linux-kernel - Re: [PATCH] sched/fair: improve spreading of utilization

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Date:   Fri, 13 Mar 2020 15:47:17 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: improve spreading of utilization


On Fri, Mar 13 2020, Vincent Guittot wrote:
>> > And with more coffee that's another Doh, ASYM_PACKING would end up as
>> > migrate_task. So this only affects the reduced capacity migration, which
>>
>> yes  ASYM_PACKING uses migrate_task and the case of reduced capacity
>> would use it too and would not be impacted by this patch. I say
>> "would" because the original rework of load balance got rid of this
>> case. I'm going to prepare a separate fix  for this
>
> After more thought, I think that we are safe for reduced capacity too
> because this is handled in the migrate_load case. In my previous
> reply, I was thinking of  the case where rq is not overloaded but cpu
> has reduced capacity which is not handled. But in such case, we don't
> have to force the migration of the task because there is still enough
> capacity otherwise rq would be overloaded and we are back to the case
> already handled
>

Good point on the capacity reduction vs group_is_overloaded.

That said, can't we also reach this with migrate_task? Say the local
group is entirely idle, and the busiest group has a few non-idle CPUs
but they all have at most 1 running task. AFAICT we would still go to
calculate_imbalance(), and try to balance out the number of idle CPUs.

If the migration_type is migrate_util, that can't happen because of this
change. Since we have this progressive balancing strategy (tasks -> util
-> load), it's a bit odd to have this "gap" in the middle where we get
one less possibility to trigger active balance, don't you think? That
is, providing I didn't say nonsense again :)

It's not a super big deal, but I think it's nice if we can maintain a
consistent / gradual migration policy.

>>
>> > might be hard to notice in benchmarks.