Re: [RFC][PATCH] sched: Avoid select_idle_sibling() for wake_affine(.sync=true)

[Mike Galbraith <bitbucket@online.de>]

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;
>  	}