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Date:   Thu, 17 May 2018 07:43:59 -0700
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Juri Lelli <juri.lelli@redhat.com>
Cc:     Viresh Kumar <viresh.kumar@linaro.org>,
        linux-kernel@vger.kernel.org,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Luca Abeni <luca.abeni@santannapisa.it>,
        linux-pm@vger.kernel.org, kernel-team@android.com
Subject: Re: [PATCH RFC] schedutil: Allow cpufreq requests to be made even
 when kthread kicked
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On Thu, May 17, 2018 at 04:28:23PM +0200, Juri Lelli wrote:
[...]
> > > > We would need more locking stuff in the work handler in that case and
> > > > I think there maybe a chance of missing the request in that solution
> > > > if the request happens right at the end of when sugov_work returns.
> > > 
> > > Mmm, true. Ideally we might want to use some sort of queue where to
> > > atomically insert requests and then consume until queue is empty from
> > > sugov kthread.
> > 
> > IMO we don't really need a queue or anything, we should need the kthread to
> > process the *latest* request it sees since that's the only one that matters.
> 
> Yep, makes sense.
> 
> > > But, I guess that's going to be too much complexity for an (hopefully)
> > > corner case.
> > 
> > I thought of this corner case too, I'd argue its still an improvement over
> > not doing anything, but we could tighten this up a bit more if you wanted by
> 
> Indeed! :)
> 
> > doing something like this on top of my patch. Thoughts?
> > 
> > ---8<-----------------------
> > 
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index a87fc281893d..e45ec24b810b 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -394,6 +394,7 @@ static void sugov_work(struct kthread_work *work)
> >  	unsigned int freq;
> >  	unsigned long flags;
> >  
> > +redo_work:
> >  	/*
> >  	 * Hold sg_policy->update_lock shortly to handle the case where:
> >  	 * incase sg_policy->next_freq is read here, and then updated by
> > @@ -409,6 +410,9 @@ static void sugov_work(struct kthread_work *work)
> >  	__cpufreq_driver_target(sg_policy->policy, freq,
> >  				CPUFREQ_RELATION_L);
> >  	mutex_unlock(&sg_policy->work_lock);
> > +
> > +	if (sg_policy->work_in_progress)
> > +		goto redo_work;
> 
> Didn't we already queue up another irq_work at this point?

Oh yeah, so the case I was thinking was if the kthread was active, while the
new irq_work raced and finished.

Since that would just mean a new kthread_work for the worker, the loop I
mentioned above isn't needed. Infact there's already a higher level loop
taking care of it in kthread_worker_fn as below. So the governor thread
will not sleep and we'll keep servicing all pending requests till
they're done. So I think we're good with my original patch.

repeat:
[...]
if (!list_empty(&worker->work_list)) {
		work = list_first_entry(&worker->work_list,
					struct kthread_work, node);
		list_del_init(&work->node);
	}
	worker->current_work = work;
	spin_unlock_irq(&worker->lock);

	if (work) {
		__set_current_state(TASK_RUNNING);
		work->func(work);
	} else if (!freezing(current))
		schedule();

	try_to_freeze();
	cond_resched();
	goto repeat;