linux-kernel - Re: [PATCH]sched: convert wall-time to vruntime for check_preempt

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Message-ID: <1302240665.7551.48.camel@marge.simson.net>
Date:	Fri, 08 Apr 2011 07:31:05 +0200
From:	Mike Galbraith <efault@....de>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"Yan, Zheng Z" <zheng.z.yan@...el.com>
Subject: Re: [PATCH]sched: convert wall-time to vruntime for
 check_preempt_tick

On Fri, 2011-04-08 at 12:55 +0800, Shaohua Li wrote:
> On Fri, 2011-04-08 at 10:49 +0800, Mike Galbraith wrote:
> > 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.

> But we scale wakeup_granularity too for wakeup preemption, see
> wakeup_gran(), am I missing anything?

Only because we want lighter tasks to be easier to preempt, and heavier
tasks harder.  Weight is already built into vruntime.

	-Mike

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