mm, compaction: sample pageblocks for free pages

Once fast searching finishes, there is a possibility that the linear
scanner is scanning full blocks found by the fast scanner earlier.  This
patch uses an adaptive stride to sample pageblocks for free pages.  The
more consecutive full pageblocks encountered, the larger the stride
until a pageblock with free pages is found.  The scanners might meet
slightly sooner but it is an acceptable risk given that the search of
the free lists may still encounter the pages and adjust the cached PFN
of the free scanner accordingly.

                                     5.0.0-rc1              5.0.0-rc1
                              roundrobin-v3r17       samplefree-v3r17
Amean     fault-both-1         0.00 (   0.00%)        0.00 *   0.00%*
Amean     fault-both-3      2752.37 (   0.00%)     2729.95 (   0.81%)
Amean     fault-both-5      4341.69 (   0.00%)     4397.80 (  -1.29%)
Amean     fault-both-7      6308.75 (   0.00%)     6097.61 (   3.35%)
Amean     fault-both-12    10241.81 (   0.00%)     9407.15 (   8.15%)
Amean     fault-both-18    13736.09 (   0.00%)    10857.63 *  20.96%*
Amean     fault-both-24    16853.95 (   0.00%)    13323.24 *  20.95%*
Amean     fault-both-30    15862.61 (   0.00%)    17345.44 (  -9.35%)
Amean     fault-both-32    18450.85 (   0.00%)    16892.00 (   8.45%)

The latency is mildly improved offseting some overhead from earlier
patches that are prerequisites for the rest of the series.  However, a
major impact is on the free scan rate with an 82% reduction.

                                5.0.0-rc1      5.0.0-rc1
                         roundrobin-v3r17 samplefree-v3r17
Compaction migrate scanned    21607271            20116887
Compaction free scanned       95336406            16668703

It's also the first time in the series where the number of pages scanned
by the migration scanner is greater than the free scanner due to the
increased search efficiency.

Link: http://lkml.kernel.org/r/20190118175136.31341-21-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: David Rientjes <rientjes@google.com>
Cc: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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