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

[zhengzucheng]

On 2022/3/11 23:52, Rafael J. Wysocki wrote:
> On Fri, Mar 11, 2022 at 9:11 AM z00314508 <zhengzucheng@xxxxxxxxxx> wrote:
> > From: Zucheng Zheng <zhengzucheng@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > ---
> >   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.
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);
> >
> > --
> .