linux-kernel - [PATCH 3/3] sched/fair: Do not raise overutilized for idle CPUs

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Date:   Mon, 20 Dec 2021 12:43:23 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org
Cc:     linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        valentin.schneider@....com, morten.rasmussen@....com,
        qperret@...gle.com, Vincent Donnefort <vincent.donnefort@....com>
Subject: [PATCH 3/3] sched/fair: Do not raise overutilized for idle CPUs

During a migration, the lock for the previous runqueue is not taken and
hence, the task contribution isn't directly removed from that runqueue
utilization but instead temporarily saved, until the next PELT signals
update where it would be accounted. There is then a window in which a
CPU can ben idle be nonetheless overutilized.

The load balancer wouldn't be able to do anything to help a sleeping CPU,
it brings then no gain to raise overutilized there, only the risk of
spuriously doing it.

Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 51f6f55abb37..37f737c5f0b8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8641,26 +8641,28 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 
 		nr_running = rq->nr_running;
 		sgs->sum_nr_running += nr_running;
-
-		if (nr_running > 1)
-			*sg_status |= SG_OVERLOAD;
-
-		if (cpu_overutilized(i))
-			*sg_status |= SG_OVERUTILIZED;
-
 #ifdef CONFIG_NUMA_BALANCING
 		sgs->nr_numa_running += rq->nr_numa_running;
 		sgs->nr_preferred_running += rq->nr_preferred_running;
 #endif
+		if (nr_running > 1)
+			*sg_status |= SG_OVERLOAD;
+
 		/*
 		 * No need to call idle_cpu() if nr_running is not 0
 		 */
 		if (!nr_running && idle_cpu(i)) {
 			sgs->idle_cpus++;
-			/* Idle cpu can't have misfit task */
+			/*
+			 * Idle cpu can neither be overutilized nor have a
+			 * misfit task.
+			 */
 			continue;
 		}
 
+		if (cpu_overutilized(i))
+			*sg_status |= SG_OVERUTILIZED;
+
 		if (local_group)
 			continue;
 
-- 
2.25.1