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From:   Peng Wang <rocking@linux.alibaba.com>
To:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de
Cc:     linux-kernel@vger.kernel.org
Subject: [PATCH v2] sched/fair: Optimize dequeue_task_fair()
Date:   Tue, 16 Jun 2020 14:04:07 +0800
Message-Id: <701eef9a40de93dcf5fe7063fd607bca5db38e05.1592287263.git.rocking@linux.alibaba.com>
In-Reply-To: <6f2f195aea48bc50187dfb064aa530ba132be01b.1592230286.git.rocking@linux.alibaba.com>
References: <6f2f195aea48bc50187dfb064aa530ba132be01b.1592230286.git.rocking@linux.alibaba.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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>
---
 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 cbcb2f7..05242b7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5614,14 +5614,14 @@ 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);
 }
-- 
2.9.5