[tip:sched/core] sched/fair: Optimize find_idlest_cpu() when there is no choice

[tip-bot for Morten Rasmussen]

Commit-ID:  eaecf41f5abf80b63c8e025fcb9ee4aa203c3038
Gitweb:     http://git.kernel.org/tip/eaecf41f5abf80b63c8e025fcb9ee4aa203c3038
Author:     Morten Rasmussen <morten.rasmussen@xxxxxxx>
AuthorDate: Wed, 22 Jun 2016 18:03:14 +0100
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
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@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: dietmar.eggemann@xxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: mgalbraith@xxxxxxx
Cc: vincent.guittot@xxxxxxxxxx
Cc: yuyang.du@xxxxxxxxx
Link: http://lkml.kernel.org/r/1466615004-3503-4-git-send-email-morten.rasmussen@xxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 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)) {