			     Event Tracing

		Documentation written by Theodore Ts'o
			Updated by Li Zefan

1. 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
tracing information should be printed.

2. Using Event Tracing
======================

2.1 Via the 'set_event' interface
---------------------------------

The events which are available for tracing can be found in the file
/debug/tracing/available_events.

To enable a particular event, such as 'sched_wakeup', simply echo it
to /debug/tracing/set_event. For example:

	# echo sched_wakeup >> /debug/tracing/set_event

[ Note: '>>' is necessary, otherwise it will firstly disable
  all the events. ]

To disable an event, echo the event name to the set_event file prefixed
with an exclamation point:

	# echo '!sched_wakeup' >> /debug/tracing/set_event

To disable all events, echo an empty line to the set_event file:

	# echo > /debug/tracing/set_event

To enable all events, echo '*:*' or '*:' to the set_event file:

	# echo *:* > /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:*' > /debug/tracing/set_event

2.2 Via the 'enable' toggle
---------------------------

The events available are also listed in /debug/tracing/events/ hierarchy
of directories.

To enable event 'sched_wakeup':

	# echo 1 > /debug/tracing/events/sched/sched_wakeup/enable

To disable it:

	# echo 0 > /debug/tracing/events/sched/sched_wakeup/enable

To enable all events in sched subsystem:

	# echo 1 > /debug/tracing/events/sched/enable

To eanble all events:

	# echo 1 > /debug/tracing/events/enable

When reading one of these enable files, there are four results:

 0 - all events this file affects are disabled
 1 - all events this file affects are enabled
 X - there is a mixture of events enabled and disabled
 ? - this file does not affect any event

2.3 Boot option
---------------

In order to facilitate early boot debugging, use boot option:

	trace_event=[event-list]

The format of this boot option is the same as described in section 2.1.

3. Defining an event-enabled tracepoint
=======================================

See The example provided in samples/trace_events