Messages in this thread | | | Subject | Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping for RT tasks | From | Dietmar Eggemann <> | Date | Thu, 16 Aug 2018 12:34:05 +0200 |
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On 08/13/2018 05:01 PM, Patrick Bellasi wrote: > On 13-Aug 16:06, Vincent Guittot wrote: >> On Mon, 13 Aug 2018 at 14:49, Patrick Bellasi <patrick.bellasi@arm.com> wrote: >>> On 13-Aug 14:07, Vincent Guittot wrote: >>>> On Mon, 13 Aug 2018 at 12:12, Patrick Bellasi <patrick.bellasi@arm.com> wrote: > > [...] > >>> Yes I agree that the current behavior is not completely clean... still >>> the question is: do you reckon the problem I depicted above, i.e. RT >>> workloads eclipsing the min_util required by lower priority classes? >> >> As said above, I don't think that there is a problem that is specific >> to cross class scheduling that can't also happen in the same class. >> >> Regarding your example: >> task TA util=40% with uclamp_min 50% >> task TB util=10% with uclamp_min 0% >> >> If TA and TB are cfs, util=50% and it doesn't seem to be a problem >> whereas TB will steal some bandwidth to TA and delay it (and i even >> don't speak about the impact of the nice priority of TB) >> If TA is cfs and TB is rt, Why util=50% is now a problem for TA ? > > You right, in the current implementation, where we _do not_ > distinguish among scheduling classes it's not possible to get a > reasonable implementation of a per sched class clamping. > >>> To a certain extend I see this problem similar to the rt/dl/irq pressure >>> in defining cpu_capacity, isn't it? > > However, I still think that higher priority classes eclipsing the > clamping of lower priority classes can still be a problem. > > In your example above, the main difference between TA and TB being on > the same class or different classes is that in the second case TB > is granted to always preempt TA. We can end up with a non boosted RT > task consuming all the boosted bandwidth required by a CFS task. > > This does not happen, apart maybe for the corner case of really > different nice values, if the tasks are both CFS, since the fair > scheduler will grant some progress for both of them. > > Thus, given the current implementation, I think it makes sense to drop > the UCLAMP_SCHED_CLASS policy and stick with a more clean and > consistent design.
I agree with everything said in this thread so far. So in case you skip UCLAMP_SCHED_CLASS [(B) combine the clamped utilizations] in v4, you will only provide [A) clamp the combined utilization]?
I assume that we don't have to guard the util clamping for rt tasks behind a disabled by default sched feature because all runnable rt tasks will have util_min = SCHED_CAPACITY_SCALE by default?
> I'll then see if it makes sense to add a dedicated patch on top of the > series to add a proper per-class clamp tracking.
I assume if you introduce this per-class clamping you will switch to use the UCLAMP_SCHED_CLASS approach?
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