Re: [patch] mm, memcg: avoid unnecessary function call when memcg is disabled

[Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>]

On Mon, 19 Nov 2012 17:44:34 -0800 (PST)
David Rientjes <rientjes-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:

> While profiling numa/core v16 with cgroup_disable=memory on the command 
> line, I noticed mem_cgroup_count_vm_event() still showed up as high as 
> 0.60% in perftop.
> 
> This occurs because the function is called extremely often even when memcg 
> is disabled.
> 
> To fix this, inline the check for mem_cgroup_disabled() so we avoid the 
> unnecessary function call if memcg is disabled.
> 
> ...
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -181,7 +181,14 @@ unsigned long mem_cgroup_soft_limit_reclaim(struct zone *zone, int order,
>  						gfp_t gfp_mask,
>  						unsigned long *total_scanned);
>  
> -void mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx);
> +void __mem_cgroup_count_vm_event(struct mm_struct *mm, enum vm_event_item idx);
> +static inline void mem_cgroup_count_vm_event(struct mm_struct *mm,
> +					     enum vm_event_item idx)
> +{
> +	if (mem_cgroup_disabled() || !mm)
> +		return;
> +	__mem_cgroup_count_vm_event(mm, idx);
> +}

Does the !mm case occur frequently enough to justify inlining it, or
should that test remain out-of-line?