mm: reduce lock contention of pcp buffer refill

rmqueue_bulk() batches the allocation of multiple elements to refill the
per-CPU buffers into a single hold of the zone lock.  Each element is
allocated and checked using check_pcp_refill().  The check touches every
related struct page which is especially expensive for higher order
allocations (huge pages).

This patch reduces the time holding the lock by moving the check out of
the critical section similar to rmqueue_buddy() which allocates a single
element.

Measurements of parallel allocation-heavy workloads show a reduction of
the average huge page allocation latency of 50 percent for two cores and
nearly 90 percent for 24 cores.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Alexander Halbuer <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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