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

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Date:	Tue, 12 Mar 2013 14:00:30 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	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 03/11/2013 05:40 PM, Ingo Molnar wrote:
> 
> * Michael Wang <wangyun@...ux.vnet.ibm.com> wrote:
> 
>> Hi, Ingo
>>
>> On 03/11/2013 04:21 PM, Ingo Molnar wrote:
>> [snip]
>>>
>>> I have actually written the prctl() approach before, for instrumentation 
>>> purposes, and it does wonders to system analysis.
>>
>> The idea sounds great, we could get many new info to implement more
>> smart scheduler, that's amazing :)
>>
>>>
>>> Any objections?
>>
>> Just one concern, may be I have misunderstand you, but will it cause 
>> trouble if the prctl() was indiscriminately used by some applications, 
>> will we get fake data?
> 
> It's their problem: overusing it will increase their CPU overhead. The two 
> boundary worst-cases are that they either call it too frequently or too 
> rarely:
> 
>  - too frequently: it approximates the current cpu-runtime work metric
> 
>  - too infrequently: we just ignore it and fall back to a runtime metric
>    if it does not change.
> 
> It's not like it can be used to get preferential treatment - we don't ever 
> balance other tasks against these tasks based on work throughput, we try 
> to maximize this workload's work throughput.
> 
> What could happen is if an app is 'optimized' for a buggy scheduler by 
> changing the work metric frequency. We offer no guarantee - apps will be 
> best off (and users will be least annoyed) if apps honestly report their 
> work metric.
> 
> Instrumentation/stats/profiling will also double check the correctness of 
> this data: if developers/users start relying on the work metric as a 
> substitute benchmark number, then app writers will have an additional 
> incentive to make them correct.

I see, I could not figure out how to wisely using the info currently,
but I have the feeling that it will make scheduler very different ;-)

May be we could implement the API and get those info ready firstly
(along with the new sched-pipe which provide work tick info), then think
about the way to use them in scheduler, is there any patches on the way?

Regards,
Michael Wang

> 
> Thanks,
> 
> 	Ingo
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