Documentation/trace/events.txt

   1                              Event Tracing
   2
   3                 Documentation written by Theodore Ts'o
   4                 Updated by Li Zefan and Tom Zanussi
   5
   6 1. Introduction
   7 ===============
   8
   9 Tracepoints (see Documentation/trace/tracepoints.txt) can be used
  10 without creating custom kernel modules to register probe functions
  11 using the event tracing infrastructure.
  12
  13 Not all tracepoints can be traced using the event tracing system;
  14 the kernel developer must provide code snippets which define how the
  15 tracing information is saved into the tracing buffer, and how the
  16 tracing information should be printed.
  17
  18 2. Using Event Tracing
  19 ======================
  20
  21 2.1 Via the 'set_event' interface
  22 ---------------------------------
  23
  24 The events which are available for tracing can be found in the file
  25 /sys/kernel/debug/tracing/available_events.
  26
  27 To enable a particular event, such as 'sched_wakeup', simply echo it
  28 to /sys/kernel/debug/tracing/set_event. For example:
  29
  30         # echo sched_wakeup >> /sys/kernel/debug/tracing/set_event
  31
  32 [ Note: '>>' is necessary, otherwise it will firstly disable
  33   all the events. ]
  34
  35 To disable an event, echo the event name to the set_event file prefixed
  36 with an exclamation point:
  37
  38         # echo '!sched_wakeup' >> /sys/kernel/debug/tracing/set_event
  39
  40 To disable all events, echo an empty line to the set_event file:
  41
  42         # echo > /sys/kernel/debug/tracing/set_event
  43
  44 To enable all events, echo '*:*' or '*:' to the set_event file:
  45
  46         # echo *:* > /sys/kernel/debug/tracing/set_event
  47
  48 The events are organized into subsystems, such as ext4, irq, sched,
  49 etc., and a full event name looks like this: <subsystem>:<event>.  The
  50 subsystem name is optional, but it is displayed in the available_events
  51 file.  All of the events in a subsystem can be specified via the syntax
  52 "<subsystem>:*"; for example, to enable all irq events, you can use the
  53 command:
  54
  55         # echo 'irq:*' > /sys/kernel/debug/tracing/set_event
  56
  57 2.2 Via the 'enable' toggle
  58 ---------------------------
  59
  60 The events available are also listed in /sys/kernel/debug/tracing/events/ hierarchy
  61 of directories.
  62
  63 To enable event 'sched_wakeup':
  64
  65         # echo 1 > /sys/kernel/debug/tracing/events/sched/sched_wakeup/enable
  66
  67 To disable it:
  68
  69         # echo 0 > /sys/kernel/debug/tracing/events/sched/sched_wakeup/enable
  70
  71 To enable all events in sched subsystem:
  72
  73         # echo 1 > /sys/kernel/debug/tracing/events/sched/enable
  74
  75 To enable all events:
  76
  77         # echo 1 > /sys/kernel/debug/tracing/events/enable
  78
  79 When reading one of these enable files, there are four results:
  80
  81  0 - all events this file affects are disabled
  82  1 - all events this file affects are enabled
  83  X - there is a mixture of events enabled and disabled
  84  ? - this file does not affect any event
  85
  86 2.3 Boot option
  87 ---------------
  88
  89 In order to facilitate early boot debugging, use boot option:
  90
  91         trace_event=[event-list]
  92
  93 event-list is a comma separated list of events. See section 2.1 for event
  94 format.
  95
  96 3. Defining an event-enabled tracepoint
  97 =======================================
  98
  99 See The example provided in samples/trace_events
 100
 101 4. Event formats
 102 ================
 103
 104 Each trace event has a 'format' file associated with it that contains
 105 a description of each field in a logged event.  This information can
 106 be used to parse the binary trace stream, and is also the place to
 107 find the field names that can be used in event filters (see section 5).
 108
 109 It also displays the format string that will be used to print the
 110 event in text mode, along with the event name and ID used for
 111 profiling.
 112
 113 Every event has a set of 'common' fields associated with it; these are
 114 the fields prefixed with 'common_'.  The other fields vary between
 115 events and correspond to the fields defined in the TRACE_EVENT
 116 definition for that event.
 117
 118 Each field in the format has the form:
 119
 120      field:field-type field-name; offset:N; size:N;
 121
 122 where offset is the offset of the field in the trace record and size
 123 is the size of the data item, in bytes.
 124
 125 For example, here's the information displayed for the 'sched_wakeup'
 126 event:
 127
 128 # cat /sys/kernel/debug/tracing/events/sched/sched_wakeup/format
 129
 130 name: sched_wakeup
 131 ID: 60
 132 format:
 133         field:unsigned short common_type;       offset:0;       size:2;
 134         field:unsigned char common_flags;       offset:2;       size:1;
 135         field:unsigned char common_preempt_count;       offset:3;       size:1;
 136         field:int common_pid;   offset:4;       size:4;
 137         field:int common_tgid;  offset:8;       size:4;
 138
 139         field:char comm[TASK_COMM_LEN]; offset:12;      size:16;
 140         field:pid_t pid;        offset:28;      size:4;
 141         field:int prio; offset:32;      size:4;
 142         field:int success;      offset:36;      size:4;
 143         field:int cpu;  offset:40;      size:4;
 144
 145 print fmt: "task %s:%d [%d] success=%d [%03d]", REC->comm, REC->pid,
 146            REC->prio, REC->success, REC->cpu
 147
 148 This event contains 10 fields, the first 5 common and the remaining 5
 149 event-specific.  All the fields for this event are numeric, except for
 150 'comm' which is a string, a distinction important for event filtering.
