sched/fair: Let low-priority cores help high-priority busy SMT cores

Using asym_packing priorities within an SMT core is straightforward. Just
follow the priorities that hardware indicates.

When balancing load from an SMT core, also consider the idle state of its
siblings. Priorities do not reflect that an SMT core divides its throughput
among all its busy siblings. They only makes sense when exactly one sibling
is busy.

Indicate that active balance is needed if the destination CPU has lower
priority than the source CPU but the latter has busy SMT siblings.

Make find_busiest_queue() not skip higher-priority SMT cores with more than
busy sibling.

Suggested-by: Valentin Schneider <[email protected]>
Signed-off-by: Ricardo Neri <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Tested-by: Zhang Rui <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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