Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity
From: Peter Zijlstra
Date: Tue Dec 15 2015 - 07:20:36 EST
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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