linux-kernel - Re: [PATCH v7 1/4] cpufreq: Introduce an optional cpuinfo_avg

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Message-ID: <20241029070429.m7q5dkumitoyqxq2@vireshk-i7>
Date: Tue, 29 Oct 2024 12:34:29 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: Beata Michalska <beata.michalska@....com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-pm@...r.kernel.org, ionela.voinescu@....com,
	sudeep.holla@....com, will@...nel.org, catalin.marinas@....com,
	rafael@...nel.org, sumitg@...dia.com, yang@...amperecomputing.com,
	vanshikonda@...amperecomputing.com, lihuisong@...wei.com,
	zhanjie9@...ilicon.com
Subject: Re: [PATCH v7 1/4] cpufreq: Introduce an optional cpuinfo_avg_freq
 sysfs entry

Apologies for the delay from my side. September was mostly holidays
for me and then I was stuck with other stuff plus email backlog and
this series was always a painful point to return to :(

On 13-09-24, 14:29, Beata Michalska wrote:
> Currently the CPUFreq core exposes two sysfs attributes that can be used
> to query current frequency of a given CPU(s): namely cpuinfo_cur_freq
> and scaling_cur_freq. Both provide slightly different view on the
> subject and they do come with their own drawbacks.
> 
> cpuinfo_cur_freq provides higher precision though at a cost of being
> rather expensive. Moreover, the information retrieved via this attribute
> is somewhat short lived as frequency can change at any point of time
> making it difficult to reason from.
> 
> scaling_cur_freq, on the other hand, tends to be less accurate but then
> the actual level of precision (and source of information) varies between
> architectures making it a bit ambiguous.
> 
> The new attribute, cpuinfo_avg_freq, is intended to provide more stable,
> distinct interface, exposing an average frequency of a given CPU(s), as
> reported by the hardware, over a time frame spanning no more than a few
> milliseconds. As it requires appropriate hardware support, this
> interface is optional.

>From what I recall, the plan is to:
- keep cpuinfo_cur_freq as it is, not expose for x86 and call ->get()
  for ARM.

- introduce cpuinfo_avg_freq() and make it return frequency from hw
  counters for both ARM and Intel and others who provide the API.

- update scaling_cur_freq() to only return the requested frequency or
  error in case of X86 and update documentation to reflect the same.
  Right now or after some time ? How much time ?

  Rafael ?

> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 04fc786dd2c0..3493e5a9500d 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -752,6 +752,16 @@ __weak unsigned int arch_freq_get_on_cpu(int cpu)
>  	return 0;
>  }
>  
> +__weak int arch_freq_avg_get_on_cpu(int cpu)
> +{
> +	return -EOPNOTSUPP;
> +}
> +
> +static inline bool cpufreq_avg_freq_supported(struct cpufreq_policy *policy)
> +{
> +	return arch_freq_avg_get_on_cpu(policy->cpu) >= 0;
> +}

And why aren't we simply reusing arch_freq_get_on_cpu() here ?

-- 
viresh