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

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Date:	Thu, 23 Oct 2014 04:12:40 +0200
From:	Mike Galbraith <efault@....de>
To:	Rik van Riel <riel@...hat.com>
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

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?

-Mike

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