--- /dev/null
+From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
+Subject: Tracepoints Documentation
+
+Documentation of tracepoint usage.
+
+Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
+Acked-by: 'Peter Zijlstra' <peterz@infradead.org>
+CC: Masami Hiramatsu <mhiramat@redhat.com>
+CC: "Frank Ch. Eigler" <fche@redhat.com>
+CC: 'Ingo Molnar' <mingo@elte.hu>
+CC: 'Hideo AOKI' <haoki@redhat.com>
+CC: Takashi Nishiie <t-nishiie@np.css.fujitsu.com>
+CC: 'Steven Rostedt' <rostedt@goodmis.org>
+CC: Eduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
+Acked-by: Jan Blunck <jblunck@suse.de>
+---
+ Documentation/tracepoints.txt | 101 ++++++++++++++++++++++++++++++++++++++++++
+ 1 file changed, 101 insertions(+)
+
+Index: linux-2.6-lttng/Documentation/tracepoints.txt
+===================================================================
+--- /dev/null 1970-01-01 00:00:00.000000000 +0000
++++ linux-2.6-lttng/Documentation/tracepoints.txt 2008-07-18 12:15:32.000000000 -0400
+@@ -0,0 +1,101 @@
++ 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 functions.
++
++
++* 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>
++
++DEFINE_TRACE(subsys_eventname,
++ TPPTOTO(int firstarg, struct task_struct *p),
++ TPARGS(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.
++- TPPTOTO(int firstarg, struct task_struct *p) is the prototype of the function
++ called by this tracepoint.
++- TPARGS(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 sure there is no
++caller left using the probe when it returns. Probe removal is preempt-safe
++because preemption is disabled around the probe call. 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.
++
++
++* Probe / tracepoint example
++
++See the example provided in samples/tracepoints/src
++
++Compile them with your kernel.
++
++Run, as root :
++modprobe tracepoint-example (insmod order is not important)
++modprobe tracepoint-probe-example
++cat /proc/tracepoint-example (returns an expected error)
++rmmod tracepoint-example tracepoint-probe-example
++dmesg
projects / people / pmueller / ipfire-2.x.git / blobdiff