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Based on kernel version 2.6.29. Page generated on 2009-03-25 22:23 EST.

1		             Using the Linux Kernel Tracepoints
2	
3				    Mathieu Desnoyers
4	
5	
6	This document introduces Linux Kernel Tracepoints and their use. It
7	provides examples of how to insert tracepoints in the kernel and
8	connect probe functions to them and provides some examples of probe
9	functions.
10	
11	
12	* Purpose of tracepoints
13	
14	A tracepoint placed in code provides a hook to call a function (probe)
15	that you can provide at runtime. A tracepoint can be "on" (a probe is
16	connected to it) or "off" (no probe is attached). When a tracepoint is
17	"off" it has no effect, except for adding a tiny time penalty
18	(checking a condition for a branch) and space penalty (adding a few
19	bytes for the function call at the end of the instrumented function
20	and adds a data structure in a separate section).  When a tracepoint
21	is "on", the function you provide is called each time the tracepoint
22	is executed, in the execution context of the caller. When the function
23	provided ends its execution, it returns to the caller (continuing from
24	the tracepoint site).
25	
26	You can put tracepoints at important locations in the code. They are
27	lightweight hooks that can pass an arbitrary number of parameters,
28	which prototypes are described in a tracepoint declaration placed in a
29	header file.
30	
31	They can be used for tracing and performance accounting.
32	
33	
34	* Usage
35	
36	Two elements are required for tracepoints :
37	
38	- A tracepoint definition, placed in a header file.
39	- The tracepoint statement, in C code.
40	
41	In order to use tracepoints, you should include linux/tracepoint.h.
42	
43	In include/trace/subsys.h :
44	
45	#include <linux/tracepoint.h>
46	
47	DECLARE_TRACE(subsys_eventname,
48		TPPROTO(int firstarg, struct task_struct *p),
49		TPARGS(firstarg, p));
50	
51	In subsys/file.c (where the tracing statement must be added) :
52	
53	#include <trace/subsys.h>
54	
55	DEFINE_TRACE(subsys_eventname);
56	
57	void somefct(void)
58	{
59		...
60		trace_subsys_eventname(arg, task);
61		...
62	}
63	
64	Where :
65	- subsys_eventname is an identifier unique to your event
66	    - subsys is the name of your subsystem.
67	    - eventname is the name of the event to trace.
68	
69	- TPPROTO(int firstarg, struct task_struct *p) is the prototype of the
70	  function called by this tracepoint.
71	
72	- TPARGS(firstarg, p) are the parameters names, same as found in the
73	  prototype.
74	
75	Connecting a function (probe) to a tracepoint is done by providing a
76	probe (function to call) for the specific tracepoint through
77	register_trace_subsys_eventname().  Removing a probe is done through
78	unregister_trace_subsys_eventname(); it will remove the probe.
79	
80	tracepoint_synchronize_unregister() must be called before the end of
81	the module exit function to make sure there is no caller left using
82	the probe. This, and the fact that preemption is disabled around the
83	probe call, make sure that probe removal and module unload are safe.
84	See the "Probe example" section below for a sample probe module.
85	
86	The tracepoint mechanism supports inserting multiple instances of the
87	same tracepoint, but a single definition must be made of a given
88	tracepoint name over all the kernel to make sure no type conflict will
89	occur. Name mangling of the tracepoints is done using the prototypes
90	to make sure typing is correct. Verification of probe type correctness
91	is done at the registration site by the compiler. Tracepoints can be
92	put in inline functions, inlined static functions, and unrolled loops
93	as well as regular functions.
94	
95	The naming scheme "subsys_event" is suggested here as a convention
96	intended to limit collisions. Tracepoint names are global to the
97	kernel: they are considered as being the same whether they are in the
98	core kernel image or in modules.
99	
100	If the tracepoint has to be used in kernel modules, an
101	EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be
102	used to export the defined tracepoints.
103	
104	* Probe / tracepoint example
105	
106	See the example provided in samples/tracepoints/src
107	
108	Compile them with your kernel.
109	
110	Run, as root :
111	modprobe tracepoint-example (insmod order is not important)
112	modprobe tracepoint-probe-example
113	cat /proc/tracepoint-example (returns an expected error)
114	rmmod tracepoint-example tracepoint-probe-example
115	dmesg
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