impending issues with RT_GROUP_SCHED kernel option

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impending issues with RT_GROUP_SCHED kernel option

Paul Davis
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.
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Re: impending issues with RT_GROUP_SCHED kernel option

Fons Adriaensen-2
On Mon, Dec 13, 2010 at 05:37:54PM -0500, 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.

Who's pushing this ?

--
FA

There are three of them, and Alleline.

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Re: impending issues with RT_GROUP_SCHED kernel option

Paul Davis
On Mon, Dec 13, 2010 at 6:23 PM,  <[hidden email]> wrote:

> On Mon, Dec 13, 2010 at 05:37:54PM -0500, 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.
>
> Who's pushing this ?

RT_GROUP_SCHED has existed for several years, but is rarely turned on.
I guess some embedded systems probably use it to good effect, but take
complete control over everything that gets executed. AFAIK, Ubuntu is
(so far) the only distribution to have turned it on in a release
kernel.
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Re: impending issues with RT_GROUP_SCHED kernel option

Robin Gareus
In reply to this post by Paul Davis
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.

robin

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Re: impending issues with RT_GROUP_SCHED kernel option

Paul Davis
On Mon, Dec 13, 2010 at 7:05 PM, Robin Gareus <[hidden email]> 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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Re: impending issues with RT_GROUP_SCHED kernel option

David Henningsson-3
In reply to this post by Paul Davis
On 2010-12-13 23:37, 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.

Thanks for bringing this up, I was unaware of the issue. I don't know
much about what advantages this brings, i e why it was turned on in the
first place (will try to figure out during the day).

In the same breath though, I'd like to say that I have had
plans/thoughts about contributing some piece of code to jackdbus (and
possibly ffado as well) that uses rtkit to get real-time prio instead. I
assume that will solve this issue as well, at least for dbus-enabled
versions of jack. What do you folks think of that?


--
David Henningsson, Canonical Ltd.
http://launchpad.net/~diwic
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Re: impending issues with RT_GROUP_SCHED kernel option

Robin Gareus
In reply to this post by Paul Davis
On 12/14/2010 02:37 AM, Paul Davis wrote:
> On Mon, Dec 13, 2010 at 7:05 PM, Robin Gareus <[hidden email]> wrote:
>> On 12/13/2010 11:37 PM, Paul Davis wrote:
[snip]
>> 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

Might be PAM?! pam-cgroup that is. or /etc/init.d/cgconfig

Did someone who can reproduce the problem try:
  sudo dpkg -P cgroup-bin libpam-cgroup

2c,
robin
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Re: impending issues with RT_GROUP_SCHED kernel option

torbenh
On Tue, Dec 14, 2010 at 11:53:24AM +0100, Robin Gareus wrote:

> On 12/14/2010 02:37 AM, Paul Davis wrote:
> > On Mon, Dec 13, 2010 at 7:05 PM, Robin Gareus <[hidden email]> wrote:
> >> On 12/13/2010 11:37 PM, Paul Davis wrote:
> [snip]
> >> 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
>
> Might be PAM?! pam-cgroup that is. or /etc/init.d/cgconfig
>
> Did someone who can reproduce the problem try:
>   sudo dpkg -P cgroup-bin libpam-cgroup

yeah... its probably the cgroup management and pam.
i have a kvm with such a kernel, but i am not really keen on setting
this up properly.
it depends too much on the default config files in ubuntu.
and my kvm is running debian.

dunno... maybe i get motivated at some point.
but maybe the ubuntu guys can find this out for themselves.

>
> 2c,
> robin
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Re: impending issues with RT_GROUP_SCHED kernel option

Lennart Poettering-16
In reply to this post by Paul Davis
On Mon, 13.12.10 18:31, Paul Davis ([hidden email]) wrote:

>
> On Mon, Dec 13, 2010 at 6:23 PM,  <[hidden email]> wrote:
> > On Mon, Dec 13, 2010 at 05:37:54PM -0500, 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.
> >
> > Who's pushing this ?
>
> RT_GROUP_SCHED has existed for several years, but is rarely turned on.
> I guess some embedded systems probably use it to good effect, but take
> complete control over everything that gets executed. AFAIK, Ubuntu is
> (so far) the only distribution to have turned it on in a release
> kernel.

No, it's on on Fedora too. However, on FEdora we do not ship a broken
default config that would sort processes into a cgroup that isn't
"/". Ubuntu just needs to fix its default configuration. It should
refrain from touching any controller by default.

Lennart

--
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