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    Subject[PATCH -mm 0/8] memcg/slab: reintroduce dead cache self-destruction

    When a memcg is turned offline, some of its kmem caches can still have
    active objects and therefore cannot be destroyed immediately. Currently,
    we simply leak such caches along with the owner memcg, which is bad and
    should be resolved.

    It would be perfect if we could move all slab pages of such dead caches
    to the root/parent cache on memcg offline. However, when I tried to
    implement such re-parenting, I was pointed out by Christoph that the
    overhead of this would be unacceptable, at least for SLUB (see

    The problem with re-parenting of individual slabs is that it requires
    tracking of all slabs allocated to a cache, but SLUB doesn't track full
    slabs if !debug. Changing this behavior would result in significant
    performance degradation of regular alloc/free paths, because it would
    make alloc/free take per node list locks more often.

    After pondering about this problem for some time, I think we should
    return to dead caches self-destruction, i.e. scheduling cache
    destruction work when the last slab page is freed.

    This is the behavior we had before commit 5bd93da9917f ("memcg, slab:
    simplify synchronization scheme"). The reason why it was removed was that
    it simply didn't work, because SL[AU]B are implemented in such a way
    that they don't discard empty slabs immediately, but prefer keeping them
    cached for indefinite time to speed up further allocations.

    However, we can change this w/o noticeable performance impact for both
    SLAB and SLUB. For SLAB, we can make internal cache reaper shrink dead
    caches aggressively so that they will die quickly after the last object
    is freed. For SLUB, we can make free path drop free slabs as soon as
    they become empty. Since dead caches should never be allocated from,
    removing empty slabs from them shouldn't degrade performance.

    So, this patch set reintroduces dead cache self-destruction and adds
    some tweaks to SL[AU]B to prevent dead caches from hanging around
    indefinitely. It is organized as follows:

    - patches 1-3 reintroduce dead memcg cache self-destruction;
    - patches 4-5 do some cleanup in kmem_cache_shrink;
    - patch 6 is a minor optimization for SLUB, which makes the following
    work a bit easier for me;
    - patches 7 and 8 solves the problem with dead memcg caches for SLUB
    and SLAB respectively.

    Even if the whole approach is NAK'ed, patches 4, 5, and 6 are worth
    applying, IMO, provided Christoph doesn't mind, of course. They don't
    depend on the rest of the set, BTW.

    Note, this doesn't resolve the problem of memcgs pinned by dead kmem
    caches. I'm planning to solve this by re-parenting dead kmem caches to
    the parent memcg.


    Vladimir Davydov (8):
    memcg: cleanup memcg_cache_params refcnt usage
    memcg: destroy kmem caches when last slab is freed
    memcg: mark caches that belong to offline memcgs as dead
    slub: never fail kmem_cache_shrink
    slab: remove kmem_cache_shrink retval
    slub: do not use cmpxchg for adding cpu partials when irqs disabled
    slub: make dead caches discard free slabs immediately
    slab: reap dead memcg caches aggressively

    include/linux/slab.h | 10 ++-
    mm/memcontrol.c | 25 +++++-
    mm/slab.c | 28 +++++--
    mm/slab.h | 12 ++-
    mm/slab_common.c | 8 +-
    mm/slob.c | 3 +-
    mm/slub.c | 212 ++++++++++++++++++++++++++++++++++----------------
    7 files changed, 207 insertions(+), 91 deletions(-)


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