sched/deadline: Improve admission control for asymmetric CPU capacities

sched/deadline: Improve admission control for asymmetric CPU capacities

The current SCHED_DEADLINE (DL) admission control ensures that

    sum of reserved CPU bandwidth < x * M

where

    x = /proc/sys/kernel/sched_rt_{runtime,period}_us
    M = # CPUs in root domain.

DL admission control works well for homogeneous systems where the
capacity of all CPUs are equal (1024). I.e. bounded tardiness for DL
and non-starvation of non-DL tasks is guaranteed.

But on heterogeneous systems where capacity of CPUs are different it
could fail by over-allocating CPU time on smaller capacity CPUs.

On an Arm big.LITTLE/DynamIQ system DL tasks can easily starve other
tasks making it unusable.

Fix this by explicitly considering the CPU capacity in the DL admission
test by replacing M with the root domain CPU capacity sum.

Signed-off-by: Luca Abeni <luca.abeni@santannapisa.it>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20200520134243.19352-4-dietmar.eggemann@arm.com