + ctf_integer(int, recv_size, my_custom_structure->recv_size) + ctf_integer(int, send_size, my_custom_structure->send_size) + ) +) +---- + +Refer to this tracepoint definition with the `tracepoint()` macro in +your application's source code like this: + +[source,c] +---- +tracepoint(my_provider, my_tracepoint, + my_structure, some_ratio, the_query); +---- +==== + +NOTE: The LTTng tracer only evaluates tracepoint arguments at run time +if they satisfy an enabled <>. + + +[[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 tracing. + +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] +---- +TRACEPOINT_EVENT( + my_app, + get_account, + TP_ARGS( + int, userid, + size_t, len + ), + TP_FIELDS( + ctf_integer(int, userid, userid) + ctf_integer(size_t, len, len) + ) +) + +TRACEPOINT_EVENT( + my_app, + get_settings, + TP_ARGS( + int, userid, + size_t, len + ), + TP_FIELDS( + ctf_integer(int, userid, userid) + ctf_integer(size_t, len, len) + ) +) + +TRACEPOINT_EVENT( + my_app, + get_transaction, + TP_ARGS( + int, userid, + size_t, len + ), + TP_FIELDS( + ctf_integer(int, userid, userid) + ctf_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 */ +TRACEPOINT_EVENT_CLASS( + /* Tracepoint provider name */ + my_app, + + /* Tracepoint class name */ + my_class, + + /* Input arguments */ + TP_ARGS( + int, userid, + size_t, len + ), + + /* Output event fields */ + TP_FIELDS( + ctf_integer(int, userid, userid) + ctf_integer(size_t, len, len) + ) +) + +/* The tracepoint instances */ +TRACEPOINT_EVENT_INSTANCE( + /* Tracepoint provider name */ + my_app, + + /* Tracepoint class name */ + my_class, + + /* Tracepoint name */ + get_account, + + /* Input arguments */ + TP_ARGS( + int, userid, + size_t, len + ) +) +TRACEPOINT_EVENT_INSTANCE( + my_app, + my_class, + get_settings, + TP_ARGS( + int, userid, + size_t, len + ) +) +TRACEPOINT_EVENT_INSTANCE( + my_app, + my_class, + get_transaction, + TP_ARGS( + int, userid, + size_t, len + ) +) +---- +==== + + +[[assigning-log-levels]] +===== Assign a log level to a tracepoint definition + +Assign a _log level_ to a <> +with the `TRACEPOINT_LOGLEVEL()` macro. + +Assigning different levels of severity to tracepoint definitions can +be useful: when you <>, +you can target tracepoints having a log level 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 `tracepoint()` macro invocation which refers to + this definition has this log level. + +You must use `TRACEPOINT_LOGLEVEL()` _after_ the +<> or +<> macro for a given +tracepoint. + +The syntax of the `TRACEPOINT_LOGLEVEL()` macro is: + +[source,c] +.`TRACEPOINT_LOGLEVEL()` macro syntax. +---- +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 `TRACE_DEBUG_UNIT` log level to a tracepoint definition. +==== +[source,c] +---- +/* Tracepoint definition */ +TRACEPOINT_EVENT( + my_app, + get_transaction, + TP_ARGS( + int, userid, + size_t, len + ), + TP_FIELDS( + ctf_integer(int, userid, userid) + ctf_integer(size_t, len, len) + ) +) + +/* Log level assignment */ +TRACEPOINT_LOGLEVEL(my_app, get_transaction, TRACE_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 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 an application's source code + +Once you <>, +use the `tracepoint()` macro in your application's source code to insert +the tracepoints that this header <>. + +The `tracepoint()` macro takes at least two parameters: the tracepoint +provider name and the tracepoint name. The corresponding tracepoint +definition defines the other parameters. + +.`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" + +TRACEPOINT_EVENT( + my_provider, + my_tracepoint, + TP_ARGS( + int, argc, + const char*, cmd_name + ), + TP_FIELDS( + ctf_string(cmd_name, cmd_name) + ctf_integer(int, number_of_args, argc) + ) +) +---- + +Refer to this tracepoint definition with the `tracepoint()` macro in +your application's source code like this: + +[source,c] +.Application's source file. +---- +#include "tp.h" + +int main(int argc, char* argv[]) +{ + tracepoint(my_provider, my_tracepoint, argc, argv[0]); + + return 0; +} +---- + +Note how the application's source code includes +the tracepoint provider header file containing the tracepoint +definitions to use, path:{tp.h}. +==== + +.`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 + +TRACEPOINT_EVENT( + my_provider, + my_tracepoint, + TP_ARGS( + int, my_int_arg, + char*, my_str_arg, + struct stat*, st + ), + TP_FIELDS( + ctf_integer(int, my_constant_field, 23 + 17) + ctf_integer(int, my_int_arg_field, my_int_arg) + ctf_integer(int, my_int_arg_field2, my_int_arg * my_int_arg) + ctf_integer(int, sum4_field, my_str_arg[0] + my_str_arg[1] + + my_str_arg[2] + my_str_arg[3]) + ctf_string(my_str_arg_field, my_str_arg) + ctf_integer_hex(off_t, size_field, st->st_size) + ctf_float(double, size_dbl_field, (double) st->st_size) + ctf_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 `tracepoint()` macro in +your application's source code like this: + +[source,c] +.Application's source file. +---- +#define TRACEPOINT_DEFINE +#include "tp.h" + +int main(void) +{ + struct stat s; + + stat("/etc/fstab", &s); + tracepoint(my_provider, my_tracepoint, 23, "Hello, World!", &s); + + return 0; +} +---- + +If you look at the event record that LTTng writes when tracing this +program, assuming the file size of path:{/etc/fstab} is 301{nbsp}bytes, +it should look like this: + +.Event record fields +|==== +|Field's name |Field's 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 `tracepoint()` are expensive to +compute--they use the call stack, for example. To avoid this computation +when the tracepoint is disabled, use the `tracepoint_enabled()` and +`do_tracepoint()` macros. + +The syntax of the `tracepoint_enabled()` and `do_tracepoint()` macros +is: + +[source,c] +.`tracepoint_enabled()` and `do_tracepoint()` macros syntax. +---- +tracepoint_enabled(provider_name, tracepoint_name) +do_tracepoint(provider_name, tracepoint_name, ...) +---- + +Replace: + +* `provider_name` with the tracepoint provider name. +* `tracepoint_name` with the tracepoint name. + +`tracepoint_enabled()` returns a non-zero value if the tracepoint named +`tracepoint_name` from the provider named `provider_name` is enabled +**at run time**. + +`do_tracepoint()` is like `tracepoint()`, except that it doesn't check +if the tracepoint is enabled. Using `tracepoint()` with +`tracepoint_enabled()` is dangerous since `tracepoint()` also contains +the `tracepoint_enabled()` check, thus a race condition is +possible in this situation: + +[source,c] +.Possible race condition when using `tracepoint_enabled()` with `tracepoint()`. +---- +if (tracepoint_enabled(my_provider, my_tracepoint)) { + stuff = prepare_stuff(); +} + +tracepoint(my_provider, my_tracepoint, stuff); +---- + +If the tracepoint is enabled after the condition, then `stuff` isn't +prepared: the emitted event will either contain wrong data, or the whole +application could crash (segmentation fault, for example). + +NOTE: Neither `tracepoint_enabled()` nor `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's 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 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 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 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 TRACEPOINT_DEFINE +#define 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 TRACEPOINT_DEFINE +#define 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 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 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 TRACEPOINT_DEFINE +#define 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 TRACEPOINT_DEFINE +#define 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 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 TRACEPOINT_DEFINE +#define 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 TRACEPOINT_DEFINE +#define 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 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 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 $(mktemp -d) && +wget http://lttng.org/files/urcu/userspace-rcu-latest-0.9.tar.bz2 && +tar -xf userspace-rcu-latest-0.9.tar.bz2 && +cd userspace-rcu-0.9.