Re: [PATCH 04/16] sched/fair: Optimize find_idlest_cpu() when there is no choice

[Mike Galbraith]

On Mon, 2016-05-23 at 11:58 +0100, Morten Rasmussen wrote:
> 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@xxxxxxxxxx>
> cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> 
> Signed-off-by: Morten Rasmussen <morten.rasmussen@xxxxxxx>
> ---
>  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));
> +> 	> }
> +

Hm, if task isn't allowed there, too bad?

>  	/* Traverse only the allowed CPUs */
>  > 	> for_each_cpu_and(i, sched_group_cpus(group), tsk_cpus_allowed(p)) {
>  > 	> 	> if (idle_cpu(i)) {