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

Glauber Costa
2012-11-07T07:04:30Z

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