Subject: Re: [PATCH]sched: convert wall-time to vruntime for
 check_preempt_tick
From: Mike Galbraith <efault@gmx.de>
To: Shaohua Li <shaohua.li@intel.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "mingo@elte.hu" <mingo@elte.hu>,
        "Yan, Zheng Z" <zheng.z.yan@intel.com>
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	 <1302183260.8065.9.camel@marge.simson.net>
	 <1302222953.3981.88.camel@sli10-conroe>
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Date: Fri, 08 Apr 2011 04:49:19 +0200
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On Fri, 2011-04-08 at 08:35 +0800, Shaohua Li wrote:
> On Thu, 2011-04-07 at 21:34 +0800, Mike Galbraith wrote:
> > On Thu, 2011-04-07 at 20:43 +0800, Shaohua Li wrote:
> > > In check_preempt_tick(), delta is vruntime and ideal_runtime is wall runtime.
> > > Comparing vruntime and ideal_runtime looks buggy.
> > 
> > Why is that buggy?  It's a distance in units ns, ala wakeup_granularity,
> > a number.  This number just happens to be variable.
> vruntime is scaled wall-time. In all other places we do the scale from
> my understanding. I'm wondering why not do it here.

The purpose was to ensure that there is not too much spread, just like
wakeup preemption.  Using the number that determines tick induced spread
as the spread caliper seems perfectly fine to me.

	-Mike

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