tracing: Remove markers

 	             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

#include <linux/poll.h>

#include <linux/ptrace.h>

#include <linux/seq_file.h>

#include <linux/marker.h>

#include <asm/io.h>

#include <asm/time.h>

#include <linux/pid_namespace.h>

#include <linux/proc_fs.h>

#include <linux/seq_file.h>

#include <linux/marker.h>

#include <asm/io.h>

#include <asm/mmu_context.h>

#include <linux/sched.h>

#include <linux/mm.h>

#include <linux/preempt.h>

#include <linux/marker.h>

#include <linux/msi.h>

#include <asm/signal.h>

#ifndef _LINUX_MARKER_H

#define _LINUX_MARKER_H

/*

 * Code markup for dynamic and static tracing.

 *

 * See Documentation/marker.txt.

 *

 * (C) Copyright 2006 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>

 *

 * This file is released under the GPLv2.

 * See the file COPYING for more details.

 */

#include <stdarg.h>

#include <linux/types.h>

struct module;

struct marker;

/**

 * marker_probe_func - Type of a marker probe function

 * @probe_private: probe private data

 * @call_private: call site private data

 * @fmt: format string

 * @args: variable argument list pointer. Use a pointer to overcome C's

 *        inability to pass this around as a pointer in a portable manner in

 *        the callee otherwise.

 *

 * Type of marker probe functions. They receive the mdata and need to parse the

 * format string to recover the variable argument list.

 */

typedef void marker_probe_func(void *probe_private, void *call_private,

		const char *fmt, va_list *args);

struct marker_probe_closure {

	marker_probe_func *func;	/* Callback */

	void *probe_private;		/* Private probe data */

};

struct marker {

	const char *name;	/* Marker name */

	const char *format;	/* Marker format string, describing the

				 * variable argument list.

				 */

	char state;		/* Marker state. */

	char ptype;		/* probe type : 0 : single, 1 : multi */

				/* Probe wrapper */

	void (*call)(const struct marker *mdata, void *call_private, ...);

	struct marker_probe_closure single;

	struct marker_probe_closure *multi;

	const char *tp_name;	/* Optional tracepoint name */

	void *tp_cb;		/* Optional tracepoint callback */

} __attribute__((aligned(8)));

#ifdef CONFIG_MARKERS

#define _DEFINE_MARKER(name, tp_name_str, tp_cb, format)		\

		static const char __mstrtab_##name[]			\

		__attribute__((section("__markers_strings")))		\

		= #name "\0" format;					\

		static struct marker __mark_##name			\

		__attribute__((section("__markers"), aligned(8))) =	\

		{ __mstrtab_##name, &__mstrtab_##name[sizeof(#name)],	\

		  0, 0, marker_probe_cb, { __mark_empty_function, NULL},\

		  NULL, tp_name_str, tp_cb }

#define DEFINE_MARKER(name, format)					\

		_DEFINE_MARKER(name, NULL, NULL, format)

#define DEFINE_MARKER_TP(name, tp_name, tp_cb, format)			\

		_DEFINE_MARKER(name, #tp_name, tp_cb, format)

/*

 * Note : the empty asm volatile with read constraint is used here instead of a

 * "used" attribute to fix a gcc 4.1.x bug.

 * Make sure the alignment of the structure in the __markers section will

 * not add unwanted padding between the beginning of the section and the

 * structure. Force alignment to the same alignment as the section start.

 *

 * The "generic" argument controls which marker enabling mechanism must be used.

 * If generic is true, a variable read is used.

 * If generic is false, immediate values are used.

