• Vlastimil Babka's avatar
    mm: when stealing freepages, also take pages created by splitting buddy page · 99592d59
    Vlastimil Babka authored
    When studying page stealing, I noticed some weird looking decisions in
    try_to_steal_freepages().  The first I assume is a bug (Patch 1), the
    following two patches were driven by evaluation.
    
    Testing was done with stress-highalloc of mmtests, using the
    mm_page_alloc_extfrag tracepoint and postprocessing to get counts of how
    often page stealing occurs for individual migratetypes, and what
    migratetypes are used for fallbacks.  Arguably, the worst case of page
    stealing is when UNMOVABLE allocation steals from MOVABLE pageblock.
    RECLAIMABLE allocation stealing from MOVABLE allocation is also not ideal,
    so the goal is to minimize these two cases.
    
    The evaluation of v2 wasn't always clear win and Joonsoo questioned the
    results.  Here I used different baseline which includes RFC compaction
    improvements from [1].  I found that the compaction improvements reduce
    variability of stress-highalloc, so there's less noise in the data.
    
    First, let's look at stress-highalloc configured to do sync compaction,
    and how these patches reduce page stealing events during the test.  First
    column is after fresh reboot, other two are reiterations of test without
    reboot.  That was all accumulater over 5 re-iterations (so the benchmark
    was run 5x3 times with 5 fresh restarts).
    
    Baseline:
    
                                                       3.19-rc4        3.19-rc4        3.19-rc4
                                                      5-nothp-1       5-nothp-2       5-nothp-3
    Page alloc extfrag event                               10264225     8702233    10244125
    Extfrag fragmenting                                    10263271     8701552    10243473
    Extfrag fragmenting for unmovable                         13595       17616       15960
    Extfrag fragmenting unmovable placed with movable          7989       12193        8447
    Extfrag fragmenting for reclaimable                         658        1840        1817
    Extfrag fragmenting reclaimable placed with movable         558        1677        1679
    Extfrag fragmenting for movable                        10249018     8682096    10225696
    
    With Patch 1:
                                                       3.19-rc4        3.19-rc4        3.19-rc4
                                                      6-nothp-1       6-nothp-2       6-nothp-3
    Page alloc extfrag event                               11834954     9877523     9774860
    Extfrag fragmenting                                    11833993     9876880     9774245
    Extfrag fragmenting for unmovable                          7342       16129       11712
    Extfrag fragmenting unmovable placed with movable          4191       10547        6270
    Extfrag fragmenting for reclaimable                         373        1130         923
    Extfrag fragmenting reclaimable placed with movable         302         906         738
    Extfrag fragmenting for movable                        11826278     9859621     9761610
    
    With Patch 2:
                                                       3.19-rc4        3.19-rc4        3.19-rc4
                                                      7-nothp-1       7-nothp-2       7-nothp-3
    Page alloc extfrag event                                4725990     3668793     3807436
    Extfrag fragmenting                                     4725104     3668252     3806898
    Extfrag fragmenting for unmovable                          6678        7974        7281
    Extfrag fragmenting unmovable placed with movable          2051        3829        4017
    Extfrag fragmenting for reclaimable                         429        1208        1278
    Extfrag fragmenting reclaimable placed with movable         369         976        1034
    Extfrag fragmenting for movable                         4717997     3659070     3798339
    
    With Patch 3:
                                                       3.19-rc4        3.19-rc4        3.19-rc4
                                                      8-nothp-1       8-nothp-2       8-nothp-3
    Page alloc extfrag event                                5016183     4700142     3850633
    Extfrag fragmenting                                     5015325     4699613     3850072
    Extfrag fragmenting for unmovable                          1312        3154        3088
    Extfrag fragmenting unmovable placed with movable          1115        2777        2714
    Extfrag fragmenting for reclaimable                         437        1193        1097
    Extfrag fragmenting reclaimable placed with movable         330         969         879
    Extfrag fragmenting for movable                         5013576     4695266     3845887
    
    In v2 we've seen apparent regression with Patch 1 for unmovable events,
    this is now gone, suggesting it was indeed noise.  Here, each patch
    improves the situation for unmovable events.  Reclaimable is improved by
    patch 1 and then either the same modulo noise, or perhaps sligtly worse -
    a small price for unmovable improvements, IMHO.  The number of movable
    allocations falling back to other migratetypes is most noisy, but it's
    reduced to half at Patch 2 nevertheless.  These are least critical as
    compaction can move them around.
    