* && +./configure --libdir=/usr/local/lib32 CFLAGS=-m32 && +make && +sudo make install && +sudo ldconfig +---- +-- + +. Using your distribution's package manager, or from source, install + the following 32-bit versions of the following dependencies of + LTTng-tools and LTTng-UST: ++ +-- +* https://sourceforge.net/projects/libuuid/[libuuid] +* http://directory.fsf.org/wiki/Popt[popt] +* http://www.xmlsoft.org/[libxml2] +-- + +. Download, build, and install a 32-bit version of the latest + LTTng-UST{nbsp}{revision}: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && +wget http://lttng.org/files/lttng-ust/lttng-ust-latest-2.12.tar.bz2 && +tar -xf lttng-ust-latest-2.12.tar.bz2 && +cd lttng-ust-2.12.* && +./configure --libdir=/usr/local/lib32 \ + CFLAGS=-m32 CXXFLAGS=-m32 \ + LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' && +make && +sudo make install && +sudo ldconfig +---- +-- ++ +[NOTE] +==== +Depending on your distribution, +32-bit libraries could be installed at a different location than +`/usr/lib32`. For example, Debian is known to install +some 32-bit libraries in `/usr/lib/i386-linux-gnu`. + +In this case, make sure to set `LDFLAGS` to all the +relevant 32-bit library paths, for example: + +[role="term"] +---- +$ 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 $(mktemp -d) && +wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.12.tar.bz2 && +tar -xf lttng-tools-latest-2.12.tar.bz2 && +cd lttng-tools-2.12.* && +./configure --libdir=/usr/local/lib32 CFLAGS=-m32 CXXFLAGS=-m32 \ + LDFLAGS='-L/usr/local/lib32 -L/usr/lib32' \ + --disable-bin-lttng --disable-bin-lttng-crash \ + --disable-bin-lttng-relayd --disable-bin-lttng-sessiond && +make && +cd src/bin/lttng-consumerd && +sudo make install && +sudo ldconfig +---- +-- + +. From your distribution or from source, + <> the 64-bit versions of + LTTng-UST and Userspace RCU. +. Download, build, and install the 64-bit version of the + latest LTTng-tools{nbsp}{revision}: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && +wget http://lttng.org/files/lttng-tools/lttng-tools-latest-2.12.tar.bz2 && +tar -xf lttng-tools-latest-2.12.tar.bz2 && +cd lttng-tools-2.12.* && +./configure --with-consumerd32-libdir=/usr/local/lib32 \ + --with-consumerd32-bin=/usr/local/lib32/lttng/libexec/lttng-consumerd && +make && +sudo make install && +sudo ldconfig +---- +-- + +. Pass the following options to man:gcc(1), man:g++(1), or man:clang(1) + when linking your 32-bit application: ++ +---- +-m32 -L/usr/lib32 -L/usr/local/lib32 \ +-Wl,-rpath,/usr/lib32,-rpath,/usr/local/lib32 +---- ++ +For example, let's rebuild the quick start example in +<> as an +instrumented 32-bit application: ++ +-- +[role="term"] +---- +$ 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 +to trace it: use the command-line man:lttng(1) tool as usual. + + +[role="since-2.5"] +[[tracef]] +==== Use `tracef()` + +man: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 `tracef()` in your application: + +. In the C or C++ source files where you need to use `tracef()`, + include ``: ++ +-- +[source,c] +---- +#include +---- +-- + +. In the application's source code, use `tracef()` like you would use + man:printf(3): ++ +-- +[source,c] +---- + /* ... */ + + 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 trace the events that `tracef()` calls emit: + +* <> which matches the + `lttng_ust_tracef:*` event name: ++ +-- +[role="term"] +---- +$ lttng enable-event --userspace 'lttng_ust_tracef:*' +---- +-- + +[IMPORTANT] +.Limitations of `tracef()` +==== +The `tracef()` utility function was developed to make user space tracing +super simple, albeit with notable disadvantages compared to +<>: + +* All the emitted events have the same tracepoint provider and + tracepoint names, respectively `lttng_ust_tracef` and `event`. +* There is no static type checking. +* The only event record field you actually get, named `msg`, is a string + potentially containing the values you passed to `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 `tracef()` uses the C standard library's man:vasprintf(3) + function 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, `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 `tracelog()` + +The man:tracelog(3) API is very similar to <>, with +the difference that it accepts an additional log level parameter. + +The goal of `tracelog()` is to ease the migration from logging to +tracing. + +To use `tracelog()` in your application: + +. In the C or C++ source files where you need to use `tracelog()`, + include ``: ++ +-- +[source,c] +---- +#include +---- +-- + +. In the application's source code, use `tracelog()` like you would use + man:printf(3), except for the first parameter which is the log + level: ++ +-- +[source,c] +---- + /* ... */ + + tracelog(TRACE_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 trace the events that `tracelog()` calls emit with a log level +_as severe as_ a specific log level: + +* <> which matches the + `lttng_ust_tracelog:*` event name and a minimum level + of severity: ++ +-- +[role="term"] +---- +$ lttng enable-event --userspace 'lttng_ust_tracelog:*' + --loglevel=TRACE_WARNING +---- +-- + +To trace the events that `tracelog()` calls emit with a +_specific log level_: + +* Create an event rule which matches the `lttng_ust_tracelog:*` + event name and a specific log level: ++ +-- +[role="term"] +---- +$ lttng enable-event --userspace 'lttng_ust_tracelog:*' + --loglevel-only=TRACE_INFO +---- +-- + + +[[prebuilt-ust-helpers]] +=== Prebuilt user space tracing helpers + +The LTTng-UST package provides a few helpers in the form or 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 are 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 helper's name). + +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 `TRACE_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]] +=== User space Java agent + +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. +* http://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 http://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 Java application's source code, import the LTTng-UST + log handler package for `java.util.logging`: ++ +-- +[source,java] +---- +import org.lttng.ust.agent.jul.LttngLogHandler; +---- +-- + +. Create an LTTng-UST JUL log handler: ++ +-- +[source,java] +---- +Handler lttngUstLogHandler = new LttngLogHandler(); +---- +-- + +. Add this handler to the JUL 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 is recommended for a clean +disposal of the handler's resources. + +. Include the LTTng-UST Java agent's common and JUL-specific JAR files, + 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 the +`jello` JUL logger, 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 <> generated by a Java +application using `java.util.logging` is named `lttng_jul:event` and +has the following fields: + +`msg`:: + Log record's 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 JUL log levels +or a specific JUL 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 Java application's source code, 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 is recommended for a clean +disposal of the appender's resources. + +. Include the LTTng-UST Java agent's common and log4j-specific JAR + files, 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:$LOG4JPATH Test.java +---- + +<>, +<> matching the +`jello` log4j logger, and <>: + +[role="term"] +---- +$ 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 <> generated by a Java +application using log4j is named `lttng_log4j:event` and +has the following fields: + +`msg`:: + Log record's 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 provided +by the application which <>, using the +man:lttng-add-context(1) command, to each <> +produced by the log statements of this application. + +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 to +the JUL or log4j <>. + +To provide application-specific context fields in a Java application: + +. In the Java application's source code, 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 is recommended for a clean +disposal of some manager's resources. + +. 