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