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SubjectRe: [RFC][PATCH] sched: Avoid select_idle_sibling() for wake_affine(.sync=true)
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On Wed, 2013-09-25 at 09:53 +0200, Peter Zijlstra wrote: 
> Subject: sched: Avoid select_idle_sibling() for wake_affine(.sync=true)
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Wed Sep 25 08:28:39 CEST 2013
>
> When a task is the only running task and does a sync wakeup; avoid
> going through select_idle_sibling() as it doesn't know the current CPU
> is going to be idle shortly.
>
> Without this two sync wakers will ping-pong between CPUs for no
> reason.

That will make pipe-test go fugly -> pretty, and help very fast/light
localhost network, but eat heavier localhost overlap recovery. We need
a working (and cheap) overlap detector scheme, so we can know when there
is enough to be worth going after.

(I sent you some lmbench numbers offline a while back showing the
two-faced little <b-word> in action, doing both good and evil)
> Suggested-by: Rik van Riel <riel@redhat.com>
> Signed-off-by: Peter Zijlstra <peterz@infradead.org>
> ---
> kernel/sched/fair.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -3461,6 +3461,16 @@ select_task_rq_fair(struct task_struct *
> if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
> prev_cpu = cpu;
>
> + /*
> + * Don't bother with select_idle_sibling() in the case of a sync wakeup
> + * where we know the only running task will soon go away. Going
> + * through select_idle_sibling will only lead to pointless ping-pong.
> + */
> + if (sync && prev_cpu == cpu && cpu_rq(cpu)->nr_running == 1) {
> + new_cpu = cpu;
> + goto unlock;
> + }
> +
> new_cpu = select_idle_sibling(p, prev_cpu);
> goto unlock;
> }




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