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 10:54:24 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
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

On Tue, Sep 11, 2012 at 10:51:06AM -0400, Vivek Goyal wrote:
> On Mon, Sep 10, 2012 at 03:31:25PM -0700, Tejun Heo wrote:
> > Currently, cgroup hierarchy support is a mess.  cpu related subsystems
> > behave correctly - configuration, accounting and control on a parent
> > properly cover its children.  blkio and freezer completely ignore
> > hierarchy and treat all cgroups as if they're directly under the root
> > cgroup.  Others show yet different behaviors.
> > 
> > These differing interpretations of cgroup hierarchy make using cgroup
> > confusing and it impossible to co-mount controllers into the same
> > hierarchy and obtain sane behavior.
> > 
> > Eventually, we want full hierarchy support from all subsystems and
> > probably a unified hierarchy.  Users using separate hierarchies
> > expecting completely different behaviors depending on the mounted
> > subsystem is deterimental to making any progress on this front.
> > 
> > This patch adds cgroup_subsys.broken_hierarchy and sets it to %true
> > for controllers which are lacking in hierarchy support.  The goal of
> > this patch is two-fold.
> > 
> > * Move users away from using hierarchy on currently non-hierarchical
> >   subsystems, so that implementing proper hierarchy support on those
> >   doesn't surprise them.
> 
> I know two current/potential users. systemd and libvirt. They are
> anyway going to create hierarchy irrespective of the fact whether
> controller supports it or not.

Just to be little precise here. AFAIK, systemd only mounts blkio
controller on a separate hiearchy but does not create child cgroups
under it. So all the services should still run in root cgroup.

For libvirt, I think by default they had enabled using blkio
controller and they were creating hiearchies under that for each
virtual machine/container.

Thanks
Vivek
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