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

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Date:	Fri, 29 Jul 2011 15:03:57 +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 02:49:40PM +0800, Lin Ming wrote:
> On Fri, 2011-07-29 at 14:21 +0800, Yong Zhang wrote:
> > On Thu, Jul 28, 2011 at 05:43:23PM +0800, Lin Ming wrote:
> > > Currently, entity_tick calls check_preempt_tick if WAKEUP_PREEMPT feature is
> > > disabled. That's wrong. It should do that if the feature is enabled.
> > 
> > Why is it wrong?
> > check_preempt_wakeup() is used for wakeup.
> 
> I guess you mean "check_preempt_tick" here, yes?

check_preempt_wakeup() excactly.
try_to_wake_up()
  check_preempt_curr()
    sched_fair->check_preempt_wakeup()  <========== [1]

> 
> in entity_tick(...):
>         if (cfs_rq->nr_running > 1 || !sched_feat(WAKEUP_PREEMPT))
>                 check_preempt_tick(cfs_rq, curr);
> 
> Note that, above "if" statement says "if WAKEUP_PREEMPT feature is
> *disabled* then calls check_preempt_tick".

Yeah, if !sched_feat(WAKEUP_PREEMPT) [1] will just return;
thus new waked task will wait until the next tick to schedule.

> 
> Shouldn't it be "if WAKEUP_PREEMPT feature is *enabled* then ...."?

So no IMHO.

> 
> > 
> > > 
> > > And actually the check is duplicate because check_preempt_tick will do
> > > that. So just remove it from entity_tick.
> > 
> > It's not exactly duplicated. entity_tick() will resched_task(*p)
> > if p's slice is over. So if there is an following wakeup(say X),
> > then there is an opportunity for X to schedule quickly.
> 
> Understood this.
> 
> But what I mean is both "entity_tick" and "check_preempt_tick" check
> WAKEUP_PREEMPT feature. That's duplicated.
> 
> Only need to check it in "check_preempt_tick".

I think we need that check(!sched_feat(WAKEUP_PREEMPT)) in entity_tick()
to give new waked task better opportunity.

Thanks,
Yong

-- 
Only stand for myself
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