linux-kernel - Re: [PATCH] sched: Remove WAKEUP_PREEMPT feature check in entity

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Message-ID: <20110729074635.GB10420@zhy>
Date:	Fri, 29 Jul 2011 15:46:35 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Lin Ming <ming.m.lin@...el.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"mingo@...e.hu" <mingo@...e.hu>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Remove WAKEUP_PREEMPT feature check in entity_tick

On Fri, Jul 29, 2011 at 03:36:15PM +0800, Lin Ming wrote:
> >From another point of view, below !sched_feat(WAKEUP_PREEMPT) still
> looks like duplicated.
> 
>         if (cfs_rq->nr_running > 1 || !sched_feat(WAKEUP_PREEMPT))
>                 check_preempt_tick(cfs_rq, curr);
> 
> if "!sched_feat(WAKEUP_PREEMPT)" is run,
> that implies cfs_rq->nr_running == 1.

That's true.

> 
> Why do we need to call check_preempt_tick when there is only 1 task
> runnable?

Just set_tsk_need_resched(p) if p's slice is over, thus:

	(n tick)	--->		(n+1 tick)
set_tsk_need_resched(p);
		another task Q is awaked

If we don't have !sched_feat(WAKEUP_PREEMPT), Q maybe will wait
for tick coming to get scheduled. If we have
!sched_feat(WAKEUP_PREEMPT), Q will get scheduled when some event
happen, like IRQ.

Thanks,
Yong

-- 
Only stand for myself
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