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Subject[PATCH v2] tracing/events: Documentation updates
- fix some typos
- document the difference between '>' and '>>'
- document the 'enable' toggle
- remove section "Defining an event-enabled tracepoint", since it's
out-dated and sample/trace_events/ already serves this purpose.

v2: add "Updated by Li Zefan"

[ Impact: make the document up-to-date ]

Signed-off-by: Li Zefan <>
and forgot to CC LKML..

diff --git a/Documentation/trace/events.txt b/Documentation/trace/events.txt
index abdee66..f157d75 100644
--- a/Documentation/trace/events.txt
+++ b/Documentation/trace/events.txt
@@ -1,9 +1,10 @@
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
@@ -12,30 +13,37 @@ 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.
+tracing information should be printed.

-Using Event Tracing
+2. Using Event Tracing
+2.1 Via the 'set_event' interface

The events which are available for tracing can be found in the file

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

- # echo sched_wakeup > /sys/kernel/debug/tracing/set_event
+ # echo sched_wakeup >> /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. ]
+[ 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' >> /sys/kernel/debug/tracing/set_event
+ # 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 disable events, echo an empty line to the set_event file:
+To enable all events, echo '*:*' or '*:' to the set_event file:

- # echo > /sys/kernel/debug/tracing/set_event
+ # 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
@@ -44,92 +52,39 @@ 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

- # 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>
-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>
-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.
-#define TRACE_SYSTEM jbd2
-#include <linux/jbd2.h>
- 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_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
-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.
+ # 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
+3. Defining an event-enabled tracepoint
+See The example provided in samples/trace_events

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