Re: [PATCH 1/2] cpufreq: schedutil: Fix superfluous updates caused by need_freq_update

[Sultan Alsawaf <sultan@kerneltoast.com>]

On Thu, Apr 10, 2025 at 10:13:04AM +0800, Xuewen Yan wrote:
> On Thu, Apr 10, 2025 at 10:09 AM Sultan Alsawaf <sultan@kerneltoast.com> wrote:
> >
> > On Thu, Apr 10, 2025 at 10:06:41AM +0800, Xuewen Yan wrote:
> > > On Thu, Apr 10, 2025 at 9:49 AM Sultan Alsawaf <sultan@kerneltoast.com> wrote:
> > > >
> > > > On Wed, Apr 09, 2025 at 07:48:05PM +0800, Xuewen Yan wrote:
> > > > > Or can we modify it as follows?
> > > > >
> > > > > -->8--
> > > > >
> > > > > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > > > > index 1a19d69b91ed..0e8d3b92ffe7 100644
> > > > > --- a/kernel/sched/cpufreq_schedutil.c
> > > > > +++ b/kernel/sched/cpufreq_schedutil.c
> > > > > @@ -83,7 +83,7 @@ static bool sugov_should_update_freq(struct
> > > > > sugov_policy *sg_policy, u64 time)
> > > > >
> > > > >         if (unlikely(sg_policy->limits_changed)) {
> > > > >                 sg_policy->limits_changed = false;
> > > > > -               sg_policy->need_freq_update =
> > > > > cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
> > > > > +               sg_policy->need_freq_update = true;
> > > > >                 return true;
> > > > >         }
> > > > >
> > > > > @@ -95,11 +95,15 @@ static bool sugov_should_update_freq(struct
> > > > > sugov_policy *sg_policy, u64 time)
> > > > >  static bool sugov_update_next_freq(struct sugov_policy *sg_policy, u64 time,
> > > > >                                    unsigned int next_freq)
> > > > >  {
> > > > > -       if (sg_policy->need_freq_update)
> > > > > +       if (sg_policy->need_freq_update) {
> > > > >                 sg_policy->need_freq_update = false;
> > > > > -       else if (sg_policy->next_freq == next_freq)
> > > > > -               return false;
> > > > > +               if (cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
> > > > > +                       goto change;
> > > > > +       }
> > > > >
> > > > > +       if (sg_policy->next_freq == next_freq)
> > > > > +               return false;
> > > > > +change:
> > > > >         sg_policy->next_freq = next_freq;
> > > > >         sg_policy->last_freq_update_time = time;
> > > >
> > > > If CPUFREQ_NEED_UPDATE_LIMITS isn't specified, then there's no need to request a
> > > > frequency switch from the driver when the current frequency is exactly the same
> > > > as the next frequency.
> > >
> > > Yes, the following check would return false:
> > >
> > >  +       if (sg_policy->next_freq == next_freq)
> > >  +               return false;
> >
> > But what does that change fix? In fact, that change causes a limits update to
> > trigger a frequency switch request to the driver even when the new frequency is
> > the same as the current one.
> 
> We set the sg_policy->need_freq_update = false instead of
> sg_policy->need_freq_update =
> cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS),
> to fix the original issue, and then add the
> +               if (cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
> +                       goto change;
> 
> to allow cpufreq to update when CPUFREQ_NEED_UPDATE_LIMITS is set.

Please take a closer look at this snippet in sugov_update_next_freq():

	if (sg_policy->need_freq_update)
		sg_policy->need_freq_update = false;
	else if (sg_policy->next_freq == next_freq)
		return false;

The 2nd if-statement is an else-if. Therefore, when need_freq_update is true, it
is set to false *and* skips the (sg_policy->next_freq == next_freq) check.

Sultan