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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 | TP_PROTO(int firstarg, struct task_struct *p), |
49 | TP_ARGS(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 | - TP_PROTO(int firstarg, struct task_struct *p) is the prototype of the |
70 | function called by this tracepoint. |
71 | |
72 | - TP_ARGS(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 |
107 | |
108 | Compile them with your kernel. They are built during 'make' (not |
109 | 'make modules') when CONFIG_SAMPLE_TRACEPOINTS=m. |
110 | |
111 | Run, as root : |
112 | modprobe tracepoint-sample (insmod order is not important) |
113 | modprobe tracepoint-probe-sample |
114 | cat /proc/tracepoint-sample (returns an expected error) |
115 | rmmod tracepoint-sample tracepoint-probe-sample |
116 | dmesg |
117 |
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