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

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Message-ID: <CAKfTPtDBba=XnVV_z8ahFk1BznkdoWPtBTEp1BnUM3MOsGFrfA@mail.gmail.com>
Date:	Tue, 15 Dec 2015 13:56:03 +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 15 December 2015 at 13:41, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Dec 15, 2015 at 05:43:44AM +0100, Vincent Guittot wrote:
>> On 14 December 2015 at 17:51, Peter Zijlstra <peterz@...radead.org> wrote:
>
>> > 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
>
> Only if you've stuffed your machine full of deadline tasks, if you take
> Luca's example of the I/B frame decoder thingy, then even the WCET for
> the I frames should not be very much (albeit significantly more than B
> frames).

But in this case, the impact of deadline scheduler on the remaining
capacity for CFS should not be that much as well. This will not
prevent a CFS task but only will only make it a bit smaller than the
other

>
> So while the WCET is pessimistic compared to the avg case, most CPUs can
> do video decoding without much effort at all, so even the WCET for the
> I-frames might allow us to drop to the lowest cpufreq.
>
> Now, if you were to decode 10 streams at the same time, different story
> of course ;-)
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