[lkml]   [2009]   [Apr]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
Patch in this message
Subject[tip:tracing/core] tracing: Document the event tracing system
Commit-ID:  1542263998a2d143596972b2776c59c9247578a7
Author: Theodore Ts'o <>
AuthorDate: Sat, 11 Apr 2009 15:51:18 -0400
Committer: Ingo Molnar <>
CommitDate: Sun, 12 Apr 2009 11:23:48 +0200

tracing: Document the event tracing system

Signed-off-by: "Theodore Ts'o" <>
Cc: Theodore Ts'o <>
Cc: Steven Rostedt <>
LKML-Reference: <>
Signed-off-by: Ingo Molnar <>

Documentation/trace/events.txt | 132 ++++++++++++++++++++++++++++++++++++++++
1 files changed, 132 insertions(+), 0 deletions(-)

diff --git a/Documentation/trace/events.txt b/Documentation/trace/events.txt
new file mode 100644
index 0000000..122522f
--- /dev/null
+++ b/Documentation/trace/events.txt
@@ -0,0 +1,132 @@
+ Event Tracing
+ Documentation written by Theodore Ts'o
+Tracepoints (see Documentation/tracing/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
+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
+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
+ # 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 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.

 \ /
  Last update: 2009-04-12 11:33    [W:0.103 / U:0.984 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site