linux-kernel - Re: [PATCH] memcg: put soft limit reclaim out of way if the excess tree is empty

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Date:	Mon, 1 Aug 2016 16:57:57 +0300
From:	Vladimir Davydov <vdavydov@...tuozzo.com>
To:	Michal Hocko <mhocko@...nel.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>, <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] memcg: put soft limit reclaim out of way if the excess
 tree is empty

On Mon, Aug 01, 2016 at 12:00:21PM +0200, Michal Hocko wrote:
...
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index c265212bec8c..eb7e39c2d948 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2543,6 +2543,11 @@ static int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
>  	return ret;
>  }
>  
> +static inline bool soft_limit_tree_empty(struct mem_cgroup_tree_per_node *mctz)
> +{
> +	return rb_last(&mctz->rb_root) == NULL;
> +}
> +

I don't think traversing rb tree as rb_last() does w/o holding the lock
is a good idea. Why is RB_EMPTY_ROOT() insufficient here?