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

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Date:	Tue, 15 Dec 2015 14:41:53 +0100
From:	Luca Abeni <luca.abeni@...tn.it>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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>
Subject: Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in
 scale_rt_capacity

On 12/15/2015 01:58 PM, Vincent Guittot wrote:
> On 15 December 2015 at 09:50, Luca Abeni <luca.abeni@...tn.it> wrote:
>> On 12/15/2015 05:59 AM, Vincent Guittot wrote:
>> [...]
>>>>>>
>>>>>> So I don't think this is right. AFAICT this projects the WCET as the
>>>>>> amount of time actually used by DL. This will, under many
>>>>>> circumstances, vastly overestimate the amount of time actually
>>>>>> spend on it. Therefore unduly pessimisme the fair capacity of this
>>>>>> CPU.
>>>>>
>>>>>
>
> [snip]
>
>>> 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.
>
> In this case, it's probably easier to split what is already done into
> a rt_avg metric  and a dl_avg metric
Yes, I think this could be the best approach for what concerns the average
utilisation used by CFS.



				Luca
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