[PATCH v3 0/8] try to reduce fragmenting fallbacks

[Vlastimil Babka]

Last year, Johannes Weiner has reported a regression in page mobility
grouping [1] and while the exact cause was not found, I've come up with some
ways to improve it by reducing the number of allocations falling back to
different migratetype and causing permanent fragmentation.

Changes since v2:
- incorporated feedback (nothing major)
- updated to current mmotm
- dropped the more intrusive RFC patches 9-10

The series was tested with mmtests stress-highalloc modified to do GFP_KERNEL
order-4 allocations, on 4.9 with "mm, vmscan: fix zone balance check in
prepare_kswapd_sleep" (without that, kcompactd indeed wasn't woken up) on UMA
machine with 4GB memory. There were 5 repeats of each run, as the extfrag stats
are quite volatile (note the stats below are sums, not averages, as it was less
perl hacking for me).

Success rate are the same, already high due to the low allocation order used,
so I'm not including them.

Compaction stats:
(the patches are stacked, and I haven't measured the non-functional-changes
patches separately)

                                   patch 1     patch 2     patch 3     patch 4     patch 7     patch 8
Compaction stalls                    22449       24680       24846       19765       22059       17480
Compaction success                   12971       14836       14608       10475       11632        8757
Compaction failures                   9477        9843       10238        9290       10426        8722
Page migrate success               3109022     3370438     3312164     1695105     1608435     2111379
Page migrate failure                911588     1149065     1028264     1112675     1077251     1026367
Compaction pages isolated          7242983     8015530     7782467     4629063     4402787     5377665
Compaction migrate scanned       980838938   987367943   957690188   917647238   947155598  1018922197
Compaction free scanned          557926893   598946443   602236894   594024490   541169699   763651731
Compaction cost                      10243       10578       10304        8286        8398        9440

Compaction stats are mostly within noise until patch 4, which decreases the
number of compactions, and migrations. Part of that could be due to more
pageblocks marked as unmovable, and async compaction skipping those. This
changes a bit with patch 7, but not so much. Patch 8 increases free scanner
stats and migrations, which comes from the changed termination criteria.
Interestingly number of compactions decreases - probably the fully compacted
pageblock satisfies multiple subsequent allocations, so it amortizes.

Next comes the extfrag tracepoint, where "fragmenting" means that an allocation
had to fallback to a pageblock of another migratetype which wasn't fully free
(which is almost all of the fallbacks). I have locally added another tracepoint
for "Page steal" into steal_suitable_fallback() which triggers in situations
where we are allowed to do move_freepages_block(). If we decide to also do
set_pageblock_migratetype(), it's "Pages steal with pageblock" with break down
for which allocation migratetype we are stealing and from which fallback
migratetype. The last part "due to counting" comes from patch 4 and counts the
events where the counting of movable pages allowed us to change pageblock's
migratetype, while the number of free pages alone wouldn't be enough to cross
the threshold.

                                                     patch 1     patch 2     patch 3     patch 4     patch 7     patch 8
Page alloc extfrag event                            10155066     8522968    10164959    15622080    13727068    13140319
Extfrag fragmenting                                 10149231     8517025    10159040    15616925    13721391    13134792
Extfrag fragmenting for unmovable                     159504      168500      184177       97835       70625       56948
Extfrag fragmenting unmovable placed with movable     153613      163549      172693       91740       64099       50917
Extfrag fragmenting unmovable placed with reclaim.      5891        4951       11484        6095        6526        6031
Extfrag fragmenting for reclaimable                     4738        4829        6345        4822        5640        5378
Extfrag fragmenting reclaimable placed with movable     1836        1902        1851        1579        1739        1760
Extfrag fragmenting reclaimable placed with unmov.      2902        2927        4494        3243        3901        3618
Extfrag fragmenting for movable                      9984989     8343696     9968518    15514268    13645126    13072466
Pages steal                                           179954      192291      210880      123254       94545       81486
Pages steal with pageblock                             22153       18943       20154       33562       29969       33444
Pages steal with pageblock for unmovable               14350       12858       13256       20660       19003       20852
Pages steal with pageblock for unmovable from mov.     12812       11402       11683       19072       17467       19298
Pages steal with pageblock for unmovable from recl.     1538        1456        1573        1588        1536        1554
Pages steal with pageblock for movable                  7114        5489        5965       11787       10012       11493
Pages steal with pageblock for movable from unmov.      6885        5291        5541       11179        9525       10885
Pages steal with pageblock for movable from recl.        229         198         424         608         487         608
Pages steal with pageblock for reclaimable               689         596         933        1115         954        1099
Pages steal with pageblock for reclaimable from unmov.   273         219         537         658         547         667
Pages steal with pageblock for reclaimable from mov.     416         377         396         457         407         432
Pages steal with pageblock due to counting                                                 11834       10075        7530
... for unmovable                                                                           8993        7381        4616
... for movable                                                                             2792        2653        2851
... for reclaimable                                                                           49          41          63

What we can see is that "Extfrag fragmenting for unmovable" and "... placed with
movable" drops with almost each patch, which is good as we are polluting less
movable pageblocks with unmovable pages.
The most significant change is patch 4 with movable page counting. On the other
hand it increases "Extfrag fragmenting for movable" by 50%. "Pages steal" drops
though, so these movable allocation fallbacks find only small free pages and are
not allowed to steal whole pageblocks back. "Pages steal with pageblock" raises,
because the patch increases the chances of pageblock migratetype changes to
happen. This affects all migratetypes.
The summary is that patch 4 is not a clear win wrt these stats, but I believe
that the tradeoff it makes is a good one. There's less pollution of movable
pageblocks by unmovable allocations. There's less stealing between pageblock,
and those that remain have higher chance of changing migratetype also the
pageblock itself, so it should more faithfully reflect the migratetype of the
pages within the pageblock. The increase of movable allocations falling back to
unmovable pageblock might look dramatic, but those allocations can be migrated
by compaction when needed, and other patches in the series (7-9) improve that
aspect.
Patches 7 and 8 continue the trend of reduced unmovable fallbacks and also
reduce the impact on movable fallbacks from patch 4.

[1] https://www.spinics.net/lists/linux-mm/msg114237.html

Vlastimil Babka (8):
  mm, compaction: reorder fields in struct compact_control
  mm, compaction: remove redundant watermark check in compact_finished()
  mm, page_alloc: split smallest stolen page in fallback
  mm, page_alloc: count movable pages when stealing from pageblock
  mm, compaction: change migrate_async_suitable() to
    suitable_migration_source()
  mm, compaction: add migratetype to compact_control
  mm, compaction: restrict async compaction to pageblocks of same
    migratetype
  mm, compaction: finish whole pageblock to reduce fragmentation

 include/linux/mmzone.h         |   5 ++
 include/linux/page-isolation.h |   5 +-
 mm/compaction.c                |  83 ++++++++++++++++-------
 mm/internal.h                  |  12 ++--
 mm/page_alloc.c                | 148 ++++++++++++++++++++++++++++-------------
 mm/page_isolation.c            |   5 +-
 6 files changed, 177 insertions(+), 81 deletions(-)

-- 
2.12.0