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

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Date:	Thu, 28 Feb 2013 15:40:04 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Mike Galbraith <efault@....de>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>, Alex Shi <alex.shi@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ram Pai <linuxram@...ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [RFC PATCH] sched: wakeup buddy

Hi, Mike

Thanks for your reply.

On 02/28/2013 03:18 PM, Mike Galbraith wrote:
> On Thu, 2013-02-28 at 14:38 +0800, Michael Wang wrote:
> 
>> +				/*
>> +				 * current is the only task on rq and it is
>> +				 * going to sleep, current cpu will be a nice
>> +				 * candidate for p to run on.
>> +				 */
> 
> The sync hint only means it might be going to sleep soon, and even then,
> there can still be enough execution overlap to be a win to schedule
> cross core.  Sched pipe numbers will always be much prettier if you do
> wakeup cpu affine, as it's ~100% scheduler and ~100% sync.

Hmm.. so it's the comparison between 'cache benefit - execution overlap'
and 'latency - execution overlap'?

I could not estimate how many latency will be added to wait for current
going to sleep (it should be faster than access cold data, isn't it?),
but I really like the cache benefit, unless sync doesn't means current
is going to sleep every time, but that's the promise of WF_SYNC, isn't it?

You may lose
> a lot on other stuff if you interpret the hint as gospel truth.

Could you please give more details on this point?

> 
> IMHO, sched pipe is a "how fat have I become" benchmark, not "how well
> do I perform".  The scheduler performs well when it makes more work
> happen.  Playing ping-pong with yourself is _exercise_, not a job :)

That's right, may be I'm using the wrong description, it's the ops/sec
which has been doubled, that means 'fat', correct?

Regards,
Michael Wang

> 
> -Mike
> 
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