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

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Date:   Tue, 15 Mar 2022 10:30:23 +0800
From:   zhengzucheng <zhengzucheng@...wei.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
CC:     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 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.
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);
>>
>> --
> .
>