linux-kernel - Re: [PATCH v3 05/22] sched: remove domain iterations in fork/exec/wake

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Date:	Mon, 14 Jan 2013 17:18:51 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	li guang <lig.fnst@...fujitsu.com>
CC:	Morten Rasmussen <Morten.Rasmussen@....com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"bp@...en8.de" <bp@...en8.de>, "pjt@...gle.com" <pjt@...gle.com>,
	"namhyung@...nel.org" <namhyung@...nel.org>,
	"efault@....de" <efault@....de>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 05/22] sched: remove domain iterations in fork/exec/wake

On 01/14/2013 04:55 PM, li guang wrote:
>>>>>> > > >> > -		/* while loop will break here if sd == NULL */
>>>> > > > I agree that this should be a major optimization. I just can't figure
>>>> > > > out why the existing recursive search for an idle cpu switches to the
>>>> > > > new cpu near the end and then starts a search for an idle cpu in the new
>>>> > > > cpu's domain. Is this to handle some exotic sched domain configurations?
>>>> > > > If so, they probably wouldn't work with your optimizations.
>>> > > 
>>> > > I did not find odd configuration that asking for old logical.
>>> > > 
>>> > > According to Documentation/scheduler/sched-domains.txt, Maybe never.
>>> > > "A domain's span MUST be a superset of it child's span (this restriction
>>> > > could be relaxed if the need arises), and a base domain for CPU i MUST
>>> > > span at least i."  etc. etc.
>> > 
>> > The reason for my suspicion is the SD_OVERLAP flag, which has something
>> > to do overlapping sched domains. I haven't looked into what it does or
>> > how it works. I'm just wondering if this optimization will affect the
>> > use of that flag.
> seems it did, SD_OVERLAP will not work after this change,
> though this flag is maybe scarcely used.
> because, this optimization assume all sched-domains span 
> is super-set over child domain.
> isn't it? Alex.
> 

As my understanding, overlap just said some cpu may appears in 2 or more
same level sub domains. If so, this change won't miss cpus.

Am I right, Peter?

-- 
Thanks Alex
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