[paul at linuxaudiosystems.com: Re: [Jack-Devel] impending issues with RT_GROUP_SCHED kernel option]

[Adrian Knoth]

----- Forwarded message from Paul Davis <paul at linuxaudiosystems.com> -----

Date: Mon, 13 Dec 2010 20:37:56 -0500
From: Paul Davis <paul at linuxaudiosystems.com>
To: Robin Gareus <robin at gareus.org>
Cc: JACK <jack-devel at lists.jackaudio.org>
Subject: Re: [Jack-Devel] impending issues with RT_GROUP_SCHED kernel option

On Mon, Dec 13, 2010 at 7:05 PM, Robin Gareus <robin at gareus.org> wrote:
> On 12/13/2010 11:37 PM, Paul Davis wrote:
>> I've written up the issues with RT_GROUP_SCHED and JACK here:
>>
>>   http://jackaudio.org/linux_group_sched
>>
>> Somehow, a much wider discussion needs to occur with various people
>> about this issue, which is already present in Ubuntu 10.10 and will
>> likely start showing up in other distributions as well.
>
> AFAICT this is only a problem with linux >= 2.6.35.
>
> RT_GROUP_SCHED=y on linux-2.6.33-rt29 (latest RT kernel patch) does not
> cause the issue described at http://jackaudio.org/linux_group_sched
> jackd starts and runs just fine and is very stable.

its not a function of the kernel option itself, i think.

apparently the root cgroup is properly RT enabled. but something
(upstart?) is creating new cgroups for users (this is NOT the
much-heralded per-TTY cgroup patch) and *those* cgroups are not
RT-enabled. i don't know yet (because i don't have access to 10.10)
whether these are per-user or just for "any login" or something else.

so you can run a kernel with RT_GROUP_SCHED and if your tasks remain
in the root cgroup, it will work. otherwise it requires that the child
cgroups are correctly configured.
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