Messages in this thread |  | | Date | Mon, 28 Oct 2019 15:37:49 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2] sched: rt: Make RT capacity aware |
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On Wed, Oct 09, 2019 at 11:46:11AM +0100, Qais Yousef wrote: > Capacity Awareness refers to the fact that on heterogeneous systems > (like Arm big.LITTLE), the capacity of the CPUs is not uniform, hence > when placing tasks we need to be aware of this difference of CPU > capacities. > > In such scenarios we want to ensure that the selected CPU has enough > capacity to meet the requirement of the running task. Enough capacity > means here that capacity_orig_of(cpu) >= task.requirement. > > The definition of task.requirement is dependent on the scheduling class. > > For CFS, utilization is used to select a CPU that has >= capacity value > than the cfs_task.util. > > capacity_orig_of(cpu) >= cfs_task.util > > DL isn't capacity aware at the moment but can make use of the bandwidth > reservation to implement that in a similar manner CFS uses utilization. > The following patchset implements that: > > https://lore.kernel.org/lkml/20190506044836.2914-1-luca.abeni@santannapisa.it/ > > capacity_orig_of(cpu)/SCHED_CAPACITY >= dl_deadline/dl_runtime > > For RT we don't have a per task utilization signal and we lack any > information in general about what performance requirement the RT task > needs. But with the introduction of uclamp, RT tasks can now control > that by setting uclamp_min to guarantee a minimum performance point. > > ATM the uclamp value are only used for frequency selection; but on > heterogeneous systems this is not enough and we need to ensure that the > capacity of the CPU is >= uclamp_min. Which is what implemented here. > > capacity_orig_of(cpu) >= rt_task.uclamp_min > > Note that by default uclamp.min is 1024, which means that RT tasks will > always be biased towards the big CPUs, which make for a better more > predictable behavior for the default case. > > Must stress that the bias acts as a hint rather than a definite > placement strategy. For example, if all big cores are busy executing > other RT tasks we can't guarantee that a new RT task will be placed > there. > > On non-heterogeneous systems the original behavior of RT should be > retained. Similarly if uclamp is not selected in the config. > > Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Works for me; Steve you OK with this?
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