From: Valentin Schneider <valentin.schneider@arm.com>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Juri Lelli <juri.lelli@redhat.com>,
Dietmar Eggemann <dietmar.eggemann@arm.com>,
Steven Rostedt <rostedt@goodmis.org>,
Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] sched/fair: improve spreading of utilization
Date: Fri, 13 Mar 2020 15:47:17 +0000 [thread overview]
Message-ID: <jhj36acp88q.mognet@arm.com> (raw)
In-Reply-To: <CAKfTPtBZgvTBYR+kYjj9dHq8_25mG19CZmYzY5s33ijSHdLGyQ@mail.gmail.com>
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.
next prev parent reply other threads:[~2020-03-13 15:47 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-12 16:54 [PATCH] sched/fair: improve spreading of utilization Vincent Guittot
2020-03-13 10:26 ` Peter Zijlstra
2020-03-13 11:00 ` Valentin Schneider
2020-03-13 11:24 ` Vincent Guittot
2020-03-13 11:28 ` Valentin Schneider
2020-03-13 12:42 ` Valentin Schneider
2020-03-13 12:55 ` Vincent Guittot
2020-03-13 14:26 ` Vincent Guittot
2020-03-13 15:47 ` Valentin Schneider [this message]
2020-03-13 16:09 ` Vincent Guittot
2020-03-13 16:57 ` Valentin Schneider
2020-03-13 17:12 ` Vincent Guittot
2020-03-13 17:34 ` Valentin Schneider
2020-03-20 12:58 ` [tip: sched/core] sched/fair: Improve " tip-bot2 for Vincent Guittot
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