linux-kernel - Re: [PATCH v2] sched: rt: Make RT capacity aware

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Date:   Tue, 29 Oct 2019 14:02:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Qais Yousef <qais.yousef@....com>, Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched: rt: Make RT capacity aware

On Tue, Oct 29, 2019 at 01:54:24PM +0100, Vincent Guittot wrote:
> On Tue, 29 Oct 2019 at 13:46, Qais Yousef <qais.yousef@....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

FIFO/RR tasks don't have a bandwidth specification (barring uclamp),
therefore we must assume the worst. This is the same principle that has
them select max_freq all the time.

I think it is a very natural extention of that very principle to place
(otherwise unconstrained RT tasks) on big cores.