sched/cfs: Make util/load_avg more stable

In the current implementation of load/util_avg, we assume that the
ongoing time segment has fully elapsed, and util/load_sum is divided
by LOAD_AVG_MAX, even if part of the time segment still remains to
run. As a consequence, this remaining part is considered as idle time
and generates unexpected variations of util_avg of a busy CPU in the
range [1002..1024[ whereas util_avg should stay at 1023.

In order to keep the metric stable, we should not consider the ongoing
time segment when computing load/util_avg but only the segments that
have already fully elapsed. But to not consider the current time
segment adds unwanted latency in the load/util_avg responsivness
especially when the time is scaled instead of the contribution.

Instead of waiting for the current time segment to have fully elapsed
before accounting it in load/util_avg, we can already account the
elapsed part but change the range used to compute load/util_avg
accordingly.

At the very beginning of a new time segment, the past segments have
been decayed and the max value is LOAD_AVG_MAX*y. At the very end of
the current time segment, the max value becomes:

  LOAD_AVG_MAX*y + 1024(us)  (== LOAD_AVG_MAX)

In fact, the max value is:

  LOAD_AVG_MAX*y + sa->period_contrib

at any time in the time segment.

Taking advantage of the fact that:

  LOAD_AVG_MAX*y == LOAD_AVG_MAX-1024

the range becomes [0..LOAD_AVG_MAX-1024+sa->period_contrib].

As the elapsed part is already accounted in load/util_sum, we update
the max value according to the current position in the time segment
instead of removing its contribution.

Suggested-by: Peter Zijlstra <[email protected]>
Signed-off-by: Vincent Guittot <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: [email protected]
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

kernel/sched/fair.c

@@ -2916,12 +2916,12 @@ ___update_load_avg(u64 now, int cpu, struct sched_avg *sa,
 	/*
 	 * Step 2: update *_avg.
 	 */
-	sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX);
+	sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX - 1024 + sa->period_contrib);
 	if (cfs_rq) {
 		cfs_rq->runnable_load_avg =
-			div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX);
+			div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX - 1024 + sa->period_contrib);
 	}
-	sa->util_avg = sa->util_sum / LOAD_AVG_MAX;
+	sa->util_avg = sa->util_sum / (LOAD_AVG_MAX - 1024 + sa->period_contrib);
 
 	return 1;
 }