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Subject: Re: [PATCH v3 09/16] sl[au]b: always get the cache from its page in kfree
From: Pekka Enberg <penberg@kernel.org>
To: Glauber Costa <glommer@parallels.com>
Cc: Christoph Lameter <cl@linux.com>, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, kamezawa.hiroyu@jp.fujitsu.com,
        devel@openvz.org, Tejun Heo <tj@kernel.org>, linux-mm@kvack.org,
        Suleiman Souhlal <suleiman@google.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, Mel Gorman <mgorman@suse.de>,
        David Rientjes <rientjes@google.com>, Ingo Molnar <mingo@elte.hu>,
        Steven Rostedt <rostedt@goodmis.org>
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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?