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

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Message-ID: <20110801013341.GA26768@zhy>
Date:	Mon, 1 Aug 2011 09:33:41 +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 01:45:33PM +0200, Peter Zijlstra wrote:
> On Fri, 2011-07-29 at 16:46 +0800, Yong Zhang wrote:
> > Let's take UP for example, we have cpu-hug task A and threadirq B.
> > 
> >         n tick          --->            n+1 tick
> > set_tsk_need_resched(A);
> >                 B comes in and
> >                 wake up thread-B;
> > 
> > So for system on which we disable WAKEUP_PREEMPT,
> > if we don't have that check, thread-B will wait until n+1 tick comes
> > to get to run.
> > But if we have that check, thread-B will get to run after IRQ-B returns.
> 
> But that's exactly what wakeup preemption is about, waking tasks don't
> get to preempt running tasks. So no doing that preemption is exactly
> right for !WAKEUP_PREEMPT.
> 
> Anyway, I've queued the removal patch since that removes all
> confusion ;-)

Yup, that sounds good. Thanks Peter.

-Yong

-- 
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