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

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Date:	Tue, 1 Mar 2016 14:17:06 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	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 01/03/16 14:58, Peter Zijlstra wrote:
> On Fri, Feb 12, 2016 at 03:48:54PM +0100, Vincent Guittot wrote:
> 
> > Another point to take into account is that the RT tasks will "steal"
> > the compute capacity that has been requested by the cfs tasks.
> > 
> > Let takes the example of a CPU with 3 OPP on which run 2 rt tasks A
> > and B and 1 cfs task C.
> 
> > Let assume that the real time constraint of RT task A is too agressive
> > for the lowest OPP0 and that the change of the frequency of the core
> > is too slow compare to this constraint but the real time constraint of
> > RT task B can be handle whatever the OPP. System don't have other
> > choice than setting the cpufreq min freq to OPP1 to be sure that
> > constraint of task A will be covered at anytime.
> 
> > Then, we still have 2
> > possible OPPs. The CFS task asks for compute capacity that fits in
> > OPP1 but a part of this capacity will be stolen by RT tasks. If we
> > monitor the load of RT tasks and request capacity for these RT tasks
> > according to their current utilization, we can decide to switch to
> > highest OPP2 to ensure that task C will have enough remaining
> > capacity. A lot of embedded platform faces such kind of use cases
> 
> Still doesn't make sense. How would you know the constraint of RT task
> A, and that it cannot be satisfied by OPP0 ? The only information you
> have in the task model is a static priority.
> 

But, can't we have the problem Vincent describes if we s/RT/DL/ ?

Thanks,

- Juri