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 the +`jello` JUL logger: + +[role="term"] +---- +$ lttng create +$ lttng enable-event --jul jello +---- + +<> to the +JUL 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]] +=== User space Python agent + +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 emits 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 Python application's source code, 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. ++ +Any log statement that the application executes before this import does +not emit 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, you can trace any log statement from any logger. + +.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 the +`my-logger` Python 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 <> generated by a Python +application is named `lttng_python:event` and has the following fields: + +`asctime`:: + Logging time (string). + +`msg`:: + Log record's 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]] +=== 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 emit an LTTng event. + +[role="img-100"] +.An application writes to the LTTng logger file to emit an LTTng event. +image::lttng-logger.png[] + +The LTTng logger is the quickest method--not the most efficient, +however--to add instrumentation to an application. It is designed +mostly to instrument shell scripts: + +[role="term"] +---- +$ echo "Some message, some $variable" > /dev/lttng-logger +---- + +Any event that the LTTng logger emits is named `lttng_logger` and +belongs to the Linux kernel <>. However, unlike +other instrumentation points in the kernel tracing domain, **any Unix +user** can <> which +matches its event name, not only the root user or users in the +<>. + +To use the LTTng logger: + +* From any application, write text data to the path:{/dev/lttng-logger} + file. + +The `msg` field of `lttng_logger` event records contains the +recorded message. + +NOTE: The maximum message length of an LTTng logger event is +1024{nbsp}bytes. Writing more than this makes the LTTng logger emit more +than one event to contain the remaining data. + +You shouldn't use the LTTng logger to trace a user application which +can be instrumented in a more efficient way, namely: + +* <>. +* <>. +* <>. + +.Use the LTTng logger. +==== +[source,bash] +.path:{test.bash} +---- +echo 'Hello, World!' > /dev/lttng-logger +sleep 2 +df --human-readable --print-type / > /dev/lttng-logger +---- + +<>, +<> matching the +`lttng_logger` Linux kernel tracepoint, and +<>: + +[role="term"] +---- +$ 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]] +=== LTTng kernel tracepoints + +NOTE: This section shows how to _add_ instrumentation points to the +Linux kernel. The kernel's subsystems are already thoroughly +instrumented at strategic places for LTTng when you +<> the <> +package. + +//// +There are two methods to instrument the Linux kernel: + +. <> over an existing ftrace + tracepoint which uses the `TRACE_EVENT()` API. ++ +Choose this if you want to instrumentation a Linux kernel tree with an +instrumentation point compatible with ftrace, perf, and SystemTap. + +. Use an <> to + instrument an out-of-tree kernel module. ++ +Choose this if you don't need ftrace, perf, or SystemTap support. +//// + + +[[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: + +* http://lwn.net/Articles/379903/[Using the TRACE_EVENT() macro (Part{nbsp}1)] +* http://lwn.net/Articles/381064/[Using the TRACE_EVENT() macro (Part{nbsp}2)] +* http://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 are 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's name. + +. Get a copy of the latest LTTng-modules{nbsp}{revision}: ++ +-- +[role="term"] +---- +$ cd $(mktemp -d) && +wget http://lttng.org/files/lttng-modules/lttng-modules-latest-2.12.tar.bz2 && +tar -xf lttng-modules-latest-2.12.tar.bz2 && +cd lttng-modules-2.12.* +---- +-- + +. 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 TRACE_EVENT()'s 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 +ftrace's `TRACE_EVENT()` macro. ++ +See <> for a +complete description of the available `ctf_*()` macros. + +. Create the LTTng-modules probe's kernel module C source file, + +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 ftrace's `TRACE_EVENT()` macro. + +Note that you can also use the +<> +instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and +C code that need to be executed before the event fields are recorded. + +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 man:modprobe(8)'s `--remove` option 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. + +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 use _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 is most of the time the first operation that you do, sometimes it's +not. Some examples are: + +* <>. +* <>. + +[[tracing-group]] Each Unix user must have its own running session +daemon to trace user applications. The session daemon that the root user +starts is the only one allowed to control the LTTng kernel tracer. Users +that are part of the _tracing group_ can control the root session +daemon. The default tracing group name is `tracing`; set it to something +else with the opt:lttng-sessiond(8):--group option when you start the +root session daemon. + +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, use man:kill(1) on its process ID (standard +`TERM` signal). + +Note that some Linux distributions could manage the LTTng session daemon +as a service. In this case, you should use the service manager to +start, restart, and stop session daemons. + + +[[creating-destroying-tracing-sessions]] +=== Create and destroy a tracing session + +Almost all the LTTng control operations happen in the scope of +a <>, which is the dialogue between the +<> and you. + +To create a tracing session with a generated name: + +* Use the man:lttng-create(1) command: ++ +-- +[role="term"] +---- +$ lttng create +---- +-- + +The created tracing session's name is `auto` followed by the +creation date. + +To create a tracing session with a specific name: + +* Use the optional argument of the man:lttng-create(1) command: ++ +-- +[role="term"] +---- +$ lttng create my-session +---- +-- ++ +Replace `my-session` with the specific tracing session name. + +LTTng appends the creation date to the created tracing session's name. + +LTTng writes the traces of a tracing session in ++$LTTNG_HOME/lttng-trace/__name__+ by default, where +__name__+ is the +name of the tracing session. Note that the env:LTTNG_HOME environment +variable defaults to `$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 tracing sessions as you wish. + +To list all the existing tracing sessions for your Unix user: + +* Use the man:lttng-list(1) command: ++ +-- +[role="term"] +---- +$ lttng list +---- +-- + +[[cur-tracing-session]]When you create a tracing session, it is set as +the _current tracing session_. The following man:lttng(1) commands +operate on the current tracing 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 tracing session: + +* Use the man:lttng-set-session(1) command: ++ +-- +[role="term"] +---- +$ lttng set-session new-session +---- +-- ++ +Replace `new-session` by the name of the new current tracing session. + +When you're done tracing in a given tracing session, destroy it. This +operation frees the resources taken by the