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    Subject[PATCH 03/16] mm: slub: Optimise the SLUB fast path to avoid pfmemalloc checks
    Date
    From: Christoph Lameter <cl@linux.com>

    This patch removes the check for pfmemalloc from the alloc hotpath and
    puts the logic after the election of a new per cpu slab. For a pfmemalloc
    page we do not use the fast path but force the use of the slow path which
    is also used for the debug case.

    This has the side-effect of weakening pfmemalloc processing in the
    following way;

    1. A process that is allocating for network swap calls __slab_alloc.
    pfmemalloc_match is true so the freelist is loaded and c->freelist is
    now pointing to a pfmemalloc page.

    2. A process that is attempting normal allocations calls slab_alloc,
    finds the pfmemalloc page on the freelist and uses it because it did
    not check pfmemalloc_match()

    The patch allows non-pfmemalloc allocations to use pfmemalloc pages with
    the kmalloc slabs being the most vunerable caches on the grounds they
    are most likely to have a mix of pfmemalloc and !pfmemalloc requests. A
    later patch will still protect the system as processes will get throttled
    if the pfmemalloc reserves get depleted but performance will not degrade
    as smoothly.

    [mgorman@suse.de: Expanded changelog]
    Signed-off-by: Christoph Lameter <cl@linux.com>
    Signed-off-by: Mel Gorman <mgorman@suse.de>
    ---
    mm/slub.c | 7 +++----
    1 file changed, 3 insertions(+), 4 deletions(-)

    diff --git a/mm/slub.c b/mm/slub.c
    index f0909bf..f8cbec4 100644
    --- a/mm/slub.c
    +++ b/mm/slub.c
    @@ -2298,11 +2298,11 @@ new_slab:
    }
    }

    - if (likely(!kmem_cache_debug(s)))
    + if (likely(!kmem_cache_debug(s) && pfmemalloc_match(c, gfpflags)))
    goto load_freelist;

    /* Only entered in the debug case */
    - if (!alloc_debug_processing(s, c->page, object, addr))
    + if (kmem_cache_debug(s) && !alloc_debug_processing(s, c->page, object, addr))
    goto new_slab; /* Slab failed checks. Next slab needed */

    c->freelist = get_freepointer(s, object);
    @@ -2352,8 +2352,7 @@ redo:
    barrier();

    object = c->freelist;
    - if (unlikely(!object || !node_match(c, node) ||
    - !pfmemalloc_match(c, gfpflags)))
    + if (unlikely(!object || !node_match(c, node)))
    object = __slab_alloc(s, gfpflags, node, addr, c);

    else {
    --
    1.7.9.2


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