linux-kernel - [PATCH 04/16] sched/fair: Optimize find_idlest

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Message-Id: <1464001138-25063-5-git-send-email-morten.rasmussen@arm.com>
Date:	Mon, 23 May 2016 11:58:46 +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,
	linux-kernel@...r.kernel.org,
	Morten Rasmussen <morten.rasmussen@....com>
Subject: [PATCH 04/16] 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>

Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
---
 kernel/sched/fair.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0fe3020..564215d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5155,6 +5155,11 @@ 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