    If we look at success rates, the patches don't affect them, that didn't change.
    
    Baseline:
                                 3.19-rc4              3.19-rc4              3.19-rc4
                                5-nothp-1             5-nothp-2             5-nothp-3
    Success 1 Min         49.00 (  0.00%)       42.00 ( 14.29%)       41.00 ( 16.33%)
    Success 1 Mean        51.00 (  0.00%)       45.00 ( 11.76%)       42.60 ( 16.47%)
    Success 1 Max         55.00 (  0.00%)       51.00 (  7.27%)       46.00 ( 16.36%)
    Success 2 Min         53.00 (  0.00%)       47.00 ( 11.32%)       44.00 ( 16.98%)
    Success 2 Mean        59.60 (  0.00%)       50.80 ( 14.77%)       48.20 ( 19.13%)
    Success 2 Max         64.00 (  0.00%)       56.00 ( 12.50%)       52.00 ( 18.75%)
    Success 3 Min         84.00 (  0.00%)       82.00 (  2.38%)       78.00 (  7.14%)
    Success 3 Mean        85.60 (  0.00%)       82.80 (  3.27%)       79.40 (  7.24%)
    Success 3 Max         86.00 (  0.00%)       83.00 (  3.49%)       80.00 (  6.98%)
    
    Patch 1:
                                 3.19-rc4              3.19-rc4              3.19-rc4
                                6-nothp-1             6-nothp-2             6-nothp-3
    Success 1 Min         49.00 (  0.00%)       44.00 ( 10.20%)       44.00 ( 10.20%)
    Success 1 Mean        51.80 (  0.00%)       46.00 ( 11.20%)       45.80 ( 11.58%)
    Success 1 Max         54.00 (  0.00%)       49.00 (  9.26%)       49.00 (  9.26%)
    Success 2 Min         58.00 (  0.00%)       49.00 ( 15.52%)       48.00 ( 17.24%)
    Success 2 Mean        60.40 (  0.00%)       51.80 ( 14.24%)       50.80 ( 15.89%)
    Success 2 Max         63.00 (  0.00%)       54.00 ( 14.29%)       55.00 ( 12.70%)
    Success 3 Min         84.00 (  0.00%)       81.00 (  3.57%)       79.00 (  5.95%)
    Success 3 Mean        85.00 (  0.00%)       81.60 (  4.00%)       79.80 (  6.12%)
    Success 3 Max         86.00 (  0.00%)       82.00 (  4.65%)       82.00 (  4.65%)
    
    Patch 2:
    
                                 3.19-rc4              3.19-rc4              3.19-rc4
                                7-nothp-1             7-nothp-2             7-nothp-3
    Success 1 Min         50.00 (  0.00%)       44.00 ( 12.00%)       39.00 ( 22.00%)
    Success 1 Mean        52.80 (  0.00%)       45.60 ( 13.64%)       42.40 ( 19.70%)
    Success 1 Max         55.00 (  0.00%)       46.00 ( 16.36%)       47.00 ( 14.55%)
    Success 2 Min         52.00 (  0.00%)       48.00 (  7.69%)       45.00 ( 13.46%)
    Success 2 Mean        53.40 (  0.00%)       49.80 (  6.74%)       48.80 (  8.61%)
    Success 2 Max         57.00 (  0.00%)       52.00 (  8.77%)       52.00 (  8.77%)
    Success 3 Min         84.00 (  0.00%)       81.00 (  3.57%)       79.00 (  5.95%)
    Success 3 Mean        85.00 (  0.00%)       82.40 (  3.06%)       79.60 (  6.35%)
    Success 3 Max         86.00 (  0.00%)       83.00 (  3.49%)       80.00 (  6.98%)
    
