path: root/Documentation/trace/tracepoints.txt
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authorLi Zefan <lizf@cn.fujitsu.com>2009-04-09 11:40:27 +0800
committerIngo Molnar <mingo@elte.hu>2009-04-09 07:28:10 +0200
commit66bb74888eb4bef4ba7c87c931ecb7ecca3a240c (patch)
tree567e333523e16ba7285deac89efb645f72a97529 /Documentation/trace/tracepoints.txt
parent9eb85125ce218a8b8d9a7c982510388e227adbec (diff)
tracing: consolidate documents
Move kmemtrace.txt, tracepoints.txt, ftrace.txt and mmiotrace.txt to the new trace/ directory. I didnt find any references to those documents in both source files and documents, so no extra work needs to be done. Signed-off-by: Li Zefan <lizf@cn.fujitsu.com> Acked-by: Pekka Paalanen <pq@iki.fi> Cc: Steven Rostedt <rostedt@goodmis.org> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> LKML-Reference: <49DD6E2B.6090200@cn.fujitsu.com> Signed-off-by: Ingo Molnar <mingo@elte.hu>
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+ Using the Linux Kernel Tracepoints
+ Mathieu Desnoyers
+This document introduces Linux Kernel Tracepoints and their use. It
+provides examples of how to insert tracepoints in the kernel and
+connect probe functions to them and provides some examples of probe
+* Purpose of tracepoints
+A tracepoint placed in code provides a hook to call a function (probe)
+that you can provide at runtime. A tracepoint can be "on" (a probe is
+connected to it) or "off" (no probe is attached). When a tracepoint is
+"off" it has no effect, except for adding a tiny time penalty
+(checking a condition for a branch) and space penalty (adding a few
+bytes for the function call at the end of the instrumented function
+and adds a data structure in a separate section). When a tracepoint
+is "on", the function you provide is called each time the tracepoint
+is executed, in the execution context of the caller. When the function
+provided ends its execution, it returns to the caller (continuing from
+the tracepoint site).
+You can put tracepoints at important locations in the code. They are
+lightweight hooks that can pass an arbitrary number of parameters,
+which prototypes are described in a tracepoint declaration placed in a
+header file.
+They can be used for tracing and performance accounting.
+* Usage
+Two elements are required for tracepoints :
+- A tracepoint definition, placed in a header file.
+- The tracepoint statement, in C code.
+In order to use tracepoints, you should include linux/tracepoint.h.
+In include/trace/subsys.h :
+#include <linux/tracepoint.h>
+ TP_PROTO(int firstarg, struct task_struct *p),
+ TP_ARGS(firstarg, p));
+In subsys/file.c (where the tracing statement must be added) :
+#include <trace/subsys.h>
+void somefct(void)
+ ...
+ trace_subsys_eventname(arg, task);
+ ...
+Where :
+- subsys_eventname is an identifier unique to your event
+ - subsys is the name of your subsystem.
+ - eventname is the name of the event to trace.
+- TP_PROTO(int firstarg, struct task_struct *p) is the prototype of the
+ function called by this tracepoint.
+- TP_ARGS(firstarg, p) are the parameters names, same as found in the
+ prototype.
+Connecting a function (probe) to a tracepoint is done by providing a
+probe (function to call) for the specific tracepoint through
+register_trace_subsys_eventname(). Removing a probe is done through
+unregister_trace_subsys_eventname(); it will remove the probe.
+tracepoint_synchronize_unregister() must be called before the end of
+the module exit function to make sure there is no caller left using
+the probe. This, and the fact that preemption is disabled around the
+probe call, make sure that probe removal and module unload are safe.
+See the "Probe example" section below for a sample probe module.
+The tracepoint mechanism supports inserting multiple instances of the
+same tracepoint, but a single definition must be made of a given
+tracepoint name over all the kernel to make sure no type conflict will
+occur. Name mangling of the tracepoints is done using the prototypes
+to make sure typing is correct. Verification of probe type correctness
+is done at the registration site by the compiler. Tracepoints can be
+put in inline functions, inlined static functions, and unrolled loops
+as well as regular functions.
+The naming scheme "subsys_event" is suggested here as a convention
+intended to limit collisions. Tracepoint names are global to the
+kernel: they are considered as being the same whether they are in the
+core kernel image or in modules.
+If the tracepoint has to be used in kernel modules, an
+used to export the defined tracepoints.
+* Probe / tracepoint example
+See the example provided in samples/tracepoints
+Compile them with your kernel. They are built during 'make' (not
+'make modules') when CONFIG_SAMPLE_TRACEPOINTS=m.
+Run, as root :
+modprobe tracepoint-sample (insmod order is not important)
+modprobe tracepoint-probe-sample
+cat /proc/tracepoint-sample (returns an expected error)
+rmmod tracepoint-sample tracepoint-probe-sample