Messages in this thread | | | Date | Tue, 1 Mar 2016 14:17:06 +0000 | From | Juri Lelli <> | 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
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