linux-kernel - Re: [PATCH v2 2/2] cpufreq: CPPC: Dont read counters for idle CPUs

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Message-ID: <CAFivqmKgiVEWMQ90Lh6T+Y44E6m4jmdF5sUFfVNTmgVHOMtZsw@mail.gmail.com>
Date: Mon, 4 Aug 2025 13:55:37 -0700
From: Prashant Malani <pmalani@...gle.com>
To: Beata Michalska <beata.michalska@....com>
Cc: Viresh Kumar <viresh.kumar@...aro.org>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Jie Zhan <zhanjie9@...ilicon.com>, Ionela Voinescu <ionela.voinescu@....com>, 
	Ben Segall <bsegall@...gle.com>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>, 
	open list <linux-kernel@...r.kernel.org>, 
	"open list:CPU FREQUENCY SCALING FRAMEWORK" <linux-pm@...r.kernel.org>, Mel Gorman <mgorman@...e.de>, 
	Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Valentin Schneider <vschneid@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, 
	z00813676 <zhenglifeng1@...wei.com>, sudeep.holla@....com
Subject: Re: [PATCH v2 2/2] cpufreq: CPPC: Dont read counters for idle CPUs

On Fri, 1 Aug 2025 at 02:16, Prashant Malani <pmalani@...gle.com> wrote:
> On Thu, 31 Jul 2025 at 13:31, Beata Michalska <beata.michalska@....com> wrote:
> > Thank you for the info, but I'm exploring ways that will not increase the time
> > window between the reads.
>
> IMO this issue is intractable on non-RT OSes like Linux (at least,
> Linux when it is not compiled for RT), since we basically need to
> ensure atomicity for the reading of both ref and del registers together.
> We can't disable preemption here, since some of
> the code paths (like PCC regs) acquire semaphores [2].

Actually, minor correction here. The PCC path is not the issue.
It's the ffh read path[3], which requires interrupts to be enabled.
Larger point still stands.

[3] https://elixir.bootlin.com/linux/v6.16-rc7/source/arch/arm64/kernel/topology.c#L451

BR,

-Prashant