The LTTng Documentation ======================= Philippe Proulx v2.12, 28 November 2023 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 the tracing data of a tracing session 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 opt:lttng-relayd(8):--group-output-by-session option of the relay daemon. + 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 latency of the network stack thanks to those event records. * The `thread` field of the `irqaction` structure, 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 `architecture` field of the tracepoint is statically defined and exists for all architecture implementations. Analysis tools can therefore anticipate the layout of the event record. + 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' DTrace to Linux. + The cmd:dtrace tool interprets user scripts and is responsible for loading code into the Linux kernel for further execution and collecting the outputted data. https://en.wikipedia.org/wiki/Berkeley_Packet_Filter[eBPF]:: A subsystem in the Linux kernel in which a virtual machine can execute programs passed from the user space to the kernel. + You can attach such programs to tracepoints and kprobes thanks to a system call, and they can output data to the user space when executed thanks to different mechanisms (pipe, VM register values, and eBPF maps, to name a few). https://www.kernel.org/doc/Documentation/trace/ftrace.txt[ftrace]:: The de facto function tracer of the Linux kernel. + Its user interface is a set of special files in sysfs. https://perf.wiki.kernel.org/[perf]:: A performance analysis tool for Linux which supports hardware performance counters, tracepoints, as well as other counters and types of probes. + The controlling utility of perf is the cmd:perf command line/text UI tool. 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 the jargon of sysdig, in Lua and sysdig executes them while it traces the system or afterwards. The interface of sysdig is the cmd:sysdig command-line tool as well as the text UI-based cmd:csysdig tool. https://sourceware.org/systemtap/[SystemTap]:: A Linux kernel and user space tracer which uses custom user scripts to produce plain text traces. + SystemTap converts the scripts to the C language, and then compiles them as Linux kernel modules which are loaded to produce trace data. The primary user interface of SystemTap is the cmd:stap command-line tool. The main distinctive features of LTTng is that it produces correlated kernel and user space traces, as well as doing so with the lowest overhead amongst other solutions. It produces trace files in the 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. [role="growable"] .Availability of LTTng{nbsp}{revision} for major Linux distributions as of 3{nbsp}November{nbsp}2023. |==== |Distribution |Available in releases |https://www.ubuntu.com/[Ubuntu] |Ubuntu{nbsp}16.04 _Xenial Xerus_, Ubuntu{nbsp}18.04 _Bionic Beaver_, Ubuntu{nbsp}20.04 _Focal Fossa_, and Ubuntu{nbsp}22.04 _Jammy Jellyfish_: <>. |https://www.debian.org/[Debian] |<>. |https://www.redhat.com/[RHEL] and https://www.suse.com/[SLES] |See http://packages.efficios.com/[EfficiOS Enterprise Packages]. |https://alpinelinux.org/[Alpine Linux] |xref:alpine-linux[Alpine Linux{nbsp}3.12, Alpine Linux{nbsp}3.13, Alpine Linux{nbsp}3.14, and Alpine Linux{nbsp}3.15]. |https://buildroot.org/[Buildroot] |xref:buildroot[Buildroot{nbsp}2020.08, Buildroot{nbsp}2020.11, Builroot{nbsp}2021.02, Buildroot{nbsp}2021.05, Buildroot{nbsp}2021.08, and Builroot{nbsp}2021.11]. |https://www.openembedded.org/wiki/Main_Page[OpenEmbedded] and https://www.yoctoproject.org/[Yocto] |xref:oe-yocto[Yocto Project{nbsp}3.2 _Gatesgarth_ and Yocto Project{nbsp}3.3 _Hardknott_]. |==== [[ubuntu-ppa]] === Ubuntu: noch:{LTTng} Stable {revision} PPA The https://launchpad.net/~lttng/+archive/ubuntu/stable-{revision}[LTTng Stable{nbsp}{revision} PPA] offers the latest stable LTTng{nbsp}{revision} packages for Ubuntu{nbsp}16.04 _Xenial Xerus_, Ubuntu{nbsp}18.04 _Bionic Beaver_, Ubuntu{nbsp}20.04 _Focal Fossa_, and Ubuntu{nbsp}22.04 _Jammy Jellyfish_. To install LTTng{nbsp}{revision} from the LTTng Stable{nbsp}{revision} PPA: . Add the LTTng Stable{nbsp}{revision} PPA repository and update the list of packages: + -- [role="term"] ---- # apt-add-repository ppa:lttng/stable-2.12 # apt-get update ---- -- . Install the main LTTng{nbsp}{revision} packages: + -- [role="term"] ---- # apt-get install lttng-tools # apt-get install lttng-modules-dkms # apt-get install liblttng-ust-dev ---- -- . **If you need to instrument and trace <>**, install the LTTng-UST Java agent: + -- [role="term"] ---- # apt-get install