linux-kernel - Re: [PATCH v6 19/29] memcg: infrastructure to match an allocation to the right cache

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Message-ID: <509A07E3.5090700@parallels.com>
Date:	Wed, 7 Nov 2012 08:04:03 +0100
From:	Glauber Costa <glommer@...allels.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Tejun Heo <tj@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	Christoph Lameter <cl@...ux.com>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Suleiman Souhlal <suleiman@...gle.com>,
	JoonSoo Kim <js1304@...il.com>
Subject: Re: [PATCH v6 19/29] memcg: infrastructure to match an allocation
 to the right cache

On 11/06/2012 01:28 AM, Andrew Morton wrote:
> On Thu,  1 Nov 2012 16:07:35 +0400
> Glauber Costa <glommer@...allels.com> wrote:
> 
>> +static __always_inline struct kmem_cache *
>> +memcg_kmem_get_cache(struct kmem_cache *cachep, gfp_t gfp)
> 
> I still don't understand why this code uses __always_inline so much.
> 
> I don't recall seeing the compiler producing out-of-line versions of
> "static inline" functions (and perhaps it has special treatment for
> functions which were defined in a header file?).
> 
> And if the compiler *does* decide to uninline the function, perhaps it
> knows best, and the function shouldn't have been declared inline in the
> first place.
> 
> 
> If it is indeed better to use __always_inline in this code then we have
> a heck of a lot of other "static inline" definitions whcih we need to
> convert!  So, what's going on here?
> 

The original motivation is indeed performance related. We want to make
sure it is inline so it will figure out quickly the "I am not a memcg
user" case and keep it going. The slub, for instance, is full of
__always_inline functions to make sure that the fast path contains
absolutely no function calls. So I was just following this here.

I can remove the marker without a problem and leave it to the compiler
if you think it is best

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