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

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Date:   Wed, 9 May 2018 11:48:06 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Claudio Scordino <claudio@...dence.eu.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Joel Fernandes <joelaf@...gle.com>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [RFC PATCH] sched/cpufreq/schedutil: handling urgent frequency requests

On Wed, May 9, 2018 at 11:39 AM, Joel Fernandes <joel@...lfernandes.org> wrote:
> On Wed, May 09, 2018 at 11:06:24AM +0200, Rafael J. Wysocki wrote:
>> On Wed, May 9, 2018 at 10:51 AM, Joel Fernandes <joel@...lfernandes.org> 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@...lfernandes.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.
>>
>> I'm not sure if taking new requests with the irq_work in flight is a good idea.
>
> That's the point of the original $SUBJECT patch posted by Claudio :) In that
> you can see if urgent_request, then work_in_progress isn't checked.
>
> Also I don't see why we cannot do this with this small tweak as in my diff.
> It solves a real problem seen with frequency updates done with the
> slow-switch as we discussed at OSPM.

OK

> But let me know if I missed your point or something ;)
>
>>
>> >>
>> >> 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?
>
> We are not doing that. If you see in my diff, I am not queuing an irq_work if
> one was already queued. What we're allowing is an update to next_freq. We
> still use work_in_progress but don't use it to ban all incoming update
> requests as done previously. Instead we use work_in_progress to make sure
> that we dont unnecessarily increase the irq pressure and have excessive wake
> ups (as Juri suggested).
>
> I can clean it up and post it as a patch next week after some testing incase
> that's less confusing.

Yeah, that would help. :-)

> This week I'm actually on vacation and the diff was pure vacation hacking ;-)

No worries.

Thanks,
Rafael