Re: [patch 07/10] Linux Kernel Markers - Documentation

Scott Preece
2007-05-10T15:51:54Z

Hi,

Here is a patch to marker.txt to make the English read a little
better. I didn't change the references to out-of-tree packages.

@@ -3,33 +3,30 @@
Mathieu Desnoyers

- This document introduces to markers and discusses its purpose. It
-shows some usage examples of the Linux Kernel Markers : how to insert markers
-within the kernel and how to connect probes to a marker. Finally, it has some
-probe module examples. This is what connects to a marker.
-
+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 your code provides a hook to a function (probe) that
+A marker placed in your 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). An "off" marker has no effect. When turned 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 (marker site).
+or "off" (no probe is attached). When a marker is "off" it has no
+effect. 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. They act as
+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 a function whenever the marker
-code is reached.
+described in a printk-like format string, to the attached probe function.

-They can be used for tracing (LTTng, LKET over SystemTAP), overall performance
-accounting (SystemTAP). They could also be used to implement
-efficient hooks for SELinux or any other subsystem that would have this
-kind of need.
+Markers can be used for tracing (LTTng, LKET over SystemTAP), overall
+performance accounting (SystemTAP), or to hook to monitoring
+subsystems (like SELinux) that need to observe kernel behavior.

-Using the markers for system audit (SELinux) would require to pass a
-variable by address that would be later checked by the marked routine.
+Using the markers for system audit (SELinux) would require passing a
+variable by address so that it could be checked by the marked routine.

* Usage
@@ -52,15 +49,15 @@ Where :
Connecting a function (probe) to a marker is done by providing a probe
(function to call) for the specific marker through marker_set_probe(). It will
automatically connect the function and enable the marker site. Removing a probe
-is done through marker_remove_probe(). Probe removal is preempt safe because
+is done through marker_remove_probe(). 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 multiple instances of the same marker.
-Markers can be put in inline functions, inlined static functions and
+The marker mechanism supports inserting multiple instances of the same marker.
+Markers can be put in inline functions, inlined static functions, and
unrolled loops.

-Note : It is safe to put markers within preempt-safe code : preempt_enable()
+Note: It is safe to put markers within preempt-safe code : preempt_enable()
will not call the scheduler due to the tests in preempt_schedule().

@@ -68,23 +65,23 @@ will not call the scheduler due to the t

You will find, in asm-*/marker.h, optimisations for given architectures
(currently i386 and powerpc). They use a load immediate instead of a data load,
-which saves a data cache hit, but also requires cross CPU code modification. In
-order to support embedded systems which use read-only memory for
their code, the
+which saves a data cache hit, but also requires cross-CPU code modification. In
+order to support embedded systems that use read-only memory for their code, the
optimization can be disabled through menu options.

The MF_* flags can be used to control the type of marker. See the
include/marker.h header for the list of flags. They can be specified as the
-first parameter of the _trace_mark() macro, such as the following example which
+first parameter of the _trace_mark() macro, as in the following example, which
is safe with respect to lockdep.c (useful for marking lockdep.c and printk
functions).

_trace_mark(MF_DEFAULT | ~MF_LOCKDEP, subsystem_eventb, MARK_NOARGS);

-Another example is to specify that a specific marker must never call printk :
+Another example is to specify that a specific marker must never call printk:
_trace_mark(MF_DEFAULT | ~MF_PRINTK, subsystem_eventc,
"%d %s", someint, somestring,);

-Flag compatibility is checked before connecting the probe to the marker : the
+Flag compatibility is checked before connecting the probe to the marker: the
right flags must be given to _marker_set_enable().