linux-kernel - [PATCH] sched/fair: tweak pick_next

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Message-Id: <20200930173532.1069092-1-posk@google.com>
Date:   Wed, 30 Sep 2020 10:35:32 -0700
From:   Peter Oskolkov <posk@...gle.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org
Cc:     Peter Oskolkov <posk@...k.io>, Peter Oskolkov <posk@...gle.com>
Subject: [PATCH] sched/fair: tweak pick_next_entity

Currently, pick_next_entity(...) has the following structure
(simplified):

[...]
if (last_buddy_ok())
  result = last_buddy;
if (next_buddy_ok())
  result = next_buddy;
[...]

The intended behavior is to prefer next buddy over last buddy;
the current code somewhat obfuscates this, and also wastes
cycles checking the last buddy when eventually the next buddy is
picked up.

So this patch refactors two 'ifs' above into

[...]
if (next_buddy_ok())
    result = next_buddy;
else if (last_buddy_ok())
    result = last_buddy;
[...]

Signed-off-by: Peter Oskolkov <posk@...gle.com>
---
 kernel/sched/fair.c | 20 ++++++++++----------
 1 file changed, 10 insertions(+), 10 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fc3410b8b990..cec6cf9b2bb3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4465,17 +4465,17 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 			se = second;
 	}
 
-	/*
-	 * Prefer last buddy, try to return the CPU to a preempted task.
-	 */
-	if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
-		se = cfs_rq->last;
-
-	/*
-	 * Someone really wants this to run. If it's not unfair, run it.
-	 */
-	if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
+	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;
+	}
 
 	clear_buddies(cfs_rq, se);
 
-- 
2.28.0.709.gb0816b6eb0-goog