• Kirill Tkhai's avatar
    mm/vmscan.c: clear shrinker bit if there are no objects related to memcg · f90280d6
    Kirill Tkhai authored
    To avoid further unneed calls of do_shrink_slab() for shrinkers, which
    already do not have any charged objects in a memcg, their bits have to
    be cleared.
    
    This patch introduces a lockless mechanism to do that without races
    without parallel list lru add.  After do_shrink_slab() returns
    SHRINK_EMPTY the first time, we clear the bit and call it once again.
    Then we restore the bit, if the new return value is different.
    
    Note, that single smp_mb__after_atomic() in shrink_slab_memcg() covers
    two situations:
    
    1)list_lru_add()     shrink_slab_memcg
        list_add_tail()    for_each_set_bit() <--- read bit
                             do_shrink_slab() <--- missed list update (no barrier)
        <MB>                 <MB>
        set_bit()            do_shrink_slab() <--- seen list update
    
    This situation, when the first do_shrink_slab() sees set bit, but it
    doesn't see list update (i.e., race with the first element queueing), is
    rare.  So we don't add <MB> before the first call of do_shrink_slab()
    instead of this to do not slow down generic case.  Also, it's need the
    second call as seen in below in (2).
    
    2)list_lru_add()      shrink_slab_memcg()
        list_add_tail()     ...
        set_bit()           ...
      ...                   for_each_set_bit()
      do_shrink_slab()        do_shrink_slab()
        clear_bit()           ...
      ...                     ...
      list_lru_add()          ...
        list_add_tail()       clear_bit()
        <MB>                  <MB>
        set_bit()             do_shrink_slab()
    
    The barriers guarantee that the second do_shrink_slab() in the right
    side task sees list update if really cleared the bit.  This case is
    drawn in the code comment.
    
    [Results/performance of the patchset]
    
    After the whole patchset applied the below test shows signify increase
    of performance:
    
      $echo 1 > /sys/fs/cgroup/memory/memory.use_hierarchy
      $mkdir /sys/fs/cgroup/memory/ct
      $echo 4000M > /sys/fs/cgroup/memory/ct/memory.kmem.limit_in_bytes
          $for i in `seq 0 4000`; do mkdir /sys/fs/cgroup/memory/ct/$i;
    			    echo $$ > /sys/fs/cgroup/memory/ct/$i/cgroup.procs;
    			    mkdir -p s/$i; mount -t tmpfs $i s/$i;
    			    touch s/$i/file; done
    
    Then, 5 sequential calls of drop caches:
    
      $time echo 3 > /proc/sys/vm/drop_caches
    
    1)Before:
      0.00user 13.78system 0:13.78elapsed 99%CPU
      0.00user 5.59system 0:05.60elapsed 99%CPU
      0.00user 5.48system 0:05.48elapsed 99%CPU
      0.00user 8.35system 0:08.35elapsed 99%CPU
      0.00user 8.34system 0:08.35elapsed 99%CPU
    
    2)After
      0.00user 1.10system 0:01.10elapsed 99%CPU
      0.00user 0.00system 0:00.01elapsed 64%CPU
      0.00user 0.01system 0:00.01elapsed 82%CPU
      0.00user 0.00system 0:00.01elapsed 64%CPU
      0.00user 0.01system 0:00.01elapsed 82%CPU
    
    The results show the performance increases at least in 548 times.
    
    Shakeel Butt tested this patchset with fork-bomb on his configuration:
    
     > I created 255 memcgs, 255 ext4 mounts and made each memcg create a
     > file containing few KiBs on corresponding mount. Then in a separate
     > memcg of 200 MiB limit ran a fork-bomb.
     >
     > I ran the "perf record -ag -- sleep 60" and below are the results:
     >
     > Without the patch series:
     > Samples: 4M of event 'cycles', Event count (approx.): 3279403076005
     > +  36.40%            fb.sh  [kernel.kallsyms]    [k] shrink_slab
     > +  18.97%            fb.sh  [kernel.kallsyms]    [k] list_lru_count_one
     > +   6.75%            fb.sh  [kernel.kallsyms]    [k] super_cache_count
     > +   0.49%            fb.sh  [kernel.kallsyms]    [k] down_read_trylock
     > +   0.44%            fb.sh  [kernel.kallsyms]    [k] mem_cgroup_iter
     > +   0.27%            fb.sh  [kernel.kallsyms]    [k] up_read
     > +   0.21%            fb.sh  [kernel.kallsyms]    [k] osq_lock
     > +   0.13%            fb.sh  [kernel.kallsyms]    [k] shmem_unused_huge_count
     > +   0.08%            fb.sh  [kernel.kallsyms]    [k] shrink_node_memcg
     > +   0.08%            fb.sh  [kernel.kallsyms]    [k] shrink_node
     >
     > With the patch series:
     > Samples: 4M of event 'cycles', Event count (approx.): 2756866824946
     > +  47.49%            fb.sh  [kernel.kallsyms]    [k] down_read_trylock
     > +  30.72%            fb.sh  [kernel.kallsyms]    [k] up_read
     > +   9.51%            fb.sh  [kernel.kallsyms]    [k] mem_cgroup_iter
     > +   1.69%            fb.sh  [kernel.kallsyms]    [k] shrink_node_memcg
     > +   1.35%            fb.sh  [kernel.kallsyms]    [k] mem_cgroup_protected
     > +   1.05%            fb.sh  [kernel.kallsyms]    [k] queued_spin_lock_slowpath
     > +   0.85%            fb.sh  [kernel.kallsyms]    [k] _raw_spin_lock
     > +   0.78%            fb.sh  [kernel.kallsyms]    [k] lruvec_lru_size
     > +   0.57%            fb.sh  [kernel.kallsyms]    [k] shrink_node
     > +   0.54%            fb.sh  [kernel.kallsyms]    [k] queue_work_on
     > +   0.46%            fb.sh  [kernel.kallsyms]    [k] shrink_slab_memcg
    
    [ktkhai@virtuozzo.com: v9]
      Link: http://lkml.kernel.org/r/153112561772.4097.11011071937553113003.stgit@localhost.localdomain
    Link: http://lkml.kernel.org/r/153063070859.1818.11870882950920963480.stgit@localhost.localdomainSigned-off-by: default avatarKirill Tkhai <ktkhai@virtuozzo.com>
    Acked-by: default avatarVladimir Davydov <vdavydov.dev@gmail.com>
    Tested-by: default avatarShakeel Butt <shakeelb@google.com>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
    Cc: Chris Wilson <chris@chris-wilson.co.uk>
    Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Cc: Guenter Roeck <linux@roeck-us.net>
    Cc: "Huang, Ying" <ying.huang@intel.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Josef Bacik <jbacik@fb.com>
    Cc: Li RongQing <lirongqing@baidu.com>
    Cc: Matthew Wilcox <willy@infradead.org>
    Cc: Matthias Kaehlcke <mka@chromium.org>
    Cc: Mel Gorman <mgorman@techsingularity.net>
    Cc: Michal Hocko <mhocko@kernel.org>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Philippe Ombredanne <pombredanne@nexb.com>
    Cc: Roman Gushchin <guro@fb.com>
    Cc: Sahitya Tummala <stummala@codeaurora.org>
    Cc: Stephen Rothwell <sfr@canb.auug.org.au>
    Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Waiman Long <longman@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    f90280d6
vmscan.c 120 KB