liblttng-ust-agent-java ---- -- . **If you need to instrument and trace <>**, install the LTTng-UST Python agent: + -- [role="term"] ---- # apt-get install python3-lttngust ---- -- [[debian]] === Debian To install LTTng{nbsp}{revision} on Debian{nbsp}11 _bullseye_: . Install the main LTTng{nbsp}{revision} packages: + -- [role="term"] ---- # apt-get install lttng-modules-dkms # apt-get install liblttng-ust-dev # apt-get install lttng-tools ---- -- . **If you need to instrument and trace <>**, install the LTTng-UST Java agent: + -- [role="term"] ---- # apt-get install liblttng-ust-agent-java ---- -- . **If you need to instrument and trace <>**, install the LTTng-UST Python agent: + -- [role="term"] ---- # apt-get install python3-lttngust ---- -- [[alpine-linux]] === Alpine Linux To install LTTng-tools{nbsp}{revision} and LTTng-UST{nbsp}{revision} on Alpine Linux{nbsp}3.12, Alpine Linux{nbsp}3.13, Alpine Linux{nbsp}3.14, or Alpine Linux{nbsp}3.15: . Add the LTTng packages: + -- [role="term"] ---- # apk add lttng-tools # apk add lttng-ust-dev ---- -- . 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 ---- -- [[buildroot]] === Buildroot To install LTTng{nbsp}{revision} on Buildroot{nbsp}2020.08, Buildroot{nbsp}2020.11, Buildroot{nbsp}2021.02, Buildroot{nbsp}2021.05, Buildroot{nbsp}2021.08, or Buildroot{nbsp}2021.11: . Launch the Buildroot configuration tool: + -- [role="term"] ---- $ make menuconfig ---- -- . In **Kernel**, check **Linux kernel**. . In **Toolchain**, check **Enable WCHAR support**. . In **Target packages**{nbsp}→ **Debugging, profiling and benchmark**, check **lttng-modules** and **lttng-tools**. . In **Target packages**{nbsp}→ **Libraries**{nbsp}→ **Other**, check **lttng-libust**. [[oe-yocto]] === OpenEmbedded and Yocto LTTng{nbsp}{revision} recipes are available in the https://layers.openembedded.org/layerindex/branch/master/layer/openembedded-core/[`openembedded-core`] layer for Yocto Project{nbsp}3.2 _Gatesgarth_ and Yocto Project{nbsp}3.3 _Hardknott_ under the following names: * `lttng-tools` * `lttng-modules` * `lttng-ust` With BitBake, the simplest way to include LTTng recipes in your target image is to add them to `IMAGE_INSTALL_append` in path:{conf/local.conf}: ---- IMAGE_INSTALL_append = " lttng-tools lttng-modules lttng-ust" ---- If you use Hob: . Select a machine and an image recipe. . Click **Edit image recipe**. . Under the **All recipes** tab, search for **lttng**. . Check the desired LTTng recipes. [[building-from-source]] === Build from source To build and install LTTng{nbsp}{revision} from source: . Using the package manager of your distribution, or from source, install the following dependencies of LTTng-tools and LTTng-UST: + -- * https://sourceforge.net/projects/libuuid/[libuuid] * 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. [[linux-kernel-sig]] === Linux kernel module signature Linux kernel modules require trusted signatures in order to be loaded when any of the following is true: * The system boots with https://uefi.org/specs/UEFI/2.10/32_Secure_Boot_and_Driver_Signing.html#secure-boot-and-driver-signing[Secure Boot] enabled. * The Linux kernel which boots is configured with `CONFIG_MODULE_SIG_FORCE`. * The Linux kernel boots with a command line containing `module.sig_enforce=1`. .`root` user running <> which fails to load a required <> due to the signature enforcement policies. ==== [role="term"] ---- # lttng-sessiond Warning: No tracing group detected modprobe: ERROR: could not insert 'lttng_ring_buffer_client_discard': Key was rejected by service Error: Unable to load required module lttng-ring-buffer-client-discard Warning: No kernel tracer available ---- ==== There are several methods to enroll trusted keys for signing modules that are built from source. The precise details vary from one Linux version to another, and distributions may have their own mechanisms. For example, https://github.com/dell/dkms[DKMS] may autogenerate a key and sign modules, but the key isn't automatically enrolled. See https://www.kernel.org/doc/html/latest/admin-guide/module-signing.html[Kernel module signing facility] and the documentation of your distribution to learn more about signing Linux kernel modules. [[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"] .Build steps of the user space tracing tutorial. 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 service manager of the distribution started. . List the available user space tracepoints: + -- [role="term"] ---- $ lttng list --userspace ---- -- + You see the `hello_world:my_first_tracepoint` tracepoint listed under the `./hello` process. . Create a <>: + -- [role="term"] ---- $ lttng create my-user-space-session ---- -- . Create 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 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 those of LTTng. 