linux-kernel - Re: [PATCH 2/2] slab: move kmem_cache

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Message-ID: <50865024.60309@parallels.com>
Date:	Tue, 23 Oct 2012 12:07:00 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	JoonSoo Kim <js1304@...il.com>
CC:	Christoph Lameter <cl@...ux.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, Pekka Enberg <penberg@...nel.org>,
	"David Rientjes" <rientjes@...gle.com>
Subject: Re: [PATCH 2/2] slab: move kmem_cache_free to common code

On 10/23/2012 04:48 AM, JoonSoo Kim wrote:
> Hello, Glauber.
> 
> 2012/10/23 Glauber Costa <glommer@...allels.com>:
>> On 10/22/2012 06:45 PM, Christoph Lameter wrote:
>>> On Mon, 22 Oct 2012, Glauber Costa wrote:
>>>
>>>> + * kmem_cache_free - Deallocate an object
>>>> + * @cachep: The cache the allocation was from.
>>>> + * @objp: The previously allocated object.
>>>> + *
>>>> + * Free an object which was previously allocated from this
>>>> + * cache.
>>>> + */
>>>> +void kmem_cache_free(struct kmem_cache *s, void *x)
>>>> +{
>>>> +    __kmem_cache_free(s, x);
>>>> +    trace_kmem_cache_free(_RET_IP_, x);
>>>> +}
>>>> +EXPORT_SYMBOL(kmem_cache_free);
>>>> +
>>>
>>> This results in an additional indirection if tracing is off. Wonder if
>>> there is a performance impact?
>>>
>> if tracing is on, you mean?
>>
>> Tracing already incurs overhead, not sure how much a function call would
>> add to the tracing overhead.
>>
>> I would not be concerned with this, but I can measure, if you have any
>> specific workload in mind.
> 
> With this patch, kmem_cache_free() invokes __kmem_cache_free(),
> that is, it add one more "call instruction" than before.
> 
> I think that Christoph's comment means above fact.

Ah, this. Ok, I got fooled by his mention to tracing.

I do agree, but since freeing is ultimately dependent on the allocator
layout, I don't see a clean way of doing this without dropping tears of
sorrow around. The calls in slub/slab/slob would have to be somehow
inlined. Hum... maybe it is possible to do it from
include/linux/sl*b_def.h...

Let me give it a try and see what I can come up with.

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