Here is some documentation explaining what is/how to use the Linux Kernel Markers. Signed-off-by: Mathieu Desnoyers --- Documentation/marker.txt | 244 +++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 244 insertions(+) Index: linux-2.6-lttng/Documentation/marker.txt =================================================================== --- /dev/null 1970-01-01 00:00:00.000000000 +0000 +++ linux-2.6-lttng/Documentation/marker.txt 2007-07-13 20:58:32.000000000 -0400 @@ -0,0 +1,244 @@ + 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 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). 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. 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 + +Add, in your code : + +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 disactivation 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. + + +* Optimization for a given architecture + +One can implement optimized markers for a given architecture by replacing +asm-$ARCH/marker.h. + +To force use of a non-optimized version of the markers, _trace_mark() should be +used. It takes the same parameters as the normal markers, but it does not use +the immediate values based on code patching. + + +* Probe example + +You can build the kernel modules, probe-example.ko and marker-example.ko, +using the following Makefile: +------------------------------ CUT ------------------------------------- +obj-m := probe-example.o marker-example.o +KDIR := /lib/modules/$(shell uname -r)/build +PWD := $(shell pwd) +default: + $(MAKE) -C $(KDIR) SUBDIRS=$(PWD) modules +clean: + rm -f *.mod.c *.ko *.o +------------------------------ CUT ------------------------------------- +/* probe-example.c + * + * Connects two functions to marker call sites. + * + * (C) Copyright 2007 Mathieu Desnoyers + * + * This file is released under the GPLv2. + * See the file COPYING for more details. + */ + +#include +#include +#include +#include +#include + +struct probe_data { + const char *name; + const char *format; + marker_probe_func *probe_func; +}; + +void probe_subsystem_event(const struct __mark_marker *mdata, + 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, + 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"); +------------------------------ CUT ------------------------------------- +/* marker-example.c + * + * Executes a marker when /proc/marker-example is opened. + * + * (C) Copyright 2007 Mathieu Desnoyers + * + * This file is released under the GPLv2. + * See the file COPYING for more details. + */ + +#include +#include +#include +#include + +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("Linux Trace Toolkit example"); +------------------------------ CUT ------------------------------------- +Sequence of operations : (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 +------------------------------ CUT ------------------------------------- -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/