• Roman Gushchin's avatar
    mm: slowly shrink slabs with a relatively small number of objects · 172b06c3
    Roman Gushchin authored
    9092c71b ("mm: use sc->priority for slab shrink targets") changed the
    way that the target slab pressure is calculated and made it
        delta = freeable >> priority;
        delta *= 4;
        do_div(delta, shrinker->seeks);
    The problem is that on a default priority (which is 12) no pressure is
    applied at all, if the number of potentially reclaimable objects is less
    than 4096 (1<<12).
    This causes the last objects on slab caches of no longer used cgroups to
    (almost) never get reclaimed.  It's obviously a waste of memory.
    It can be especially painful, if these stale objects are holding a
    reference to a dying cgroup.  Slab LRU lists are reparented on memcg
    offlining, but corresponding objects are still holding a reference to the
    dying cgroup.  If we don't scan these objects, the dying cgroup can't go
    away.  Most likely, the parent cgroup hasn't any directly charged objects,
    only remaining objects from dying children cgroups.  So it can easily hold
    a reference to hundreds of dying cgroups.
    If there are no big spikes in memory pressure, and new memory cgroups are
    created and destroyed periodically, this causes the number of dying
    cgroups grow steadily, causing a slow-ish and hard-to-detect memory
    "leak".  It's not a real leak, as the memory can be eventually reclaimed,
    but it could not happen in a real life at all.  I've seen hosts with a
    steadily climbing number of dying cgroups, which doesn't show any signs of
    a decline in months, despite the host is loaded with a production
    It is an obvious waste of memory, and to prevent it, let's apply a minimal
    pressure even on small shrinker lists.  E.g.  if there are freeable
    objects, let's scan at least min(freeable, scan_batch) objects.
    This fix significantly improves a chance of a dying cgroup to be
    reclaimed, and together with some previous patches stops the steady growth
    of the dying cgroups number on some of our hosts.
    Link: http://lkml.kernel.org/r/20180905230759.12236-1-guro@fb.com
    Fixes: 9092c71b
     ("mm: use sc->priority for slab shrink targets")
    Signed-off-by: default avatarRoman Gushchin <guro@fb.com>
    Acked-by: default avatarRik van Riel <riel@surriel.com>
    Cc: Josef Bacik <jbacik@fb.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Shakeel Butt <shakeelb@google.com>
    Cc: Michal Hocko <mhocko@kernel.org>
    Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
vmscan.c 120 KB