linux-kernel - Re: [PATCH 4/4] sched: fix wakeup preemption

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Date:	Mon, 20 Oct 2008 15:57:21 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>
Subject: Re: [PATCH 4/4] sched: fix wakeup preemption

Peter Zijlstra wrote:

 > In my recent wakeup preempt rework I messed up the asym wakeup.
 > The idea is that it should be easier to preempt lighter tasks
 > but not harder to preempt heavier tasks.

 > Index: linux-2.6/kernel/sched_fair.c
 > ===================================================================
 > --- linux-2.6.orig/kernel/sched_fair.c
 > +++ linux-2.6/kernel/sched_fair.c
 > @@ -1243,8 +1243,8 @@ static unsigned long wakeup_gran(struct
 >   	 * More easily preempt - nice tasks, while not making it harder for
 >   	 * + nice tasks.
 >   	 */
 > -	if (sched_feat(ASYM_GRAN))
 > -		gran = calc_delta_mine(gran, NICE_0_LOAD, &se->load);
 > +	if (sched_feat(ASYM_GRAN) && se->load.weight < NICE_0_LOAD)
 > +		gran = (gran * se->load.weight) >> NICE_0_SHIFT;
 >
 >   	return gran;
 >   }

Setting aside whether the asym wakeup is desirable, the code looks 
reasonable but I think you need to change the code comments as well.

The proposed code only affects with a weight of less than NICE_0_LOAD, 
ie. +nice tasks.  The comment suggests the opposite.

Chris
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