linux-kernel - Re: [PATCH] cpufreq: CPPC: Increase delay between perf counter reads

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Message-ID: <aKRDxhirzwEPxaqd@arm.com>
Date: Tue, 19 Aug 2025 11:28:38 +0200
From: Beata Michalska <beata.michalska@....com>
To: Prashant Malani <pmalani@...gle.com>
Cc: Yang Shi <yang@...amperecomputing.com>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:CPU FREQUENCY SCALING FRAMEWORK" <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Ionela Voinescu <Ionela.Voinescu@....com>
Subject: Re: [PATCH] cpufreq: CPPC: Increase delay between perf counter reads

On Tue, Aug 05, 2025 at 06:00:09PM -0700, Prashant Malani wrote:
> Thanks Yang,
> 
> On Tue, 5 Aug 2025 at 17:26, Yang Shi <yang@...amperecomputing.com> wrote:
> > Thank you for cc'ing me the patch. I posted the similar patch ago and
> > had some discussion on the mailing list. Then someone else from ARM
> > pursued a different way to solve it. But I didn't follow very closely.
> > If I remember correctly, a new sysfs interface, called cpuinfo_avg_freq
> > was added. It should be the preferred way to get cpu frequency. Please
> > see
> > https://github.com/torvalds/linux/commit/fbb4a4759b541d09ebb8e391d5fa7f9a5a0cad61.
> >
> > Added Beata Michalska in the loop too, who is the author of the patch.
> > Please feel free to correct me, if I'm wrong.
> 
> Thanks for the additional context. Yeah, the issue is that :
> - The new sysfs node is sampling period is too long (20ms) [1]
> That makes it problematic for userspace use cases, so we need something
> which takes less time.
That actually specifies the duration, for which the most recently acquired
sample is considered valid. Sampling is tick-based.

> - The central accuracy issue behind cpuinfo_cur_freq still needs to be handled.
I'd really discourage increasing the sampling period for cppc.

---
BR
Beata
> 
> [1] https://elixir.bootlin.com/linux/v6.16/source/arch/arm64/kernel/topology.c#L283
> 
> 
> -- 
> -Prashant