RT_GROUP_SCHED kernel option makes JACK unusable in Ubuntu Natty

[Ronan Jouchet]

Hi Ubuntustudio-devel, Ubuntu-kernel, JACK-devel, Alessio,

After talking with adi in #ffado and las in #jack, it appears that 
Natty's kernel needs adjustment to be usable for audio work. Currently 
the kernel flavors shipped by Ubuntu have the RT_GROUP_SCHED option 
activated, which makes JACK unusable (las put up 
http://jackaudio.org/linux_group_sched to sum up the situation).

*Until this problem is solved, JACK is unusable under Natty*. Las 
mentioned several solutions:
   0. Disabling RT_GROUP_SCHED for all kernels
   1. Disabling RT_GROUP_SCHED for the kernel targeting audio users 
(-lowlatency, currently living in abogani's PPA)
   2. Set up a cgroup and invoke JACK and every client from a shell 
script that joins the cgroup first
   3. Use cgrules.conf to at least put processes into the right group by 
default
   4. Use Ubuntu's specific mechanism to put user tasks into a 
RT-enabled cgroup

Quoting las: "(1) is the desirable option; (2) would be next and (3) 
would be hard, because it would enforce 'application <foobar> always run 
in the <bazbar> cgroup' which is not really very useful or correct". (0) 
may also be considered since this kernel option is apparently very 
specialized for embedded systems and doesn't make sense for a desktop 
distro.

Which path should be followed is beyond my knowledge. I just hope this 
discussion will pave the way to a usable audio stack in Natty.

Resources:
   LP bug: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/690010
   IRC : #jack, #ubuntustudio,  #ubuntustudio-devel
   IRC log of my discussion with las: http://pastebin.com/2yw7aPhr

All the best,

-- 
Ronan Jouchet