Re: [RFC PATCH] sched/cpufreq/schedutil: handling urgent frequency requests

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, May 09, 2018 at 10:30:37AM +0200, Rafael J. Wysocki wrote:
> On Wed, May 9, 2018 at 10:06 AM, Joel Fernandes <joel@joelfernandes.org> wrote:
> > On Wed, May 09, 2018 at 08:45:30AM +0200, Juri Lelli wrote:
> >> On 08/05/18 21:54, Joel Fernandes wrote:
> >>
> >> [...]
> >>
> >> > Just for discussion sake, is there any need for work_in_progress? If we can
> >> > queue multiple work say kthread_queue_work can handle it, then just queuing
> >> > works whenever they are available should be Ok and the kthread loop can
> >> > handle them. __cpufreq_driver_target is also protected by the work lock if
> >> > there is any concern that can have races... only thing is rate-limiting of
> >> > the requests, but we are doing a rate limiting, just not for the "DL
> >> > increased utilization" type requests (which I don't think we are doing at the
> >> > moment for urgent DL requests anyway).
> >> >
> >> > Following is an untested diff to show the idea. What do you think?
> >> >
> >> > thanks,
> >> >
> >> > - Joel
> >> >
> >> > ----8<---
> >> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> >> > index d2c6083304b4..862634ff4bf3 100644
> >> > --- a/kernel/sched/cpufreq_schedutil.c
> >> > +++ b/kernel/sched/cpufreq_schedutil.c
> >> > @@ -38,7 +38,6 @@ struct sugov_policy {
> >> >     struct                  mutex work_lock;
> >> >     struct                  kthread_worker worker;
> >> >     struct task_struct      *thread;
> >> > -   bool                    work_in_progress;
> >> >
> >> >     bool                    need_freq_update;
> >> >  };
> >> > @@ -92,16 +91,8 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
> >> >         !cpufreq_can_do_remote_dvfs(sg_policy->policy))
> >> >             return false;
> >> >
> >> > -   if (sg_policy->work_in_progress)
> >> > -           return false;
> >> > -
> >> >     if (unlikely(sg_policy->need_freq_update)) {
> >> >             sg_policy->need_freq_update = false;
> >> > -           /*
> >> > -            * This happens when limits change, so forget the previous
> >> > -            * next_freq value and force an update.
> >> > -            */
> >> > -           sg_policy->next_freq = UINT_MAX;
> >> >             return true;
> >> >     }
> >> >
> >> > @@ -129,7 +120,6 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
> >> >             policy->cur = next_freq;
> >> >             trace_cpu_frequency(next_freq, smp_processor_id());
> >> >     } else {
> >> > -           sg_policy->work_in_progress = true;
> >> >             irq_work_queue(&sg_policy->irq_work);
> >>
> >> Isn't this potentially introducing unneeded irq pressure (and doing the
> >> whole wakeup the kthread thing), while the already active kthread could
> >> simply handle multiple back-to-back requests before going to sleep?
> >
> > How about this? Will use the latest request, and also doesn't do unnecessary
> > irq_work_queue:
> >
> > (untested)
> > -----8<--------
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index d2c6083304b4..6a3e42b01f52 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -38,7 +38,7 @@ struct sugov_policy {
> >         struct                  mutex work_lock;
> >         struct                  kthread_worker worker;
> >         struct task_struct      *thread;
> > -       bool                    work_in_progress;
> > +       bool                    work_in_progress; /* Has kthread been kicked */
> >
> >         bool                    need_freq_update;
> >  };
> > @@ -92,9 +92,6 @@ static bool sugov_should_update_freq(struct sugov_policy *sg_policy, u64 time)
> >             !cpufreq_can_do_remote_dvfs(sg_policy->policy))
> >                 return false;
> >
> > -       if (sg_policy->work_in_progress)
> > -               return false;
> > -
> 
> Why this change?
> 
> Doing the below is rather pointless if work_in_progress is set, isn't it?

The issue being discussed is that if a work was already in progress, then new
frequency updates will be dropped. So say even if DL increased in
utilization, nothing will happen because if work_in_progress = true and
need_freq_update = true, we would skip an update.  In this diff, I am
allowing the frequency request to be possible while work_in_progress is true.
In the end the latest update will be picked.

> 
> You'll drop the results of it on the floor going forward anyway then AFAICS.

Why? If sg_policy->need_freq_update = true, sugov_should_update_freq() will
return true.

thanks,

- Joel


> 
> >         if (unlikely(sg_policy->need_freq_update)) {
> >                 sg_policy->need_freq_update = false;
> >                 /*
> > @@ -129,8 +126,11 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
> >                 policy->cur = next_freq;
> >                 trace_cpu_frequency(next_freq, smp_processor_id());
> >         } else {
> > -               sg_policy->work_in_progress = true;
> > -               irq_work_queue(&sg_policy->irq_work);
> > +               /* work_in_progress helps us not queue unnecessarily */
> > +               if (!sg_policy->work_in_progress) {
> > +                       sg_policy->work_in_progress = true;
> > +                       irq_work_queue(&sg_policy->irq_work);
> > +               }
> >         }
> >  }
> >
> > @@ -381,13 +381,23 @@ sugov_update_shared(struct update_util_data *hook, u64 time, unsigned int flags)
> >  static void sugov_work(struct kthread_work *work)
> >  {
> >         struct sugov_policy *sg_policy = container_of(work, struct sugov_policy, work);
> > +       unsigned int freq;
> > +
> > +       /*
> > +        * Hold sg_policy->update_lock just enough to handle the case where:
> > +        * if sg_policy->next_freq is updated before work_in_progress is set to
> > +        * false, we may miss queueing the new update request since
> > +        * work_in_progress would appear to be true.
> > +        */
> > +       raw_spin_lock(&sg_policy->update_lock);
> > +       freq = sg_policy->next_freq;
> > +       sg_policy->work_in_progress = false;
> > +       raw_spin_unlock(&sg_policy->update_lock);
> >
> >         mutex_lock(&sg_policy->work_lock);
> > -       __cpufreq_driver_target(sg_policy->policy, sg_policy->next_freq,
> > +       __cpufreq_driver_target(sg_policy->policy, freq,
> >                                 CPUFREQ_RELATION_L);
> >         mutex_unlock(&sg_policy->work_lock);
> > -
> > -       sg_policy->work_in_progress = false;
> >  }
> >
> >  static void sugov_irq_work(struct irq_work *irq_work)