From: Peng Wang <rocking@linux.alibaba.com>
Date: Mon, 22 Jun 2020 10:18:37 +0100
Subject: [PATCH] sched/fair: Optimize dequeue_task_fair()

References: bnc#1155798 (CPU scheduler functional and performance backports)
Patch-mainline: v5.9-rc1
Git-commit: 423d02e1463b21109106f52d94f7396b63731f3b

While looking at enqueue_task_fair and dequeue_task_fair, it occurred
to me that dequeue_task_fair can also be optimized as Vincent described
in commit 7d148be69e3a ("sched/fair: Optimize enqueue_task_fair()").

When encountering throttled cfs_rq, dequeue_throttle label can ensure
se not to be NULL, and rq->nr_running remains unchanged, so we can also
skip the early balance check.

Signed-off-by: Peng Wang <rocking@linux.alibaba.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 kernel/sched/fair.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a71106f5ee04..18e86a70144b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5591,14 +5591,14 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 
 	}
 
-dequeue_throttle:
-	if (!se)
-		sub_nr_running(rq, 1);
+	/* At this point se is NULL and we are at root level*/
+	sub_nr_running(rq, 1);
 
 	/* balance early to pull high priority tasks */
 	if (unlikely(!was_sched_idle && sched_idle_rq(rq)))
 		rq->next_balance = jiffies;
 
+dequeue_throttle:
 	util_est_dequeue(&rq->cfs, p, task_sleep);
 	hrtick_update(rq);
 }