linux-kernel - [tip:sched/core] sched/fair: Optimize find_idlest

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Date:	Wed, 10 Aug 2016 11:03:57 -0700
From:	tip-bot for Morten Rasmussen <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	tglx@...utronix.de, hpa@...or.com, efault@....de,
	torvalds@...ux-foundation.org, peterz@...radead.org,
	morten.rasmussen@....com, mingo@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [tip:sched/core] sched/fair: Optimize find_idlest_cpu() when there
 is no choice

Commit-ID:  eaecf41f5abf80b63c8e025fcb9ee4aa203c3038
Gitweb:     http://git.kernel.org/tip/eaecf41f5abf80b63c8e025fcb9ee4aa203c3038
Author:     Morten Rasmussen <morten.rasmussen@....com>
AuthorDate: Wed, 22 Jun 2016 18:03:14 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 10 Aug 2016 14:03:32 +0200

sched/fair: Optimize find_idlest_cpu() when there is no choice

In the current find_idlest_group()/find_idlest_cpu() search we end up
calling find_idlest_cpu() in a sched_group containing only one CPU in
the end. Checking idle-states becomes pointless when there is no
alternative, so bail out instead.

Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mike Galbraith <efault@....de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: dietmar.eggemann@....com
Cc: linux-kernel@...r.kernel.org
Cc: mgalbraith@...e.de
Cc: vincent.guittot@...aro.org
Cc: yuyang.du@...el.com
Link: http://lkml.kernel.org/r/1466615004-3503-4-git-send-email-morten.rasmussen@arm.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/fair.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d819da6..acdc351 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5239,6 +5239,10 @@ find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
 	int shallowest_idle_cpu = -1;
 	int i;
 
+	/* Check if we have any choice: */
+	if (group->group_weight == 1)
+		return cpumask_first(sched_group_cpus(group));
+
 	/* Traverse only the allowed CPUs */
 	for_each_cpu_and(i, sched_group_cpus(group), tsk_cpus_allowed(p)) {
 		if (idle_cpu(i)) {