linux-kernel - Re: [v10 2/6] mm: implement mem_cgroup_scan

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Date:   Wed, 4 Oct 2017 13:10:44 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Roman Gushchin <guro@...com>
cc:     linux-mm@...ck.org, Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, kernel-team@...com,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v10 2/6] mm: implement mem_cgroup_scan_tasks() for the root
 memory cgroup

On Wed, 4 Oct 2017, Roman Gushchin wrote:

> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index d5f3a62887cf..b4de17a78dc1 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -917,7 +917,8 @@ static void invalidate_reclaim_iterators(struct mem_cgroup *dead_memcg)
>   * value, the function breaks the iteration loop and returns the value.
>   * Otherwise, it will iterate over all tasks and return 0.
>   *
> - * This function must not be called for the root memory cgroup.
> + * If memcg is the root memory cgroup, this function will iterate only
> + * over tasks belonging directly to the root memory cgroup.
>   */
>  int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
>  			  int (*fn)(struct task_struct *, void *), void *arg)
> @@ -925,8 +926,6 @@ int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
>  	struct mem_cgroup *iter;
>  	int ret = 0;
>  
> -	BUG_ON(memcg == root_mem_cgroup);
> -
>  	for_each_mem_cgroup_tree(iter, memcg) {
>  		struct css_task_iter it;
>  		struct task_struct *task;
> @@ -935,7 +934,7 @@ int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
>  		while (!ret && (task = css_task_iter_next(&it)))
>  			ret = fn(task, arg);
>  		css_task_iter_end(&it);
> -		if (ret) {
> +		if (ret || memcg == root_mem_cgroup) {
>  			mem_cgroup_iter_break(memcg, iter);
>  			break;
>  		}

I think this is fine, but a little strange to start an iteration that 
never loops :)  No objection to the patch but it could also be extracted 
into a new mem_cgroup_scan_tasks() which actually scans the tasks in that 
mem cgroup and then a wrapper that does the iteration that calls into it, 
say, mem_cgroup_scan_tasks_tree().