3 Documentation written by Theodore Ts'o
8 Tracepoints (see Documentation/trace/tracepoints.txt) can be used
9 without creating custom kernel modules to register probe functions
10 using the event tracing infrastructure.
12 Not all tracepoints can be traced using the event tracing system;
13 the kernel developer must provide code snippets which define how the
14 tracing information is saved into the tracing buffer, and how the
15 the tracing information should be printed.
20 The events which are available for tracing can be found in the file
21 /sys/kernel/debug/tracing/available_events.
23 To enable a particular event, such as 'sched_wakeup', simply echo it
24 to /sys/debug/tracing/set_event. For example:
26 # echo sched_wakeup > /sys/kernel/debug/tracing/set_event
28 [ Note: events can also be enabled/disabled via the 'enabled' toggle
29 found in the /sys/kernel/tracing/events/ hierarchy of directories. ]
31 To disable an event, echo the event name to the set_event file prefixed
32 with an exclamation point:
34 # echo '!sched_wakeup' >> /sys/kernel/debug/tracing/set_event
36 To disable events, echo an empty line to the set_event file:
38 # echo > /sys/kernel/debug/tracing/set_event
40 The events are organized into subsystems, such as ext4, irq, sched,
41 etc., and a full event name looks like this: <subsystem>:<event>. The
42 subsystem name is optional, but it is displayed in the available_events
43 file. All of the events in a subsystem can be specified via the syntax
44 "<subsystem>:*"; for example, to enable all irq events, you can use the
47 # echo 'irq:*' > /sys/kernel/debug/tracing/set_event
49 Defining an event-enabled tracepoint
50 ------------------------------------
52 A kernel developer which wishes to define an event-enabled tracepoint
53 must declare the tracepoint using TRACE_EVENT instead of DECLARE_TRACE.
54 This is done via two header files in include/trace. For example, to
55 event-enable the jbd2 subsystem, we must create two files,
56 include/trace/jbd2.h and include/trace/jbd2_event_types.h. The
57 include/trace/jbd2.h file should be included by kernel source files that
58 will have a tracepoint inserted, and might look like this:
63 #include <linux/jbd2.h>
64 #include <linux/tracepoint.h>
66 #include <trace/jbd2_event_types.h>
70 In a file that utilizes a jbd2 tracepoint, this header file would be
71 included. Note that you still have to use DEFINE_TRACE(). So for
72 example, if fs/jbd2/commit.c planned to use the jbd2_start_commit
73 tracepoint, it would have the following near the beginning of the file:
75 #include <trace/jbd2.h>
77 DEFINE_TRACE(jbd2_start_commit);
79 Then in the function that would call the tracepoint, it would call the
80 tracepoint function. (For more information, please see the tracepoint
81 documentation in Documentation/trace/tracepoints.txt):
83 trace_jbd2_start_commit(journal, commit_transaction);
85 The code snippets which allow jbd2_start_commit to be an event-enabled
86 tracepoint are placed in the file include/trace/jbd2_event_types.h:
88 /* use <trace/jbd2.h> instead */
90 # error Do not include this file directly.
91 # error Unless you know what you are doing.
95 #define TRACE_SYSTEM jbd2
97 #include <linux/jbd2.h>
99 TRACE_EVENT(jbd2_start_commit,
100 TP_PROTO(journal_t *journal, transaction_t *commit_transaction),
101 TP_ARGS(journal, commit_transaction),
103 __array( char, devname, BDEVNAME_SIZE+24 )
104 __field( int, transaction )
107 memcpy(__entry->devname, journal->j_devname, BDEVNAME_SIZE+24);
108 __entry->transaction = commit_transaction->t_tid;
110 TP_printk("dev %s transaction %d",
111 __entry->devname, __entry->transaction)
114 The TP_PROTO and TP_ARGS are unchanged from DECLARE_TRACE. The new
115 arguments to TRACE_EVENT are TP_STRUCT__entry, TP_fast_assign, and
118 TP_STRUCT__entry defines the data structure which will be stored in the
119 trace buffer. Normally, fields in __entry will be arrays or simple
120 types. It is possible to place data structures in __entry --- however,
121 pointers in the data structure can not be trusted, since they will be
122 accessed sometime later by TP_printk, and if the data structure contains
123 fields that will not or cannot be used by TP_printk, this will waste
124 space in the trace buffer. In general, data structures should be
125 avoided, unless they do only contain non-pointer types and all of the
126 fields will be used by TP_printk.
128 TP_fast_assign defines the code snippet which saves information into the
129 __entry data structure, using the passed-in arguments defined in
130 TP_PROTO and TP_ARGS.
132 Finally, TP_printk will print the __entry data structure. At the time
133 when the code snippet defined by TP_printk is executed, it will not have
134 access to the TP_ARGS arguments; it can only use the information saved
135 in the __entry data structure.