linux-kernel - Re: [PATCH v2 2/2] cpufreq: Wire-up arch-flavored freq info into cpufreq_verify_current

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Message-ID: <3e6077bb-907c-057f-0896-d0a5814a4229@nvidia.com>
Date:   Fri, 1 Dec 2023 18:32:10 +0530
From:   Sumit Gupta <sumitg@...dia.com>
To:     Ionela Voinescu <ionela.voinescu@....com>,
        Beata Michalska <beata.michalska@....com>
CC:     <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>, <linux-pm@...r.kernel.org>,
        <sudeep.holla@....covm>, <will@...nel.org>,
        <catalin.marinas@....com>, <viresh.kumar@...aro.org>,
        <rafael@...nel.org>, <yang@...amperecomputing.com>,
        <linux-tegra@...r.kernel.org>, Sumit Gupta <sumitg@...dia.com>
Subject: Re: [PATCH v2 2/2] cpufreq: Wire-up arch-flavored freq info into
 cpufreq_verify_current_freq

Hi Ionela,

>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -1756,7 +1756,8 @@ static unsigned int cpufreq_verify_current_freq(struct cpufreq_policy *policy, b
>>   {
>>        unsigned int new_freq;
>>
>> -     new_freq = cpufreq_driver->get(policy->cpu);
>> +     new_freq = arch_freq_get_on_cpu(policy->cpu);
>> +     new_freq = new_freq ?: cpufreq_driver->get(policy->cpu);
> 
> Given that arch_freq_get_on_cpu() is an average frequency, it does not
> seem right to me to trigger the sync & update process of
> cpufreq_verify_current_freq() based on it.
> 
> cpufreq_verify_current_freq() will at least modify the internal state of
> the policy and send PRE and POST notifications, if not do a full frequency
> update, based on this average frequency, which is likely different from
> the current frequency, even beyond the 1MHz threshold.
> 
> While I believe it's okay to return this average frequency in
> cpuinfo_cur_freq, I don't think it should be used as an indication of
> an accurate current frequency, which is what
> cpufreq_verify_current_freq() expects.
> 
> Sumit, can you give more details on the issue at [1] and why this change
> fixes it?
> 
> [1] https://lore.kernel.org/lkml/6a5710f6-bfbb-5dfd-11cd-0cd02220cee7@nvidia.com/
> 
> Thank you,
> Ionela.
>
cpufreq_verify_current_freq() also updates 'policy->cur' in POST
notification if the frequency from hardware has more delta (out of sync).

As the value from 'cpufreq_driver->get()' is not reliable due to [1],
calling the 'get' hook can update the 'policy->cur' with a wrong value 
when governor starts in cpufreq_start_governor().
And if the frequency is never changed after the governor starts during
boot e.g. when performance governor is set as default, then 
'scaling_cur_freq' always returns wrong value.

Instead, the arch_freq_get_on_cpu() API updates 'policy->cur' with a 
more stable freq value.

[1] https://lore.kernel.org/lkml/20230418113459.12860-7-sumitg@nvidia.com/

Best regards,
Sumit Gupta

>>        if (!new_freq)
>>                return 0;
>>
>> --
>> 2.25.1
>>