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
