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

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Message-ID: <20110729081800.GA12106@zhy>
Date:	Fri, 29 Jul 2011 16:18:00 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Lin Ming <ming.m.lin@...el.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, Jul 29, 2011 at 09:56:58AM +0200, Peter Zijlstra wrote:
> On Fri, 2011-07-29 at 15:46 +0800, Yong Zhang wrote:
> > 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.
> 
> 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.

Thanks,
Yong

> 
> I've queued Lin's patch as I don't see the point of this thing either,
> normally WAKEUP_PREEMPT is enabled so it says || 0 which is kinda
> useless :-)
> 
> And I'm starting to think we should just kill all of WAKEUP_PREEMPT I
> don't think we ever want to disable it anyway.. 
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