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 of the event message. 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 # (Short) name of the previous task command. 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]]Local mode:: LTTng writes the traces to the file system of the machine it traces (target system). [[net-streaming-mode]]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 current sub-buffers of the tracing session, and to write it to the file system of the target or to send it over the network to a <> running on a remote system. [[live-mode]]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 the ring buffer of a specific channel. 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 execution of the instrumented application. 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 _event record loss mode_ of the channel 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. Since LTTng{nbsp}2.8, in overwrite mode, LTTng writes to a given sub-buffer its sequence number within its data stream. With a <>, <>, or <> <>, a trace reader can use such sequence numbers to report lost packets. In overwrite mode, LTTng doesn't write to the trace the exact number of lost event records in those lost sub-buffers. Trace analyses can use saved discarded event record and sub-buffer (packet) counts of the trace 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 overhead of the tracer shouldn't be a problem. * **Low memory system**: If your target system has a low memory limit, prefer fewer first, then smaller sub-buffers. + Even if the system is limited in memory, you want to keep the sub-buffers as 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 overhead of the tracer 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 _read timer_ of the channel 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 fixed maximum size of the channel, LTTng creates another file to contain the next event records. LTTng appends a file count to each trace file name in this case. If you set the trace file size attribute when you create a channel, the maximum number of trace files that LTTng creates is _unlimited_ by default. To limit them, set a maximum number of trace files. When the number of trace files reaches the fixed maximum count of the channel, 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 sub-buffers of the attached channel. The available conditions, as of LTTng{nbsp}{revision}, are: * The event rule _is enabled_. * The type of the instrumentation point _is{nbsp}T_. * The name of the instrumentation point (sometimes called _event name_) _matches{nbsp}N_, but _isn't{nbsp}E_. * The log level of the instrumentation point _is as severe as{nbsp}L_, or _is exactly{nbsp}L_. * The fields of the payload of the event _satisfy_ a filter expression{nbsp}__F__. As you can see, all the conditions but the dynamic filter are related to the status of the event rule 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 payload of the event 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 ID of the event, and its 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 details of the protocol. `liblttng-ctl` is part of LTTng-tools. The <> is linked with `liblttng-ctl`. Use `liblttng-ctl` in C or $$C++$$ source code by including its ``master'' header: [source,c] ---- #include ---- 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 log handler of the agent. The log handler of the agent calls a native function in a tracepoint provider package shared library linked with <>, passing the formatted log message and other fields, like its logger name and its log level. This native function contains a user space instrumentation point, 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. See also <>. 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 child process of the session daemon. Command-line options of man:lttng-sessiond(8) target the consumer daemon process. There are up to two running consumer daemons per Unix user, whereas only one session daemon can run per user. This is because each process can be either 32-bit or 64-bit: if the target system runs a mixture of 32-bit and 64-bit processes, it 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 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 source code of the software. 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 the built-in <> of LTTng. 