linux-kernel - Re: [PATCH v3 01/12] sched: fix imbalance flag reset

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Date:	Tue, 08 Jul 2014 23:36:26 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
	mingo@...nel.org, linux-kernel@...r.kernel.org,
	linux@....linux.org.uk, linux-arm-kernel@...ts.infradead.org
CC:	preeti@...ux.vnet.ibm.com, Morten.Rasmussen@....com, efault@....de,
	nicolas.pitre@...aro.org, linaro-kernel@...ts.linaro.org,
	daniel.lezcano@...aro.org, dietmar.eggemann@....com
Subject: Re: [PATCH v3 01/12] sched: fix imbalance flag reset

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On 07/08/2014 11:05 PM, Rik van Riel wrote:
> On 06/30/2014 12:05 PM, Vincent Guittot wrote:
>> The imbalance flag can stay set whereas there is no imbalance.
> 
>> Let assume that we have 3 tasks that run on a dual cores /dual 
>> cluster system. We will have some idle load balance which are 
>> triggered during tick. Unfortunately, the tick is also used to 
>> queue background work so we can reach the situation where short 
>> work has been queued on a CPU which already runs a task. The
>> load balance will detect this imbalance (2 tasks on 1 CPU and an
>> idle CPU) and will try to pull the waiting task on the idle CPU.
>> The waiting task is a worker thread that is pinned on a CPU so
>> an imbalance due to pinned task is detected and the imbalance
>> flag is set. Then, we will not be able to clear the flag because
>> we have at most 1 task on each CPU but the imbalance flag will
>> trig to useless active load balance between the idle CPU and the
>> busy CPU.
> 
>> We need to reset of the imbalance flag as soon as we have reached
>> a balanced state.
> 
>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> 
> Acked-by: Rik van Riel <riel@...hat.com>

Never mind that, Preeti explained the failure mode in more detail
on irc, and I am no longer convinced this change is a good idea.

- -- 
All rights reversed
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