[Pkg-libvirt-maintainers] Bug#707201: Bug#707201: update cgroup layout

[Lorenzo M. Catucci]

On 30/10/2013 20:39, Guido Günther wrote:
> On Wed, Oct 30, 2013 at 05:41:08PM +0100, Lorenzo M. Catucci wrote:
>> Hi all;
>> 	after getting the same cgroup errors after a test-upgrade of libvirt,
>> I stumbled upon the wishlist report from Daniel Baumann archived at
>>
>> http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=601757
> 
> This looks a bit like what we're adding to libvirt-bin atm:
> 
>     http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=725261
>>
>> and tried the mountcgroupsubsys.sh script proposed by Marcus Osdoba as
>> archived at
>>
>> http://bugs.debian.org/cgi-bin/bugreport.cgi?msg=45;filename=mountcgroupsubsys.sh;att=1;bug=601757
>>
>> At least on my system, this restored libvirt/qemu functionality
>> without doing any harm to lxc guests.
> 
> The problem here is different though, it's not the mounting per se
> that's the problem - it's the newer libvirt that requires a different
> cgroup layout. So if somebody wants to help fixing this (I haven't had
> time to even reproduce it) one would need to check what we need to do in
> the preinst/postinst to make the upgrade work automatically. If this
> can't be done we need to abort the upgrade and ask the user to shutdown
> all vms.
> Cheers,
>  -- Guido
> 

In my (very limited) experience, the upgrade can work automatically if
and only if the system was not using a comounted cgroup hierarchy;
otherwise, a reboot is needed before trying to restart the VMs, and
the VM shutdown before upgrade should be mandatory.

To reproduce the problem, it's enough if you run a VM on a system
whose fstab contains the entry:

cgroup          /sys/fs/cgroup         cgroup  defaults   0  0

in this case libvirt-bin will not start any VM after the upgrade UNTIL
THE SYSTEM IS REBOOTED without such fstab line in effect, since

  "When a cgroup filesystem is unmounted, if there are any
   child cgroups created below the top-level cgroup, that hierarchy
   will remain active even though unmounted; if there are no
   child cgroups then the hierarchy will be deactivated."

https://www.kernel.org/doc/Documentation/cgroups/cgroups.txt

Thank you very much, yours

	lorenzo m catucci