 151
 152 5. Event filtering
 153 ==================
 154
 155 Trace events can be filtered in the kernel by associating boolean
 156 'filter expressions' with them.  As soon as an event is logged into
 157 the trace buffer, its fields are checked against the filter expression
 158 associated with that event type.  An event with field values that
 159 'match' the filter will appear in the trace output, and an event whose
 160 values don't match will be discarded.  An event with no filter
 161 associated with it matches everything, and is the default when no
 162 filter has been set for an event.
 163
 164 5.1 Expression syntax
 165 ---------------------
 166
 167 A filter expression consists of one or more 'predicates' that can be
 168 combined using the logical operators '&&' and '||'.  A predicate is
 169 simply a clause that compares the value of a field contained within a
 170 logged event with a constant value and returns either 0 or 1 depending
 171 on whether the field value matched (1) or didn't match (0):
 172
 173           field-name relational-operator value
 174
 175 Parentheses can be used to provide arbitrary logical groupings and
 176 double-quotes can be used to prevent the shell from interpreting
 177 operators as shell metacharacters.
 178
 179 The field-names available for use in filters can be found in the
 180 'format' files for trace events (see section 4).
 181
 182 The relational-operators depend on the type of the field being tested:
 183
 184 The operators available for numeric fields are:
 185
 186 ==, !=, <, <=, >, >=
 187
 188 And for string fields they are:
 189
 190 ==, !=
 191
 192 Currently, only exact string matches are supported.
 193
 194 5.2 Setting filters
 195 -------------------
 196
 197 A filter for an individual event is set by writing a filter expression
 198 to the 'filter' file for the given event.
 199
 200 For example:
 201
 202 # cd /sys/kernel/debug/tracing/events/sched/sched_wakeup
 203 # echo "common_preempt_count > 4" > filter
 204
 205 A slightly more involved example:
 206
 207 # cd /sys/kernel/debug/tracing/events/signal/signal_generate
 208 # echo "((sig >= 10 && sig < 15) || sig == 17) && comm != bash" > filter
 209
 210 If there is an error in the expression, you'll get an 'Invalid
 211 argument' error when setting it, and the erroneous string along with
 212 an error message can be seen by looking at the filter e.g.:
 213
 214 # cd /sys/kernel/debug/tracing/events/signal/signal_generate
 215 # echo "((sig >= 10 && sig < 15) || dsig == 17) && comm != bash" > filter
 216 -bash: echo: write error: Invalid argument
 217 # cat filter
 218 ((sig >= 10 && sig < 15) || dsig == 17) && comm != bash
 219 ^
 220 parse_error: Field not found
 221
 222 Currently the caret ('^') for an error always appears at the beginning of
 223 the filter string; the error message should still be useful though
 224 even without more accurate position info.
 225
 226 5.3 Clearing filters
 227 --------------------
 228
 229 To clear the filter for an event, write a '0' to the event's filter
 230 file.
 231
 232 To clear the filters for all events in a subsystem, write a '0' to the
 233 subsystem's filter file.
 234
 235 5.3 Subsystem filters
 236 ---------------------
 237
 238 For convenience, filters for every event in a subsystem can be set or
 239 cleared as a group by writing a filter expression into the filter file
 240 at the root of the subsystem.  Note however, that if a filter for any
 241 event within the subsystem lacks a field specified in the subsystem
 242 filter, or if the filter can't be applied for any other reason, the
 243 filter for that event will retain its previous setting.  This can
 244 result in an unintended mixture of filters which could lead to
 245 confusing (to the user who might think different filters are in
 246 effect) trace output.  Only filters that reference just the common
 247 fields can be guaranteed to propagate successfully to all events.
 248
 249 Here are a few subsystem filter examples that also illustrate the
 250 above points:
 251
 252 Clear the filters on all events in the sched subsystem:
 253
 254 # cd /sys/kernel/debug/tracing/events/sched
 255 # echo 0 > filter
 256 # cat sched_switch/filter
 257 none
 258 # cat sched_wakeup/filter
 259 none
 260
 261 Set a filter using only common fields for all events in the sched
 262 subsystem (all events end up with the same filter):
 263
 264 # cd /sys/kernel/debug/tracing/events/sched
 265 # echo common_pid == 0 > filter
 266 # cat sched_switch/filter
 267 common_pid == 0
 268 # cat sched_wakeup/filter
 269 common_pid == 0
 270
 271 Attempt to set a filter using a non-common field for all events in the
 272 sched subsystem (all events but those that have a prev_pid field retain
 273 their old filters):
 274
 275 # cd /sys/kernel/debug/tracing/events/sched
 276 # echo prev_pid == 0 > filter
 277 # cat sched_switch/filter
 278 prev_pid == 0
 279 # cat sched_wakeup/filter
 280 common_pid == 0