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diff --git a/Documentation/trace/events.txt b/Documentation/trace/events.txt index 2bcc8d4dea29..6e5f35ebb9c1 100644 --- a/Documentation/trace/events.txt +++ b/Documentation/trace/events.txt @@ -1,7 +1,7 @@ Event Tracing Documentation written by Theodore Ts'o - Updated by Li Zefan + Updated by Li Zefan and Tom Zanussi 1. Introduction =============== @@ -97,3 +97,185 @@ The format of this boot option is the same as described in section 2.1. See The example provided in samples/trace_events +4. Event formats +================ + +Each trace event has a 'format' file associated with it that contains +a description of each field in a logged event. This information can +be used to parse the binary trace stream, and is also the place to +find the field names that can be used in event filters (see section 5). + +It also displays the format string that will be used to print the +event in text mode, along with the event name and ID used for +profiling. + +Every event has a set of 'common' fields associated with it; these are +the fields prefixed with 'common_'. The other fields vary between +events and correspond to the fields defined in the TRACE_EVENT +definition for that event. + +Each field in the format has the form: + + field:field-type field-name; offset:N; size:N; + +where offset is the offset of the field in the trace record and size +is the size of the data item, in bytes. + +For example, here's the information displayed for the 'sched_wakeup' +event: + +# cat /debug/tracing/events/sched/sched_wakeup/format + +name: sched_wakeup +ID: 60 +format: + field:unsigned short common_type; offset:0; size:2; + field:unsigned char common_flags; offset:2; size:1; + field:unsigned char common_preempt_count; offset:3; size:1; + field:int common_pid; offset:4; size:4; + field:int common_tgid; offset:8; size:4; + + field:char comm[TASK_COMM_LEN]; offset:12; size:16; + field:pid_t pid; offset:28; size:4; + field:int prio; offset:32; size:4; + field:int success; offset:36; size:4; + field:int cpu; offset:40; size:4; + +print fmt: "task %s:%d [%d] success=%d [%03d]", REC->comm, REC->pid, + REC->prio, REC->success, REC->cpu + +This event contains 10 fields, the first 5 common and the remaining 5 +event-specific. All the fields for this event are numeric, except for +'comm' which is a string, a distinction important for event filtering. + +5. Event filtering +================== + +Trace events can be filtered in the kernel by associating boolean +'filter expressions' with them. As soon as an event is logged into +the trace buffer, its fields are checked against the filter expression +associated with that event type. An event with field values that +'match' the filter will appear in the trace output, and an event whose +values don't match will be discarded. An event with no filter +associated with it matches everything, and is the default when no +filter has been set for an event. + +5.1 Expression syntax +--------------------- + +A filter expression consists of one or more 'predicates' that can be +combined using the logical operators '&&' and '||'. A predicate is +simply a clause that compares the value of a field contained within a +logged event with a constant value and returns either 0 or 1 depending +on whether the field value matched (1) or didn't match (0): + + field-name relational-operator value + +Parentheses can be used to provide arbitrary logical groupings and +double-quotes can be used to prevent the shell from interpreting +operators as shell metacharacters. + +The field-names available for use in filters can be found in the +'format' files for trace events (see section 4). + +The relational-operators depend on the type of the field being tested: + +The operators available for numeric fields are: + +==, !=, <, <=, >, >= + +And for string fields they are: + +==, != + +Currently, only exact string matches are supported. + +Currently, the maximum number of predicates in a filter is 16. + +5.2 Setting filters +------------------- + +A filter for an individual event is set by writing a filter expression +to the 'filter' file for the given event. + +For example: + +# cd /debug/tracing/events/sched/sched_wakeup +# echo "common_preempt_count > 4" > filter + +A slightly more involved example: + +# cd /debug/tracing/events/sched/sched_signal_send +# echo "((sig >= 10 && sig < 15) || sig == 17) && comm != bash" > filter + +If there is an error in the expression, you'll get an 'Invalid +argument' error when setting it, and the erroneous string along with +an error message can be seen by looking at the filter e.g.: + +# cd /debug/tracing/events/sched/sched_signal_send +# echo "((sig >= 10 && sig < 15) || dsig == 17) && comm != bash" > filter +-bash: echo: write error: Invalid argument +# cat filter +((sig >= 10 && sig < 15) || dsig == 17) && comm != bash +^ +parse_error: Field not found + +Currently the caret ('^') for an error always appears at the beginning of +the filter string; the error message should still be useful though +even without more accurate position info. + +5.3 Clearing filters +-------------------- + +To clear the filter for an event, write a '0' to the event's filter +file. + +To clear the filters for all events in a subsystem, write a '0' to the +subsystem's filter file. + +5.3 Subsystem filters +--------------------- + +For convenience, filters for every event in a subsystem can be set or +cleared as a group by writing a filter expression into the filter file +at the root of the subsytem. Note however, that if a filter for any +event within the subsystem lacks a field specified in the subsystem +filter, or if the filter can't be applied for any other reason, the +filter for that event will retain its previous setting. This can +result in an unintended mixture of filters which could lead to +confusing (to the user who might think different filters are in +effect) trace output. Only filters that reference just the common +fields can be guaranteed to propagate successfully to all events. + +Here are a few subsystem filter examples that also illustrate the +above points: + +Clear the filters on all events in the sched subsytem: + +# cd /sys/kernel/debug/tracing/events/sched +# echo 0 > filter +# cat sched_switch/filter +none +# cat sched_wakeup/filter +none + +Set a filter using only common fields for all events in the sched +subsytem (all events end up with the same filter): + +# cd /sys/kernel/debug/tracing/events/sched +# echo common_pid == 0 > filter +# cat sched_switch/filter +common_pid == 0 +# cat sched_wakeup/filter +common_pid == 0 + +Attempt to set a filter using a non-common field for all events in the +sched subsytem (all events but those that have a prev_pid field retain +their old filters): + +# cd /sys/kernel/debug/tracing/events/sched +# echo prev_pid == 0 > filter +# cat sched_switch/filter +prev_pid == 0 +# cat sched_wakeup/filter +common_pid == 0 |