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   perf-script-python    ( 1 )

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EVENT HANDLERS

When perf script is invoked using a trace script, a user-defined handler function is called for each event in the trace. If there's no handler function defined for a given event type, the event is ignored (or passed to a trace_unhandled function, see below) and the next event is processed.

Most of the event's field values are passed as arguments to the handler function; some of the less common ones aren't - those are available as calls back into the perf executable (see below).

As an example, the following perf record command can be used to record all sched_wakeup events in the system:

# perf record -a -e sched:sched_wakeup

Traces meant to be processed using a script should be recorded with the above option: -a to enable system-wide collection.

The format file for the sched_wakep event defines the following fields (see /sys/kernel/debug/tracing/events/sched/sched_wakeup/format):

.ft C format: field:unsigned short common_type; field:unsigned char common_flags; field:unsigned char common_preempt_count; field:int common_pid;

field:char comm[TASK_COMM_LEN]; field:pid_t pid; field:int prio; field:int success; field:int target_cpu; .ft

The handler function for this event would be defined as:

.ft C def sched__sched_wakeup(event_name, context, common_cpu, common_secs, common_nsecs, common_pid, common_comm, comm, pid, prio, success, target_cpu): pass .ft

The handler function takes the form subsystem__event_name.

The common_* arguments in the handler's argument list are the set of arguments passed to all event handlers; some of the fields correspond to the common_* fields in the format file, but some are synthesized, and some of the common_* fields aren't common enough to to be passed to every event as arguments but are available as library functions.

Here's a brief description of each of the invariant event args:

event_name the name of the event as text context an opaque 'cookie' used in calls back into perf common_cpu the cpu the event occurred on common_secs the secs portion of the event timestamp common_nsecs the nsecs portion of the event timestamp common_pid the pid of the current task common_comm the name of the current process

All of the remaining fields in the event's format file have counterparts as handler function arguments of the same name, as can be seen in the example above.

The above provides the basics needed to directly access every field of every event in a trace, which covers 90% of what you need to know to write a useful trace script. The sections below cover the rest.