Messages in this thread |  | | From | Vincent Guittot <> | Date | Mon, 13 Aug 2018 16:06:23 +0200 | Subject | Re: [PATCH v3 06/14] sched/cpufreq: uclamp: add utilization clamping for RT tasks |
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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: > > > > > > Hi Vincent! > > > > > > On 09-Aug 18:03, Vincent Guittot wrote: > > > > > On 07-Aug 15:26, Juri Lelli wrote: > > > > > > [...] > > > > > > > > > > + util_cfs = cpu_util_cfs(rq); > > > > > > > + util_rt = cpu_util_rt(rq); > > > > > > > + if (sched_feat(UCLAMP_SCHED_CLASS)) { > > > > > > > + util = 0; > > > > > > > + if (util_cfs) > > > > > > > + util += uclamp_util(cpu_of(rq), util_cfs); > > > > > > > + if (util_rt) > > > > > > > + util += uclamp_util(cpu_of(rq), util_rt); > > > > > > > + } else { > > > > > > > + util = cpu_util_cfs(rq); > > > > > > > + util += cpu_util_rt(rq); > > > > > > > + util = uclamp_util(cpu_of(rq), util); > > > > > > > + } > > > > > > > > > > Regarding the two policies, do you have any comment? > > > > > > > > Does the policy for (sched_feat(UCLAMP_SCHED_CLASS)== true) really > > > > make sense as it is ? > > > > I mean, uclamp_util doesn't make any difference between rt and cfs > > > > tasks when clamping the utilization so why should be add twice the > > > > returned value ? > > > > IMHO, this policy would make sense if there were something like > > > > uclamp_util_rt() and a uclamp_util_cfs() > > > > > > The idea for the UCLAMP_SCHED_CLASS policy is to improve fairness on > > > low-priority classese, especially when we have high RT utilization. > > > > > > Let say we have: > > > > > > util_rt = 40%, util_min=0% > > > util_cfs = 10%, util_min=50% > > > > > > the two policies will select: > > > > > > UCLAMP_SCHED_CLASS: util = uclamp(40) + uclamp(10) = 50 + 50 = 100% > > > !UCLAMP_SCHED_CLASS: util = uclamp(40 + 10) = uclmp(50) = 50% > > > > > > Which means that, despite the CPU's util_min will be set to 50% when > > > CFS is running, these tasks will have almost no boost at all, since > > > their bandwidth margin is eclipsed by RT tasks. > > > > Hmm ... At the opposite, even if there is no running rt task but only > > some remaining blocked rt utilization, > > even if util_rt = 10%, util_min=0% > > and util_cfs = 40%, util_min=50% > > the UCLAMP_SCHED_CLASS: util = uclamp(10) + uclamp(40) = 50 + 50 = 100% > > Yes, that's true... since now I clamp util_rt if it's non zero. > Perhaps this can be fixed by clamping util_rt only: > if (rt_rq_is_runnable(&rq->rt)) > ? > > > So cfs task can get double boosted by a small rt task. > > Well, in principle we don't know if the 50% clamp was asserted by the > RT or the CFS task, since in the current implementation we max > aggregate clamp values across all RT and CFS tasks.
Yes it was just the assumption of your example above. IMHO, having util = 100% for your use case looks more like a bug than a feature
As you said below: "what utilization clamping aims to do is to defined the minimum capacity to run _all_ the RUNNABLE tasks... not the minimum capacity for _each_ one of them "
> > > Furthermore, if there is no rt task but 2 cfs tasks of 40% and 10% > > the UCLAMP_SCHED_CLASS: util = uclamp(0) + uclamp(40) = 50 = 50% > > True, but here we are within the same class and what utilization > clamping aims to do is to defined the minimum capacity to run _all_ > the RUNNABLE tasks... not the minimum capacity for _each_ one of them.
I fully agree and that's exactly what I want to highlight: With UCLAMP_SCHED_CLASS policy, you try (but fail because the clamping is not done per class) to distinguish rt and cfs as different kind of runnable tasks.
> > > So in this case cfs tasks don't get more boost and have to share the > > bandwidth and you don't ensure 50% for each unlike what you try to do > > for rt. > > Above I'm not trying to fix a per-task issue. The UCLAMP_SCHED_CLASS > policy is just "trying" to fix a cross-class issue... if we agree > there can be a cross-class issue worth to be fixed.
But the cross class issue that you are describing can also exists between cfs tasks with different uclamp_min So I'm not sure that's there is more cross-class issue than in class issue
> > > You create a difference in the behavior depending of the class of the > > others co-schedule tasks which is not sane IMHO > > 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 ?
> > To a certain extend I see this problem similar to the rt/dl/irq pressure > in defining cpu_capacity, isn't it? > > Maybe we can make use of (cpu_capacity_orig - cpu_capacity) to factor > in a util_min compensation for CFS tasks? > > -- > #include <best/regards.h> > > Patrick Bellasi
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