1 Using the Linux Kernel Tracepoints 2 3 Mathieu Desnoyers 4 5 6This document introduces Linux Kernel Tracepoints and their use. It 7provides examples of how to insert tracepoints in the kernel and 8connect probe functions to them and provides some examples of probe 9functions. 10 11 12* Purpose of tracepoints 13 14A tracepoint placed in code provides a hook to call a function (probe) 15that you can provide at runtime. A tracepoint can be "on" (a probe is 16connected 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 19bytes for the function call at the end of the instrumented function 20and adds a data structure in a separate section). When a tracepoint 21is "on", the function you provide is called each time the tracepoint 22is executed, in the execution context of the caller. When the function 23provided ends its execution, it returns to the caller (continuing from 24the tracepoint site). 25 26You can put tracepoints at important locations in the code. They are 27lightweight hooks that can pass an arbitrary number of parameters, 28which prototypes are described in a tracepoint declaration placed in a 29header file. 30 31They can be used for tracing and performance accounting. 32 33 34* Usage 35 36Two elements are required for tracepoints : 37 38- A tracepoint definition, placed in a header file. 39- The tracepoint statement, in C code. 40 41In order to use tracepoints, you should include linux/tracepoint.h. 42 43In include/trace/subsys.h : 44 45#include <linux/tracepoint.h> 46 47DECLARE_TRACE(subsys_eventname, 48 TPPROTO(int firstarg, struct task_struct *p), 49 TPARGS(firstarg, p)); 50 51In subsys/file.c (where the tracing statement must be added) : 52 53#include <trace/subsys.h> 54 55DEFINE_TRACE(subsys_eventname); 56 57void somefct(void) 58{ 59 ... 60 trace_subsys_eventname(arg, task); 61 ... 62} 63 64Where : 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 75Connecting a function (probe) to a tracepoint is done by providing a 76probe (function to call) for the specific tracepoint through 77register_trace_subsys_eventname(). Removing a probe is done through 78unregister_trace_subsys_eventname(); it will remove the probe. 79 80tracepoint_synchronize_unregister() must be called before the end of 81the module exit function to make sure there is no caller left using 82the probe. This, and the fact that preemption is disabled around the 83probe call, make sure that probe removal and module unload are safe. 84See the "Probe example" section below for a sample probe module. 85 86The tracepoint mechanism supports inserting multiple instances of the 87same tracepoint, but a single definition must be made of a given 88tracepoint name over all the kernel to make sure no type conflict will 89occur. Name mangling of the tracepoints is done using the prototypes 90to make sure typing is correct. Verification of probe type correctness 91is done at the registration site by the compiler. Tracepoints can be 92put in inline functions, inlined static functions, and unrolled loops 93as well as regular functions. 94 95The naming scheme "subsys_event" is suggested here as a convention 96intended to limit collisions. Tracepoint names are global to the 97kernel: they are considered as being the same whether they are in the 98core kernel image or in modules. 99 100If the tracepoint has to be used in kernel modules, an 101EXPORT_TRACEPOINT_SYMBOL_GPL() or EXPORT_TRACEPOINT_SYMBOL() can be 102used to export the defined tracepoints. 103 104* Probe / tracepoint example 105 106See the example provided in samples/tracepoints/src 107 108Compile them with your kernel. 109 110Run, as root : 111modprobe tracepoint-example (insmod order is not important) 112modprobe tracepoint-probe-example 113cat /proc/tracepoint-example (returns an expected error) 114rmmod tracepoint-example tracepoint-probe-example 115dmesg 116