linux-kernel - Re: [PATCH] sched: wake up task on prev_cpu if not in SD_WAKE

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Date:	Fri, 09 May 2014 10:22:06 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, morten.rasmussen@....com,
	mingo@...nel.org, george.mccollister@...il.com,
	ktkhai@...allels.com
Subject: Re: [PATCH] sched: wake up task on prev_cpu if not in SD_WAKE_AFFINE
 domain with cpu

On 05/09/2014 03:34 AM, Mike Galbraith wrote:
> On Fri, 2014-05-09 at 01:27 -0400, Rik van Riel wrote: 
>> On Thu, 08 May 2014 22:20:25 -0400
>> Rik van Riel <riel@...hat.com> wrote:
>>
>>> Looks like SD_BALANCE_WAKE is not gotten from the sd flags at
>>> all, but passed into select_task_rq by try_to_wake_up, as a
>>> hard coded sd_flags argument.
>>
>>> Should we do that, if SD_WAKE_BALANCE is not set for any sched domain?
>>
>> I answered my own question. The sd_flag SD_WAKE_BALANCE simply means
>> "this is a wakeup of a previously existing task, please place it
>> properly".
>>
>> However, it appears that the current code will fall back to the large
>> loop with select_idlest_group and friends, if prev_cpu and cpu are not
>> part of the same SD_WAKE_AFFINE sched domain. That is a bug...
> 
> ttwu(): cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);
> 
> We pass SD_BALANCE_WAKE for a normal wakeup, so sd will only be set if
> we encounter a domain during traversal where Joe User has told us to do
> (expensive) wake balancing before we hit a domain shared by waker/wakee.
> 
> The user can turn SD_WAKE_AFFINE off beyond socket, and we'll not pull
> cross node on wakeup.
> 
> Or, you could create an override button to say despite SD_WAKE_AFFINE
> perhaps having been set during domain construction (because of some
> pseudo-random numbers), don't do that if we have a preferred node, or
> just make that automatically part of having numa scheduling enabled, and
> don't bother wasting cycles if preferred && this != preferred.

That's not the problem.

The problem is that if we do not do an affine wakeup, due to
SD_WAKE_AFFINE not being set on a top level domain, we will
not try to run p on prev_cpu, but we will fall through into
the loop with find_idlest_group, etc...

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