linux-kernel - Re: [RFC][PATCH] sched: Avoid select_idle_sibling() for wake

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Date:	Thu, 26 Sep 2013 15:26:45 +0800
From:	Michael wang <wangyun@...ux.vnet.ibm.com>
To:	Mike Galbraith <bitbucket@...ine.de>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>, Paul Turner <pjt@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] sched: Avoid select_idle_sibling() for wake_affine(.sync=true)

On 09/26/2013 03:09 PM, Mike Galbraith wrote:
[snip]
>>
>> Ok, a double-edged sword I see :)
>>
>> May be we can wave it carefully here, give the discount to a bigger
>> scope not the sync cpu, for example:
>>
>> 	sg1				sg2
>> 	cpu0	cpu1	cpu2	cpu3	cpu4	cpu5	cpu6	cpu7
>> 	waker	idle	idle	idle	idle	idle	idle	idle
>>
>> If it's sync wakeup on cpu0 (only waker), and the sg is wide enough,
>> which means one cpu is not so influencial, then suppose cpu0 to be idle
>> could be more safe, also prefer sg1 than sg2 is more likely to be right.
>>
>> And we can still choose idle-cpu at final step, like cpu1 in this case,
>> to avoid the risk that waker don't get off as it said.
>>
>> The key point is to reduce the influence of sync, trust a little but not
>> totally ;-)
> 
> What we need is a dirt cheap way to fairly accurately predict overlap
> potential (todo: write omniscience().. patent, buy planet).

Agree, solutions for such cases are usually incredible ;-)

Regards,
Michael Wang

> 
> -Mike
> 
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