Re: [PATCH REPOST RFC cgroup/for-3.7] cgroup: mark subsystems with broken hierarchy support and whine if cgroups are nested for them

[Michal Hocko <mhocko-AlSwsSmVLrQ@public.gmane.org>]

I like the approach in general but see the comments bellow:

On Mon 10-09-12 15:33:55, Tejun Heo wrote:
[...]
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3855,12 +3855,17 @@ static int mem_cgroup_hierarchy_write(st
>  	 */
>  	if ((!parent_memcg || !parent_memcg->use_hierarchy) &&
>  				(val == 1 || val == 0)) {
> -		if (list_empty(&cont->children))
> +		if (list_empty(&cont->children)) {
>  			memcg->use_hierarchy = val;
> -		else
> +			/* we're fully hierarchical iff root uses hierarchy */
> +			if (mem_cgroup_is_root(memcg))
> +				mem_cgroup_subsys.broken_hierarchy = !val;
> +		} else {
>  			retval = -EBUSY;
> -	} else
> +		}
> +	} else {
>  		retval = -EINVAL;
> +	}
>  
>  out:
>  	cgroup_unlock();
> @@ -4953,6 +4958,7 @@ mem_cgroup_create(struct cgroup *cont)
>  						&per_cpu(memcg_stock, cpu);
>  			INIT_WORK(&stock->work, drain_local_stock);
>  		}
> +		mem_cgroup_subsys.broken_hierarchy = !memcg->use_hierarchy;

Hmmm, this will warn even if we have
root (default use_hierarchy=0)
 \
  A (use_hierarchy=1)
   \
    B <- here

which is unfortunate because it will add a noise to a reasonable
configuration.
I think this is fixable if you move the warning after 
cgroup_subsys_state::create and broken_hierarchy would be set only if
parent is not root and use_hierarchy==0 in mem_cgroup_create. Something
like:

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 795e525..d5c93ab 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -4973,6 +4973,13 @@ mem_cgroup_create(struct cgroup *cont)
 	} else {
 		res_counter_init(&memcg->res, NULL);
 		res_counter_init(&memcg->memsw, NULL);
+		/*
+		 * Deeper hierachy with use_hierarchy == false doesn't make
+		 * much sense so let cgroup subsystem know about this unfortunate
+		 * state in our controller.
+		 */
+		if (parent && parent != root_mem_cgroup)
+			mem_cgroup_subsys.broken_hierarchy = true;
 	}
 	memcg->last_scanned_node = MAX_NUMNODES;
 	INIT_LIST_HEAD(&memcg->oom_notify);

What do you think?

>  		hotcpu_notifier(memcg_cpu_hotplug_callback, 0);
>  	} else {
>  		parent = mem_cgroup_from_cont(cont->parent);
> --- a/net/core/netprio_cgroup.c
> +++ b/net/core/netprio_cgroup.c
> @@ -330,7 +330,17 @@ struct cgroup_subsys net_prio_subsys = {
>  	.subsys_id	= net_prio_subsys_id,
>  #endif
>  	.base_cftypes	= ss_files,
> -	.module		= THIS_MODULE
> +	.module		= THIS_MODULE,
> +
> +	/*
> +	 * net_prio has artificial limit on the number of cgroups and
> +	 * disallows nesting making it impossible to co-mount it with other
> +	 * hierarchical subsystems.  Remove the artificially low PRIOIDX_SZ
> +	 * limit and properly nest configuration such that children follow
> +	 * their parents' configurations by default and are allowed to
> +	 * override and remove the following.
> +	 */
> +	.broken_hierarchy = trye,

typo

-- 
Michal Hocko
SUSE Labs