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

[Peter Zijlstra]

On Tue, Mar 01, 2016 at 03:24:59PM +0100, Peter Zijlstra wrote:
> On Tue, Mar 01, 2016 at 02:17:06PM +0000, Juri Lelli wrote:
> > 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/ ?
> 
> Still not sure I actually see a problem. With DL you have a minimal OPP
> required to guarantee correct execution of the DL tasks. For CFS you
> have an average util reflecting its workload.
> 
> Add the two and you've got an effective OPP request. Or in CPPC terms:
> we request a min freq of the DL and a max freq of DL+avg_CFS.
> 
> We could probably improve upon that by also tracking an avg DL and
> lowering the max freq request to: min(DL, avg_DL + avg_CFS). The

max(DL, avg_DL + avg_CFS) obviously! ;-)

> consequence is that when the DL tasks hit peaks (over their avg) the CFS
> tasks get a little more delay. But this might be a worthwhile trade-off.