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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Tue, 29 Oct 2019 13:54:24 +0100
Message-ID: <CAKfTPtDnt6oh7X6dGnPUn70sLJXAQoxdkn0GCwdPvA8G4Wg0fA@mail.gmail.com>
Subject: Re: [PATCH v2] sched: rt: Make RT capacity aware
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Tue, 29 Oct 2019 at 13:46, Qais Yousef <qais.yousef@arm.com> wrote:
>
> On 10/29/19 13:20, Vincent Guittot wrote:
> > > > Making big cores the default CPUs for all RT tasks is not a minor
> > > > change and IMO locality should stay the default behavior when there is
> > > > no uclamp constraint
> > >
> > > How this is affecting locality? The task will always go to the big core, so it
> > > should be local.
> >
> > local with the waker
> > You will force rt task to run on big cluster although waker, data and
> > interrupts can be on little one.
> > So making big core as default is far from always being the best choice
>
> This is loaded with assumptions IMO. AFAICT we don't know what's the best
> choice.
>
> First, the value of uclamp.min is outside of the scope of this patch. Unless
> what you're saying is that when uclamp.min is 1024 then we should NOT choose a
> big cpu then there's no disagreement about what this patch do. If that's what
> you're objecting to please be more specific about how do you see this working
> instead.

My point is that this patch makes the big cores the default CPUs for
RT tasks which is far from being a minor change and far from being an
obvious default good choice

>
> If your objection is purely based on the choice of uclamp.min then while
> I agree that on modern systems the little cores are good enough for the
> majority of RT tasks in average Android systems. But I don't feel confident to
> reach this conclusion on low end systems where the little core doesn't have
> enough grunt in many cases. So I see the current default is adequate and the
> responsibility of further tweaking lies within the hands of the system admin.
>
> --
> Qais Yousef