    Patch 3:
                                 3.19-rc4              3.19-rc4              3.19-rc4
                                8-nothp-1             8-nothp-2             8-nothp-3
    Success 1 Min         46.00 (  0.00%)       44.00 (  4.35%)       42.00 (  8.70%)
    Success 1 Mean        50.20 (  0.00%)       45.60 (  9.16%)       44.00 ( 12.35%)
    Success 1 Max         52.00 (  0.00%)       47.00 (  9.62%)       47.00 (  9.62%)
    Success 2 Min         53.00 (  0.00%)       49.00 (  7.55%)       48.00 (  9.43%)
    Success 2 Mean        55.80 (  0.00%)       50.60 (  9.32%)       49.00 ( 12.19%)
    Success 2 Max         59.00 (  0.00%)       52.00 ( 11.86%)       51.00 ( 13.56%)
    Success 3 Min         84.00 (  0.00%)       80.00 (  4.76%)       79.00 (  5.95%)
    Success 3 Mean        85.40 (  0.00%)       81.60 (  4.45%)       80.40 (  5.85%)
    Success 3 Max         87.00 (  0.00%)       83.00 (  4.60%)       82.00 (  5.75%)
    
    While there's no improvement here, I consider reduced fragmentation events
    to be worth on its own.  Patch 2 also seems to reduce scanning for free
    pages, and migrations in compaction, suggesting it has somewhat less work
    to do:
    
    Patch 1:
    
    Compaction stalls                 4153        3959        3978
    Compaction success                1523        1441        1446
    Compaction failures               2630        2517        2531
    Page migrate success           4600827     4943120     5104348
    Page migrate failure             19763       16656       17806
    Compaction pages isolated      9597640    10305617    10653541
    Compaction migrate scanned    77828948    86533283    87137064
    Compaction free scanned      517758295   521312840   521462251
    Compaction cost                   5503        5932        6110
    
    Patch 2:
    
    Compaction stalls                 3800        3450        3518
    Compaction success                1421        1316        1317
    Compaction failures               2379        2134        2201
    Page migrate success           4160421     4502708     4752148
    Page migrate failure             19705       14340       14911
    Compaction pages isolated      8731983     9382374     9910043
    Compaction migrate scanned    98362797    96349194    98609686
    Compaction free scanned      496512560   469502017   480442545
    Compaction cost                   5173        5526        5811
    
    As with v2, /proc/pagetypeinfo appears unaffected with respect to numbers
    of unmovable and reclaimable pageblocks.
    
    Configuring the benchmark to allocate like THP page fault (i.e.  no sync
    compaction) gives much noisier results for iterations 2 and 3 after
    reboot.  This is not so surprising given how [1] offers lower improvements
    in this scenario due to less restarts after deferred compaction which
    would change compaction pivot.
    
    Baseline:
                                                       3.19-rc4        3.19-rc4        3.19-rc4
                                                        5-thp-1         5-thp-2         5-thp-3
    Page alloc extfrag event                                8148965     6227815     6646741
    Extfrag fragmenting                                     8147872     6227130     6646117
    Extfrag fragmenting for unmovable                         10324       12942       15975
    Extfrag fragmenting unmovable placed with movable          5972        8495       10907
    Extfrag fragmenting for reclaimable                         601        1707        2210
    Extfrag fragmenting reclaimable placed with movable         520        1570        2000
    Extfrag fragmenting for movable                         8136947     6212481     6627932
    
    Patch 1:
                                                       3.19-rc4        3.19-rc4        3.19-rc4
                                                        6-thp-1         6-thp-2         6-thp-3
    Page alloc extfrag event                                8345457     7574471     7020419
    Extfrag fragmenting                                     8343546     7573777     7019718
    Extfrag fragmenting for unmovable                         10256       18535       30716
    Extfrag fragmenting unmovable placed with movable          6893       11726       22181
    Extfrag fragmenting for reclaimable                         465        1208        1023
    Extfrag fragmenting reclaimable placed with movable         353         996         843
    Extfrag fragmenting for movable                         8332825     7554034     6987979
    
