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@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dietmar.eggemann@arm.com
Cc: linux-kernel@vger.kernel.org
Cc: mgalbraith@suse.de
Cc: vincent.guittot@linaro.org
Cc: yuyang.du@intel.com
Link: http://lkml.kernel.org/r/1466615004-3503-4-git-send-email-morten.rasmussen@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d819da68857fe8b6f14b6adf0fb74e2bac8e30ee..acdc351d23864b7e14eb545efc1a6360bc005c6b 100644 (file)
--- 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)) {