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 source code of the user application. * 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 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 source code of the application. This expression provides the source of data of a field. The argument expression can include input argument names listed in the `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 the source code of your application 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 the source code of an application Once you <>, use the `tracepoint()` macro in the source code of your application 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 the source code of your application like this: [source,c] .Application source file. ---- #include "tp.h" int main(int argc, char* argv[]) { tracepoint(my_provider, my_tracepoint, argc, argv[0]); return 0; } ---- Note how the source code of the application includes the tracepoint provider header file containing the tracepoint definitions to use, path:{tp.h}. ==== .`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 the source code of your application like this: [source,c] .Application 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 name |Field value |`my_constant_field` |40 |`my_int_arg_field` |23 |`my_int_arg_field2` |529 |`sum4_field` |389 |`my_str_arg_field` |`Hello, World!` |`size_field` |0x12d |`size_dbl_field` |301.0 |`half_my_str_arg_field` |`Hello,` |==== ==== Sometimes, the arguments you pass to `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 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 the package manager of your distribution, 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 source code of the application, 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 man:vasprintf(3) function of the C{nbsp}standard library behind the scenes to format the strings at run time, its expected performance is lower than with user-defined tracepoints, which don't require a conversion to a string. Taking this into consideration, `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 source code of the application, 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 of preloadable shared objects which automatically instrument system functions and calls. The helper shared objects are normally found in dir:{/usr/lib}. If you built LTTng-UST <>, they 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 name of the helper). To use the LTTng-UST function tracing helper, the source files to instrument must be built using the `-finstrument-functions` compiler flag. There are two versions of the LTTng-UST function tracing helper: * **path:{liblttng-ust-cyg-profile-fast.so}** is a lightweight variant that you should only use when it can be _guaranteed_ that the complete event stream is recorded without any lost event record. Any kind of duplicate information is left out. + Assuming no event record is lost, having only the function addresses on entry is enough to create a call graph, since an event record always contains the ID of the CPU that generated it. + Use a tool like man:addr2line(1) to convert function addresses back to source file names and line numbers. * **path:{liblttng-ust-cyg-profile.so}** is a more robust variant which also works in use cases where event records might get discarded or not recorded from application startup. In these cases, the trace analyzer needs more information to be able to reconstruct the program flow. See man:lttng-ust-cyg-profile(3) to learn more about the instrumentation points of this helper. All the tracepoints that this helper provides have the log level `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 source code of the Java application, import the LTTng-UST log handler package for `java.util.logging`: + -- [source,java] ---- import org.lttng.ust.agent.jul.LttngLogHandler; ---- -- . Create an LTTng-UST 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 resources of the handler. . Include the common and JUL-specific JAR files of the LTTng-UST Java agent, path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-jul.jar}, in the https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class path] when you build the Java application. + The JAR files are typically located in dir:{/usr/share/java}. + IMPORTANT: The LTTng-UST Java agent must be <> for the logging framework your application uses. .Use the LTTng-UST Java agent for `java.util.logging`. ==== [source,java] .path:{Test.java} ---- import java.io.IOException; import java.util.logging.Handler; import java.util.logging.Logger; import org.lttng.ust.agent.jul.LttngLogHandler; public class Test { private static final int answer = 42; public static void main(String[] argv) throws Exception { // Create a logger