Message-ID: <509A07E3.5090700@parallels.com>
Date: Wed, 7 Nov 2012 08:04:03 +0100
From: Glauber Costa <glommer@parallels.com>
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To: Andrew Morton <akpm@linux-foundation.org>
CC: <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        <kamezawa.hiroyu@jp.fujitsu.com>, Johannes Weiner <hannes@cmpxchg.org>,
        Tejun Heo <tj@kernel.org>, Michal Hocko <mhocko@suse.cz>,
        Christoph Lameter <cl@linux.com>, Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@cs.helsinki.fi>,
        Suleiman Souhlal <suleiman@google.com>, JoonSoo Kim <js1304@gmail.com>
Subject: Re: [PATCH v6 19/29] memcg: infrastructure to match an allocation
 to the right cache
References: <1351771665-11076-1-git-send-email-glommer@parallels.com> <1351771665-11076-20-git-send-email-glommer@parallels.com> <20121105162837.5fdac20c.akpm@linux-foundation.org>
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On 11/06/2012 01:28 AM, Andrew Morton wrote:
> On Thu,  1 Nov 2012 16:07:35 +0400
> Glauber Costa <glommer@parallels.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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