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

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Message-ID: <20160211152625.GM6357@twins.programming.kicks-ass.net>
Date:	Thu, 11 Feb 2016 16:26:25 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Juri Lelli <juri.lelli@....com>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	"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>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update
 callbacks

On Thu, Feb 11, 2016 at 12:24:29PM +0000, Juri Lelli wrote:
> Hi Peter,
> 
> On 11/02/16 12:59, Peter Zijlstra wrote:
> > 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.
> > 
> 
> Vincent is trying to play with rt_avg (in the last sched-freq thread) to
> see if we can get some information about RT as well. I understand that
> from a theoretical perspective that's not much we can say of such tasks,
> and bumping to max can be the only sensible thing to do, but there are
> users of RT (ehm, Android) that will probably see differences in energy
> consumption if we do so. Yeah, maybe the should use a different policy,
> yes.

Can't we just leave broken people get broken results? Trying to use
rt_avg for this is just insane. We should ensure that people using this
thing correctly get correct results, the rest can take a hike.

Using rt_avg gets us to the place where people who want to do the right
thing cannot, and that is bad.

> > For DL it basically wants to set a minimum freq based on reserved
> > utilization, so that is __setparam_dl() or somewhere around there.
> > 
> 
> I think we could do better than this once Luca's reclaiming stuff gets
> in. The reserved bw is usually somewhat pessimistic. But this is a
> different discussion, maybe.

Sure, there's cleverer things that can be done. But a simple one would
indeed be the min guarantee based on accepted bandwidth.