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Date: Wed, 2 May 2012 12:40:37 -0300
From: Glauber Costa <glommer@parallels.com>
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To: Suleiman Souhlal <suleiman@google.com>
CC: <cgroups@vger.kernel.org>, <linux-mm@kvack.org>,
        <linux-kernel@vger.kernel.org>, <devel@openvz.org>,
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        Greg Thelen <gthelen@google.com>, Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@cs.helsinki.fi>
Subject: Re: [PATCH 19/23] slab: per-memcg accounting of slab caches
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>> @@ -3834,11 +3866,15 @@ static inline void __cache_free(struct kmem_cache *cachep, void *objp,
>>   */
>>   void *kmem_cache_alloc(struct kmem_cache *cachep, gfp_t flags)
>>   {
>> -       void *ret = __cache_alloc(cachep, flags, __builtin_return_address(0));
>> +       void *ret;
>> +
>> +       rcu_read_lock();
>> +       cachep = mem_cgroup_get_kmem_cache(cachep, flags);
>> +       rcu_read_unlock();
>
> Don't we need to check in_interrupt(), current, __GFP_NOFAIL every
> time we call mem_cgroup_cgroup_get_kmem_cache()?
>
> I would personally prefer if those checks were put inside
> mem_cgroup_get_kmem_cache() instead of having to check for every
> caller.
>

in_interrupt() yes, __GFP_NOFAIL I don't think so.

__GFP_NOFAIL should lead to a res_counter_charge_nofail() in the end. 
The name similarity is no coincidence...

 From a code style PoV, it makes sense to bundle an in_interrupt() check 
here, but from a performance PoV, putting it in the callers can help us 
avoid the price of a function call.

But well, looking at the code, I see it is not there as well... =(

I plan to change memcontrol.h to look like this:

static __always_inline struct kmem_cache *
mem_cgroup_get_kmem_cache(struct kmem_cache *cachep, gfp_t gfp)
{
         if (mem_cgroup_kmem_on && current->mm && !in_interrupt())
                 return __mem_cgroup_get_kmem_cache(cachep, gfp);
         return cachep;
}