sched/topology: improve topology_span_sane speed

Use a different approach to topology_span_sane(), that checks for the
same constraint of no partial overlaps for any two CPU sets for
non-NUMA topology levels, but does so in a way that is O(N) rather
than O(N^2).

Instead of comparing with all other masks to detect collisions, keep
one mask that includes all CPUs seen so far and detect collisions with
a single cpumask_intersects test.

If the current mask has no collisions with previously seen masks, it
should be a new mask, which can be uniquely identified by the lowest
bit set in this mask.  Keep a pointer to this mask for future
reference (in an array indexed by the lowest bit set), and add the
CPUs in this mask to the list of those seen.

If the current mask does collide with previously seen masks, it should
be exactly equal to a mask seen before, looked up in the same array
indexed by the lowest bit set in the mask, a single comparison.

Move the topology_span_sane() check out of the existing topology level
loop, let it use its own loop so that the array allocation can be done
only once, shared across levels.

On a system with 1920 processors (16 sockets, 60 cores, 2 threads),
the average time to take one processor offline is reduced from 2.18
seconds to 1.01 seconds.  (Off-lining 959 of 1920 processors took
34m49.765s without this change, 16m10.038s with this change in place.)

Signed-off-by: Steve Wahl <steve.wahl@hpe.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Reviewed-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Tested-by: Valentin Schneider <vschneid@redhat.com>
Tested-by: Madadi Vineeth Reddy <vineethr@linux.ibm.com>
Link: https://lore.kernel.org/r/20250304160844.75373-2-steve.wahl@hpe.com

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