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

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Message-ID: <CAKfTPtBnj9_oHoXEZFnYnsxb5a3KwPcVHYydRVQEzypkCbJakQ@mail.gmail.com>
Date:	Fri, 4 Sep 2015 09:26:42 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
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 3 September 2015 at 21:58, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> Hi Vincent,
>
> On 02/09/15 10:31, Vincent Guittot wrote:
>> Hi Morten,
>>
>> On 14 August 2015 at 18:23, Morten Rasmussen <morten.rasmussen@....com> wrote:
>>> Bring arch_scale_cpu_capacity() in line with the recent change of its
>>> arch_scale_freq_capacity() sibling in commit dfbca41f3479 ("sched:
>>> Optimize freq invariant accounting") from weak function to #define to
>>> allow inlining of the function.
>>>
>>> While at it, remove the ARCH_CAPACITY sched_feature as well. With the
>>> change to #define there isn't a straightforward way to allow runtime
>>> switch between an arch implementation and the default implementation of
>>> arch_scale_cpu_capacity() using sched_feature. The default was to use
>>> the arch-specific implementation, but only the arm architecture provides
>>> one and that is essentially equivalent to the default implementation.
>
> [...]
>
>>
>> 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

>
> 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.
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.

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

>
> To connect the cpu invariant engine (scale_cpu_capacity()
> [arch/arm/kernel/topology.c]) with the scheduler, something like this is
> missing:
>
> diff --git a/arch/arm/include/asm/topology.h b/arch/arm/include/asm/topology.h
> index 370f7a732900..17c6b3243196 100644
> --- a/arch/arm/include/asm/topology.h
> +++ b/arch/arm/include/asm/topology.h
> @@ -24,6 +24,10 @@ void init_cpu_topology(void);
>  void store_cpu_topology(unsigned int cpuid);
>  const struct cpumask *cpu_coregroup_mask(int cpu);
>
> +#define arch_scale_cpu_capacity scale_cpu_capacity
> +struct sched_domain;
> +extern unsigned long scale_cpu_capacity(struct sched_domain *sd, int cpu);
> +
>  #else
>
>  static inline void init_cpu_topology(void) { }
> diff --git a/arch/arm/kernel/topology.c b/arch/arm/kernel/topology.c
> index 08b7847bf912..907e0d2d9b82 100644
> --- a/arch/arm/kernel/topology.c
> +++ b/arch/arm/kernel/topology.c
> @@ -42,7 +42,7 @@
>   */
>  static DEFINE_PER_CPU(unsigned long, cpu_scale);
>
> -unsigned long arch_scale_cpu_capacity(struct sched_domain *sd, int cpu)
> +unsigned long scale_cpu_capacity(struct sched_domain *sd, int cpu)
>  {
>         return per_cpu(cpu_scale, cpu);
>  }
> @@ -166,7 +166,7 @@ static void update_cpu_capacity(unsigned int cpu)
>         set_capacity_scale(cpu, cpu_capacity(cpu) / middle_capacity);
>
>         pr_info("CPU%u: update cpu_capacity %lu\n",
> -               cpu, arch_scale_cpu_capacity(NULL, cpu));
> +               cpu, scale_cpu_capacity(NULL, cpu));
>  }
>
> -- Dietmar
>
> [...]
>
--
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