sched/fair: Reduce cases for active balance

Active balance is triggered for a number of voluntary cases like misfit
or pinned tasks cases but also after that a number of load balance
attempts failed to migrate a task. There is no need to use active load
balance when the group is overloaded because an overloaded state means
that there is at least one waiting task. Nevertheless, the waiting task
is not selected and detached until the threshold becomes higher than its
load. This threshold increases with the number of failed lb (see the
condition if ((load >> env->sd->nr_balance_failed) > env->imbalance) in
detach_tasks()) and the waiting task will end up to be selected after a
number of attempts.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Link: https://lkml.kernel.org/r/20210107103325.30851-4-vincent.guittot@linaro.org

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