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24b8d831 MD |
1 | Using the Linux Kernel Tracepoints |
2 | ||
3 | Mathieu Desnoyers | |
4 | ||
5 | ||
6 | This document introduces Linux Kernel Tracepoints and their use. It provides | |
7 | examples of how to insert tracepoints in the kernel and connect probe functions | |
8 | to them and provides some examples of probe functions. | |
9 | ||
10 | ||
11 | * Purpose of tracepoints | |
12 | ||
13 | A tracepoint placed in code provides a hook to call a function (probe) that you | |
14 | can provide at runtime. A tracepoint can be "on" (a probe is connected to it) or | |
15 | "off" (no probe is attached). When a tracepoint is "off" it has no effect, | |
16 | except for adding a tiny time penalty (checking a condition for a branch) and | |
17 | space penalty (adding a few bytes for the function call at the end of the | |
18 | instrumented function and adds a data structure in a separate section). When a | |
19 | tracepoint is "on", the function you provide is called each time the tracepoint | |
20 | is executed, in the execution context of the caller. When the function provided | |
21 | ends its execution, it returns to the caller (continuing from the tracepoint | |
22 | site). | |
23 | ||
24 | You can put tracepoints at important locations in the code. They are | |
25 | lightweight hooks that can pass an arbitrary number of parameters, | |
26 | which prototypes are described in a tracepoint declaration placed in a header | |
27 | file. | |
28 | ||
29 | They can be used for tracing and performance accounting. | |
30 | ||
31 | ||
32 | * Usage | |
33 | ||
34 | Two elements are required for tracepoints : | |
35 | ||
36 | - A tracepoint definition, placed in a header file. | |
37 | - The tracepoint statement, in C code. | |
38 | ||
39 | In order to use tracepoints, you should include linux/tracepoint.h. | |
40 | ||
41 | In include/trace/subsys.h : | |
42 | ||
43 | #include <linux/tracepoint.h> | |
44 | ||
45 | DEFINE_TRACE(subsys_eventname, | |
46 | TPPTOTO(int firstarg, struct task_struct *p), | |
47 | TPARGS(firstarg, p)); | |
48 | ||
49 | In subsys/file.c (where the tracing statement must be added) : | |
50 | ||
51 | #include <trace/subsys.h> | |
52 | ||
53 | void somefct(void) | |
54 | { | |
55 | ... | |
56 | trace_subsys_eventname(arg, task); | |
57 | ... | |
58 | } | |
59 | ||
60 | Where : | |
61 | - subsys_eventname is an identifier unique to your event | |
62 | - subsys is the name of your subsystem. | |
63 | - eventname is the name of the event to trace. | |
64 | - TPPTOTO(int firstarg, struct task_struct *p) is the prototype of the function | |
65 | called by this tracepoint. | |
66 | - TPARGS(firstarg, p) are the parameters names, same as found in the prototype. | |
67 | ||
68 | Connecting a function (probe) to a tracepoint is done by providing a probe | |
69 | (function to call) for the specific tracepoint through | |
70 | register_trace_subsys_eventname(). Removing a probe is done through | |
71 | unregister_trace_subsys_eventname(); it will remove the probe sure there is no | |
72 | caller left using the probe when it returns. Probe removal is preempt-safe | |
73 | because preemption is disabled around the probe call. See the "Probe example" | |
74 | section below for a sample probe module. | |
75 | ||
76 | The tracepoint mechanism supports inserting multiple instances of the same | |
77 | tracepoint, but a single definition must be made of a given tracepoint name over | |
78 | all the kernel to make sure no type conflict will occur. Name mangling of the | |
79 | tracepoints is done using the prototypes to make sure typing is correct. | |
80 | Verification of probe type correctness is done at the registration site by the | |
81 | compiler. Tracepoints can be put in inline functions, inlined static functions, | |
82 | and unrolled loops as well as regular functions. | |
83 | ||
84 | The naming scheme "subsys_event" is suggested here as a convention intended | |
85 | to limit collisions. Tracepoint names are global to the kernel: they are | |
86 | considered as being the same whether they are in the core kernel image or in | |
87 | modules. | |
88 | ||
89 | ||
90 | * Probe / tracepoint example | |
91 | ||
92 | See the example provided in samples/tracepoints/src | |
93 | ||
94 | Compile them with your kernel. | |
95 | ||
96 | Run, as root : | |
97 | modprobe tracepoint-example (insmod order is not important) | |
98 | modprobe tracepoint-probe-example | |
99 | cat /proc/tracepoint-example (returns an expected error) | |
100 | rmmod tracepoint-example tracepoint-probe-example | |
101 | dmesg |