linux-kernel - Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks

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Date:	Thu, 11 Feb 2016 12:59:59 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Juri Lelli <juri.lelli@....com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update
 callbacks

On Tue, Feb 09, 2016 at 05:02:33PM -0800, Steve Muckle wrote:
> > Index: linux-pm/kernel/sched/deadline.c
> > ===================================================================
> > --- linux-pm.orig/kernel/sched/deadline.c
> > +++ linux-pm/kernel/sched/deadline.c
> > @@ -1197,6 +1197,9 @@ static void task_tick_dl(struct rq *rq,
> >  {
> >  	update_curr_dl(rq);
> >  
> > +	/* Kick cpufreq to prevent it from stalling. */
> > +	cpufreq_kick();
> > +
> >  	/*
> >  	 * Even when we have runtime, update_curr_dl() might have resulted in us
> >  	 * not being the leftmost task anymore. In that case NEED_RESCHED will
> 
> I think additional hooks such as enqueue/dequeue would be needed in
> RT/DL. The task tick callbacks will only run if a task in that class is
> executing at the time of the tick. There could be intermittent RT/DL
> task activity in a frequency domain (the only task activity there, no
> CFS tasks) that doesn't happen to overlap the tick. Worst case the task
> activity could be periodic in such a way that it never overlaps the tick
> and the update is never made.

No, for RT (RR/FIFO) we do not have enough information to do anything
useful. Basically RR/FIFO should result in running 100% whenever we
schedule such a task.

That means RR/FIFO want a hook in pick_next_task_rt() to bump the freq
to 100% and leave it there until something else gets to run.

For DL it basically wants to set a minimum freq based on reserved
utilization, so that is __setparam_dl() or somewhere around there.

And we should either use CPPC hints for min freq or manually ensure that
the CFS callback will not select something less than this.