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1 | Using the Linux Kernel Tracepoints |
2 | ||
3 | Mathieu Desnoyers | |
4 | ||
5 | ||
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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. | |
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10 | |
11 | ||
12 | * Purpose of tracepoints | |
13 | ||
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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). | |
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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, | |
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28 | which prototypes are described in a tracepoint declaration placed in a |
29 | header file. | |
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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 | ||
7e066fb8 | 47 | DECLARE_TRACE(subsys_eventname, |
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48 | TP_PROTO(int firstarg, struct task_struct *p), |
49 | TP_ARGS(firstarg, p)); | |
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50 | |
51 | In subsys/file.c (where the tracing statement must be added) : | |
52 | ||
53 | #include <trace/subsys.h> | |
54 | ||
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55 | DEFINE_TRACE(subsys_eventname); |
56 | ||
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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. | |
24b8d831 | 68 | |
2939b046 | 69 | - TP_PROTO(int firstarg, struct task_struct *p) is the prototype of the |
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70 | function called by this tracepoint. |
71 | ||
2939b046 | 72 | - TP_ARGS(firstarg, p) are the parameters names, same as found in the |
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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 | |
24b8d831 | 77 | register_trace_subsys_eventname(). Removing a probe is done through |
8fd88d15 | 78 | unregister_trace_subsys_eventname(); it will remove the probe. |
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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. | |
24b8d831 | 99 | |
7e066fb8 | 100 | If the tracepoint has to be used in kernel modules, an |
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101 | EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be |
102 | used to export the defined tracepoints. | |
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103 | |
104 | * Probe / tracepoint example | |
105 | ||
0a5d6490 | 106 | See the example provided in samples/tracepoints |
24b8d831 | 107 | |
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108 | Compile them with your kernel. They are built during 'make' (not |
109 | 'make modules') when CONFIG_SAMPLE_TRACEPOINTS=m. | |
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110 | |
111 | Run, as root : | |
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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 | |
24b8d831 | 116 | dmesg |