[tip:sched/core] sched/util_est: Fix util_est_dequeue() for throttled cfs_rq

[tip-bot for Vincent Guittot]

Commit-ID:  3482d98bbc730758b63a5d1cf41d05ea17481412
Gitweb:     https://git.kernel.org/tip/3482d98bbc730758b63a5d1cf41d05ea17481412
Author:     Vincent Guittot <vincent.guittot@xxxxxxxxxx>
AuthorDate: Thu, 14 Jun 2018 12:33:00 +0200
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 3 Jul 2018 09:17:30 +0200

sched/util_est: Fix util_est_dequeue() for throttled cfs_rq

When a cfs_rq is throttled, parent cfs_rq->nr_running is decreased and
everything happens at cfs_rq level. Currently util_est stays unchanged
in such case and it keeps accounting the utilization of throttled tasks.
This can somewhat make sense as we don't dequeue tasks but only throttled
cfs_rq.

If a task of another group is enqueued/dequeued and root cfs_rq becomes
idle during the dequeue, util_est will be cleared whereas it was
accounting util_est of throttled tasks before. So the behavior of util_est
is not always the same regarding throttled tasks and depends of side
activity. Furthermore, util_est will not be updated when the cfs_rq is
unthrottled as everything happens at cfs_rq level. Main results is that
util_est will stay null whereas we now have running tasks. We have to wait
for the next dequeue/enqueue of the previously throttled tasks to get an
up to date util_est.

Remove the assumption that cfs_rq's estimated utilization of a CPU is 0
if there is no running task so the util_est of a task remains until the
latter is dequeued even if its cfs_rq has been throttled.

Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Patrick Bellasi <patrick.bellasi@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Fixes: 7f65ea42eb00 ("sched/fair: Add util_est on top of PELT")
Link: http://lkml.kernel.org/r/1528972380-16268-1-git-send-email-vincent.guittot@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/fair.c | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 840b92ee6f89..2f0a0be4d344 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3982,18 +3982,10 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
 	if (!sched_feat(UTIL_EST))
 		return;
 
-	/*
-	 * Update root cfs_rq's estimated utilization
-	 *
-	 * If *p is the last task then the root cfs_rq's estimated utilization
-	 * of a CPU is 0 by definition.
-	 */
-	ue.enqueued = 0;
-	if (cfs_rq->nr_running) {
-		ue.enqueued  = cfs_rq->avg.util_est.enqueued;
-		ue.enqueued -= min_t(unsigned int, ue.enqueued,
-				     (_task_util_est(p) | UTIL_AVG_UNCHANGED));
-	}
+	/* Update root cfs_rq's estimated utilization */
+	ue.enqueued  = cfs_rq->avg.util_est.enqueued;
+	ue.enqueued -= min_t(unsigned int, ue.enqueued,
+			     (_task_util_est(p) | UTIL_AVG_UNCHANGED));
 	WRITE_ONCE(cfs_rq->avg.util_est.enqueued, ue.enqueued);
 
 	/*