linux-kernel - Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt

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Date:	Tue, 15 Dec 2015 05:43:44 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>,
	Luca Abeni <luca.abeni@...tn.it>
Subject: Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity

On 14 December 2015 at 17:51, Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Dec 14, 2015 at 04:56:17PM +0100, Vincent Guittot wrote:
>> I agree that if the WCET is far from reality, we will underestimate
>> available capacity for CFS. Have you got some use case in mind which
>> overestimates the WCET ?
>
> Pretty much any 'correct' WCET is pessimistic. There's heaps of smart
> people working on improving WCET bounds, but they're still out there.
> This is mostly because of the .00001% tail cases that 'never' happen but
> would make your tokamak burn a hole just when you're outside.
>
>> If we can't rely on this parameters to evaluate the amount of capacity
>> used by deadline scheduler on a core, this will imply that we can't
>> also use it for requesting capacity to cpufreq and we should fallback
>> on a monitoring mechanism which reacts to a change instead of
>> anticipating it.
>
> No, since the WCET can and _will_ happen, its the best you can do with
> cpufreq. If you were to set it lower you could not be able to execute
> correctly in your 'never' tail cases.

In the context of frequency scaling, This mean that we will never
reach low frequency


>
> There 'might' be smart pants ways around this, where you run part of the
> execution at lower speed and switch to a higher speed to 'catch' up if
> you exceed some boundary, such that, on average, you run at the same
> speed the WCET mandates, but I'm not sure that's worth it. Juri/Luca
> might know.
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