Re: [PATCH] cpufreq: schedutil: Don't skip freq update if need_freq_update is set

[Rafael J. Wysocki]

On Fri, Oct 30, 2020 at 4:07 PM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
>
> On Fri, Oct 30, 2020 at 8:31 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> >
> > The cpufreq policy's frequency limits (min/max) can get changed at any
> > point of time, while schedutil is trying to update the next frequency.
> > Though the schedutil governor has necessary locking and support in place
> > to make sure we don't miss any of those updates, there is a corner case
> > where the governor will find that the CPU is already running at the
> > desired frequency and so may skip an update.
> >
> > For example, consider that the CPU can run at 1 GHz, 1.2 GHz and 1.4 GHz
> > and is running at 1 GHz currently. Schedutil tries to update the
> > frequency to 1.2 GHz, during this time the policy limits get changed as
> > policy->min = 1.4 GHz. As schedutil (and cpufreq core) does clamp the
> > frequency at various instances, we will eventually set the frequency to
> > 1.4 GHz, while we will save 1.2 GHz in sg_policy->next_freq.
> >
> > Now lets say the policy limits get changed back at this time with
> > policy->min as 1 GHz. The next time schedutil is invoked by the
> > scheduler, we will reevaluate the next frequency (because
> > need_freq_update will get set due to limits change event) and lets say
> > we want to set the frequency to 1.2 GHz again. At this point
> > sugov_update_next_freq() will find the next_freq == current_freq and
> > will abort the update, while the CPU actually runs at 1.4 GHz.
> >
> > Until now need_freq_update was used as a flag to indicate that the
> > policy's frequency limits have changed, and that we should consider the
> > new limits while reevaluating the next frequency.
> >
> > This patch fixes the above mentioned issue by extending the purpose of
> > the need_freq_update flag. If this flag is set now, the schedutil
> > governor will not try to abort a frequency change even if next_freq ==
> > current_freq.
> >
> > As similar behavior is required in the case of
> > CPUFREQ_NEED_UPDATE_LIMITS flag as well, need_freq_update will never be
> > set to false if that flag is set for the driver.
> >
> > We also don't need to consider the need_freq_update flag in
> > sugov_update_single() anymore to handle the special case of busy CPU, as
> > we won't abort a frequency update anymore.
> >
> > Reported-by: zhuguangqing <zhuguangqing@xxxxxxxxxx>
> > Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>
> Thanks for following my suggestion!
>
> > Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> > ---
> >  kernel/sched/cpufreq_schedutil.c | 22 ++++++++++------------
> >  1 file changed, 10 insertions(+), 12 deletions(-)
> >
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index c03a5775d019..c6861be02c86 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -102,9 +102,12 @@ 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->next_freq == next_freq &&
> > -           !cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
> > -               return false;
> > +       if (!sg_policy->need_freq_update) {
> > +               if (sg_policy->next_freq == next_freq)
> > +                       return false;
> > +       } else if (!cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS)) {
> > +               sg_policy->need_freq_update = false;

One nit, though.

This can be changed into

} else {
      sg_policy->need_freq_update =
cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
}

to save a branch and because need_freq_update is there in the cache
already, this should be a fast update.

So I'm going to make this change while applying the patch.

> > +       }
> >
> >         sg_policy->next_freq = next_freq;
> >         sg_policy->last_freq_update_time = time;
> > @@ -162,11 +165,9 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
> >
> >         freq = map_util_freq(util, freq, max);
> >
> > -       if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update &&
> > -           !cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS))
> > +       if (freq == sg_policy->cached_raw_freq && !sg_policy->need_freq_update)
> >                 return sg_policy->next_freq;
> >
> > -       sg_policy->need_freq_update = false;
> >         sg_policy->cached_raw_freq = freq;
> >         return cpufreq_driver_resolve_freq(policy, freq);
> >  }
> > @@ -442,7 +443,6 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> >         struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> >         unsigned long util, max;
> >         unsigned int next_f;
> > -       bool busy;
> >         unsigned int cached_freq = sg_policy->cached_raw_freq;
> >
> >         sugov_iowait_boost(sg_cpu, time, flags);
> > @@ -453,9 +453,6 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> >         if (!sugov_should_update_freq(sg_policy, time))
> >                 return;
> >
> > -       /* Limits may have changed, don't skip frequency update */
> > -       busy = !sg_policy->need_freq_update && sugov_cpu_is_busy(sg_cpu);
> > -
> >         util = sugov_get_util(sg_cpu);
> >         max = sg_cpu->max;
> >         util = sugov_iowait_apply(sg_cpu, time, util, max);
> > @@ -464,7 +461,7 @@ static void sugov_update_single(struct update_util_data *hook, u64 time,
> >          * Do not reduce the frequency if the CPU has not been idle
> >          * recently, as the reduction is likely to be premature then.
> >          */
> > -       if (busy && next_f < sg_policy->next_freq) {
> > +       if (sugov_cpu_is_busy(sg_cpu) && next_f < sg_policy->next_freq) {
> >                 next_f = sg_policy->next_freq;
> >
> >                 /* Restore cached freq as next_freq has changed */
> > @@ -829,9 +826,10 @@ static int sugov_start(struct cpufreq_policy *policy)
> >         sg_policy->next_freq                    = 0;
> >         sg_policy->work_in_progress             = false;
> >         sg_policy->limits_changed               = false;
> > -       sg_policy->need_freq_update             = false;
> >         sg_policy->cached_raw_freq              = 0;
> >
> > +       sg_policy->need_freq_update = cpufreq_driver_test_flags(CPUFREQ_NEED_UPDATE_LIMITS);
> > +
> >         for_each_cpu(cpu, policy->cpus) {
> >                 struct sugov_cpu *sg_cpu = &per_cpu(sugov_cpu, cpu);
> >
> > --
>
> I'll queue it up for -rc3 next week, thanks!