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

From: Rik van Riel
Date: Tue Jul 08 2014 - 23:37:52 EST

Hash: SHA1

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

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.

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