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

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Date:	Tue, 11 Sep 2012 11:22:10 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Michal Hocko <mhocko@...e.cz>,
	Li Zefan <lizf@...fujitsu.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>
Subject: Re: [PATCH RFC cgroup/for-3.7] cgroup: mark subsystems with broken
 hierarchy support and whine if cgroups are nested for them

Hello, Vivek.

On Tue, Sep 11, 2012 at 02:16:00PM -0400, Vivek Goyal wrote:
> Ok, so whole point of warning seems to be so that we can change the
> behavior in future and say to user space they few kernel releases back we
> had started printing a warning that creating hierarchy is wrong and 
> move to a flat setup. So don't complain to us now.?

Yes, pretty much.  At the moment, it's simply broken.

> Are you planning to get rid of .user_hierarchy file from memory cgroup
> too? If you are planning not to put such a file in blkio controller,
> then it will make sense to remove it from mem_cgorup too.

Yes, or at least make it RO 1 eventually.

> The point I am trying to make is that deep hierarchies (5-6 levels) are
> /going to be a reality and if accounting overhead is not manageable then
> enabling hierarchy by default might not be a practical solution even
> if you implement hierarchy support (like memory cgroup), and in that
> case retaining .use_hierarchy will make sense.

That doesn't make any sense to me.  If you don't want feature and
overhead of hierarchy, you just need to not create a hierarchy.  If
hierarchical behavior isn't needed, why create hierarchy at all?

> IIUC, are you saying that now none of the controller will have flat
> hiearchy support because there is no way to be able to create flat 
> hierarchy. (Any new group is child of root group). So are we moving
> towards a model where every controller is hierarhical and there is
> no concept of flat hierarchy.

Yeap.

Thanks.

-- 
tejun
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