linux-kernel - Re: [PATCH 2.6.32] memcg: warn on deeper hierarchies with use

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Date:	Wed, 19 Sep 2012 12:38:18 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Michal Hocko <mhocko@...e.cz>
cc:	Tejun Heo <tj@...nel.org>, containers@...ts.linux-foundation.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	Li Zefan <lizefan@...wei.com>,
	Glauber Costa <glommer@...allels.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Turner <pjt@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Graf <tgraf@...g.ch>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Neil Horman <nhorman@...driver.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	Dave Jones <davej@...hat.com>,
	Ben Hutchings <ben@...adent.org.uk>
Subject: Re: [PATCH 2.6.32] memcg: warn on deeper hierarchies with
 use_hierarchy==0

On Wed, 19 Sep 2012, Michal Hocko wrote:

> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f99f599..b61c34b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3106,6 +3106,11 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
>  	} else {
>  		parent = mem_cgroup_from_cont(cont->parent);
>  		mem->use_hierarchy = parent->use_hierarchy;
> +		WARN_ONCE(!mem->use_hierarchy && parent != root_mem_cgroup,
> +				"Creating hierarchies with use_hierarchy==0 "
> +				"(flat hierarchy) is considered deprecated. "
> +				"If you believe that your setup is correct, "
> +				"we kindly ask you to contact linux-mm@...ck.org and let us know");

When I deprecated /proc/pid/oom_adj (now removed), we did a WARN_ONCE() 
and others complained that this unnecessarily alters userspace scripts 
that a serious issue has occurred and Linus agreed that we shouldn't do 
deprecation in this way.  The alternative is to use printk_once() instead.

This applies to all three patches for this one, 3.0, and 3.2+.
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