Date: Tue, 15 Dec 2015 13:20:15 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Luca Abeni <luca.abeni@unitn.it>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
        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>
Subject: Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in
 scale_rt_capacity
Message-ID: <20151215122015.GA6357@twins.programming.kicks-ass.net>
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 <20151214221231.39b5bc4e@luca-1225C>
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On Tue, Dec 15, 2015 at 09:50:14AM +0100, Luca Abeni wrote:
> On 12/15/2015 05:59 AM, Vincent Guittot wrote:

> >The 2nd definition is used to compute the remaining capacity for the
> >CFS scheduler. This one doesn't need to be updated at each wake/sleep
> >of a deadline task but should reflect the capacity used by deadline in
> >a larger time scale. The latter will be used by the CFS scheduler  at
> >the periodic load balance pace

> Ok, so as I wrote above this really looks like an average utilisation.
> My impression (but I do not know the CFS code too much) is that the mainline
> kernel is currently doing the right thing to compute it, so maybe there is no
> need to change the current code in this regard.
> If the current code is not acceptable for some reason, an alternative would
> be to measure the active utilisation for frequency scaling, and then apply a
> low-pass filter to it for CFS.

So CFS really only needs a 'vague' average idea on how much time it will
not get. Its best effort etc., so being a little wrong isn't a problem.

The current code suffices, but I think the reason its been changed in
this series is that they want/need separate tracking for fifo/rr and
deadline in the next patch, and taking out deadline like proposed was
the easiest way of achieving that.


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