linux-kernel - Re: [PATCH 3/3] mm: kmem: switch to static_branch_likely() in memcg_kmem

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Date:   Wed, 8 Jul 2020 13:17:09 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        kernel-team@...com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm: kmem: switch to static_branch_likely() in
 memcg_kmem_enabled()

On 7/7/20 7:36 PM, Roman Gushchin wrote:
> Currently memcg_kmem_enabled() is optimized for the kernel memory
> accounting being off. It was so for a long time, and arguably the
> reason behind was that the kernel memory accounting was initially an
> opt-in feature. However, now it's on by default on both cgroup v1
> and cgroup v2, and it's on for all cgroups. So let's switch over
> to static_branch_likely() to reflect this fact.
> 
> Unlikely there is a significant performance difference, as the cost
> of a memory allocation and its accounting significantly exceeds the
> cost of a jump. However, the conversion makes the code look more
> logically.
> 
> Signed-off-by: Roman Gushchin <guro@...com>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  include/linux/memcontrol.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index b8f52a3fed90..ab9322215b2e 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -1456,7 +1456,7 @@ void memcg_put_cache_ids(void);
>  
>  static inline bool memcg_kmem_enabled(void)
>  {
> -	return static_branch_unlikely(&memcg_kmem_enabled_key);
> +	return static_branch_likely(&memcg_kmem_enabled_key);
>  }
>  
>  static inline bool memcg_kmem_bypass(void)
>