Re: [PATCH] sched: fix spurious active migration

[Rik van Riel]

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On 09/30/2014 04:41 AM, Vincent Guittot wrote:
> Since commit caeb178c60f4 ("sched/fair: Make
> update_sd_pick_busiest() ...") sd_pick_busiest returns a group that
> can be neither imbalanced nor overloaded but is only more loaded
> than others. This change has been introduced to ensure a better
> load balance in system that are not overloaded but as a side
> effect, it can also generate useless active migration between
> groups.
> 
> Let take the example of 3 tasks on a quad cores system. We will
> always have an idle core so the load balance will find a busiest
> group (core) whenever an ILB is triggered and it will force an
> active migration (once above nr_balance_failed threshold) so the
> idle core becomes busy but another core will become idle. With the
> next ILB, the freshly idle core will try to pull the task of a busy
> CPU. The number of spurious active migration is not so huge in quad
> core system because the ILB is not triggered so much. But it
> becomes significant as soon as you have more than one sched_domain
> level like on a dual cluster of quad cores where the ILB is
> triggered every tick when you have more than 1 busy_cpu
> 
> We need to ensure that the migration generate a real improveÃent
> and will not only move the avg_load imbalance on another CPU.

Good catch.

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

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


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