Logger logger = Logger.getLogger("jello"); // Create an LTTng-UST log handler Handler lttngUstLogHandler = new LttngLogHandler(); // Add the LTTng-UST log handler to our logger logger.addHandler(lttngUstLogHandler); // Log at will! logger.info("some info"); logger.warning("some warning"); Thread.sleep(500); logger.finer("finer information; the answer is " + answer); Thread.sleep(123); logger.severe("error!"); // Not mandatory, but cleaner logger.removeHandler(lttngUstLogHandler); lttngUstLogHandler.close(); } } ---- Build this example: [role="term"] ---- $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java ---- <>, <> matching 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 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 source code of the Java application, import the LTTng-UST log appender package for Apache log4j: + -- [source,java] ---- import org.lttng.ust.agent.log4j.LttngLogAppender; ---- -- . Create an LTTng-UST log4j log appender: + -- [source,java] ---- Appender lttngUstLogAppender = new LttngLogAppender(); ---- -- . Add this appender to the log4j loggers which should emit LTTng events: + -- [source,java] ---- Logger myLogger = Logger.getLogger("some-logger"); myLogger.addAppender(lttngUstLogAppender); ---- -- . Use Apache log4j log statements and configuration as usual. The loggers with an attached LTTng-UST log appender can emit LTTng events. . Before exiting the application, remove the LTTng-UST log appender from the loggers attached to it and call its `close()` method: + -- [source,java] ---- myLogger.removeAppender(lttngUstLogAppender); lttngUstLogAppender.close(); ---- -- + This isn't strictly necessary, but it is recommended for a clean disposal of the resources of the appender. . Include the common and log4j-specific JAR files of the LTTng-UST Java agent, path:{lttng-ust-agent-common.jar} and path:{lttng-ust-agent-log4j.jar}, in the https://docs.oracle.com/javase/tutorial/essential/environment/paths.html[class path] when you build the Java application. + The JAR files are typically located in dir:{/usr/share/java}. + IMPORTANT: The LTTng-UST Java agent must be <> for the logging framework your application uses. .Use the LTTng-UST Java agent for Apache log4j. ==== [source,java] .path:{Test.java} ---- import org.apache.log4j.Appender; import org.apache.log4j.Logger; import org.lttng.ust.agent.log4j.LttngLogAppender; public class Test { private static final int answer = 42; public static void main(String[] argv) throws Exception { // Create a logger Logger logger = Logger.getLogger("jello"); // Create an LTTng-UST log appender Appender lttngUstLogAppender = new LttngLogAppender(); // Add the LTTng-UST log appender to our logger logger.addAppender(lttngUstLogAppender); // Log at will! logger.info("some info"); logger.warn("some warning"); Thread.sleep(500); logger.debug("debug information; the answer is " + answer); Thread.sleep(123); logger.fatal("error!"); // Not mandatory, but cleaner logger.removeAppender(lttngUstLogAppender); lttngUstLogAppender.close(); } } ---- Build this example (`$LOG4JPATH` is the path to the Apache log4j JAR file): [role="term"] ---- $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-log4j.jar:$LOG4JPATH Test.java ---- <>, <> matching 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 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 source code of the Java application, import the LTTng-UST Java agent context classes and interfaces: + -- [source,java] ---- import org.lttng.ust.agent.context.ContextInfoManager; import org.lttng.ust.agent.context.IContextInfoRetriever; ---- -- . Create a context information retriever class, that is, a class which implements the `IContextInfoRetriever` interface: + -- [source,java] ---- class MyContextInfoRetriever implements IContextInfoRetriever { @Override public Object retrieveContextInfo(String key) { if (key.equals("intCtx")) { return (short) 17; } else if (key.equals("strContext")) { return "context value!"; } else { return null; } } } ---- -- + This `retrieveContextInfo()` method is the only member of the `IContextInfoRetriever` interface. Its role is to return the current value of a state by name to create a context field. The names of the context fields and which state variables they return depends on your specific scenario. + All primitive types and objects are supported as context fields. When `retrieveContextInfo()` returns an object, the context field serializer calls its `toString()` method to add a string field to event records. The method can also return `null`, which means that no context field is available for the required name. . Register an instance of your