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

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Date:   Tue, 15 Mar 2022 11:47:35 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     zhengzucheng <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 Tue, Mar 15, 2022 at 3:30 AM zhengzucheng <zhengzucheng@...wei.com> wrote:
>
>
>
> On 2022/3/11 23:52, Rafael J. Wysocki wrote:
> > 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?
> Thank you for review.
>
> In some scenarios, cpufreq driver ->get is not defined,
> some driver get the CPU frequency by calling cpufreq_get() will return 0.

Which driver?  Again, there is only one calling cpufreq_get() in the
kernel tree and it is not on x86.

> The modification to this problem is inspired by the implementation of
> the show_scaling_cur_freq().
> >
> >>                          ret_freq = __cpufreq_get(policy);
> >>                  up_read(&policy->rwsem);
> >>
> >> --

The answer to my question appears to be that you want cpufreq_get() to
be consistent with show_scaling_cur_freq().

Fair enough, but in that case please make them both call the same
lower-level routine implementing the desired behavior so as to avoid
code duplication.