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

[Joel Fernandes]

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?

Yes, I was also thinking the same. I think we can come up with a better
mechanism that still doesn't use work_in_progress. I am cooking up a patch
but may take a bit longer since I'm traveling. I'll share it once I have
something :)

thanks,

- Joel