Messages in this thread | | | Date | Tue, 30 Sep 2014 06:07:48 -0400 | From | Rik van Riel <> | Subject | Re: [PATCH] sched: fix spurious active migration |
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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 improveùent > and will not only move the avg_load imbalance on another CPU.
Good catch.
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
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