linux-kernel - Re: [PATCH 5/6] memcg: optimize memcg_get_hierarchical

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Date:	Sat, 23 Jun 2012 11:39:34 +0200
From:	Johannes Weiner <hannes@...xchg.org>
To:	Wanpeng Li <liwp.linux@...il.com>
Cc:	linux-mm@...ck.org, Michal Hocko <mhocko@...e.cz>,
	Balbir Singh <bsingharora@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, Minchan Kim <minchan@...nel.org>,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	Gavin Shan <shangw@...ux.vnet.ibm.com>
Subject: Re: [PATCH 5/6] memcg: optimize memcg_get_hierarchical_limit

On Sat, Jun 23, 2012 at 02:18:17PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <liwp@...ux.vnet.ibm.com>
> 
> Optimize memcg_get_hierarchical_limit to save cpu cycle.
> 
> Signed-off-by: Wanpeng Li <liwp.linux@...il.com>

I really would have thought the compiler would detect it, but this
patch actually does switch around move and jump.

But this is miniscule and anything but a fastpath...

> ---
>  mm/memcontrol.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c821e36..1ca79e2 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3917,9 +3917,9 @@ static void memcg_get_hierarchical_limit(struct mem_cgroup *memcg,
>  
>  	min_limit = res_counter_read_u64(&memcg->res, RES_LIMIT);
>  	min_memsw_limit = res_counter_read_u64(&memcg->memsw, RES_LIMIT);
> -	cgroup = memcg->css.cgroup;
>  	if (!memcg->use_hierarchy)
>  		goto out;
> +	cgroup = memcg->css.cgroup;
>  
>  	while (cgroup->parent) {
>  		cgroup = cgroup->parent;
> -- 
> 1.7.9.5
> 
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