Re: [PATCH] sched: Skip double execution of pick_next_task_fair

[Tim Chen]

On Thu, 2014-04-24 at 12:00 +0200, Peter Zijlstra wrote:
> On Wed, Apr 23, 2014 at 03:31:57PM -0700, Tim Chen wrote:
> > The current code will call pick_next_task_fair a second time
> > in the slow path if we did not pull any task in our first try.
> > This is really unnecessary as we already know no task can
> > be pulled and it doubles the delay for the cpu to enter idle.
> > 
> > We instrumented some network workloads and that saw that
> > pick_next_task_fair is frequently called twice before a cpu enters idle.
> > The call to pick_next_task_fair can add
> > non trivial latency as it calls load_balance which runs find_busiest_group
> > on an hierarchy of sched domains spanning the cpus for a large system.  For
> > some 4 socket systems, we saw almost 0.25 msec spent per call
> > of pick_next_task_fair before a cpu can be idled.
> > 
> > This patch skips pick_next_task_fair in the slow path if it
> > has already been invoked.
> 
> How about something like so?

Yes, this version is more concise.

> 
> Its a little more contained.
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2636,8 +2636,14 @@ pick_next_task(struct rq *rq, struct tas
>  	if (likely(prev->sched_class == class &&
>  		   rq->nr_running == rq->cfs.h_nr_running)) {
>  		p = fair_sched_class.pick_next_task(rq, prev);
> -		if (likely(p && p != RETRY_TASK))
> -			return p;
> +		if (unlikely(p == RETRY_TASK))
> +			goto again;
> +
> +		/* assumes fair_sched_class->next == idle_sched_class */
> +		if (unlikely(!p))
> +			p = pick_next_task_idle(rq, prev);
Should be 
			  p = idle_sched_class.pick_next_task(rq, prev);
> +
> +		return p;
>  	}
>  
>  again:

I'll respin the patch with these changes.

Thanks.

Tim


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