tracing session to destroy; +it doesn't destroy the trace data that LTTng wrote for this tracing +session (see <> for one way to do this). + +To destroy the current tracing 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 tracing 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. For the Linux kernel <>, +they are tracepoints and system calls. For the user space tracing +domain, they are tracepoints. For the other tracing domains, they are +logger names. + +To list the available instrumentation points: + +* Use the man:lttng-list(1) command with the requested tracing domain's + option amongst: ++ +-- +opt:lttng-list(1):--kernel:: + Linux kernel tracepoints (your Unix user must be a root user, or it + must be a member of the <>). + +opt:lttng-list(1):--kernel with opt:lttng-list(1):--syscall:: + Linux kernel system calls (your Unix user must be a root user, or it + must be a member of the tracing group). + +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 call tracepoints. +==== +[role="term"] +---- +$ lttng list --kernel --syscall +---- +==== + + +[[enabling-disabling-events]] +=== Create and enable an event rule + +Once you <>, you can create <> with the +man:lttng-enable-event(1) command. + +You specify each condition with a command-line option. The available +condition arguments are shown in the following table. + +[role="growable",cols="asciidoc,asciidoc,default"] +.Condition command-line arguments for the man:lttng-enable-event(1) command. +|==== +|Argument |Description |Applicable tracing domains + +| +One of: + +. `--syscall` +. +--probe=__ADDR__+ +. +--function=__ADDR__+ +. +--userspace-probe=__PATH__:__SYMBOL__+ +. +--userspace-probe=sdt:__PATH__:__PROVIDER__:__NAME__+ + +| +Instead of using the default _tracepoint_ instrumentation type, use: + +. A Linux system call (entry and exit). +. A Linux https://lwn.net/Articles/132196/[kprobe] (symbol or address). +. The entry and return points of a Linux function (symbol or address). +. The entry point of a user application or library function (path to + application/library and symbol). +. A https://www.sourceware.org/systemtap/wiki/AddingUserSpaceProbingToApps[SystemTap + Statically Defined Tracing] (USDT) probe (path to application/library, + provider and probe names). + +|Linux kernel. + +|First positional argument. + +| +Tracepoint or system call name. + +With the opt:lttng-enable-event(1):--probe, +opt:lttng-enable-event(1):--function, and +opt:lttng-enable-event(1):--userspace-probe options, this is a custom +name given to the event rule. With the JUL, log4j, and Python domains, +this is a logger name. + +With a tracepoint, logger, or system call name, use the special +`*` globbing character to match anything (for example, `sched_*`, +`my_comp*:*msg_*`). + +|All. + +| +One of: + +. +--loglevel=__LEVEL__+ +. +--loglevel-only=__LEVEL__+ + +| +. Match only tracepoints or log statements with a logging level at + least as severe as +__LEVEL__+. +. Match only tracepoints or log statements with a logging level + equal to +__LEVEL__+. + +See man:lttng-enable-event(1) for the list of available logging level +names. + +|User space, JUL, log4j, and Python. + +|+--exclude=__EXCLUSIONS__+ + +| +When you use a `*` character at the end of the tracepoint or logger +name (first positional argument), exclude the specific names in the +comma-delimited list +__EXCLUSIONS__+. + +| +User space, JUL, log4j, and Python. + +|+--filter=__EXPR__+ + +| +Match only events which satisfy the expression +__EXPR__+. + +See man:lttng-enable-event(1) to learn more about the syntax of a +filter expression. + +|All. + +|==== + +You attach an event rule to a <> on creation. If you do +not specify the channel with the opt:lttng-enable-event(1):--channel +option, and if the event rule to create is the first in its +<> for a given tracing session, then LTTng +creates a _default channel_ for you. This default channel is reused in +subsequent invocations of the man:lttng-enable-event(1) command for the +same tracing domain. + +An event rule is always enabled at creation time. + +The following examples show how to combine the previous +command-line options to create simple to more complex event rules. + +.Create an event rule targetting a Linux kernel tracepoint (default channel). +==== +[role="term"] +---- +$ lttng enable-event --kernel sched_switch +---- +==== + +.Create an event rule matching four Linux kernel system calls (default channel). +==== +[role="term"] +---- +$ lttng enable-event --kernel --syscall open,write,read,close +---- +==== + +.Create event rules matching tracepoints with 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='$app.retriever:cur_msg_id > 3' +---- + +IMPORTANT: Make sure to always quote the filter string when you +use man:lttng(1) from a shell. + +See also <> which offers another, +more efficient filtering mechanism for process ID, user ID, and group +ID attributes. +==== + +.Create an event rule matching any user space tracepoint of a given tracepoint provider with a log level range (default channel). +==== +[role="term"] +---- +$ lttng enable-event --userspace my_app:'*' --loglevel=TRACE_INFO +---- + +IMPORTANT: Make sure to always quote the wildcard character when you +use man:lttng(1) from a shell. +==== + +.Create an event rule matching multiple Python loggers with a wildcard and with exclusions (default channel). +==== +[role="term"] +---- +$ lttng enable-event --python my-app.'*' \ + --exclude='my-app.module,my-app.hello' +---- +==== + +.Create an event rule matching any Apache log4j logger with a specific log level (default channel). +==== +[role="term"] +---- +$ lttng enable-event --log4j --all --loglevel-only=LOG4J_WARN +---- +==== + +.Create an event rule attached to a specific channel matching a specific user space tracepoint provider and tracepoint. +==== +[role="term"] +---- +$ lttng enable-event --userspace my_app:my_tracepoint --channel=my-channel +---- +==== + +.Create an event rule matching 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 an event rule matching 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 event rules of a given channel form a whitelist: as soon as an +emitted event passes one of them, LTTng can record the event. For +example, an event named `my_app:my_tracepoint` emitted from a user space +tracepoint with a `TRACE_ERROR` log level passes both of the following +rules: + +[role="term"] +---- +$ lttng enable-event --userspace my_app:my_tracepoint +$ lttng enable-event --userspace my_app:my_tracepoint \ + --loglevel=TRACE_INFO +---- + +The second event rule is redundant: the first one includes +the second one. + + +[[disable-event-rule]] +=== Disable an event rule + +To disable an event rule that you <> +previously, use the man:lttng-disable-event(1) command. This command +disables _all_ the event rules (of a given tracing domain and channel) +which match an instrumentation point. The other conditions aren't +supported as of LTTng{nbsp}{revision}. + +The LTTng tracer doesn't record an emitted event which passes +a _disabled_ event rule. + +.Disable an event rule matching a Python logger (default channel). +==== +[role="term"] +---- +$ lttng disable-event --python my-logger +---- +==== + +.Disable an event rule matching all `java.util.logging` loggers (default channel). +==== +[role="term"] +---- +$ lttng disable-event --jul '*' +---- +==== + +.Disable _all_ the event rules of the default channel. +==== +The opt:lttng-disable-event(1):--all-events option isn't, like the +opt:lttng-enable-event(1):--all option of man:lttng-enable-event(1), the +equivalent of the event name `*` (wildcard): it disables _all_ the event +rules of a given channel. + +[role="term"] +---- +$ lttng disable-event --jul --all-events +---- +==== + +NOTE: You can't delete an event rule once you create it. + + +[[status]] +=== Get the status of a tracing session + +To get the status