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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:

quoted

The imbalance flag can stay set whereas there is no imbalance.

quoted

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.

quoted

We need to reset of the imbalance flag as soon as we have reached
a balanced state.

quoted

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>

Acked-by: Rik van Riel <redacted>

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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