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    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.
    +#undef TRACE_SYSTEM
    +#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
    +TP_PROTO and TP_ARGS.
    +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.

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