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Subject: Re: [PATCH] sched: Remove WAKEUP_PREEMPT feature check in
 entity_tick
From: Lin Ming <ming.m.lin@intel.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Yong Zhang <yong.zhang0@gmail.com>, "mingo@elte.hu" <mingo@elte.hu>,
        lkml <linux-kernel@vger.kernel.org>, Mike Galbraith <efault@gmx.de>
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Date: Fri, 29 Jul 2011 16:44:03 +0800
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On Fri, 2011-07-29 at 16:20 +0800, Peter Zijlstra wrote:
> On Fri, 2011-07-29 at 16:18 +0800, Yong Zhang wrote:
> > > Nah, if there is 1 runnable task it will always run, preemption simply
> > > doesn't matter. There's nothing to preempt it with.
> > 
> > Hmmm, so the newly waked task could be scheduled a little later.
> > That means schedule tick judge everything. 
> 
> Oh, are you referring to the case where a task gets woken on an idle
> remote cpu?

Yong means the case below.
At (n tick) there is only 1 task, then another task is woken before
(n+1) tick.

On Fri, 2011-07-29 at 15:46 +0800, Yong Zhang wrote:
> 
> 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.