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

From: Tejun Heo <tj@kernel.org>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: linux-kernel@vger.kernel.org, Michal Hocko <mhocko@suse.cz>,
	Li Zefan <lizf@cn.fujitsu.com>,
	Glauber Costa <glommer@parallels.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Paul Turner <pjt@google.com>,
	Johannes Weiner <hannes@cmpxchg.org>, Thomas Graf <tgraf@suug.ch>,
	"Serge E. Hallyn" <serue@us.ibm.com>,
	Paul Mackerras <paulus@samba.org>, Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
	Neil Horman <nhorman@tuxdriver.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.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
Date: Tue, 11 Sep 2012 11:22:10 -0700	[thread overview]
Message-ID: <20120911182210.GQ7677@google.com> (raw)
In-Reply-To: <20120911181600.GK12039@redhat.com>

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