mm: vmscan: use proportional scanning during direct reclaim and full scan at DEF_PRIORITY

Commit "mm: vmscan: obey proportional scanning requirements for kswapd"
ensured that file/anon lists were scanned proportionally for reclaim from
kswapd but ignored it for direct reclaim.  The intent was to minimse
direct reclaim latency but Yuanhan Liu pointer out that it substitutes one
long stall for many small stalls and distorts aging for normal workloads
like streaming readers/writers.  Hugh Dickins pointed out that a
side-effect of the same commit was that when one LRU list dropped to zero
that the entirety of the other list was shrunk leading to excessive
reclaim in memcgs.  This patch scans the file/anon lists proportionally
for direct reclaim to similarly age page whether reclaimed by kswapd or
direct reclaim but takes care to abort reclaim if one LRU drops to zero
after reclaiming the requested number of pages.

Based on ext4 and using the Intel VM scalability test

                                              3.15.0-rc5            3.15.0-rc5
                                                shrinker            proportion
Unit  lru-file-readonce    elapsed      5.3500 (  0.00%)      5.4200 ( -1.31%)
Unit  lru-file-readonce time_range      0.2700 (  0.00%)      0.1400 ( 48.15%)
Unit  lru-file-readonce time_stddv      0.1148 (  0.00%)      0.0536 ( 53.33%)
Unit lru-file-readtwice    elapsed      8.1700 (  0.00%)      8.1700 (  0.00%)
Unit lru-file-readtwice time_range      0.4300 (  0.00%)      0.2300 ( 46.51%)
Unit lru-file-readtwice time_stddv      0.1650 (  0.00%)      0.0971 ( 41.16%)

The test cases are running multiple dd instances reading sparse files. The results are within
the noise for the small test machine. The impact of the patch is more noticable from the vmstats

                            3.15.0-rc5  3.15.0-rc5
                              shrinker  proportion
Minor Faults                     35154       36784
Major Faults                       611        1305
Swap Ins                           394        1651
Swap Outs                         4394        5891
Allocation stalls               118616       44781
Direct pages scanned           4935171     4602313
Kswapd pages scanned          15921292    16258483
Kswapd pages reclaimed        15913301    16248305
Direct pages reclaimed         4933368     4601133
Kswapd efficiency                  99%         99%
Kswapd velocity             670088.047  682555.961
Direct efficiency                  99%         99%
Direct velocity             207709.217  193212.133
Percentage direct scans            23%         22%
Page writes by reclaim        4858.000    6232.000
Page writes file                   464         341
Page writes anon                  4394        5891

Note that there are fewer allocation stalls even though the amount
of direct reclaim scanning is very approximately the same.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Tested-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Cc: Bob Liu <bob.liu@oracle.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Rik van Riel <riel@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>