On Fri, 07 Mar 2008 18:32:20 +0300
Pavel Emelyanov <xemul@openvz.org> wrote:

> This allows us two things basically:
> 
> 1. If the subgroup has the limit higher than its parent has
>    then the one will get more memory than allowed.
> 2. When we will need to account for a resource in more than
>    one place, we'll be able to use this technics.
> 
>    Look, consider we have a memory limit and swap limit. The
>    memory limit is the limit for the sum of RSS, page cache
>    and swap usage. To account for this gracefuly, we'll set
>    two counters:
> 
> 	   res_counter mem_counter;
> 	   res_counter swap_counter;
> 
>    attach mm to the swap one
> 
> 	   mm->mem_cnt = &swap_counter;
> 
>    and make the swap_counter be mem's child. That's it. If we
>    want hierarchical support, then the tree will look like this:
> 
>    mem_counter_top
>     swap_counter_top <- mm_struct living at top
>      mem_counter_sub
>       swap_counter_sub <- mm_struct living at sub
> 
Hmm? seems strange.

IMO, a parent's usage is just sum of all childs'.
And, historically, memory overcommit is done agaist "memory usage + swap".

How about this ?
    <mem_counter_top, swap_counter_top>
	<mem_counter_sub, swap_counter_sub>
	<mem_counter_sub, swap_counter_sub>
	<mem_counter_sub, swap_counter_sub>

   mem_counter_top.usage == sum of all mem_coutner_sub.usage
   swap_counter_sub.usage = sum of all swap_counter_sub.usage


> @@ -976,19 +976,22 @@ static void free_mem_cgroup_per_zone_info(struct mem_cgroup *mem, int node)
>  static struct cgroup_subsys_state *
>  mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
>  {
> -	struct mem_cgroup *mem;
> +	struct mem_cgroup *mem, *parent;
>  	int node;
>  
>  	if (unlikely((cont->parent) == NULL)) {
>  		mem = &init_mem_cgroup;
>  		init_mm.mem_cgroup = mem;
> -	} else
> +		parent = NULL;
> +	} else {
>  		mem = kzalloc(sizeof(struct mem_cgroup), GFP_KERNEL);
> +		parent = mem_cgroup_from_cont(cont->parent);
> +	}
>  
>  	if (mem == NULL)
>  		return ERR_PTR(-ENOMEM);
>  
> -	res_counter_init(&mem->res);
> +	res_counter_init(&mem->res, parent ? &parent->res : NULL);
>  
I have no objection to add some hierarchical support to res_counter.

But we should wait to add it to mem_cgroup because we have to add 
some amount of codes to handle hierarchy under mem_cgroup in reasonable way.
for example) 
	- hierarchical memory reclaim
	- keeping fairness between sub memory controllers.
	  etc...

Thanks,
-Kame

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