[CRIU] [PATCH 1/2] Re-create cgroups if necessary

[Tycho Andersen]

Hi Pavel,

On Tue, Jun 24, 2014 at 11:03:14AM +0400, Pavel Emelyanov wrote:
> On 06/23/2014 07:54 PM, Tycho Andersen wrote:
> > If a task had cgroups, these should be restored. (Also any properties of
> > cgroups, but this commit doesn't do that yet.)
> > 
> > Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>
> > ---
> >  cgroup.c       |  6 +++++-
> >  include/util.h |  5 +++++
> >  util.c         | 26 ++++++++++++++++++++++++++
> >  3 files changed, 36 insertions(+), 1 deletion(-)
> > 
> > diff --git a/cgroup.c b/cgroup.c
> > index 2d9ebad..8b2deb2 100644
> > --- a/cgroup.c
> > +++ b/cgroup.c
> > @@ -258,9 +258,13 @@ static int move_in_cgroup(CgSetEntry *se)
> >  		int fd, err;
> >  		ControllerEntry *ce = se->ctls[i];
> >  
> > +		sprintf(aux, "%s/%s", ce->name, ce->path);
> > +		if (mkdirpat(cg, aux) < 0)
> 
> Tycho, there's one thing about this mkdir-s that we've discussed with Saied (in Cc).
> The problem is that in cgroup FS there may be directories corresponding to cgroups
> in which there are no tasks, but that would appear some time soon.
> 
> When we dump such a container we don't dump the information about such empty cgroups.
> And on restore, if we create the directories by criu, we will _not_ create empty
> cgroups and further 'echo $pid > cgroup/tasks' would fail.
> 
> Thus we should chose one of two ways:
> 
> 1. We do not handle cgroup hierarchy at all in criu, asking caller to do this.
>    What we do it just populate cgroups with tasks. This is how criu works now.
> 
> 2. We state, that we take full care of cgroup hierarchy on dump and restore. This
>    is were we should go, but in that case we should also dump and restore those
>    empty cgroups I've mentioned.
> 
> And, once we start to go the route #2 we should also take care of the cgroups
> configuration.
> 
> The fix you're proposing is how we (probably) should start doing the way #2, so
> let's decide how can we handle empty cgroups (and configuration) :)

Understood. For dumping empty cgroups, is it just that the current
code only looks at the task and finds its cgroups, and we need to add
an additional step that walks /sys/fs/cgroup (or wherever)? A quick
glance through the code didn't reveal any "skip empty cgroups" type
steps.

Tycho