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

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Message-ID: <1311939933.5890.341.camel@twins>
Date:	Fri, 29 Jul 2011 13:45:33 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Yong Zhang <yong.zhang0@...il.com>
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, 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 ;-)
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