of the <>, that is, its parameters, its channels, event rules, and their +attributes: + +* Use the man:lttng-status(1) command: ++ +-- +[role="term"] +---- +$ lttng status +---- +-- + +To get the status of any tracing session: + +* Use the man:lttng-list(1) command with the tracing session's name: ++ +-- +[role="term"] +---- +$ lttng list my-session +---- +-- ++ +Replace `my-session` with the desired tracing session's name. + + +[[basic-tracing-session-control]] +=== Start and stop a tracing session + +Once you <> and +<>, +you can start and stop the tracers for this tracing session. + +To start tracing in the <>: + +* Use the man:lttng-start(1) command: ++ +-- +[role="term"] +---- +$ lttng start +---- +-- + +LTTng is very flexible: you can launch user applications before +or after the you start the tracers. The tracers only record the events +if they pass enabled event rules and if they occur while the tracers are +started. + +To stop tracing in the current tracing 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), warnings are printed when you run the +man:lttng-stop(1) command. + +IMPORTANT: You need to stop tracing 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. + +[role="since-2.12"] +[[clear]] +=== Clear a tracing 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 tracing 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 tracing sessions: + +* Use the `lttng clear` command with the opt:lttng-clear(1):--all + option: ++ +-- +[role="term"] +---- +$ lttng clear --all +---- +-- + + +[[enabling-disabling-channels]] +=== Create a channel + +Once you create a tracing session, you can create a <> +with the man:lttng-enable-channel(1) command. + +Note that LTTng automatically creates a default channel when, for a +given <>, no channels exist and you +<> the first event rule. This default +channel is named `channel0` and its attributes are set to reasonable +values. Therefore, you only need to create a channel when you need +non-default attributes. + +You specify each non-default channel attribute with a command-line +option when you use the man:lttng-enable-channel(1) command. The +available command-line options are: + +[role="growable",cols="asciidoc,asciidoc"] +.Command-line options for the man:lttng-enable-channel(1) command. +|==== +|Option |Description + +|`--overwrite` + +| +Use the _overwrite_ +<> instead +of the default _discard_ mode. + +|`--buffers-pid` (user space tracing domain only) + +| +Use the per-process <> +instead of the default per-user buffering scheme. + +|+--subbuf-size=__SIZE__+ + +| +Allocate sub-buffers of +__SIZE__+ bytes (power of two), for each CPU, +either for each Unix user (default), or for each instrumented process. + +See <>. + +|+--num-subbuf=__COUNT__+ + +| +Allocate +__COUNT__+ sub-buffers (power of two), for each CPU, either +for each Unix user (default), or for each instrumented process. + +See <>. + +|+--tracefile-size=__SIZE__+ + +| +Set the maximum size of each trace file that this channel writes within +a stream to +__SIZE__+ bytes instead of no maximum. + +See <>. + +|+--tracefile-count=__COUNT__+ + +| +Limit the number of trace files that this channel creates to ++__COUNT__+ channels instead of no limit. + +See <>. + +|+--switch-timer=__PERIODUS__+ + +| +Set the <> +to +__PERIODUS__+{nbsp}µs. + +|+--read-timer=__PERIODUS__+ + +| +Set the <> +to +__PERIODUS__+{nbsp}µs. + +|[[opt-blocking-timeout]]+--blocking-timeout=__TIMEOUTUS__+ + +| +Set the timeout of user space applications which load LTTng-UST +in blocking mode to +__TIMEOUTUS__+: + +0 (default):: + Never block (non-blocking mode). + +`inf`:: + Block forever until space is available in a sub-buffer to record + the event. + +__n__, a positive value:: + Wait for at most __n__ µs when trying to write into a sub-buffer. + +Note that, for this option to have any effect on an instrumented +user space application, you need to run the application with a set +env:LTTNG_UST_ALLOW_BLOCKING environment variable. + +|+--output=__TYPE__+ (Linux kernel tracing domain only) + +| +Set the channel's output type to +__TYPE__+, either `mmap` or `splice`. + +|==== + +You can only create a channel in the Linux kernel and user space +<>: other tracing domains have their own channel +created on the fly when <>. + +[IMPORTANT] +==== +Because of a current LTTng limitation, you must create all channels +_before_ you <> in a given +tracing session, that is, before the first time you run +man:lttng-start(1). + +Since LTTng automatically creates a default channel when you use the +man:lttng-enable-event(1) command with a specific tracing domain, you +can't, for example, create a Linux kernel event rule, start tracing, +and then create a user space event rule, because no user space channel +exists yet and it's too late to create one. + +For this reason, make sure to configure your channels properly +before starting the tracers for the first time! +==== + +The following examples show how to combine the previous +command-line options to create simple to more complex channels. + +.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-channel +$ lttng enable-event --userspace --channel=blocking-channel --all +$ lttng start +---- + +Run an application instrumented with LTTng-UST 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 overwrite (or _flight recorder_) mode. +==== +[role="term"] +---- +$ lttng enable-channel --userspace --overwrite my-channel +---- +==== + +<> the same event rule in +two different channels: + +[role="term"] +---- +$ lttng enable-event --userspace --channel=my-channel app:tp +$ lttng enable-event --userspace --channel=other-channel app:tp +---- + +If both channels are enabled, when a tracepoint named `app:tp` is +reached, LTTng records two events, one for each channel. + + +[[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 +---- +==== + +The state of a channel precedes the individual states of event rules +attached to it: event rules which belong to a disabled channel, even if +they are enabled, are also considered disabled. + + +[[adding-context]] +=== Add context fields to a channel + +Event record fields in trace files provide important information about +events that occured previously, 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 in which the event occurs. +* The **hostname** of the system on which the event occurs. +* The Linux kernel and user call stacks (since + LTTng{nbsp}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 context defined at the application level (supported for the + JUL and log4j <>). + +To get the full list of available context fields, see +`lttng add-context --list`. Some context fields are reserved for a +specific <> (Linux kernel or user space). + +You add context fields to <>. All the events +that a channel with added context fields records contain those fields. + +To add context fields to one or all the channels of a given tracing +session: + +* Use the man:lttng-add-context(1) command. + +.Add context fields to all the channels of the current tracing session. +==== +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 +<>. + +[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 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` in +the current tracing session. + +[role="term"] +---- +$ lttng add-context --kernel --channel=my-channel \ + --type=tid --type=callstack-user +---- +==== + +.Add an application-specific context field to a specific channel. +==== +The following command line adds the `cur_msg_id` context field of the +`retriever` context retriever for all the instrumented +<> recording <> +in the channel named `my-channel`: + +[role="term"] +---- +$ lttng add-context --kernel --channel=my-channel \ + --type='$app:retriever:cur_msg_id' +---- + +IMPORTANT: Make sure to always quote the `$` character when you +use man:lttng-add-context(1) from a shell. +==== + +NOTE: You can't remove