Using the Linux Kernel Markers
  
  			    Mathieu Desnoyers
  
  
  This document introduces Linux Kernel Markers and their use. It provides
  examples of how to insert markers in the kernel and connect probe functions to
  them and provides some examples of probe functions.
  
  
  * Purpose of markers
  
  A marker placed in code provides a hook to call a function (probe) that you can
  provide at runtime. A marker can be "on" (a probe is connected to it) or "off"
  (no probe is attached). When a marker 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
  marker is "on", the function you provide is called each time the marker is
  executed, in the execution context of the caller. When the function provided
  ends its execution, it returns to the caller (continuing from the marker site).
  
  You can put markers at important locations in the code. Markers are
  lightweight hooks that can pass an arbitrary number of parameters,
  described in a printk-like format string, to the attached probe function.
  
  They can be used for tracing and performance accounting.
  
  
  * Usage
  
  In order to use the macro trace_mark, you should include linux/marker.h.
  
  #include <linux/marker.h>
  
  And,

  trace_mark(subsystem_event, "myint %d mystring %s", someint, somestring);

  Where :
  - subsystem_event is an identifier unique to your event
      - subsystem is the name of your subsystem.
      - event is the name of the event to mark.

  - "myint %d mystring %s" is the formatted string for the serializer. "myint" and
    "mystring" are repectively the field names associated with the first and
    second parameter.

  - someint is an integer.
  - somestring is a char pointer.
  
  Connecting a function (probe) to a marker is done by providing a probe (function
  to call) for the specific marker through marker_probe_register() and can be
  activated by calling marker_arm(). Marker deactivation can be done by calling
  marker_disarm() as many times as marker_arm() has been called. Removing a probe

  is done through marker_probe_unregister(); it will disarm the probe.

  
  marker_synchronize_unregister() must be called between probe unregistration and
  the first occurrence of
  - the end of module exit function,
    to make sure there is no caller left using the probe;
  - the free of any resource used by the probes,
    to make sure the probes wont be accessing invalid data.
  This, and the fact that preemption is disabled around the probe call, make sure
  that probe removal and module unload are safe. See the "Probe example" section
  below for a sample probe module.

  
  The marker mechanism supports inserting multiple instances of the same marker.
  Markers can be put in inline functions, inlined static functions, and
  unrolled loops as well as regular functions.
  
  The naming scheme "subsystem_event" is suggested here as a convention intended
  to limit collisions. Marker names are global to the kernel: they are considered
  as being the same whether they are in the core kernel image or in modules.
  Conflicting format strings for markers with the same name will cause the markers
  to be detected to have a different format string not to be armed and will output
  a printk warning which identifies the inconsistency:
  
  "Format mismatch for probe probe_name (format), marker (format)"

  Another way to use markers is to simply define the marker without generating any
  function call to actually call into the marker. This is useful in combination
  with tracepoint probes in a scheme like this :
  
  void probe_tracepoint_name(unsigned int arg1, struct task_struct *tsk);
  
  DEFINE_MARKER_TP(marker_eventname, tracepoint_name, probe_tracepoint_name,
  	"arg1 %u pid %d");
  
  notrace void probe_tracepoint_name(unsigned int arg1, struct task_struct *tsk)
  {
  	struct marker *marker = &GET_MARKER(kernel_irq_entry);
  	/* write data to trace buffers ... */
  }

  
  * Probe / marker example
  
  See the example provided in samples/markers/src
  
  Compile them with your kernel.
  
  Run, as root :
  modprobe marker-example (insmod order is not important)
  modprobe probe-example
  cat /proc/marker-example (returns an expected error)
  rmmod marker-example probe-example
  dmesg