linux-kernel - Re: [PATCH] sched/fair: Don't increase sd->balance

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Message-ID: <CAKfTPtDxn4C-4wiZtyx-yAMK+pjz=AsRb1WwR5gaWuVkKD16wQ@mail.gmail.com>
Date:   Wed, 26 Sep 2018 15:56:06 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <Dietmar.Eggemann@....com>,
        Patrick Bellasi <Patrick.Bellasi@....com>
Subject: Re: [PATCH] sched/fair: Don't increase sd->balance_interval on
 newidle balance

On Wed, 26 Sep 2018 at 15:16, Valentin Schneider
<valentin.schneider@....com> wrote:
>
> On 26/09/18 11:33, Vincent Guittot wrote:
> > On Wed, 26 Sep 2018 at 11:35, Valentin Schneider
> > [...]
> >>> Can you give us details about the use case that you care about ?
> >>>
> >>
> >> It's the same as I presented last week - devlib (some python target communication
> >
> > ok. you mean at linaro connect
> >
>
> Yeah, sorry.
>
> >> library I use) has some phase where it spawns at lot of tasks at once to do
> >> some setup (busybox, shutils, bash...). Some of those tasks are pinned to a
> >> particular CPU, and that can lead to failed load_balance() - and to make things
> >> worse, there's a lot of idle_balance() in there.
> >>
> >> Eventually when I start running my actual workload a few ~100ms later, it's
> >> impacted by that balance_interval increase.
> >>
> >> Admittedly that's a specific use-case, but I don't think this quick increase
> >> is something that was intended.
> >
> > Yes, this really sounds like a specific use-case. Unluckily you find a
> > way to reach max interval quite easily/every time with your test
> > set-up but keep in mind that this can also happen in real system life
> > and without using the newly idle path.
> > So if it's a problem to have a interval at max value for your unitary
> > test, it probably means that it's a problem for the system and the max
> > value is too high
> >
>
> Limiting the max value for those tests is actually a good point, and I think
> I'll give it a shot. However...
>
> > Taking advantage of all load_balance event to update the interval
> > makes sense to me. It seems that you care about a short and regular
> > balance interval more that minimizing overhead of load balancing.
> > At the opposite, i'm sure that you don't complain if newly idle load
> > balance resets the interval to min value and overwrite what the
> > periodic load balance set up previously :-)
> >
>
> ...My concern is more about increasing balance_interval faster than we should.
> The proposed "fix" is to prevent any balance_interval increase when going
> through idle_balance(), but Patrick (added in cc) suggested offline that
> we could simply limit the rate at which we do these increases, so that they
> match what we do in rebalance_domains().
>
> We'd still increase balance_interval on failed newidle load_balance(), but
> we wouldn't increase from min to max in e.g. 50ms. Would that work better
> for you?

adding a rate limite would be a reasonnable solution.
>
> >> [...]