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Date:   Thu, 17 May 2018 17:23:12 +0200
From:   Juri Lelli <juri.lelli@redhat.com>
To:     Joel Fernandes <joel@joelfernandes.org>
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 17/05/18 07:43, Joel Fernandes wrote:
> 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;

Ah, right. Your original patch LGTM then. :)

Maybe add a comment about this higher level loop?