Re: [PATCH] cpufreq: schedutil: Don't consider freq reduction to busy CPU if need_freq_update is set

[Rafael J. Wysocki]

On Mon, Feb 22, 2021 at 2:57 PM Yue Hu <zbestahu@xxxxxxx> wrote:
>
> On Mon, 22 Feb 2021 11:00:14 +0530
> Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
>
> > On 19-02-21, 19:45, Yue Hu wrote:
> > > We will set next_f to next_freq(previous freq) if next_f is
> > > reduced for busy CPU. Then the next sugov_update_next_freq() will check
> > > if next_freq matches next_f if need_freq_update is not set.
> > > Obviously, we will do nothing for the case. And The related check to
> > > fast_switch_enabled and raw_spin_{lock,unlock} operations are
> > > unnecessary.
> >
> > Right, but we will still need sugov_update_next_freq() to have the
> > same implementation regardless and so I am not sure if we should add
>
> Yes, sugov_update_next_freq() should be keeping current logic for corner case.
>
> > this change:
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 41e498b0008a..7289e1adab73 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -362,6 +362,9 @@ static void sugov_update_single_freq(struct update_util_data *hook, u64 time,
> >          * recently, as the reduction is likely to be premature then.
> >          */
> >         if (sugov_cpu_is_busy(sg_cpu) && next_f < sg_policy->next_freq) {
> > +               if (!sg_policy->need_freq_update)
>
> The initial purpose about code of `next_f = sg_policy->next_freq` here (for special CPU busy
> case) should be skipping the freq update.
>
> Since commit 600f5badb78c ("cpufreq: schedutil: Don't skip freq update when limits change"),
> we add the check to busy CPU for not skipping the update, we need to update the freq using
> computed one because limits change.
>
> After commit 23a881852f3e ("cpufreq: schedutil: Don't skip freq update if need_freq_update
> is set"), we removed the need_freq_update check(no issue of commit 600f5badb78c anymore?)
> and introduce to always do an update in sugov_update_next_freq() if need_freq_update is set
> even though current freq == sg_policy->next_freq because of corner case issue. But that is
> conflict with original purpose of the freq skip code (next_f = sg_policy->next_freq) of
> busy CPU.

That's because we realized that it was not always a good idea to skip
the update even if next_f == sg_policy->next_freq.

That's why CPUFREQ_NEED_UPDATE_LIMITS has been introduced and the
current flow is a result of subsequent code rearrangements.