FreeBSD/Linux Kernel Cross Reference
sys/Documentation/tracepoints.txt

                          Using the Linux Kernel Tracepoints
     
                                 Mathieu Desnoyers
     
     
     This document introduces Linux Kernel Tracepoints and their use. It provides
     examples of how to insert tracepoints in the kernel and connect probe functions
     to them and provides some examples of probe functions.
     
    
    * Purpose of tracepoints
    
    A tracepoint placed in code provides a hook to call a function (probe) that you
    can provide at runtime. A tracepoint can be "on" (a probe is connected to it) or
    "off" (no probe is attached). When a tracepoint is "off" it has no effect,
    except for adding a tiny time penalty (checking a condition for a branch) and
    space penalty (adding a few bytes for the function call at the end of the
    instrumented function and adds a data structure in a separate section).  When a
    tracepoint is "on", the function you provide is called each time the tracepoint
    is executed, in the execution context of the caller. When the function provided
    ends its execution, it returns to the caller (continuing from the tracepoint
    site).
    
    You can put tracepoints at important locations in the code. They are
    lightweight hooks that can pass an arbitrary number of parameters,
    which prototypes are described in a tracepoint declaration placed in a header
    file.
    
    They can be used for tracing and performance accounting.
    
    
    * Usage
    
    Two elements are required for tracepoints :
    
    - A tracepoint definition, placed in a header file.
    - The tracepoint statement, in C code.
    
    In order to use tracepoints, you should include linux/tracepoint.h.
    
    In include/trace/subsys.h :
    
    #include <linux/tracepoint.h>
    
    DEFINE_TRACE(subsys_eventname,
            TPPTOTO(int firstarg, struct task_struct *p),
            TPARGS(firstarg, p));
    
    In subsys/file.c (where the tracing statement must be added) :
    
    #include <trace/subsys.h>
    
    void somefct(void)
    {
            ...
            trace_subsys_eventname(arg, task);
            ...
    }
    
    Where :
    - subsys_eventname is an identifier unique to your event
        - subsys is the name of your subsystem.
        - eventname is the name of the event to trace.
    - TPPTOTO(int firstarg, struct task_struct *p) is the prototype of the function
      called by this tracepoint.
    - TPARGS(firstarg, p) are the parameters names, same as found in the prototype.
    
    Connecting a function (probe) to a tracepoint is done by providing a probe
    (function to call) for the specific tracepoint through
    register_trace_subsys_eventname().  Removing a probe is done through
    unregister_trace_subsys_eventname(); it will remove the probe sure there is no
    caller left using the probe when it returns. Probe removal is preempt-safe
    because preemption is disabled around the probe call. See the "Probe example"
    section below for a sample probe module.
    
    The tracepoint mechanism supports inserting multiple instances of the same
    tracepoint, but a single definition must be made of a given tracepoint name over
    all the kernel to make sure no type conflict will occur. Name mangling of the
    tracepoints is done using the prototypes to make sure typing is correct.
    Verification of probe type correctness is done at the registration site by the
    compiler. Tracepoints can be put in inline functions, inlined static functions,
    and unrolled loops as well as regular functions.
    
    The naming scheme "subsys_event" is suggested here as a convention intended
    to limit collisions. Tracepoint names are global to the kernel: they are
    considered as being the same whether they are in the core kernel image or in
    modules.
    
    
    * Probe / tracepoint example
    
    See the example provided in samples/tracepoints/src
    
    Compile them with your kernel.
    
    Run, as root :
    modprobe tracepoint-example (insmod order is not important)
    modprobe tracepoint-probe-example
    cat /proc/tracepoint-example (returns an expected error)
   rmmod tracepoint-example tracepoint-probe-example
   dmesg

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