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

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Message-ID: <c8b4d391-681e-f4a6-2839-e5951429f043@nvidia.com>
Date:   Wed, 6 Dec 2023 18:58:17 +0530
From:   Sumit Gupta <sumitg@...dia.com>
To:     Ionela Voinescu <ionela.voinescu@....com>
CC:     Beata Michalska <beata.michalska@....com>,
        <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



On 05/12/23 16:35, Ionela Voinescu wrote:
> External email: Use caution opening links or attachments
> 
> 
> Hi Sumit,
> 
> On Friday 01 Dec 2023 at 18:32:10 (+0530), Sumit Gupta wrote:
>> 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/
> 
> Got it, many thanks!
> 
> As the code is right now in v2, arch_freq_get_on_cpu() is called on
> show_scaling_cur_freq(), so the problem you describe would not show up.
> policy->cur would still be incorrect, but 'scaling_cur_freq' would
> return the value from arch_freq_get_on_cpu().
> 
> Would it be enough if arch_freq_get_on_cpu() gets also called from
> show_cpuinfo_cur_freq() instead of cpufreq_verify_current_freq()?
> 
> Thanks,
> Ionela.
> 

Yes.
I am not sure if making both the nodes 'scaling_cur_freq' and 
'cpuinfo_cur_freq' same is fine?

Best Regards,
Sumit Gupta