linux-kernel - Re: [PATCH RFC 5/7] sched: cfs: cpu frequency scaling arch functions

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Message-ID: <54486B26.30703@redhat.com>
Date:	Wed, 22 Oct 2014 22:42:46 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mike Galbraith <efault@....de>
CC:	Mike Turquette <mturquette@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	kamalesh@...ux.vnet.ibm.com,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Paul Turner <pjt@...gle.com>,
	Benjamin Segall <bsegall@...gle.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Patch Tracking <patches@...aro.org>,
	Tuukka Tikkanen <tuukka.tikkanen@...aro.org>,
	Amit Kucheria <amit.kucheria@...aro.org>
Subject: Re: [PATCH RFC 5/7] sched: cfs: cpu frequency scaling arch functions

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On 10/22/2014 10:12 PM, Mike Galbraith wrote:
> On Wed, 2014-10-22 at 21:42 -0400, Rik van Riel wrote:
>> On 10/22/2014 07:20 PM, Mike Turquette wrote:
>>> On Wed, Oct 22, 2014 at 1:06 PM, Rik van Riel
>>> <riel@...hat.com> wrote: On 10/22/2014 02:07 AM, Mike Turquette
>>> wrote:
>>>>>> arch_eval_cpu_freq and arch_scale_cpu_freq are added to
>>>>>> allow the scheduler to evaluate if cpu frequency should
>>>>>> change and to invoke that change from a safe context.
>>>>>> 
>>>>>> They are weakly defined arch functions that do nothing
>>>>>> by default. A CPUfreq governor could use these functions
>>>>>> to implement a frequency scaling policy based on updates
>>>>>> to per-task statistics or updates to per-cpu
>>>>>> utilization.
>>>>>> 
>>>>>> As discussed at Linux Plumbers Conference 2014, the goal
>>>>>> will be to focus on a single cpu frequency scaling policy
>>>>>> that works for everyone. That may mean that the weak
>>>>>> arch functions definitions can be removed entirely and a
>>>>>> single policy implements that logic for all
>>>>>> architectures.
>>> 
>>> On virtual machines, we probably want to use both frequency and
>>>  steal time to calculate the factor.
>>> 
>>>> You mean for calculating desired cpu frequency on a virtual 
>>>> guest? Is that something we want to do?
>> 
>> A guest will be unable to set the cpu frequency, but it should 
>> know what the frequency is, so it can take the capacity of each 
>> CPU into account when doing things like load balancing.
> 
> Hm.  Why does using vaporite freq/capacity/whatever make any sense,
> the silicon under the V(aporite)PU can/does change at the drop of a
> hat, no?

It can, but IIRC that should cause the kvmclock data for that VCPU
to be regenerated, and the VCPU should be able to use that to figure
out that the frequency changed the next time it runs the scheduler
code on that VCPU.

- -- 
All rights reversed
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