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

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Message-ID: <b647ffbd0711230418i67434250t6ab0f05738eb3f5e@mail.gmail.com>
Date:	Fri, 23 Nov 2007 13:18:20 +0100
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	"Nikanth Karthikesan" <knikanth@...e.de>
Cc:	mingo@...e.hu, a.p.zijlstra@...llo.nl,
	linux-kernel@...r.kernel.org, vatsa@...ux.vnet.ibm.com
Subject: Re: [PATCH] sched: minor optimization

On 24/11/2007, Nikanth Karthikesan <knikanth@...e.de> wrote:
> As an optimization, if all tasks are in the fair class, the next task is
> directly picked from fair_sched_class. But, if it returns no task we go
> through again from sched_class_highest which could be avoided and
> instead return the idle task directly.

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'.

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).


-- 
Best regards,
Dmitry Adamushko
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