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

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Date:   Wed, 26 Sep 2018 15:58:33 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Dietmar Eggemann <Dietmar.Eggemann@....com>
Subject: Re: [PATCH] sched/fair: Don't increase sd->balance_interval on
 newidle balance

On Wed, 26 Sep 2018 at 15:17, Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Sep 26, 2018 at 12:33:25PM +0200, Vincent Guittot wrote:
> > On Wed, 26 Sep 2018 at 11:35, Valentin Schneider
>
> > > 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
> >
> > 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 :-)
>
> Well, we've excluded newidle balance from updating such stats before. So
> in that respect the patch proposed by Valentin isn't weird.
>
> Consider for example:
>
>   58b26c4c0257 ("sched: Increment cache_nice_tries only on periodic lb")
>
> In general I think it makes perfect sense to exclude newidle balance
> from such stats; you get much more stable results from the regular
> balance.

Ok so in this case we should exclude all update of  the interval
during newly idle and not only some of them