context information retriever class to the context information manager singleton: + -- [source,java] ---- IContextInfoRetriever cir = new MyContextInfoRetriever(); ContextInfoManager cim = ContextInfoManager.getInstance(); cim.registerContextInfoRetriever("retrieverName", cir); ---- -- . Before exiting the application, remove your context information retriever from the context information manager singleton: + -- [source,java] ---- ContextInfoManager cim = ContextInfoManager.getInstance(); cim.unregisterContextInfoRetriever("retrieverName"); ---- -- + This isn't strictly necessary, but it is recommended for a clean disposal of some resources of the manager. . Build your Java application with LTTng-UST Java agent support as usual, following the procedure for either the <> or <> framework. .Provide application-specific context fields in a Java application. ==== [source,java] .path:{Test.java} ---- import java.util.logging.Handler; import java.util.logging.Logger; import org.lttng.ust.agent.jul.LttngLogHandler; import org.lttng.ust.agent.context.ContextInfoManager; import org.lttng.ust.agent.context.IContextInfoRetriever; public class Test { // Our context information retriever class private static class MyContextInfoRetriever implements IContextInfoRetriever { @Override public Object retrieveContextInfo(String key) { if (key.equals("intCtx")) { return (short) 17; } else if (key.equals("strContext")) { return "context value!"; } else { return null; } } } private static final int answer = 42; public static void main(String args[]) throws Exception { // Get the context information manager instance ContextInfoManager cim = ContextInfoManager.getInstance(); // Create and register our context information retriever IContextInfoRetriever cir = new MyContextInfoRetriever(); cim.registerContextInfoRetriever("myRetriever", cir); // Create a logger Logger logger = Logger.getLogger("jello"); // Create an LTTng-UST log handler Handler lttngUstLogHandler = new LttngLogHandler(); // Add the LTTng-UST log handler to our logger logger.addHandler(lttngUstLogHandler); // Log at will! logger.info("some info"); logger.warning("some warning"); Thread.sleep(500); logger.finer("finer information; the answer is " + answer); Thread.sleep(123); logger.severe("error!"); // Not mandatory, but cleaner logger.removeHandler(lttngUstLogHandler); lttngUstLogHandler.close(); cim.unregisterContextInfoRetriever("myRetriever"); } } ---- Build this example: [role="term"] ---- $ javac -cp /usr/share/java/jarpath/lttng-ust-agent-common.jar:/usr/share/java/jarpath/lttng-ust-agent-jul.jar Test.java ---- <> and <> matching 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 source code of the Python application, import the LTTng-UST Python agent: + -- [source,python] ---- import lttngust ---- -- + The LTTng-UST Python agent automatically adds its logging handler to the root logger at import time. + 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 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 subsystems of the kernel 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 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 the TRACE_EVENT() version for the three * following macro parameters: */ my_subsys_my_event, TP_PROTO(int my_int, const char *my_string), TP_ARGS(my_int, my_string), /* LTTng-modules specific macros */ TP_FIELDS( ctf_integer(int, my_int_field, my_int) ctf_string(my_bar_field, my_bar) ) ) #endif /* !defined(_LTTNG_MY_SUBSYS_H) || defined(TRACE_HEADER_MULTI_READ) */ #include "../../../probes/define_trace.h" ---- -- + The entries in the `TP_FIELDS()` section are the list of fields for the LTTng tracepoint. This is similar to the `TP_STRUCT__entry()` part of the `TRACE_EVENT()` ftrace macro. + See <> for a complete description of the available `ctf_*()` macros. . Create the kernel module C{nbsp}source file of the LTTng-modules probe, +probes/lttng-probe-__subsys__.c+, where +__subsys__+ is your subsystem name: + -- [source,c] .path:{probes/lttng-probe-my-subsys.c} ---- #include #include "../lttng-tracer.h" /* * Build-time verification of mismatch between mainline * TRACE_EVENT() arguments and the LTTng-modules adaptation * layer LTTNG_TRACEPOINT_EVENT() arguments. */ #include /* Create LTTng tracepoint probes */ #define LTTNG_PACKAGE_BUILD #define CREATE_TRACE_POINTS #define TRACE_INCLUDE_PATH ../instrumentation/events/lttng-module #include "../instrumentation/events/lttng-module/my_subsys.h" MODULE_LICENSE("GPL and additional rights"); MODULE_AUTHOR("Your name "); MODULE_DESCRIPTION("LTTng my_subsys probes"); MODULE_VERSION(__stringify(LTTNG_MODULES_MAJOR_VERSION) "." __stringify(LTTNG_MODULES_MINOR_VERSION) "." __stringify(LTTNG_MODULES_PATCHLEVEL_VERSION) LTTNG_MODULES_EXTRAVERSION); ---- -- . Edit