Date: Tue, 1 Mar 2016 14:17:06 +0000
From: Juri Lelli <juri.lelli@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
        Steve Muckle <steve.muckle@linaro.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Linux PM list <linux-pm@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update
 callbacks
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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