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

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Date: Fri, 2 Feb 2024 10:14:47 +0100
From: Beata Michalska <beata.michalska@....com>
To: Ionela Voinescu <ionela.voinescu@....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 Ionela,

So sorry for relpying so late, lost if from my rader for a while ...
On Tue, Nov 28, 2023 at 02:01:54PM +0000, Ionela Voinescu wrote:
> 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.
> 
Noted, will drop this change.

---
BR
Beata
> 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
> >