perf/core: Rewrite event timekeeping

The current even timekeeping, which computes enabled and running
times, uses 3 distinct timestamps to reflect the various event states:
OFF (stopped), INACTIVE (enabled) and ACTIVE (running).

Furthermore, the update rules are such that even INACTIVE events need
their timestamps updated. This is undesirable because we'd like to not
touch INACTIVE events if at all possible, this makes event scheduling
(much) more expensive than needed.

Rewrite the timekeeping to directly use event->state, this greatly
simplifies the code and results in only having to update things when
we change state, or an up-to-date value is requested (read).
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/perf_event.h

@@ -588,26 +588,10 @@ struct perf_event {
 	 * has been enabled (i.e. eligible to run, and the task has
 	 * been scheduled in, if this is a per-task event)
 	 * and running (scheduled onto the CPU), respectively.
-	 *
-	 * They are computed from tstamp_enabled, tstamp_running and
-	 * tstamp_stopped when the event is in INACTIVE or ACTIVE state.
 	 */
 	u64				total_time_enabled;
 	u64				total_time_running;
-
-	/*
-	 * These are timestamps used for computing total_time_enabled
-	 * and total_time_running when the event is in INACTIVE or
-	 * ACTIVE state, measured in nanoseconds from an arbitrary point
-	 * in time.
-	 * tstamp_enabled: the notional time when the event was enabled
-	 * tstamp_running: the notional time when the event was scheduled on
-	 * tstamp_stopped: in INACTIVE state, the notional time when the
-	 *	event was scheduled off.
-	 */
-	u64				tstamp_enabled;
-	u64				tstamp_running;
-	u64				tstamp_stopped;
+	u64				tstamp;
 
 	/*
 	 * timestamp shadows the actual context timing but it can
@@ -699,7 +683,6 @@ struct perf_event {
 
 #ifdef CONFIG_CGROUP_PERF
 	struct perf_cgroup		*cgrp; /* cgroup event is attach to */
-	int				cgrp_defer_enabled;
 #endif
 
 	struct list_head		sb_list;