linux-kernel - Re: [PATCH] sched: minor optimization

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Date:	Sat, 24 Nov 2007 18:56:13 +0530
From:	Nikanth Karthikesan <knikanth@...e.de>
To:	Dmitry Adamushko <dmitry.adamushko@...il.com>
Cc:	a.p.zijlstra@...llo.nl, linux-kernel@...r.kernel.org,
	vatsa@...ux.vnet.ibm.com, mingo@...e.hu
Subject: Re: [PATCH] sched: minor optimization

> On Fri, Nov 23, 2007 at  5:48 PM, "Dmitry Adamushko"
<dmitry.adamushko@...il.com> wrote:
> The only legitimate possibility of having the fair_sched_class
> returning no task in this case is when 'rq->nr_running ==
> rq->cfs.nr_running == 0'.

Yes, I think so.

> iow, a possible optimization would be just the following check :
>
> if (rq->nr_running == 0)
>         return idle_sched_class.pick_next_task(rq);
> at the beginning of pick_next_task().
>
> (or maybe put it at the beginning of the
> if (likely(rq->nr_running == rq->cfs.nr_running)) {} block as we
> already have 'likely()' there).
>

But that might add a test before the case we want to optimize the most.
I just thought of taking advantage of a case where we know
rq->nr_running==0, instead of throwing away that information.

Thanks
Nikanth
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