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

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Message-ID: <Zyh_ACbOkSsjcPX3@arm.com>
Date: Mon, 4 Nov 2024 09:00:00 +0100
From: Beata Michalska <beata.michalska@....com>
To: Viresh Kumar <viresh.kumar@...aro.org>
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

On Tue, Oct 29, 2024 at 12:34:29PM +0530, Viresh Kumar wrote:
> 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 :(
Thanks for getting back to me on this one!
> 
> 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 ?
We need a way to discover whether the new sysfs attrib is to be enabled or not.
I guess I could change the signature for arch_freq_get_on_cpu to return an error
if that functionality is not supported for given policy ?

---
Best Regards
Beata
> 
> -- 
> viresh