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

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Date:   Wed, 26 Sep 2018 15:17:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.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, 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.