maple_tree: add benchmarking for mas_for_each

Patch series "Reduce preallocations for maple tree", v3.

Initial work on preallocations showed no regression in performance during
testing, but recently some users (both on [1] and off [android] list) have
reported that preallocating the worst-case number of nodes has caused some
slow down.  This patch set addresses the number of allocations in a few
ways.

During munmap() most munmap() operations will remove a single VMA, so
leverage the fact that the maple tree can place a single pointer at range
0 - 0 without allocating.  This is done by changing the index of the VMAs
to be indexed by the count, starting at 0.

Re-introduce the entry argument to mas_preallocate() so that a more
intelligent guess of the node count can be made.

Implement the more intelligent guess of the node count, although there is
more work to be done.

During development of v2 of this patch set, I also noticed that the number
of nodes being allocated for a rebalance was beyond what could possibly be
needed.  This is addressed in patch 0008.


This patch (of 15):

Add a way to test the speed of mas_for_each() to the testing code.

Link: https://lkml.kernel.org/r/20230724183157.3939892-1-Liam.Howlett@oracle.com
Link: https://lkml.kernel.org/r/20230724183157.3939892-2-Liam.Howlett@oracle.com


Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Peng Zhang <zhangpeng.00@bytedance.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>