path:{probes/KBuild} and add your new kernel module object next to the existing ones: + -- [source,make] .path:{probes/KBuild} ---- # ... obj-m += lttng-probe-module.o obj-m += lttng-probe-power.o obj-m += lttng-probe-my-subsys.o # ... ---- -- . Build and install the LTTng kernel modules: + -- [role="term"] ---- $ make KERNELDIR=/path/to/linux # make modules_install && depmod -a ---- -- + Replace `/path/to/linux` with the path to the Linux source tree where you defined and used tracepoints with the `TRACE_EVENT()` ftrace macro. Note that you can also use the <> instead of `LTTNG_TRACEPOINT_EVENT()` to use custom local variables and C 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 the man:modprobe(8) `--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 name of the created tracing session 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 name of the created tracing session. 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 option of the requested tracing domain 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 name of the tracing session: + -- [role="term"] ---- $ lttng list my-session ---- -- + Replace `my-session` with the desired tracing session 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 output type of the channel 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}2.11). * The current values of many possible **performance counters** using perf, for example: ** CPU cycles, stalled cycles, idle cycles, and the other cycle types. ** Cache misses. ** Branch instructions, misses, and loads. ** CPU faults. * Any 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 tracing domain of the event rule. 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 consumer daemon of the target 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 URL of the relay daemon 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 channels of the <> 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 the current sub-buffers of a tracing session to the file system or send them over the network. When you take a snapshot, LTTng doesn't clear the ring buffers of the tracing session: if you take another snapshot immediately after, both snapshots could contain overlapping trace data. Inspired by https://en.wikipedia.org/wiki/Log_rotation[log rotation], _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 creation of the tracing session or since the last rotation, and then clears those ring buffers to avoid trace data overlaps. What LTTng is about to write when performing a 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-{revision}/src/common/src/common/mi-lttng-4.0.xsd[available] to ease the integration with external tools as much as possible. [role="since-2.8"] [[metadata-regenerate]] === Regenerate the metadata of an LTTng trace An LTTng trace, which is a 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 clock offset of the trace also needs to be updated. Use the `metadata` item of the man:lttng-regenerate(1) command to do so. The main use case of this command is to allow a system to boot with an incorrect wall time and 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 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+) or http://pramfs.sourceforge.net/[PRAMFS] (requires Linux{nbsp}<{nbsp}4). 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 the buffer usage of a channel is too high or too low With the $$C/C++$$ notification and trigger API of LTTng, 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 the buffer usage of an LTTng channel is too high. ==== In this example, we create and build an application which gets notified when the buffer usage of a specific LTTng channel is higher than 75{nbsp}%. We only print that it is the case in the example, but we could as well use the API of <> to disable event rules when this happens. . Create the C{nbsp}source file of application: + -- [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 buffer usage of the channel, * 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 LTTng samples the current values of the channel properties 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 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 <> of the <> 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 its 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`:: The https://docs.oracle.com/javase/7/docs/api/java/util/logging/package-summary.html[core logging facilities] of the Java platform. 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 <>. 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.