Messages in this thread |  | | Date | Mon, 18 Nov 2019 15:43:35 +0000 | From | Qais Yousef <> | Subject | Re: [PATCH v2] sched: rt: Make RT capacity aware |
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Hi Steve
On 10/28/19 14:01, Steven Rostedt wrote: > On Mon, 28 Oct 2019 15:37:49 +0100 > Peter Zijlstra <peterz@infradead.org> wrote: > > > 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? > > Nothing against it, but I want to take a deeper look before we accept > it. Are you OK in waiting a week? I'm currently at Open Source Summit > and still have two more talks to write (giving them Thursday). I wont > have time to look till next week.
Apologies if I am being too pushy. But not sure whether this is waiting for its turn in the queue or slipped through the cracks, hence another gentle reminder in case it's the latter :-)
Many thanks
-- Qais Yousef
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