linux-kernel - Re: [PATCH] cpufreq: fix cpufreq_get() can't get correct CPU frequency

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Message-ID: <CAJZ5v0jponp=ijVx6W=eNEGrfTKh0KbGmOQG_V0P-Mq366559g@mail.gmail.com>
Date:   Fri, 11 Mar 2022 16:52:59 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     z00314508 <zhengzucheng@...wei.com>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Len Brown <len.brown@...el.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Stable <stable@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: fix cpufreq_get() can't get correct CPU frequency

On Fri, Mar 11, 2022 at 9:11 AM z00314508 <zhengzucheng@...wei.com> wrote:
>
> From: Zucheng Zheng <zhengzucheng@...wei.com>
>
> On some specific platforms, the cpufreq driver does not define
> cpufreq_driver.get() routine (eg:x86 intel_pstate driver), as a

I guess you mean the cpufreq driver ->get callback.

No, intel_pstate doesn't implement it, because it cannot reliably
return the current CPU frequency.

> result, the cpufreq_get() can't get the correct CPU frequency.

No, it can't, if intel_pstate is the driver, but what's the problem?
This function is only called in one place in the kernel and not on x8
even.

> Modern x86 processors include the hardware needed to accurately
> calculate frequency over an interval -- APERF, MPERF and the TSC.

You can compute the average frequency over an interval, but ->get is
expected to return the actual current frequency at the time call time.

> Here we use arch_freq_get_on_cpu() in preference to any driver
> driver-specific cpufreq_driver.get() routine to get CPU frequency.
>
> Fixes: f8475cef9008 ("x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF")

No kidding.

> Signed-off-by: Zucheng Zheng <zhengzucheng@...wei.com>
> ---
>  drivers/cpufreq/cpufreq.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 80f535cc8a75..d777257b4454 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -1806,10 +1806,14 @@ unsigned int cpufreq_get(unsigned int cpu)
>  {
>         struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
>         unsigned int ret_freq = 0;
> +       unsigned int freq;
>
>         if (policy) {
>                 down_read(&policy->rwsem);
> -               if (cpufreq_driver->get)
> +               freq = arch_freq_get_on_cpu(policy->cpu);
> +               if (freq)
> +                       ret_freq = freq;
> +               else if (cpufreq_driver->get)

Again, what problem exactly does this address?

>                         ret_freq = __cpufreq_get(policy);
>                 up_read(&policy->rwsem);
>
> --