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

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Message-ID: <CAFivqmKKve8f_i8Po3v3SqQ9tt68yO_qvCyogBbZ7wPE-Y_6Mg@mail.gmail.com>
Date: Thu, 26 Jun 2025 11:42:45 -0700
From: Prashant Malani <pmalani@...gle.com>
To: Jie Zhan <zhanjie9@...ilicon.com>
Cc: 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>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Steven Rostedt <rostedt@...dmis.org>, Valentin Schneider <vschneid@...hat.com>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Viresh Kumar <viresh.kumar@...aro.org>, 
	Ionela Voinescu <ionela.voinescu@....com>, Beata Michalska <beata.michalska@....com>, 
	z00813676 <zhenglifeng1@...wei.com>
Subject: Re: [PATCH v2 2/2] cpufreq: CPPC: Dont read counters for idle CPUs

(re-sending without HTML)

Hi,

Just a friendly ping on this thread/series. It would be great to get
some feedback from the maintainers/stakeholders here.

Thanks,


On Thu, 19 Jun 2025 at 22:07, Prashant Malani <pmalani@...gle.com> wrote:
>
> Hi Jie,
>
> Thanks for taking a look at the patch.
>
> On Thu, 19 Jun 2025 at 20:53, Jie Zhan <zhanjie9@...ilicon.com> wrote:
> > On 19/06/2025 08:09, Prashant Malani wrote:
> > > AMU performance counters tend to be inaccurate when measured on idle CPUs.
> > > On an idle CPU which is programmed to 3.4 GHz (verified through firmware),
> > > here is a measurement and calculation of operating frequency:
> > >
> > > t0: ref=899127636, del=3012458473
> > > t1: ref=899129626, del=3012466509
> > > perf=40
> >
> > In this case, the target cpu is mostly idle but not fully idle during the
> > sampling window since the counter grows a little bit.
> > Perhaps some interrupts happen to run on the cpu shortly.
> >
> > Thus, the actual issue is the accuracy of frequency sampling becomes poor
> > when the delta of counters are too small to obtain a reliable accuracy.
> >
> > Would it be more sensible to put a minimum threshold of the delta of
> > counters when sampling the frequency?
>
> I'm happy to throw together a patch if there is some safe
> threshold the experts here can agree on for the minimum delta for
> the ref counter. I would caution that with this sort of approach we
> start running into the familiar issue:
> - What value is appropriate? Too large and you get false
> positives (falling back to the idle invalid path when we shouldn't), and
> too less and you get false negatives (we still report inaccurate
> counter values).
> - Is the threshold the same across platforms?
> - Will it remain the same 5/10 years from now?
>
> >  BTW, that ABI
> > doesn't seem to be synchronous at all, i.e. the cpu might be busy when we
> > check and then become idle when sampling.
> >
>
> I don't think this is necessarily an issue. The ABI doesn't need to be
> synchronous; it is merely a snapshot of the scheduler view of that CPU
> at a point in time. Even the current method of perf counters sampling
> is purely hueristic. The CPU might be idle for the 2 usec the
> sampling is done, and servicing traffic before and after that.
> This is inherent whenever you are sampling any system state.
>
> I would imagine it is more reliable to trust the kernel scheduler's view
> of whether a CPU is idle, than relying on counters and a calculation
> method which are sensitive and unreliable for idle systems
> (i.e stray interrupts can throw off the calculations).
>
> That said, I'm happy to go with the approach folks on this list recommend.
>
> Cheers,
>
> --
> -Prashant



-- 
-Prashant