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    Subject[patch 3/4] 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@polymtl.ca>
    Acked-by: "Frank Ch. Eigler" <fche@redhat.com>
    CC: Christoph Hellwig <hch@infradead.org>
    ---

    Documentation/markers/markers.txt | 93 +++++++++++++++++++++++++++
    Documentation/markers/src/Makefile | 7 ++
    Documentation/markers/src/marker-example.c | 55 ++++++++++++++++
    Documentation/markers/src/probe-example.c | 98 +++++++++++++++++++++++++++++
    4 files changed, 253 insertions(+)

    Index: linux-2.6-lttng/Documentation/markers/markers.txt
    ===================================================================
    --- /dev/null 1970-01-01 00:00:00.000000000 +0000
    +++ linux-2.6-lttng/Documentation/markers/markers.txt 2007-09-07 09:17:45.000000000 -0400
    @@ -0,0 +1,93 @@
    + 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). The
    +immediate values are used to minimize the impact on data cache, encoding the
    +condition in the instruction stream. 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, "%d %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.
    +- "%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 (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 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.
    +
    +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)"
    +
    +
    +* Optimization for a given architecture
    +
    +One can implement optimized markers for a given architecture by replacing
    +asm-$ARCH/marker.h.
    +
    +To force use of a non-optimized version of the markers, _trace_mark() should be
    +used. It takes the same parameters as the normal markers, but it does not use
    +the immediate values based on code patching.
    +
    +
    +* Probe / marker example
    +
    +See the example provided in Documentation/markers/markers/src
    +
    +Run, as root :
    +
    +make
    +insmod marker-example.ko (insmod order is not important)
    +insmod probe-example.ko
    +cat /proc/marker-example (returns an expected error)
    +rmmod marker-example probe-example
    +dmesg
    Index: linux-2.6-lttng/Documentation/markers/src/Makefile
    ===================================================================
    --- /dev/null 1970-01-01 00:00:00.000000000 +0000
    +++ linux-2.6-lttng/Documentation/markers/src/Makefile 2007-09-07 09:14:38.000000000 -0400
    @@ -0,0 +1,7 @@
    +obj-m := probe-example.o marker-example.o
    +KDIR := /lib/modules/$(shell uname -r)/build
    +PWD := $(shell pwd)
    +default:
    + $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules
    +clean:
    + rm -f *.mod.c *.ko *.o
    Index: linux-2.6-lttng/Documentation/markers/src/marker-example.c
    ===================================================================
    --- /dev/null 1970-01-01 00:00:00.000000000 +0000
    +++ linux-2.6-lttng/Documentation/markers/src/marker-example.c 2007-09-07 09:16:27.000000000 -0400
    @@ -0,0 +1,55 @@
    +/* marker-example.c
    + *
    + * Executes a marker when /proc/marker-example is opened.
    + *
    + * (C) Copyright 2007 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
    + *
    + * This file is released under the GPLv2.
    + * See the file COPYING for more details.
    + */
    +
    +#include <linux/module.h>
    +#include <linux/marker.h>
    +#include <linux/sched.h>
    +#include <linux/proc_fs.h>
    +
    +struct proc_dir_entry *pentry_example = NULL;
    +
    +static int my_open(struct inode *inode, struct file *file)
    +{
    + int i;
    +
    + trace_mark(subsystem_event, "%d %s", 123, "example string");
    + for (i=0; i<10; i++) {
    + trace_mark(subsystem_eventb, MARK_NOARGS);
    + }
    + return -EPERM;
    +}
    +
    +static struct file_operations mark_ops = {
    + .open = my_open,
    +};
    +
    +static int example_init(void)
    +{
    + printk(KERN_ALERT "example init\n");
    + pentry_example = create_proc_entry("marker-example", 0444, NULL);
    + if (pentry_example)
    + pentry_example->proc_fops = &mark_ops;
    + else
    + return -EPERM;
    + return 0;
    +}
    +
    +static void example_exit(void)
    +{
    + printk(KERN_ALERT "example exit\n");
    + remove_proc_entry("marker-example", NULL);
    +}
    +
    +module_init(example_init)
    +module_exit(example_exit)
    +
    +MODULE_LICENSE("GPL");
    +MODULE_AUTHOR("Mathieu Desnoyers");
    +MODULE_DESCRIPTION("Marker example");
    Index: linux-2.6-lttng/Documentation/markers/src/probe-example.c
    ===================================================================
    --- /dev/null 1970-01-01 00:00:00.000000000 +0000
    +++ linux-2.6-lttng/Documentation/markers/src/probe-example.c 2007-09-07 09:18:55.000000000 -0400
    @@ -0,0 +1,98 @@
    +/* probe-example.c
    + *
    + * Connects two functions to marker call sites.
    + *
    + * (C) Copyright 2007 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
    + *
    + * This file is released under the GPLv2.
    + * See the file COPYING for more details.
    + */
    +
    +#include <linux/sched.h>
    +#include <linux/kernel.h>
    +#include <linux/module.h>
    +#include <linux/marker.h>
    +#include <asm/atomic.h>
    +
    +struct probe_data {
    + const char *name;
    + const char *format;
    + marker_probe_func *probe_func;
    +};
    +
    +void probe_subsystem_event(const struct __mark_marker *mdata,
    + void *private, const char *format, ...)
    +{
    + va_list ap;
    + /* Declare args */
    + unsigned int value;
    + const char *mystr;
    +
    + /* Assign args */
    + va_start(ap, format);
    + value = va_arg(ap, typeof(value));
    + mystr = va_arg(ap, typeof(mystr));
    +
    + /* Call printk */
    + printk("Value %u, string %s\n", value, mystr);
    +
    + /* or count, check rights, serialize data in a buffer */
    +
    + va_end(ap);
    +}
    +
    +atomic_t eventb_count = ATOMIC_INIT(0);
    +
    +void probe_subsystem_eventb(const struct __mark_marker *mdata,
    + void *private, const char *format, ...)
    +{
    + /* Increment counter */
    + atomic_inc(&eventb_count);
    +}
    +
    +static struct probe_data probe_array[] =
    +{
    + { .name = "subsystem_event",
    + .format = "%d %s",
    + .probe_func = probe_subsystem_event },
    + { .name = "subsystem_eventb",
    + .format = MARK_NOARGS,
    + .probe_func = probe_subsystem_eventb },
    +};
    +
    +static int __init probe_init(void)
    +{
    + int result;
    + int i;
    +
    + for (i = 0; i < ARRAY_SIZE(probe_array); i++) {
    + result = marker_probe_register(probe_array[i].name,
    + probe_array[i].format,
    + probe_array[i].probe_func, &probe_array[i]);
    + if (result)
    + printk(KERN_INFO "Unable to register probe %s\n",
    + probe_array[i].name);
    + result = marker_arm(probe_array[i].name);
    + if (result)
    + printk(KERN_INFO "Unable to arm probe %s\n",
    + probe_array[i].name);
    + }
    + return 0;
    +}
    +
    +static void __exit probe_fini(void)
    +{
    + int i;
    +
    + for (i = 0; i < ARRAY_SIZE(probe_array); i++) {
    + marker_probe_unregister(probe_array[i].name);
    + }
    + printk("Number of event b : %u\n", atomic_read(&eventb_count));
    +}
    +
    +module_init(probe_init);
    +module_exit(probe_fini);
    +
    +MODULE_LICENSE("GPL");
    +MODULE_AUTHOR("Mathieu Desnoyers");
    +MODULE_DESCRIPTION("SUBSYSTEM Probe");
    --
    Mathieu Desnoyers
    Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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