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

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Date:	Wed, 16 Mar 2016 19:44:33 +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 15/03/16 20:46, Michael Turquette wrote:
> Quoting Dietmar Eggemann (2016-03-15 12:13:58)
>> On 14/03/16 05:22, Michael Turquette wrote:

[...]

>> For me this independence of the scheduler code towards the actual
>> implementation of the Frequency Invariant Engine (FEI) was actually a
>> feature.
> 
> I do not agree that it is a strength; I think it is confusing. My
> opinion is that cpufreq drivers should implement
> arch_scale_freq_capacity. Having a sane fallback
> (cpufreq_scale_freq_capacity) simply means that you can remove the
> boilerplate from the arm32 and arm64 code, which is a win.
> 
> Furthermore, if we have multiple competing implementations of
> arch_scale_freq_invariance, wouldn't it be better for all of them to
> live in cpufreq drivers? This means we would only need to implement a
> single run-time "selector".
> 
> On the other hand, if the implementation lives in arch code and we have
> various implementations of arch_scale_freq_capacity within an
> architecture, then each arch would need to implement this selector
> function. Even worse then if we have a split where some implementations
> live in drivers/cpufreq (e.g. intel_pstate) and others in arch/arm and
> others in arch/arm64 ... now we have three selectors.

OK, now I see your point. What I don't understand is the fact why you
want different foo_scale_freq_capacity() implementations per cpufreq
drivers. IMHO we want to do the cpufreq.c based implementation to
abstract from that (at least for target_index() cpufreq drivers).

intel_pstate (setpolicy()) is an exception but my humble guess is that
systems with intel_pstate driver have X86_FEATURE_APERFMPERF support.

> Note that this has nothing to do with cpu microarch invariance. I'm
> happy for that to stay in arch code because we can have heterogeneous
> cpus that do not scale frequency, and thus would not enable cpufreq.
> But if your platform scales cpu frequency, then really cpufreq should be
> in the loop.

Agreed.

> 
>>
>> 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).
> 
> Right, sorry again about preemptively posting the patch. Total brainfart
> on my part.
> 
>>
>> #ifdef CONFIG_CPU_FREQ
>> #define arch_scale_freq_capacity cpufreq_scale_freq_capacity
>> ...
>> +#endif
> 
> The above is no longer necessary with this patch. Same question as
> above: why insist on the arch boilerplate?

OK.

[...]