linux-kernel - Re: [PATCH 2/6] sched/fair: Convert arch_scale_cpu

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Date:	Mon, 7 Sep 2015 14:25:11 +0100
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Morten Rasmussen <Morten.Rasmussen@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Yuyang Du <yuyang.du@...el.com>,
	Michael Turquette <mturquette@...libre.com>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	Sai Charan Gurrappadi <sgurrappadi@...dia.com>,
	"pang.xunlei@....com.cn" <pang.xunlei@....com.cn>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/6] sched/fair: Convert arch_scale_cpu_capacity() from
 weak function to #define

On 04/09/15 08:26, Vincent Guittot wrote:
> On 3 September 2015 at 21:58, Dietmar Eggemann <dietmar.eggemann@....com> wrote:

[...]

>>> So you change the way to declare arch_scale_cpu_capacity but i don't
>>> see the update of the arm arch which declare a
>>> arch_scale_cpu_capacity to reflect this change in your series.
>>
>> We were reluctant to do this because this functionality makes only sense
>> for ARCH=arm big.Little systems w/ cortex-a{15|7} cores and only if the
>> clock-frequency property is set in the dts file.
> 
> IMO, we should maintain the compatibility of current implementation
> instead of breaking the link and creating a dead code.
> Your proposal below fits the requirement

The only problem with this solution is that now we got a call to
arch_scale_cpu_capacity() in the hotpath whereas before it is only
called in update_cpu_capacity(). An implementation of
scale_cpu_capacity() in arch/arm/kernel/topology.c leads to a function
call in __update_load_avg. I'm in the middle of doing some performance
tests on TC2 w/ and w/o the cpu invariant implementation.

> 
>>
>> Are you planning to push for a 'struct cpu_efficiency/clock-frequency
>> property' solution for ARCH=arm64 as well?
> 
> I know that there has been some discussions aorund that but i didn't
> follow the thread in details
> 
>>
>> I'm asking because for ARCH=arm64 systems today (JUNO, Hi6220) we use the
>> capacity value of the last entry of the capacity_state vector for the cores
>> (e.g. cortex-a{57|53).
> 
> This is a struct of the eas feature ? Not sure that we should link the
> definition of  the cpu capacity to an internal struct of a feature; DT
> seems a better way to define it.

Yeah, the cpu invariant functionality should not base on EAS. We just
use the short-cut in EAS RFCv5 to get it working on ARM64.

> So if you want to revisit the way, we set the capacity of CPU for arm
> and/or arm64, I'm fully open to the discussion but this should happen
> in another thread than this one which has for only purpose the
> alignment of the arch_scale_cpu_capacity interface declaration with
> arch_scale_freq_capacity one.

Agreed.

> 
> So, with the patch below that updates the arm definition of
> arch_scale_cpu_capacity, you can add my Acked-by: Vincent Guittot
> <vincent.guittot@...aro.org> on this patch and the additional one
> below
> 
> Regards,
> Vincent

[...]

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