context fields from a channel once you add it. + + +[role="since-2.7"] +[[pid-tracking]] +=== Track process attributes + +It's often useful to only allow processes with specific attributes to +emit events. For example, you may wish to record all the system calls +which a given process makes (à la +http://linux.die.net/man/1/strace[strace]). + +The man:lttng-track(1) and man:lttng-untrack(1) commands serve this +purpose. Both commands operate on _inclusion sets_ of process attribute +values. The available process attribute types are: + +Linux kernel <> only:: ++ +* Process ID (PID). + +* Virtual process ID (VPID). ++ +This is the PID as seen by the application. + +* Unix user ID (UID) (since LTTng{nbsp}2.12). + +* Virtual Unix user ID (VUID) (since LTTng{nbsp}2.12). ++ +This is the UID as seen by the application. + +* Unix group ID (GID) (since LTTng{nbsp}2.12). + +* Virtual Unix group ID (VGID) (since LTTng{nbsp}2.12). ++ +This is the GID as seen by the application. + + +User space tracing domain:: ++ +* VPID. +* VUID (since LTTng{nbsp}2.12). +* VGID (since LTTng{nbsp}2.12). + +Each tracing domain has one inclusion set per process attribute type: +the Linux kernel tracing domain has six while the user space tracing +domain has three. + +For a given event which passes an enabled <> to be +recorded, _all_ the attributes of its executing process must be part of +the inclusion sets of the event rule's tracing domain. + +Add entries to an inclusion set with the man:lttng-track(1) command and +remove entries with the man:lttng-untrack(1) command. A process +attribute is _tracked_ when it's part of an inclusion set and +_untracked_ otherwise. + +[NOTE] +==== +The process attribute values are _numeric_. + +Should a process with a given tracked process ID, for example, exit, and +then a new process be given this ID, then the latter would also be +allowed to emit events. + +With the `lttng track` 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 `lttng track`, its user/group ID remains tracked. +==== + +.Track and untrack virtual process IDs. +==== +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"] +.All VPIDs are tracked. +image::track-all.png[] + +When the inclusion set is full and you use the man:lttng-track(1) +command to specify some VPIDs to track, LTTng first clears the inclusion +set, and then it adds the specific VPIDs to track. After: + +[role="term"] +---- +$ lttng track --userspace --vpid=3,4,7,10,13 +---- + +the VPID inclusion set is: + +[role="img-100"] +.VPIDs 3, 4, 7, 10, and 13 are tracked. +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[] + +LTTng can track all the possible VPIDs again using the +opt:lttng-track(1):--all option: + +[role="term"] +---- +$ lttng track --userspace --vpid --all +---- + +The result is, again: + +[role="img-100"] +.All VPIDs are tracked. +image::track-all.png[] +==== + +.Track only specific process attributes. +==== +A typical use case with process attribute tracking is to start with an +empty inclusion set, then <>, and then add entries 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"] +.No UIDs are tracked. +image::untrack-all.png[] + +If you trace with this inclusion set configuration, the LTTng kernel +tracer records no events within the <> because it doesn't track any UID. Use the +man:lttng-track(1) command as usual to track specific UIDs when you need +to, for example: + +[role="term"] +---- +$ lttng track --kernel --uid=http,11 +---- + +Result: + +[role="img-100"] +.UIDs 6 (`http`) and 11 are tracked. +image::track-6-11.png[] +==== + + +[role="since-2.5"] +[[saving-loading-tracing-session]] +=== Save and load tracing session configurations + +Configuring a <> can be long. Some of +the tasks involved are: + +* <> with + specific attributes. +* <> to specific channels. +* <> with specific log + level and filter conditions. + +If you use LTTng to solve real world problems, chances are you have to +record events using the same tracing session setup over and over, +modifying a few variables each time in your instrumented program +or environment. To avoid constant tracing session reconfiguration, +the man:lttng(1) command-line tool can save and load tracing session +configurations to/from XML files. + +To save a given tracing session configuration: + +* Use the man:lttng-save(1) command: ++ +-- +[role="term"] +---- +$ lttng save my-session +---- +-- ++ +Replace `my-session` with the name of the tracing session to save. + +LTTng saves tracing session configurations to +dir:{$LTTNG_HOME/.lttng/sessions} by default. Note that the +env:LTTNG_HOME environment variable defaults to `$HOME` if not set. Use +the opt:lttng-save(1):--output-path option to change this destination +directory. + +LTTng saves all configuration parameters, for example: + +* The tracing session name. +* The trace data output path. +* The channels with their state and all their attributes. +* The context fields you added to channels. +* The event rules with their state, log level and filter conditions. + +To load a tracing session: + +* Use the man:lttng-load(1) command: ++ +-- +[role="term"] +---- +$ lttng load my-session +---- +-- ++ +Replace `my-session` with the name of the tracing session to load. + +When LTTng loads a configuration, it restores your saved tracing session +as if you just configured it manually. + +See man:lttng-load(1) for the complete list of command-line options. You +can also save and load many sessions at a time, and decide in which +directory to output the XML files. + + +[[sending-trace-data-over-the-network]] +=== Send trace data over the network + +LTTng can send the recorded trace data 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 tracing session configured to + send trace data over the network: ++ +-- +[role="term"] +---- +$ lttng create my-session --set-url=net://remote-system +---- +-- ++ +Replace `remote-system` by 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 tracing is active, the target's consumer daemon sends sub-buffers + to the relay daemon running on the remote system instead of flushing + them to the local file system. The relay daemon writes the received + packets to the local file system. + +The relay daemon writes trace files to ++$LTTNG_HOME/lttng-traces/__hostname__/__session__+ by default, where ++__hostname__+ is the host name of the target system and +__session__+ +is the tracing session name. Note that the env:LTTNG_HOME environment +variable defaults to `$HOME` if not set. Use the +opt:lttng-relayd(8):--output option of man:lttng-relayd(8) to write +trace files to another base directory. + + +[role="since-2.4"] +[[lttng-live]] +=== View events as LTTng emits them (noch:{LTTng} live) + +LTTng live is a network protocol implemented by the <> (man:lttng-relayd(8)) to allow compatible trace viewers to +display events as LTTng emits them on the target system while tracing is +active. + +The relay daemon creates a _tee_: it forwards the trace data to both +the local file system and to connected live viewers: + +[role="img-90"] +.The relay daemon creates a _tee_, forwarding the trace data to both trace files and a connected live viewer. +image::live.png[] + +To use LTTng live: + +. On the _target system_, create a <> + in _live mode_: ++ +-- +[role="term"] +---- +$ lttng create my-session --live +---- +-- ++ +This spawns a local relay daemon. + +. Start the live viewer and configure it to connect to the relay + daemon. For example, with + https://babeltrace.org/docs/v2.0/man1/babeltrace2.1/[cmd:babeltrace2]: ++ +-- +[role="term"] +---- +$ babeltrace2 net://localhost/host/hostname/my-session +---- +-- ++ +Replace: ++ +-- +* `hostname` with the host name of the target system. +* `my-session` with the name of the tracing session to view. +-- + +. Configure the tracing session as usual with the man:lttng(1) + command-line tool, and <>. + +List the available live tracing sessions