linux-kernel - Re: [PATCH v2 01/11] sched: fix imbalance flag reset

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Message-ID: <5381C70E.3030605@linux.vnet.ibm.com>
Date:	Sun, 25 May 2014 16:03:50 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org
CC:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	linux@....linux.org.uk, linux-arm-kernel@...ts.infradead.org,
	Morten.Rasmussen@....com, efault@....de, nicolas.pitre@...aro.org,
	linaro-kernel@...ts.linaro.org, daniel.lezcano@...aro.org
Subject: Re: [PATCH v2 01/11] sched: fix imbalance flag reset

Hi Vincent,

On 05/23/2014 09:22 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.

Why do we do active balancing today when there is at-most 1 task on the
busiest cpu? Shouldn't we be skipping load balancing altogether? If we
do active balancing when the number of tasks = 1, it will lead to a ping
pong right?

Regards
Preeti U Murthy

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