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

[Rik van Riel]

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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@xxxxxxxxxx>

Acked-by: Rik van Riel <riel@xxxxxxxxxx>


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