[lxc-devel] cgroup V2 and LXC

[Christian Brauner]

On Wed, Feb 10, 2016 at 05:45:48PM +0000, Serge Hallyn wrote:
> Quoting Christian Brauner (christian.brauner at mailbox.org):
> > On Mon, Feb 01, 2016 at 04:56:08AM +0000, Serge Hallyn wrote:
> > > Quoting Kevin Wilson (wkevils at gmail.com):
> > > > Hi, LXC developers,
> > > > 
> > > > The latest kernel release (4.4) includes initial support to cgroup v2
> > > > with 2 controllers (memory and io). Also it seems that the PIDs
> > > > controller works in cgroup v2, but I do not know if it is officially
> > > > supported in v2.
> > > > 
> > > > Is there any intention to replace the existing cgroup v1 usage in LXC
> > > > by cgroup v2 ? or at least to enable working with both of them ?
> > > > 
> > > > Regards,
> > > > Kevin
> > > 
> > > Replace, no, support, yes.  I've added support for it to cgmanager, and have
> > > used lxc with the unified hierarchy through cgmanager.  Without cgmanager
> > > it will currently definately not work.  It's worth discussing how we should
> > > handle it - and how init wants us to handle it.   With cgmanager I actually
> > > built in the support so that you could treat it as a legacy hierarchy, and
> > > upstart was happy with that since it used cgmanager.  Systemd will not be
> > > happy with that, and it will be a problem.  The only exception to the "no
> > > tasks in a non-leaf node" rule is for the / cgroup.  So lxc would need to
> > > place init in say /lxc/c1/.leaf, and systemd would have to accept that
> > > /lxc/c1 is the container's cgroup.  A few possibilities:
> > > 
> > > 1. maybe if we place systemd in /lxc/c1/init.scope it will be happy
> > Well, here is how I thought it could go (sticking to systemd specifics here):
> >         - create a slice for all lxc "lxc.slice" (similar to "machine.slice" of
> >           systemd-nspawn backed containers)
> >         - "lxc.slice" contains a scope for each container (e.g. "c1.scope"
> >         - "c1.scope" contains an "init.scope"
> >         - "init.scope" only contains the PID of "/sbin/init" as seen from the
> >           host (obviously)
> 
> So if we are creating container c1, are you talking about
> 
> /lxc/c1/lxc.slice/c1.scope/init.scope
> 
> or are you talking about a host-global
> 
> /lxc.slice
Yes, you have lxc.slice then you have all your machines under this. This is what
systemd-nspawn does if I'm not mistaken.
> 
> with container-specific
> 
> /lxc.slice/c1.scope
> 
> per container?
> 
> ?
Yes.
> 
> >         - All other processes are put in another slice "c1-something.slice"
> 
> Which other processes?
Well, all processes, systemd starts are either put in system.slice or
user.slice. All other things we start in the container (let it be e.g. vim) is
put in a session.slice (e.g. session-0.slice, session-1000.slice).
> 
> AFAIK all other processes will be created by systemd.  The q is what will it
> do.  If we put systemd in /lxc.slice/c1.scope/init.scope, will it take that
> as its cgroup root and try to create and move itself into
> /lxc.slice/c1.scope/init.scope ?  If so it will fail since it cannot create a
> cgroup while it is in it.
I don't think so but I need to test that again. Time to boot unified.

> 
> So I think I've convinced myself that we need to collaborate with systemd
> on this.  Perhaps we can agree with it on a default cgroup in which it should
> be started to tell it "this is the leaf cgroup for your init".  So if it sees
> it is in /a/b/c/.cg_leaf, then it will know that /a/b/c is its root.
I thought the same that's why I started to read some of the code.
fwiw, systemd-nspawn already works with the unified cgroup hierarchy and I think
nesting works as well. But I'm not completely sure how nspawn handles nesting.

> 
> >         If we do not want to create scopes we are left with the option of
> >         forcing "init" in a separate cgroup from the rest of the containers
> >         processes.
> > 
> > Christian
> > 
> > 
> > > 2. maybe we can teach systemd to accept being in a leaf node
> > > 3. maybe we can build an exception into cgroup namespaces such that
> > > a cgns root also is an exception to the no-tasks-in-non-leaf-nodes
> > > rule.  But I doubt that will fly.
> > > _______________________________________________
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