linux-kernel - Re: [PATCH] sched: wakeup buddy

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Message-ID: <1362731173.31859.81.camel@marge.simpson.net>
Date:	Fri, 08 Mar 2013 09:26:13 +0100
From:	Mike Galbraith <efault@....de>
To:	Michael Wang <wangyun@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Alex Shi <alex.shi@...el.com>, Paul Turner <pjt@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Ram Pai <linuxram@...ibm.com>
Subject: Re: [PATCH] sched: wakeup buddy

On Fri, 2013-03-08 at 15:30 +0800, Michael Wang wrote: 
> On 03/08/2013 02:44 PM, Mike Galbraith wrote:

> > In general, I think things would work better if we'd just rate limit how
> > frequently we can wakeup migrate each individual task.  
> 
> Isn't the wakeup buddy already limit the rate? and by turning the knob,
> we could change the rate on our demand.

I was referring to the existing kernel, not as altered. 

> We want
> > jabbering tasks to share L3, but we don't really want to trash L2 at an
> > awesome rate.
> 
> I don't get it..., it's a task which has 'sleep' for some time, unless
> there is no task running on prev_cpu when it's sleeping, otherwise
> whatever the new cpu is, we will trash L2, isn't it?

I'm thinking if you wake it to it's old home after a microscopic sleep,
it has a good chance of evicting the current resident, rescuing its L2.
If tasks which do microscopic sleep can't move around at a high rate,
they'll poke holes in fewer preempt victims.  If they're _really_ fast
switchers, always wake affine.  They can't hurt much, they don't do much
other than schedule off.

-Mike

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