perf: Add comment to enum perf_event_state

Better describe the event states.

Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Reviewed-by: Leo Yan <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: Peter Zijlstra

include/linux/perf_event.h

@@ -635,8 +635,46 @@ struct perf_addr_filter_range {
 	unsigned long			size;
 };
 
-/**
- * enum perf_event_state - the states of an event:
+/*
+ * The normal states are:
+ *
+ *            ACTIVE    --.
+ *               ^        |
+ *               |        |
+ *       sched_{in,out}() |
+ *               |        |
+ *               v        |
+ *      ,---> INACTIVE  --+ <-.
+ *      |                 |   |
+ *      |                {dis,en}able()
+ *   sched_in()           |   |
+ *      |       OFF    <--' --+
+ *      |                     |
+ *      `--->  ERROR    ------'
+ *
+ * That is:
+ *
+ * sched_in:       INACTIVE          -> {ACTIVE,ERROR}
+ * sched_out:      ACTIVE            -> INACTIVE
+ * disable:        {ACTIVE,INACTIVE} -> OFF
+ * enable:         {OFF,ERROR}       -> INACTIVE
+ *
+ * Where {OFF,ERROR} are disabled states.
+ *
+ * Then we have the {EXIT,REVOKED,DEAD} states which are various shades of
+ * defunct events:
+ *
+ *  - EXIT means task that the even was assigned to died, but child events
+ *    still live, and further children can still be created. But the event
+ *    itself will never be active again. It can only transition to
+ *    {REVOKED,DEAD};
+ *
+ *  - REVOKED means the PMU the event was associated with is gone; all
+ *    functionality is stopped but the event is still alive. Can only
+ *    transition to DEAD;
+ *
+ *  - DEAD event really is DYING tearing down state and freeing bits.
+ *
  */
 enum perf_event_state {
 	PERF_EVENT_STATE_DEAD		= -5,