Re: [PATCH] memcg: first step towards hierarchical controller

[Michal Hocko <mhocko@suse.cz>]

On Tue 26-06-12 17:30:28, Glauber Costa wrote:
> Okay, so after recent discussions, I am proposing the following
> patch. It won't remove hierarchy, or anything like that. Just default
> to true in the root cgroup, and print a warning once if you try
> to set it back to 0.
> 
> I am not adding it to feature-removal-schedule.txt because I don't
> view it as a consensus. Rather, changing the default would allow us
> to give it a time around in the open, and see if people complain
> and what we can learn about that.
> 
> Signed-off-by: Glauber Costa <glommer@parallels.com>
> CC: Michal Hocko <mhocko@suse.cz>
> CC: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> CC: Johannes Weiner <hannes@cmpxchg.org>
> CC: Tejun Heo <tj@kernel.org>
> ---
>  mm/memcontrol.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9e710bc..037ddd4 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3949,6 +3949,8 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
>  	if (memcg->use_hierarchy == val)
>  		goto out;
>  
> +	WARN_ONCE((!parent_memcg && memcg->use_hierarchy && val == false),
> +		"Non-hierarchical memcg is considered for deprecation");
>  	/*
>  	 * If parent's use_hierarchy is set, we can't make any modifications
>  	 * in the child subtrees. If it is unset, then the change can
> @@ -5175,6 +5177,7 @@ mem_cgroup_create(struct cgroup *cont)
>  			INIT_WORK(&stock->work, drain_local_stock);
>  		}
>  		hotcpu_notifier(memcg_cpu_hotplug_callback, 0);
> +		memcg->use_hierarchy = true;

So the only way to disable hierarchies is to do it on the root first
(before any children exist) and then start creating your groups?

I think it will be much safer if we could enable it to the first floor
under the root - I know hackish - but I guess that most users don't set
anything in the root cgroup (most of the time it's EINVAL anyway) and
only set up groups they are creating.

Anyway, I guess we can give this approach a try.

>  	} else {
>  		parent = mem_cgroup_from_cont(cont->parent);
>  		memcg->use_hierarchy = parent->use_hierarchy;

Thanks
-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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