On Wed, 24 Feb 2021 13:46:11 +0100
"Rafael J. Wysocki" <raf...@kernel.org> wrote:
> On Wed, Feb 24, 2021 at 3:24 AM Yue Hu <zbest...@gmail.com> wrote:
> >
> > On Mon, 22 Feb 2021 15:30:34 +0100
> > "Rafael J. Wysocki" <raf...@kernel.org> wrote:
> >
> > > On Mon, Feb 22, 2021 at 2:57 PM Yue Hu <zbest...@163.com> wrote:
> > > >
> > > > On Mon, 22 Feb 2021 11:00:14 +0530
> > > > Viresh Kumar <viresh.ku...@linaro.org> 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.
> >
> > ok, care about unnecessary(should be) behaviors(fast_switch_enabled and
> > raw_spin_{lock,unlock})
> > if need_freq_update is unset?
> >
> > If we care, i will send another patch (which is different from above change
> > for busy CPU).
>
> Please send a patch and we'll see (this is how things go).
Already sent it("Call sugov_update_next_freq() before check to
fast_switch_enabled"). Please review.
Thank you.