with Babeltrace{nbsp}2: + +[role="term"] +---- +$ babeltrace2 net://localhost +---- + +You can start the relay daemon on another system. In this case, you need +to specify the relay daemon's URL when you create the tracing session +with the opt:lttng-create(1):--set-url option. You also need to replace +`localhost` in the procedure above with the host name of the system on +which the relay daemon is running. + +See man:lttng-create(1) and man:lttng-relayd(8) for the complete list of +command-line options. + + +[role="since-2.3"] +[[taking-a-snapshot]] +=== Take a snapshot of the current sub-buffers of a tracing 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 +that occurred on the target system, but it can +represent too much data in some situations. For example, you may wish +to trace 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 sub-buffers 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 aren't cleared after the operation. +image::snapshot.png[] + +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: + +. Create a tracing session in _snapshot mode_: ++ +-- +[role="term"] +---- +$ lttng create my-session --snapshot +---- +-- ++ +The <> of +<> created in this mode is automatically set to +_overwrite_ (flight recorder mode). + +. Configure the tracing 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 sure that the 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 +<>'s channels to +trace files on the local file system. Those trace files have +`my-first-snapshot` in their name. + +There is no difference between the format of a normal trace file and the +format of a snapshot: viewers of LTTng traces also support LTTng +snapshots. + +By default, LTTng writes snapshot files to the path shown by +`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 + `lttng snapshot add-output`. +. An output path or URL that you provide directly to the + `lttng snapshot record` command. + +Method{nbsp}3 overrides method{nbsp}2, which overrides method 1. When +you specify a URL, a relay daemon must listen on a remote system (see +<>). + + +[role="since-2.11"] +[[session-rotation]] +=== Archive the current trace chunk (rotate a tracing session) + +The <> shows how to dump +a tracing session's current sub-buffers to the file system or send them +over the network. When you take a snapshot, LTTng doesn't clear the +tracing session's ring buffers: 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], +_tracing 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 tracing session's creation 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 tracing session rotation +is called the _current trace chunk_. When this current trace chunk is +written to the file system or sent over the network, it becomes a _trace +chunk archive_. Therefore, a tracing session rotation _archives_ the +current trace chunk. + +[role="img-100"] +.A tracing 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. + +There are two methods to perform a tracing session rotation: immediately +or with a rotation schedule. + +To perform an immediate tracing session rotation: + +. <> + in _normal mode_ or _network streaming mode_ + (only those two creation modes support tracing session rotation): ++ +-- +[role="term"] +---- +$ lttng create my-session +---- +-- + +. <> + and <>: ++ +-- +[role="term"] +---- +$ lttng enable-event --kernel sched_'*' +$ lttng start +---- +-- + +. When needed, immediately rotate the + <>: ++ +-- +[role="term"] +---- +$ lttng rotate +---- +-- ++ +The cmd:lttng-rotate command prints the path to the created trace +chunk archive. See man:lttng-rotate(1) to learn about the format +of trace chunk archive directory names. ++ +Perform other immediate rotations while the tracing session is +active. It is guaranteed that all the trace chunk archives don't +contain overlapping trace data. You can also perform an immediate +rotation once you have <> the +tracing session. + +. When you're done tracing, + <>: ++ +-- +[role="term"] +---- +$ lttng destroy +---- +-- ++ +The tracing session destruction operation creates one last trace +chunk archive from the current trace chunk. + +A tracing session rotation schedule is a planned rotation which LTTng +performs automatically based on one of the following conditions: + +* A timer with a configured period times out. + +* The total size of the flushed part of the current trace chunk + becomes greater than or equal to a configured value. + +To schedule a tracing session rotation, set a _rotation schedule_: + +. <> + in _normal mode_ or _network streaming mode_ + (only those two creation modes support tracing session rotation): ++ +-- +[role="term"] +---- +$ lttng create my-session +---- +-- + +. <>: ++ +-- +[role="term"] +---- +$ lttng enable-event --kernel sched_'*' +---- +-- + +. Set a tracing session rotation schedule: ++ +-- +[role="term"] +---- +$ lttng enable-rotation --timer=10s +---- +-- ++ +In this example, we set a rotation schedule so that LTTng performs a +tracing 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 tracing session rotations automatically while the tracing +session is active thanks to the rotation schedule. + +. When you're done tracing, + <>: ++ +-- +[role="term"] +---- +$ lttng destroy +---- +-- ++ +The tracing session destruction operation creates one last trace chunk +archive from the current trace chunk. + +Use man:lttng-disable-rotation(1) to unset a tracing session +rotation schedule. + +NOTE: man:lttng-rotate(1) and man:lttng-enable-rotation(1) list +limitations regarding those two commands. + + +[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 enable-event --kernel --syscall open +---- + +A schema definition (XSD) is +https://github.com/lttng/lttng-tools/blob/stable-2.12/src/common/mi-lttng-3.0.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 http://diamon.org/ctf[CTF] trace, has both +data stream files and a metadata file. This metadata file contains, +amongst other things, information about the offset of the clock sources +used to timestamp <> when tracing. + +If, once a <> is +<>, a major +https://en.wikipedia.org/wiki/Network_Time_Protocol[NTP] correction +happens, the trace's clock offset 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 trace it with LTTng before its wall time +is corrected. Once the system is known to be in a state where its +wall time is correct, it can run `lttng regenerate metadata`. + +To regenerate the metadata of an LTTng trace: + +* Use the `metadata` item of the man:lttng-regenerate(1) command: ++ +-- +[role="term"] +---- +$ lttng regenerate metadata +---- +-- + +[IMPORTANT] +==== +`lttng regenerate metadata` has the following limitations: + +* Tracing session <> + in non-live mode. +* User space <>, if any, are using + <>. +==== + + +[role="since-2.9"] +[[regenerate-statedump]] +=== Regenerate the state dump of a tracing 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 following event records. +http://tracecompass.org/[Trace Compass] is a notable example of an +application which uses the state dump of an LTTng trace. + +When you <>, it's possible that the +state dump event records aren't included in the snapshot because they +were recorded to a sub-buffer that has been consumed or overwritten +already. + +Use the `lttng regenerate statedump` command to emit the state +dump event records again. + +To regenerate the state dump of the current tracing session, provided +create it in snapshot mode, 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 1 and 2 +as closely as possible. + +NOTE: To record the state dump events, you need to +<> which enable them. +LTTng-UST state dump tracepoints start with `lttng_ust_statedump:`. +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 create a <>, 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 tracing session with a sub-buffer shared memory path located + on an NVRAM file system: ++ +-- +[role="term"] +---- +$ lttng create my-session --shm-path=/path/to/shm +---- +-- + +. Configure the tracing 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 + view the trace data recorded on the NVRAM file system: ++ +-- +[role="term"] +---- +$ lttng-crash /path/to/shm +---- +-- + +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 your preferred trace viewer directly. + +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 +---- +-- + + +[role="since-2.10"] +[[notif-trigger-api]] +=== Get notified when a channel's buffer usage is too high or too low + +With LTTng's $$C/C++$$ notification and trigger API, your user +application can get notified when the buffer usage of one or more +<> becomes too low or too high. Use this API +and enable or disable <> during tracing to avoid +<>. + +.Have a user application get notified when an LTTng channel's buffer usage 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 is the case in the example, but we +could as well use the API of <> to +disable event rules when this happens. + +. Create the application's C source file: ++ +-- +[source,c] +.path:{notif-app.c} +---- +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include + +int main(int argc, char *argv[]) +{ + int exit_status = 0; + struct lttng_notification_channel *notification_channel; + struct lttng_condition *condition; + struct lttng_action *action; + struct lttng_trigger *trigger; + const char *tracing_session_name; + const char *channel_name; + + assert(argc >= 3); + tracing_session_name = argv[1]; + channel_name = argv[2]; + + /* + * Create a notification channel. A notification channel + * connects the user application to the LTTng session daemon. + * This notification channel can be used to listen to various + * types of notifications. + */ + notification_channel = lttng_notification_channel_create( + lttng_session_daemon_notification_endpoint); + + /* + * Create a "high buffer usage" condition. In this case, the + * condition is reached when the buffer usage is greater than or + * equal to 75 %. We create the condition for a specific tracing + * session name, channel name, and for the user space tracing + * domain. + * + * The "low buffer usage" condition type also exists. + */ + condition = lttng_condition_buffer_usage_high_create(); + lttng_condition_buffer_usage_set_threshold_ratio(condition, .75); + lttng_condition_buffer_usage_set_session_name( + condition, tracing_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 (get a notification) to take when the + * condition created above is reached. + */ + action = lttng_action_notify_create(); + + /* + * Create a trigger. A trigger associates a condition to an + * action: the action is executed when the condition is reached. + */ + trigger = lttng_trigger_create(condition, action); + + /* Register the trigger to LTTng. */ + lttng_register_trigger(trigger); + + /* + * Now that we have registered a trigger, a notification will be + * emitted everytime its condition is met. To receive this + * notification, we must subscribe to notifications that 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 notification channel has been closed by the + * session daemon. This is typically caused by a session + * daemon shutting down. + */ + goto end; + default: + /* Unhandled conditions or errors. */ + exit_status = 1; + goto end; + } + + /* + * A notification provides, amongst other things: + * + * * The condition that caused this notification to be + * emitted. + * * 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 reached. + */ + 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 channel's buffer + * usage, like disable specific events. + */ + printf("Buffer usage is %f %% in tracing session \"%s\", " + "user space channel \"%s\".\n", buffer_usage * 100, + tracing_session_name, channel_name); + 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 to `liblttng-ctl`: ++ +-- +[role="term"] +---- +$ gcc -o notif-app notif-app.c -llttng-ctl +---- +-- + +. <>, + <> matching all the + user space tracepoints, 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, control how frequently are the current values of the +channel's properties sampled to evaluate user conditions with the +opt:lttng-enable-channel(1):--monitor-timer option. + +. Run the `notif-app` application. This program accepts the + <> name and the user space channel + name 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 tracing session "my-session", user space +channel "channel0". +---- ++ +If you don't see anything, try modifying the condition in +path:{notif-app.c} to a lower value (0.1, for example), rebuilding it +(step{nbsp}2) and running 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's 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 session trace, but are +otherwise identical. This means the `_nowrite` fields won't be written +in the recorded trace. Their primary purpose is to make some +of the event context available to the +<> without having to +commit the data to sub-buffers. + + +[[glossary]] +== Glossary + +Terms related to LTTng and to tracing in general: + +Babeltrace:: + The http://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-consumer-daemon]]<>:: + A process 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 <>'s + <> and the stream files produced since the + latest event amongst: ++ +* The creation of the <>. +* The last tracing session rotation, if any. + +<>:: + The <> in which + the <> _discards_ new event records when there's no + <> space left to store them. + +[[def-event]]event:: + The consequence of the execution of an + <>, like a + <> that you manually place in some source + code, or a Linux kernel kprobe. ++ +An event is said to _occur_ at a specific time. <> can +take various actions upon the occurrence of an event, like record the +event's payload to a <>. + +[[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, in a <>, of the payload of an + <> which occured. + +[[def-event-record-loss-mode]]<>:: + The mechanism by which event records of a given + <> are lost (not recorded) when there is no + <> space left to store them. + +[[def-event-rule]]<>:: + Set of conditions which must be satisfied for one or more occuring + <> to be recorded. + +[[def-incl-set]]inclusion set:: + In the <> context: a + set of <> of a given type. + +<>:: + The use of <> probes to make a piece of software + traceable. + +[[def-instrumentation-point]]instrumentation point:: + A point in the execution path of a piece of software that, when + reached by this execution, can emit an <>. + +instrumentation point name:: + See _<>_. + +`java.util.logging`:: + Java platform's + https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities]. + +log4j:: + A http://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 viewers which makes it possible to see <> ``live'', as they are 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. + +[[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 it is + emitted. + +[[def-trace]]trace (_noun_):: + A set of: ++ +* One http://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_):: + The action of recording the <> emitted by an + application or by a system, or to initiate such recording by + controlling a <>. + +[[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 software which records emitted <>. + +<>:: + A namespace for <> sources. + +<>:: + The Unix group in which a Unix user can be to be allowed to + <> the Linux kernel. + +[[def-tracing-session]]<>:: + A stateful dialogue between you and a <>. + +[[def-tracing-session-rotation]]<>:: + The action of archiving the + <> of a + <>. + +tracked <>:: + A process attribute which is part of an <>. + +untracked process attribute:: + A process attribute which isn't part of an <>. + +[[def-user-application]]user application:: + An application running in user space, as opposed to a Linux kernel + module, for example.