    Patch 2:
                                                       3.19-rc4        3.19-rc4        3.19-rc4
                                                        7-thp-1         7-thp-2         7-thp-3
    Page alloc extfrag event                                3512847     3020756     2891625
    Extfrag fragmenting                                     3511940     3020185     2891059
    Extfrag fragmenting for unmovable                          9017        6892        6191
    Extfrag fragmenting unmovable placed with movable          1524        3053        2435
    Extfrag fragmenting for reclaimable                         445        1081        1160
    Extfrag fragmenting reclaimable placed with movable         375         918         986
    Extfrag fragmenting for movable                         3502478     3012212     2883708
    
    Patch 3:
                                                       3.19-rc4        3.19-rc4        3.19-rc4
                                                        8-thp-1         8-thp-2         8-thp-3
    Page alloc extfrag event                                3181699     3082881     2674164
    Extfrag fragmenting                                     3180812     3082303     2673611
    Extfrag fragmenting for unmovable                          1201        4031        4040
    Extfrag fragmenting unmovable placed with movable           974        3611        3645
    Extfrag fragmenting for reclaimable                         478        1165        1294
    Extfrag fragmenting reclaimable placed with movable         387         985        1030
    Extfrag fragmenting for movable                         3179133     3077107     2668277
    
    The improvements for first iteration are clear, the rest is much noisier
    and can appear like regression for Patch 1.  Anyway, patch 2 rectifies it.
    
    Allocation success rates are again unaffected so there's no point in
    making this e-mail any longer.
    
    [1] http://marc.info/?l=linux-mm&m=142166196321125&w=2
    
    This patch (of 3):
    
    When __rmqueue_fallback() is called to allocate a page of order X, it will
    find a page of order Y >= X of a fallback migratetype, which is different
    from the desired migratetype.  With the help of try_to_steal_freepages(),
    it may change the migratetype (to the desired one) also of:
    
    1) all currently free pages in the pageblock containing the fallback page
    2) the fallback pageblock itself
    3) buddy pages created by splitting the fallback page (when Y > X)
    
    These decisions take the order Y into account, as well as the desired
    migratetype, with the goal of preventing multiple fallback allocations
    that could e.g.  distribute UNMOVABLE allocations among multiple
    pageblocks.
    
    Originally, decision for 1) has implied the decision for 3).  Commit
    47118af0 ("mm: mmzone: MIGRATE_CMA migration type added") changed that
    (probably unintentionally) so that the buddy pages in case 3) are always
    changed to the desired migratetype, except for CMA pageblocks.
    
    Commit fef903ef ("mm/page_allo.c: restructure free-page stealing code
    and fix a bug") did some refactoring and added a comment that the case of
    3) is intended.  Commit 0cbef29a ("mm: __rmqueue_fallback() should
    respect pageblock type") removed the comment and tried to restore the
    original behavior where 1) implies 3), but due to the previous
    refactoring, the result is instead that only 2) implies 3) - and the
    conditions for 2) are less frequently met than conditions for 1).  This
    may increase fragmentation in situations where the code decides to steal
    all free pages from the pageblock (case 1)), but then gives back the buddy
    pages produced by splitting.
    
    This patch restores the original intended logic where 1) implies 3).
    During testing with stress-highalloc from mmtests, this has shown to
    decrease the number of events where UNMOVABLE and RECLAIMABLE allocations
    steal from MOVABLE pageblocks, which can lead to permanent fragmentation.
    In some cases it has increased the number of events when MOVABLE
    allocations steal from UNMOVABLE or RECLAIMABLE pageblocks, but these are
    fixable by sync compaction and thus less harmful.
    
    Note that evaluation has shown that the behavior introduced by
    47118af0 for buddy pages in case 3) is actually even better than the
    original logic, so the following patch will introduce it properly once
    again.  For stable backports of this patch it makes thus sense to only fix
    versions containing 0cbef29a
    
    .
    
    [iamjoonsoo.kim@lge.com: tracepoint fix]
    Signed-off-by: default avatarVlastimil Babka <vbabka@suse.cz>
    Acked-by: default avatarMel Gorman <mgorman@suse.de>
    Cc: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
    Acked-by: default avatarMinchan Kim <minchan@kernel.org>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
    Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: Michal Hocko <mhocko@suse.cz>
    Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
    Cc: <stable@vger.kernel.org>	[3.13+ containing 0cbef29a
    
    ]
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    99592d59
page_alloc.c 181 KB