• Vlastimil Babka's avatar
    mm, compaction: make whole_zone flag ignore cached scanner positions · 06ed2998
    Vlastimil Babka authored
    Patch series "make direct compaction more deterministic")
    
    This is mostly a followup to Michal's oom detection rework, which
    highlighted the need for direct compaction to provide better feedback in
    reclaim/compaction loop, so that it can reliably recognize when
    compaction cannot make further progress, and allocation should invoke
    OOM killer or fail.  We've discussed this at LSF/MM [1] where I proposed
    expanding the async/sync migration mode used in compaction to more
    general "priorities".  This patchset adds one new priority that just
    overrides all the heuristics and makes compaction fully scan all zones.
    I don't currently think that we need more fine-grained priorities, but
    we'll see.  Other than that there's some smaller fixes and cleanups,
    mainly related to the THP-specific hacks.
    
    I've tested this with stress-highalloc in GFP_KERNEL order-4 and
    THP-like order-9 scenarios.  There's some improvement for compaction
    stats for the order-4, which is likely due to the better watermarks
    handling.  In the previous version I reported mostly noise wrt
    compaction stats, and decreased direct reclaim - now the reclaim is
    without difference.  I believe this is due to the less aggressive
    compaction priority increase in patch 6.
    
    "before" is a mmotm tree prior to 4.7 release plus the first part of the
    series that was sent and merged separately
    
                                        before        after
    order-4:
    
    Compaction stalls                    27216       30759
    Compaction success                   19598       25475
    Compaction failures                   7617        5283
    Page migrate success                370510      464919
    Page migrate failure                 25712       27987
    Compaction pages isolated           849601     1041581
    Compaction migrate scanned       143146541   101084990
    Compaction free scanned          208355124   144863510
    Compaction cost                       1403        1210
    
    order-9:
    
    Compaction stalls                     7311        7401
    Compaction success                    1634        1683
    Compaction failures                   5677        5718
    Page migrate success                194657      183988
    Page migrate failure                  4753        4170
    Compaction pages isolated           498790      456130
    Compaction migrate scanned          565371      524174
    Compaction free scanned            4230296     4250744
    Compaction cost                        215         203
    
    [1] https://lwn.net/Articles/684611/
    
    This patch (of 11):
    
    A recent patch has added whole_zone flag that compaction sets when
    scanning starts from the zone boundary, in order to report that zone has
    been fully scanned in one attempt.  For allocations that want to try
    really hard or cannot fail, we will want to introduce a mode where
    scanning whole zone is guaranteed regardless of the cached positions.
    
    This patch reuses the whole_zone flag in a way that if it's already
    passed true to compaction, the cached scanner positions are ignored.
    Employing this flag during reclaim/compaction loop will be done in the
    next patch.  This patch however converts compaction invoked from
    userspace via procfs to use this flag.  Before this patch, the cached
    positions were first reset to zone boundaries and then read back from
    struct zone, so there was a window where a parallel compaction could
    replace the reset values, making the manual compaction less effective.
    Using the flag instead of performing reset is more robust.
    
    [akpm@linux-foundation.org: coding-style fixes]
    Link: http://lkml.kernel.org/r/20160810091226.6709-2-vbabka@suse.czSigned-off-by: default avatarVlastimil Babka <vbabka@suse.cz>
    Tested-by: default avatarLorenzo Stoakes <lstoakes@gmail.com>
    Acked-by: default avatarMichal Hocko <mhocko@suse.com>
    Cc: Mel Gorman <mgorman@techsingularity.net>
    Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Rik van Riel <riel@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    06ed2998
compaction.c 56.1 KB