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    SubjectRe: [PATCH v3 09/16] sl[au]b: always get the cache from its page in kfree
    On 09/21/2012 01:33 PM, Pekka Enberg wrote:
    > On Wed, Sep 19, 2012 at 10:42 AM, Glauber Costa <glommer@parallels.com> wrote:
    >>>> index f2d760c..18de3f6 100644
    >>>> --- a/mm/slab.c
    >>>> +++ b/mm/slab.c
    >>>> @@ -3938,9 +3938,12 @@ EXPORT_SYMBOL(__kmalloc);
    >>>> * Free an object which was previously allocated from this
    >>>> * cache.
    >>>> */
    >>>> -void kmem_cache_free(struct kmem_cache *cachep, void *objp)
    >>>> +void kmem_cache_free(struct kmem_cache *s, void *objp)
    >>>> {
    >>>> unsigned long flags;
    >>>> + struct kmem_cache *cachep = virt_to_cache(objp);
    >>>> +
    >>>> + VM_BUG_ON(!slab_equal_or_parent(cachep, s));
    >>>
    >>> This is an extremely hot path of the kernel and you are adding significant
    >>> processing. Check how the benchmarks are influenced by this change.
    >>> virt_to_cache can be a bit expensive.
    >>
    >> Would it be enough for you to have a separate code path for
    >> !CONFIG_MEMCG_KMEM?
    >>
    >> I don't really see another way to do it, aside from deriving the cache
    >> from the object in our case. I am open to suggestions if you do.
    >
    > We should assume that most distributions enable CONFIG_MEMCG_KMEM,
    > right? Therfore, any performance impact should be dependent on whether
    > or not kmem memcg is *enabled* at runtime or not.
    >
    > Can we use the "static key" thingy introduced by tracing folks for this?
    >
    Yes.

    I am already using static keys extensively in this patchset, and that is
    how I intend to handle this particular case.




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