linux-kernel - [PATCH v3 03/13] sched/fair: Optimize find_idlest

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Date:	Mon, 25 Jul 2016 14:34:20 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	peterz@...radead.org, mingo@...hat.com
Cc:	dietmar.eggemann@....com, yuyang.du@...el.com,
	vincent.guittot@...aro.org, mgalbraith@...e.de,
	sgurrappadi@...dia.com, freedom.tan@...iatek.com,
	keita.kobayashi.ym@...esas.com, linux-kernel@...r.kernel.org,
	Morten Rasmussen <morten.rasmussen@....com>
Subject: [PATCH v3 03/13] 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.

cc: Ingo Molnar <mingo@...hat.com>
cc: Peter Zijlstra <peterz@...radead.org>

Acked-by: Vincent Guittot <vincent.guittot@...aro.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
---
 kernel/sched/fair.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e7d50acd6028..c1c0aa8b300b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5229,6 +5229,10 @@ 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)) {
-- 
1.9.1