linux-kernel - [PATCH] sched/fair: micro-optimize pick_next

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Date:   Wed, 11 Aug 2021 19:48:02 +0800
From:   Li RongQing <lirongqing@...du.com>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org
Subject: [PATCH] sched/fair: micro-optimize pick_next_entity()

only check the skip buddy when next buddy and last buddy
are not picked up, this can save the cycles of checking
the skip buddy and compute the second buddy when next and
last buddy will be picked up

Signed-off-by: Li RongQing <lirongqing@...du.com>
---
 kernel/sched/fair.c | 33 ++++++++++++++++-----------------
 1 file changed, 16 insertions(+), 17 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 44c4520..9c6569dd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4503,11 +4503,22 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 
 	se = left; /* ideally we run the leftmost entity */
 
-	/*
-	 * Avoid running the skip buddy, if running something else can
-	 * be done without getting too unfair.
-	 */
-	if (cfs_rq->skip && cfs_rq->skip == se) {
+
+	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
+		/*
+		 * Someone really wants this to run. If it's not unfair, run it.
+		 */
+		se = cfs_rq->next;
+	} else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
+		/*
+		 * Prefer last buddy, try to return the CPU to a preempted task.
+		 */
+		se = cfs_rq->last;
+	} else if (cfs_rq->skip && cfs_rq->skip == se) {
+		/*
+		 * Avoid running the skip buddy, if running something else can
+		 * be done without getting too unfair.
+		 */
 		struct sched_entity *second;
 
 		if (se == curr) {
@@ -4522,18 +4533,6 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 			se = second;
 	}
 
-	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
-		/*
-		 * Someone really wants this to run. If it's not unfair, run it.
-		 */
-		se = cfs_rq->next;
-	} else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
-		/*
-		 * Prefer last buddy, try to return the CPU to a preempted task.
-		 */
-		se = cfs_rq->last;
-	}
-
 	return se;
 }
 
-- 
2.9.4