tracing/events: add trace-events-sample

This patch adds a sample to the samples directory on how to create
and use TRACE_EVENT trace points.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

samples/Kconfig

@@ -19,6 +19,13 @@ config SAMPLE_TRACEPOINTS
 	help
 	  This build tracepoints example modules.
 
+config SAMPLE_TRACE_EVENTS
+	tristate "Build trace_events examples"
+	depends on EVENT_TRACING
+	default m
+	help
+	  This build trace event example modules.
+
 config SAMPLE_KOBJECT
 	tristate "Build kobject examples"
 	help

samples/Makefile

 # Makefile for Linux samples code
 
-obj-$(CONFIG_SAMPLES)	+= markers/ kobject/ kprobes/ tracepoints/
+obj-$(CONFIG_SAMPLES)	+= markers/ kobject/ kprobes/ tracepoints/ trace_events/

samples/trace_events/Makefile

+# builds the trace events example kernel modules;
+# then to use one (as root):  insmod <module_name.ko>
+
+PWD := $(shell pwd)
+
+CFLAGS_trace-events-sample.o := -I$(PWD)/samples/trace_events/
+
+obj-$(CONFIG_SAMPLE_TRACE_EVENTS) += trace-events-sample.o

samples/trace_events/trace-events-sample.c

+#include <linux/module.h>
+#include <linux/kthread.h>
+
+/*
+ * Any file that uses trace points, must include the header.
+ * But only one file, must include the header by defining
+ * CREATE_TRACE_POINTS first.  This will make the C code that
+ * creates the handles for the trace points.
+ */
+#define CREATE_TRACE_POINTS
+#include "trace-events-sample.h"
+
+
+static void simple_thread_func(int cnt)
+{
+	set_current_state(TASK_INTERRUPTIBLE);
+	schedule_timeout(HZ);
+	trace_foo_bar("hello", cnt);
+
+	if (!(cnt % 10))
+		/* It is really important that I say "hi!" */
+		printk(KERN_EMERG "hi!\n");
+}
+
+static int simple_thread(void *arg)
+{
+	int cnt = 0;
+
+	while (!kthread_should_stop())
+		simple_thread_func(cnt++);
+
+	return 0;
+}
+
+static struct task_struct *simple_tsk;
+
+static int __init trace_event_init(void)
+{
+	simple_tsk = kthread_run(simple_thread, NULL, "event-sample");
+	if (IS_ERR(simple_tsk))
+		return -1;
+
+	return 0;
+}
+
+static void __exit trace_event_exit(void)
+{
+	kthread_stop(simple_tsk);
+}
+
+module_init(trace_event_init);
+module_exit(trace_event_exit);
+
+MODULE_AUTHOR("Steven Rostedt");
+MODULE_DESCRIPTION("trace-events-sample");
+MODULE_LICENSE("GPL");

samples/trace_events/trace-events-sample.h

+/*
+ * Notice that this file is not protected like a normal header.
+ * We also must allow for rereading of this file. The
+ *
+ *  || defined(TRACE_HEADER_MULTI_READ)
+ *
+ * serves this purpose.
+ */
+#if !defined(_TRACE_EVENT_SAMPLE_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_EVENT_SAMPLE_H
+
+/*
+ * All trace headers should include tracepoint.h, until we finally
+ * make it into a standard header.
+ */
+#include <linux/tracepoint.h>
+
+/*
+ * If TRACE_SYSTEM is defined, that will be the directory created
+ * in the ftrace directory under /debugfs/tracing/events/<system>
+ *
+ * The define_trace.h belowe will also look for a file name of
+ * TRACE_SYSTEM.h where TRACE_SYSTEM is what is defined here.
+ *
+ * If you want a different system than file name, you can override
+ * the header name by defining TRACE_INCLUDE_FILE
+ *
+ * If this file was called, goofy.h, then we would define:
+ *
+ * #define TRACE_INCLUDE_FILE goofy
+ *
+ */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM trace-events-sample
+
+/*
+ * The TRACE_EVENT macro is broken up into 5 parts.
+ *
+ * name: name of the trace point. This is also how to enable the tracepoint.
+ *   A function called trace_foo_bar() will be created.
+ *
+ * proto: the prototype of the function trace_foo_bar()
+ *   Here it is trace_foo_bar(char *foo, int bar).
+ *
+ * args:  must match the arguments in the prototype.
+ *    Here it is simply "foo, bar".
+ *
+ * struct:  This defines the way the data will be stored in the ring buffer.
+ *    There are currently two types of elements. __field and __array.
+ *    a __field is broken up into (type, name). Where type can be any
+ *    type but an array.
+ *    For an array. there are three fields. (type, name, size). The
+ *    type of elements in the array, the name of the field and the size
+ *    of the array.
+ *
+ *    __array( char, foo, 10) is the same as saying   char foo[10].
+ *
+ * fast_assign: This is a C like function that is used to store the items
+ *    into the ring buffer.
+ *
+ * printk: This is a way to print out the data in pretty print. This is
+ *    useful if the system crashes and you are logging via a serial line,
+ *    the data can be printed to the console using this "printk" method.
+ *
+ * Note, that for both the assign and the printk, __entry is the handler
+ * to the data structure in the ring buffer, and is defined by the
+ * TP_STRUCT__entry.
+ */
+TRACE_EVENT(foo_bar,
+
+	TP_PROTO(char *foo, int bar),
+
+	TP_ARGS(foo, bar),
+
+	TP_STRUCT__entry(
+		__array(	char,	foo,    10		)
+		__field(	int,	bar			)
+	),
+
+	TP_fast_assign(
+		strncpy(__entry->foo, foo, 10);
+		__entry->bar	= bar;
+	),
+
+	TP_printk("foo %s %d", __entry->foo, __entry->bar)
+);
+#endif
+
+/***** NOTICE! The #if protection ends here. *****/
+
+
+/*
+ * There are several ways I could have done this. If I left out the
+ * TRACE_INCLUDE_PATH, then it would default to the kernel source
+ * include/trace/events directory.
+ *
+ * I could specify a path from the define_trace.h file back to this
+ * file.
+ *
+ * #define TRACE_INCLUDE_PATH ../../samples/trace_events
+ *
+ * But I chose to simply make it use the current directory and then in
+ * the Makefile I added:
+ *
+ * CFLAGS_trace-events-sample.o := -I$(PWD)/samples/trace_events/
+ *
+ * This will make sure the current path is part of the include
+ * structure for our file so that we can find it.
+ *
+ * I could have made only the top level directory the include:
+ *
+ * CFLAGS_trace-events-sample.o := -I$(PWD)
+ *
+ * And then let the path to this directory be the TRACE_INCLUDE_PATH:
+ *
+ * #define TRACE_INCLUDE_PATH samples/trace_events
+ *
+ * But then if something defines "samples" or "trace_events" then we
+ * could risk that being converted too, and give us an unexpected
+ * result.
+ */
+#undef TRACE_INCLUDE_PATH
+#define TRACE_INCLUDE_PATH .
+#include <trace/define_trace.h>