linux-kernel - Re: [PATCH RFC/RFT] sched/fair: Improve the behavior of sync flag

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Date:   Mon, 28 Aug 2017 08:10:23 +0200
From:   Mike Galbraith <efault@....de>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Josef Bacik <jbacik@...com>, Juri Lelli <Juri.Lelli@....com>,
        Brendan Jackman <brendan.jackman@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Rik van Riel <riel@...hat.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH RFC/RFT] sched/fair: Improve the behavior of sync flag

On Sun, 2017-08-27 at 22:27 -0700, Joel Fernandes wrote:
> Hi Mike,
> 
> On Sun, Aug 27, 2017 at 11:07 AM, Mike Galbraith <efault@....de> wrote:
> > On Sat, 2017-08-26 at 23:39 -0700, Joel Fernandes wrote:
> >>
> >> Also about real world benchmarks, in Android we have usecases that
> >> show that the graphics performance and we have risk of frame drops if
> >> we don't use the sync flag so this is a real world need.
> >
> > That likely has everything to do with cpufreq not realizing that your
> > CPUs really are quite busy when scheduling cross core at fairly high
> > frequency, and not clocking up properly.
> >
> 
> I'm glad you brought this point up. Since Android O, the userspace
> processes are much more split across procedure calls due to a feature
> called treble (which does this for security, modularity etc). Due to
> this, a lot of things that were happening within a process boundary
> happen now across process boundaries over the binder bus. Early on
> folks noticed that this caused performance issues without sync flag
> being used as a more strong hint. This can happen when there are 2
> threads are in different frequency domains on different CPUs and are
> communicating over binder, due to this the combined load of both
> threads is divided between the individual CPUs and causes them to run
> at lower frequency. Where as if they are running together on the same
> CPUs, then they would run at a higher frequency and perform better as
> their combined load would run at a higher frequency. So a stronger
> sync actually helps this case if we're careful about using it when
> possible.

Sure, but isn't that really a cpufreq issue?  We schedule cross core
quite aggressively for obvious reasons.  Now on mostly idle handheld
devices, you may get better battery life by stacking tasks a bit more,
in which case a sync-me-harder flag may be what you really want/need,
but with modern CPUs, I'm kinda skeptical of that, would have to see
cold hard numbers to become a believer.  Iff deeper cstate etc for
longer does make a big difference, I can imagine wakeup time migrate
leftward if capacity exists as an "on battery" tactic. (though that
thought also invokes some unpleasant bounce fest images)

	-Mike