Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity

From: Luca Abeni
Date: Tue Dec 15 2015 - 08:18:20 EST

On 12/15/2015 01:20 PM, Peter Zijlstra wrote:
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.
Ah, ok. Thanks for explaining.
So, I agree that this patch is not a good idea for estimating the average
utilisation needed by CFS.

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