linux-kernel - Re: [PATCH V3 0/4] cpufreq: cppc: Add support for frequency invariance

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Date:   Tue, 29 Jun 2021 10:15:27 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Qian Cai <quic_qiancai@...cinc.com>
Cc:     Ionela Voinescu <ionela.voinescu@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Will Deacon <will@...nel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH V3 0/4] cpufreq: cppc: Add support for frequency
 invariance

On 25-06-21, 09:31, Qian Cai wrote:
> The problem is that all CPUs are never scaling down.
> "cpuinfo_cur_freq" and "scaling_cur_freq" are always the 2800 MHz on
> all CPUs on this idle system. This looks like a regression somewhere
> as in 5.4-based kernel, I can see "cpuinfo_cur_freq" can go down to
> 2000 MHz in the same scenario. I'll bisect a bit unless you have
> better ideas?

Few things which may let us understand the readings properly.

- cpuinfo_cur_freq: eventually makes a call to cppc_cpufreq_get_rate()
  and returns the *actual* frequency hardware is running at (based on
  counter diff around 2 us delay).

- scaling_cur_freq: is the frequency the cpufreq core thinks the
  hardware is running at, it would more in sync with what schedutil
  (or other governors) wants the CPU to run at. This can be different
  from what the hardware is running at, i.e. given by
  cpuinfo_cur_freq.

-- 
viresh