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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: Yong Zhang <yong.zhang0@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, "mingo@elte.hu" <mingo@elte.hu>,
        lkml <linux-kernel@vger.kernel.org>
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	 <20110729062158.GA8971@zhy>
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Date: Fri, 29 Jul 2011 14:49:40 +0800
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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?

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".

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

> 
> > 
> > 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".

Thanks,
Lin Ming