 */

#define __trace_mark(generic, name, call_private, format, args...)	\

	do {								\

		DEFINE_MARKER(name, format);				\

		__mark_check_format(format, ## args);			\

		if (unlikely(__mark_##name.state)) {			\

			(*__mark_##name.call)				\

				(&__mark_##name, call_private, ## args);\

		}							\

	} while (0)

#define __trace_mark_tp(name, call_private, tp_name, tp_cb, format, args...) \

	do {								\

		void __check_tp_type(void)				\

		{							\

			register_trace_##tp_name(tp_cb);		\

		}							\

		DEFINE_MARKER_TP(name, tp_name, tp_cb, format);		\

		__mark_check_format(format, ## args);			\

		(*__mark_##name.call)(&__mark_##name, call_private,	\

					## args);			\

	} while (0)

extern void marker_update_probe_range(struct marker *begin,

	struct marker *end);

#define GET_MARKER(name)	(__mark_##name)

#else /* !CONFIG_MARKERS */

#define DEFINE_MARKER(name, tp_name, tp_cb, format)

#define __trace_mark(generic, name, call_private, format, args...) \

		__mark_check_format(format, ## args)

#define __trace_mark_tp(name, call_private, tp_name, tp_cb, format, args...) \

	do {								\

		void __check_tp_type(void)				\

		{							\

			register_trace_##tp_name(tp_cb);		\

		}							\

		__mark_check_format(format, ## args);			\

	} while (0)

static inline void marker_update_probe_range(struct marker *begin,

	struct marker *end)

{ }

#define GET_MARKER(name)

#endif /* CONFIG_MARKERS */

/**

 * trace_mark - Marker using code patching

 * @name: marker name, not quoted.

 * @format: format string

 * @args...: variable argument list

 *

 * Places a marker using optimized code patching technique (imv_read())

 * to be enabled when immediate values are present.

 */

#define trace_mark(name, format, args...) \

	__trace_mark(0, name, NULL, format, ## args)

/**

 * _trace_mark - Marker using variable read

 * @name: marker name, not quoted.

 * @format: format string

 * @args...: variable argument list

 *

 * Places a marker using a standard memory read (_imv_read()) to be

 * enabled. Should be used for markers in code paths where instruction

 * modification based enabling is not welcome. (__init and __exit functions,

 * lockdep, some traps, printk).

 */

#define _trace_mark(name, format, args...) \

	__trace_mark(1, name, NULL, format, ## args)

/**

 * trace_mark_tp - Marker in a tracepoint callback

 * @name: marker name, not quoted.

 * @tp_name: tracepoint name, not quoted.

 * @tp_cb: tracepoint callback. Should have an associated global symbol so it

 *         is not optimized away by the compiler (should not be static).

 * @format: format string

 * @args...: variable argument list

 *

 * Places a marker in a tracepoint callback.

 */

#define trace_mark_tp(name, tp_name, tp_cb, format, args...)	\

	__trace_mark_tp(name, NULL, tp_name, tp_cb, format, ## args)

/**

 * MARK_NOARGS - Format string for a marker with no argument.

 */

#define MARK_NOARGS " "

/* To be used for string format validity checking with gcc */

static inline void __printf(1, 2) ___mark_check_format(const char *fmt, ...)

{

}

#define __mark_check_format(format, args...)				\

	do {								\

		if (0)							\

			___mark_check_format(format, ## args);		\

	} while (0)

extern marker_probe_func __mark_empty_function;

extern void marker_probe_cb(const struct marker *mdata,

	void *call_private, ...);

/*

 * Connect a probe to a marker.

 * private data pointer must be a valid allocated memory address, or NULL.

 */

extern int marker_probe_register(const char *name, const char *format,

				marker_probe_func *probe, void *probe_private);

/*

 * Returns the private data given to marker_probe_register.

 */

extern int marker_probe_unregister(const char *name,

	marker_probe_func *probe, void *probe_private);

/*

 * Unregister a marker by providing the registered private data.

 */

extern int marker_probe_unregister_private_data(marker_probe_func *probe,

	void *probe_private);

extern void *marker_get_private_data(const char *name, marker_probe_func *probe,

	int num);

/*

 * marker_synchronize_unregister must be called between the last marker probe

 * unregistration and the first one of

 * - the end of module exit function

 * - the free of any resource used by the probes

 * to ensure the code and data are valid for any possibly running probes.

 */

#define marker_synchronize_unregister() synchronize_sched()

#endif