Author: Mathieu Desnoyers <[EMAIL PROTECTED]>
AuthorDate: Thu Oct 18 23:41:08 2007 -0700
Committer: Linus Torvalds <[EMAIL PROTECTED]>
CommitDate: Fri Oct 19 11:53:55 2007 -0700
Linux Kernel Markers - Documentation
Here is some documentation explaining what is/how to use the Linux
Signed-off-by: Mathieu Desnoyers <[EMAIL PROTECTED]>
Acked-by: "Frank Ch. Eigler" <[EMAIL PROTECTED]>
Cc: Christoph Hellwig <[EMAIL PROTECTED]>
Signed-off-by: Andrew Morton <[EMAIL PROTECTED]>
Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]>
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+ 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.
+In order to use the macro trace_mark, you should include linux/marker.h.
+trace_mark(subsystem_event, "%d %s", someint, somestring);
+- 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.
+- "%d %s" is the formatted string for the serializer.
+- someint is an integer.
+- somestring is a char pointer.
+Connecting a function (probe) to a marker is done by providing a probe
+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 and make
+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 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
+to be detected to have a different format string not to be armed and will
+a printk warning which identifies the inconsistency:
+"Format mismatch for probe probe_name (format), marker (format)"
+* 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)
+cat /proc/marker-example (returns an expected error)
+rmmod marker-example probe-example
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