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

[Rafael J. Wysocki]

On Wed, May 9, 2018 at 10:51 AM, Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxxxxx> 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.

I'm not sure if taking new requests with the irq_work in flight is a good idea.

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

Because you cannot queue up a new irq_work before the previous one is complete?