[CRIU] Restore into different cgroups

[Tycho Andersen]

Hi Pavel, Serge,

On Tue, Aug 12, 2014 at 05:11:29PM +0000, Serge Hallyn wrote:
> Quoting Pavel Emelyanov (xemul at parallels.com):
> > On 08/12/2014 07:13 PM, Tycho Andersen wrote:
> > 
> > >> Doing regexp substitution in criu code would be confusing... I was 
> > >> thinking about treating all the paths as being relative to the init 
> > >> task's cgset and keep this task's full paths in the image. Then
> > >> introduce the --cgroup-root option that would override the init task's
> > >> path from the image.
> > >>
> > >> Would that work for you?
> > > 
> > > I don't understand exactly how that would work. It would just add a
> > > prefix, so --cgroup-root=/foo would rewrite /lxc/u1 to /foo/lxc/u1?
> > > What about other cgroups unrelated to the container? Can we avoid
> > > rewriting those?
> > 
> > OK, let me explain in more details what I mean.
> > 
> > Let's imagine we have init task living in /lxc/ct, some other task
> > in /lxc/ct/sub and there's also an empty subgroup /lxc/ct/empty.
> > 
> > When we dump this, in the image we'd get:
> > 
> > * root task lives in /lxc/ct
> > * "some other task" lives in /sub
> > * there's a subgroup called /sub
> > * there's a subgroup called /empty
> > 
> > On restore criu would prepend the /lxc/ct (path from root task)
> > to whatever path it meets in the cgroup image.
> > 
> > If we want to restore tasks into /lxc/ct2/lxc/ct, then we'd say on
> > restore --cgroup-root /lxc/ct2/lxc/ct and criu would prepend _this_
> > to all the other paths (except for the root task's, i.e. /lxc/ct
> > from the image would be ignored).
> > 
> > This will be so for every controller. Do we need per-controller
> > control? If can look like
> > 
> > --cgroup-root $path -- prepend $path for every controller
> > --cgroup-root $ctl:$path -- only for controller $ctl
> > 
> > and the option can be specified multiple times.
> 
> Again, sounds good to me.  The code shouldn't be bad, just
> grab /proc/1/cgroup contents at start, and then index into
> each tasks' cgroup (ensuring that it lives under the init
> tasks' cgroup, which i think you're already doing) to get
> the sub-cgroup to checkpoint.

I see, combined with the patch Pavel sent this morning and a good
sleep this makes more sense now. Pavel, are you implementing this or
would you like me to?

The only other question I have is what to do about cgroups where we
don't want to reset the root (e.g. the user.slice systemd groups,
presumably).

Tycho