[lxc-devel] lxc0.8 and hierarchies

[Serge Hallyn]

Quoting William Dauchy (wdauchy at gmail.com):
> Hello,
> 
> I tested lxc0.8 rc1 and saw that cgroups are now created in
> /cgroup/lxc/, so lxc-create will create the cgroups in this directory
> as a cgroups hierarchy.
> It makes the thing unusable when using cgroups capabilities that does
> not support hierarchies. I'm thinking about CONFIG_NETPRIO_CGROUP in
> the last 3.3 kernel which only support cgroups created in /cgroup
> directory.
> Is it a known issue? or is it planned to configure the directory?

The issue of what to do with control groups which do not support
hierarchies has been discussed on lkml recently.  I thought (though
maybe I'm wrong) the decision was that such a subsystem would have
its cgroups available at leaf nodes, i.e. it's /xyz cgroup, if
composed with a devices cgroup which has /abc/xyz, would be
visible at /abc/xyz.  IIRC one of the primary drivers of the need
for this was systemd support.

Putting lxc cgroups under lxc/ is the right thing to do to cooperate
with other programs using cgroups, like libvirt.  I don't think we
should punt on that.  Rather, I personally think it's reasonable to say
that if you are using a cgroup which has max depth 1, you should mount
it separately.  If you then want to use it with lxc, perhaps we should,
for now have a hack which specifis which cgroups do not support
hierarchies, and handle them specially?

-serge