tracing: Document the event tracing system

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <1239479479-2603-3-git-send-email-tytso@mit.edu>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/Documentation/trace/events.txt b/Documentation/trace/events.txt
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+                            Event Tracing
+
+               Documentation written by Theodore Ts'o
+
+Introduction
+============
+
+Tracepoints (see Documentation/trace/tracepoints.txt) can be used
+without creating custom kernel modules to register probe functions
+using the event tracing infrastructure.
+
+Not all tracepoints can be traced using the event tracing system;
+the kernel developer must provide code snippets which define how the
+tracing information is saved into the tracing buffer, and how the
+the tracing information should be printed.
+
+Using Event Tracing
+===================
+
+The events which are available for tracing can be found in the file
+/sys/kernel/debug/tracing/available_events.
+
+To enable a particular event, such as 'sched_wakeup', simply echo it
+to /sys/debug/tracing/set_event. For example:
+
+       # echo sched_wakeup > /sys/kernel/debug/tracing/set_event
+
+[ Note: events can also be enabled/disabled via the 'enabled' toggle
+  found in the /sys/kernel/tracing/events/ hierarchy of directories. ]
+
+To disable an event, echo the event name to the set_event file prefixed
+with an exclamation point:
+
+       # echo '!sched_wakeup' >> /sys/kernel/debug/tracing/set_event
+
+To disable events, echo an empty line to the set_event file:
+
+       # echo > /sys/kernel/debug/tracing/set_event
+
+The events are organized into subsystems, such as ext4, irq, sched,
+etc., and a full event name looks like this: <subsystem>:<event>.  The
+subsystem name is optional, but it is displayed in the available_events
+file.  All of the events in a subsystem can be specified via the syntax
+"<subsystem>:*"; for example, to enable all irq events, you can use the
+command:
+
+       # echo 'irq:*' > /sys/kernel/debug/tracing/set_event
+
+Defining an event-enabled tracepoint
+------------------------------------
+
+A kernel developer which wishes to define an event-enabled tracepoint
+must declare the tracepoint using TRACE_EVENT instead of DECLARE_TRACE.
+This is done via two header files in include/trace.  For example, to
+event-enable the jbd2 subsystem, we must create two files,
+include/trace/jbd2.h and include/trace/jbd2_event_types.h.  The
+include/trace/jbd2.h file should be included by kernel source files that
+will have a tracepoint inserted, and might look like this:
+
+#ifndef _TRACE_JBD2_H
+#define _TRACE_JBD2_H
+
+#include <linux/jbd2.h>
+#include <linux/tracepoint.h>
+
+#include <trace/jbd2_event_types.h>
+
+#endif
+
+In a file that utilizes a jbd2 tracepoint, this header file would be
+included.  Note that you still have to use DEFINE_TRACE().  So for
+example, if fs/jbd2/commit.c planned to use the jbd2_start_commit
+tracepoint, it would have the following near the beginning of the file:
+
+#include <trace/jbd2.h>
+
+DEFINE_TRACE(jbd2_start_commit);
+
+Then in the function that would call the tracepoint, it would call the
+tracepoint function.  (For more information, please see the tracepoint
+documentation in Documentation/trace/tracepoints.txt):
+
+       trace_jbd2_start_commit(journal, commit_transaction);
+
+The code snippets which allow jbd2_start_commit to be an event-enabled
+tracepoint are placed in the file include/trace/jbd2_event_types.h:
+
+/* use <trace/jbd2.h> instead */
+#ifndef TRACE_EVENT
+# error Do not include this file directly.
+# error Unless you know what you are doing.
+#endif
+
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM jbd2
+
+#include <linux/jbd2.h>
+
+TRACE_EVENT(jbd2_start_commit,
+       TP_PROTO(journal_t *journal, transaction_t *commit_transaction),
+       TP_ARGS(journal, commit_transaction),
+       TP_STRUCT__entry(
+               __array(        char,   devname, BDEVNAME_SIZE+24 )
+               __field(        int,    transaction               )
+       ),
+       TP_fast_assign(
+               memcpy(__entry->devname, journal->j_devname, BDEVNAME_SIZE+24);
+               __entry->transaction    = commit_transaction->t_tid;
+       ),
+       TP_printk("dev %s transaction %d",
+                 __entry->devname, __entry->transaction)
+);
+
+The TP_PROTO and TP_ARGS are unchanged from DECLARE_TRACE.  The new
+arguments to TRACE_EVENT are TP_STRUCT__entry, TP_fast_assign, and
+TP_printk.
+
+TP_STRUCT__entry defines the data structure which will be stored in the
+trace buffer.  Normally, fields in __entry will be arrays or simple
+types.  It is possible to place data structures in __entry --- however,
+pointers in the data structure can not be trusted, since they will be
+accessed sometime later by TP_printk, and if the data structure contains
+fields that will not or cannot be used by TP_printk, this will waste
+space in the trace buffer.  In general, data structures should be
+avoided, unless they do only contain non-pointer types and all of the
+fields will be used by TP_printk.
+
+TP_fast_assign defines the code snippet which saves information into the
+__entry data structure, using the passed-in arguments defined in
+TP_PROTO and TP_ARGS.
+
+Finally, TP_printk will print the __entry data structure.  At the time
+when the code snippet defined by TP_printk is executed, it will not have
+access to the TP_ARGS arguments; it can only use the information saved
+in the __entry data structure.