linux-kernel - Re: [PATCH v10 2/4] cpufreq: Introduce an optional cpuinfo_avg

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Message-ID: <Z7NQrBvnfYwXva1W@arm.com>
Date: Mon, 17 Feb 2025 15:07:24 +0000
From: Ionela Voinescu <ionela.voinescu@....com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	Beata Michalska <beata.michalska@....com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-pm@...r.kernel.org, sudeep.holla@....com, will@...nel.org,
	viresh.kumar@...aro.org, sumitg@...dia.com,
	yang@...amperecomputing.com, vanshikonda@...amperecomputing.com,
	lihuisong@...wei.com, zhanjie9@...ilicon.com,
	ptsm@...ux.microsoft.com, Jonathan Corbet <corbet@....net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H . Peter Anvin" <hpa@...or.com>, Phil Auld <pauld@...hat.com>,
	x86@...nel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v10 2/4] cpufreq: Introduce an optional cpuinfo_avg_freq
 sysfs entry

Hi Catalin,

On Monday 17 Feb 2025 at 14:57:53 (+0000), Catalin Marinas wrote:
> On Mon, Feb 17, 2025 at 12:52:44PM +0100, Rafael J. Wysocki wrote:
> > On Fri, Jan 31, 2025 at 5:25 PM Beata Michalska <beata.michalska@....com> 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.
> > >
> > > Note that under the hood, the new attribute relies on the information
> > > provided by arch_freq_get_on_cpu, which, up to this point, has been
> > > feeding data for scaling_cur_freq attribute, being the source of
> > > ambiguity when it comes to interpretation. This has been amended by
> > > restoring the intended behavior for scaling_cur_freq, with a new
> > > dedicated config option to maintain status quo for those, who may need
> > > it.
> > 
> > In case anyone is waiting for my input here
> > 
> > Acked-by: Rafael J. Wysocki <rafael@...nel.org>
> > 
> > for this and the previous patch and please feel free to route them
> > both through ARM64.
> 
> Thanks Rafael. I indeed plan to take them through the arm64 tree.

Just a mention that this set depends on the patch that Beata linked at
[6]. That patch applies cleanly on next-20250217 and it still
builds/boots/works as expected.

Thanks,
Ionela.


> 
> -- 
> Catalin