[PATCH 1/2] sched/cfs: make util/load_avg more stable

[Vincent Guittot]

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. 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.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
---

Sorry some unexpected characters appeared in the commit message of previous
version

 kernel/sched/fair.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3f83a35..f74da94 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3017,12 +3017,15 @@ ___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 - sa->period_contrib * weight),
+					(LOAD_AVG_MAX - 1024));
 	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 - sa->period_contrib * weight),
+					(LOAD_AVG_MAX - 1024));
 	}
-	sa->util_avg = sa->util_sum / LOAD_AVG_MAX;
+	sa->util_avg = (sa->util_sum - (running * sa->period_contrib << SCHED_CAPACITY_SHIFT)) /
+					(LOAD_AVG_MAX - 1024);
 
 	return 1;
 }
-- 
2.7.4