linux-kernel - Re: [PATCH 8/8] sched: prefer cpufreq_scale_freq

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Date:	Tue, 15 Mar 2016 19:13:58 +0000
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Michael Turquette <mturquette@...libre.com>, peterz@...radead.org,
	rjw@...ysocki.net
Cc:	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Juri.Lelli@....com, steve.muckle@...aro.org,
	morten.rasmussen@....com, vincent.guittot@...aro.org,
	Michael Turquette <mturquette+renesas@...libre.com>
Subject: Re: [PATCH 8/8] sched: prefer cpufreq_scale_freq_capacity

On 14/03/16 05:22, Michael Turquette wrote:
> arch_scale_freq_capacity is weird. It specifies an arch hook for an
> implementation that could easily vary within an architecture or even a
> chip family.
> 
> This patch helps to mitigate this weirdness by defaulting to the
> cpufreq-provided implementation, which should work for all cases where
> CONFIG_CPU_FREQ is set.
> 
> If CONFIG_CPU_FREQ is not set, then try to use an implementation
> provided by the architecture. Failing that, fall back to
> SCHED_CAPACITY_SCALE.
> 
> It may be desirable for cpufreq drivers to specify their own
> implementation of arch_scale_freq_capacity in the future. The same is
> true for platform code within an architecture. In both cases an
> efficient implementation selector will need to be created and this patch
> adds a comment to that effect.

For me this independence of the scheduler code towards the actual
implementation of the Frequency Invariant Engine (FEI) was actually a
feature.

In EAS RFC5.2 (linux-arm.org/linux-power.git energy_model_rfc_v5.2 ,
which hasn't been posted to LKML) we establish the link in the ARCH code
(arch/arm64/include/asm/topology.h).

#ifdef CONFIG_CPU_FREQ
#define arch_scale_freq_capacity cpufreq_scale_freq_capacity
...
+#endif

> 
> Signed-off-by: Michael Turquette <mturquette+renesas@...libre.com>
> ---
>  kernel/sched/sched.h | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 469d11d..37502ea 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1368,7 +1368,21 @@ static inline int hrtick_enabled(struct rq *rq)
>  #ifdef CONFIG_SMP
>  extern void sched_avg_update(struct rq *rq);
>  
> -#ifndef arch_scale_freq_capacity
> +/*
> + * arch_scale_freq_capacity can be implemented by cpufreq, platform code or
> + * arch code. We select the cpufreq-provided implementation first. If it
> + * doesn't exist then we default to any other implementation provided from
> + * platform/arch code. If those do not exist then we use the default
> + * SCHED_CAPACITY_SCALE value below.
> + *
> + * Note that if cpufreq drivers or platform/arch code have competing
> + * implementations it is up to those subsystems to select one at runtime with
> + * an efficient solution, as we cannot tolerate the overhead of indirect
> + * functions (e.g. function pointers) in the scheduler fast path
> + */
> +#ifdef CONFIG_CPU_FREQ
> +#define arch_scale_freq_capacity cpufreq_scale_freq_capacity
> +#elif !defined(arch_scale_freq_capacity)
>  static __always_inline
>  unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu)
>  {
>