[PATCH] Linux Kernel Markers Documentation

[Mathieu Desnoyers]

Linux Kernel Markers - Documentation

Here is some documentation explaining what is/how to use the Linux
Kernel Markers.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>

--- /dev/null
+++ b/Documentation/marker.txt
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+ 	             Using the Linux Kernel Markers
+
+			    Mathieu Desnoyers
+
+
+	This document discusses the purpose of markers. 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.
+
+
+* Purpose of markers
+
+You can put markers at important locations in the code. They act as
+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.
+
+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 the would have this
+kind of need.
+
+Using the markers for system audit (SELinux) would require to pass a
+variable by address that would be later checked by the marked routine.
+
+
+* Usage
+
+MARK(subsystem_event, "%d %s %p[struct task_struct]",
+  someint, somestring, current);
+Where :
+- Subsystem is the name of your subsystem.
+- event is the name of the event to mark.
+- "%d %s %p[struct task_struct]" is the formatted string for (printk-style).
+- someint is an integer.
+- somestring is a char pointer.
+- current is a pointer to a struct task_struct.
+
+The expression %p[struct task_struct] is a suggested marker definition
+standard that could eventually be used for pointer type checking in
+sparse. The brackets contain the type to which the pointer refer.
+
+The marker mechanism supports 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()
+will not call the scheduler due to the tests in preempt_schedule().
+
+
+* Optimization for a given architecture
+
+You will find, in asm-*/marker.h, optimisations for given architectures
+(currently i386 and powerpc). They use a load immediate instead of a data read,
+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
+optimization can be disabled through menu options.
+
+
+* Probe example
+
+------------------------------ CUT -------------------------------------
+/* probe-example.c
+ *
+ * Loads a function at a marker call site.
+ *
+ * (C) Copyright 2007 Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
+ *
+ * This file is released under the GPLv2.
+ * See the file COPYING for more details.
+ */
+
+#include <linux/sched.h>
+#include <linux/kernel.h>
+
+#define SUBSYSTEM_EVENT_FORMAT "%d %s %p[struct task_struct]"
+void probe_subsystem_event(const char *format, ...)
+{
+	va_list ap;
+	/* Declare args */
+	unsigned int value;
+	const char *mystr;
+	struct task_struct *task;
+
+	/* Assign args */
+	va_start(ap, format);
+	value = va_arg(ap, typeof(value));
+	mystr = va_arg(ap, typeof(mystr));
+	task = va_arg(ap, typeof(task));
+
+	/* Call tracer */
+	trace_subsystem_event(value, mystr, task);
+
+	/* Or call printk */
+	vprintk(format, ap);
+
+	/* or count, check rights... */
+	
+	va_end(ap);
+}
+
+static int __init probe_init(void)
+{
+	int result;
+	result = marker_set_probe("subsystem_event",
+			FS_CLOSE_FORMAT,
+			probe_fs_close);
+	if (!result)
+		goto cleanup;
+	return 0;
+
+cleanup:
+	marker_remove_probe(probe_subsystem_event);
+	return -EPERM;
+}
+
+static void __exit probe_fini(void)
+{	
+	marker_remove_probe(probe_subsystem_event);
+}
+
+module_init(probe_init);
+module_exit(probe_fini);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Mathieu Desnoyers");
+MODULE_DESCRIPTION("SUBSYSTEM Probe");
+------------------------------ CUT -------------------------------------
+
-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Candidate, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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