[CRIU] Restore into different cgroups

[Pavel Emelyanov]

On 08/14/2014 12:53 AM, Tycho Andersen wrote:
> On Wed, Aug 13, 2014 at 11:59:30PM +0400, Pavel Emelyanov wrote:
>> On 08/13/2014 06:03 PM, Tycho Andersen wrote:
>>> 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?
>>
>> I would greatly appreciate if you do on top of what's now on the
>> mailing list.
> 
> Ok, I will work on a patch.

Thank you!

>>> 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).
>>
>> I think we can do it like
>>
>>     --cgroup-root /new-root --cgroup-root name=systemd:/$orig_root
>>
>> The first option would set the root for all, the 2nd would then
>> override it for name=systemd one. $orig_root should be the proper
>> path, but maybe for convenience we can reserve some keyword, like
>> ORIG.
>>
>> Does this make sense?
> 
> Yes, it does. (I assume that if it is unspecified it stays the same,
> i.e. if there is only --cgroup-root name=systemd:/new-root, that
> everything else has the same root.)

Yup :)

> Tycho
> .
> 

Thanks,
Pavel