linux-kernel - Re: [PATCH RFC/TEST] sched: make sync affine wakeups work

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Message-ID: <536379D0.8070306@redhat.com>
Date:	Fri, 02 May 2014 06:56:16 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
CC:	linux-kernel@...r.kernel.org, morten.rasmussen@....com,
	mingo@...nel.org, peterz@...radead.org,
	george.mccollister@...il.com, ktkhai@...allels.com
Subject: Re: [PATCH RFC/TEST] sched: make sync affine wakeups work

On 05/02/2014 03:37 AM, Mike Galbraith wrote:
> On Fri, 2014-05-02 at 02:30 -0400, Rik van Riel wrote: 
>> On 05/02/2014 02:13 AM, Mike Galbraith wrote:
>>> On Fri, 2014-05-02 at 00:42 -0400, Rik van Riel wrote:
>>>
>>>> Whether or not this is the right thing to do remains to be seen,
>>>> but it does allow us to verify whether or not the wake_affine
>>>> strategy of always doing affine wakeups and only disabling them
>>>> in a specific circumstance is sound, or needs rethinking...
>>>
>>> Yes, it needs rethinking.
>>>
>>> I know why you want to try this, yes, select_idle_sibling() is very much
>>> a two faced little bitch.
>>
>> My biggest problem with select_idle_sibling and wake_affine in
>> general is that it will override NUMA placement, even when
>> processes only wake each other up infrequently...
> 
> Hm, seems the thing to do would be to tell select_task_rq_fair() to keep
> it's mitts off of tasks that the numasched stuff has placed rather than
> decapitating select_idle_sibling() or some other drastic measure.

Thing is, if tasks are waking each other up frequently enough, we
probably DO want to place them near each other with select_idle_sibling.

We just cannot afford to have it as the default behaviour for casual
wakeup activity, because it will mess up other things.

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