Date: Tue, 15 Dec 2015 13:41:44 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Steve Muckle <steve.muckle@linaro.org>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Juri Lelli <Juri.Lelli@arm.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Michael Turquette <mturquette@baylibre.com>,
        Luca Abeni <luca.abeni@unitn.it>
Subject: Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in
 scale_rt_capacity
Message-ID: <20151215124144.GD6357@twins.programming.kicks-ass.net>
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On Tue, Dec 15, 2015 at 05:43:44AM +0100, Vincent Guittot wrote:
> On 14 December 2015 at 17:51, Peter Zijlstra <peterz@infradead.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).

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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