[Devel] Re: [PATCH v3 09/16] sl[au]b: always get the cache from its page in kfree

[Pekka Enberg]

On Wed, Sep 19, 2012 at 10:42 AM, Glauber Costa <glommer at 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?