linux-kernel - Re: [PATCH RESEND v9 05/10] sched: make scale

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Message-ID: <54EC61A2.5010702@arm.com>
Date:	Tue, 24 Feb 2015 11:33:54 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"kamalesh@...ux.vnet.ibm.com" <kamalesh@...ux.vnet.ibm.com>,
	"riel@...hat.com" <riel@...hat.com>,
	"efault@....de" <efault@....de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH RESEND v9 05/10] sched: make scale_rt invariant with frequency

On 24/02/15 10:21, Vincent Guittot wrote:
> On 19 February 2015 at 18:18, Morten Rasmussen <morten.rasmussen@....com> wrote:
>> On Thu, Feb 19, 2015 at 04:52:41PM +0000, Peter Zijlstra wrote:
>>> On Thu, Jan 15, 2015 at 11:09:25AM +0100, Vincent Guittot wrote:

[...]

>> Agreed. I think it is reasonable to assume that the arch code
>> implementing arch_scale_freq_capacity() does it's best to make it fast
>> for the particular architecture. Since the scaling factor to be returned
>> by the function may be obtained in different ways for different
>> architectures the caching should be done on the arch side.
>>
>>> But lets see, I've yet to see an actual implementation of it; and its
>>> got that sd argument, curious what you're going to do with that.
>>
>> So we do have an RFC implementation for ARM already which I posted in
>> December and is also included in the rather large RFC posting I did some
>> weeks ago. That one basically reads two atomic variables and returns the
>> ratio between the two. I have yet to benchmark how horribly expensive it
>> is though. The sd argument is ignored. We might actually not need it at
>> all?
> 
> For consistency across all arch_scale_xx_capacity, i would prefer to
> keep the same prototype interface (struct sched_domain *sd, int cpu)
> even if it's not used ofr now

Agreed.

Once we call arch_scale_xx_capacity [xx = freq, cpu] in the PELT code
(i.e. w/ sd = NULL) we have to make sure that
default_scale_cpu_capacity() can be called w/ sd = NULL too for
platforms which are not providing their own arch_scale_cpu_capacity()
function.

It's already part of '[RFCv3 PATCH 00/48] sched: Energy cost model for
energy-aware scheduling'

https://lkml.org/lkml/2015/2/4/573

[...]

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