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

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Message-ID: <ZWXy0h/fFfQh+Rhy@arm.com>
Date:   Tue, 28 Nov 2023 14:01:54 +0000
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Beata Michalska <beata.michalska@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pm@...r.kernel.org, sumitg@...dia.com, sudeep.holla@....covm,
        will@...nel.org, catalin.marinas@....com, viresh.kumar@...aro.org,
        rafael@...nel.org, yang@...amperecomputing.com,
        linux-tegra@...r.kernel.org
Subject: Re: [PATCH v2 2/2] cpufreq: Wire-up arch-flavored freq info into
 cpufreq_verify_current_freq

Hi Beata, Sumit,

On Monday 27 Nov 2023 at 16:08:38 (+0000), Beata Michalska wrote:
> From: Sumit Gupta <sumitg@...dia.com>
> 
> When available, use arch_freq_get_on_cpu to obtain current frequency
> (usually an average reported over given period of time)
> to better align the cpufreq's view on the current state of affairs.
> This also automatically pulls in the update for cpuinfo_cur_freq sysfs
> attribute, aligning it with the scaling_cur_freq one, and thus providing
> consistent view on relevant platforms.
> 
> Signed-off-by: Sumit Gupta <sumitg@...dia.com>
> [BM: Subject & commit msg]
> Signed-off-by: Beata Michalska <beata.michalska@....com>
> ---
>  drivers/cpufreq/cpufreq.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 8c4f9c2f9c44..109559438f45 100644
> --- 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.

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