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

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Message-ID: <1311929043.3938.1595.camel@minggr.sh.intel.com>
Date:	Fri, 29 Jul 2011 16:44:03 +0800
From:	Lin Ming <ming.m.lin@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Yong Zhang <yong.zhang0@...il.com>,
	"mingo@...e.hu" <mingo@...e.hu>,
	lkml <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH] sched: Remove WAKEUP_PREEMPT feature check in
 entity_tick

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.


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