Re: [LAD] [ANNOUNCE] Safe real-time on the desktop by default; Desktop/audio RT developers, read this!

On Sun, 21.06.09 16:06, Fernando Lopez-Lezcano (nando@email-addr-hidden) wrote:

>
> On Fri, 2009-06-19 at 20:13 +0200, Lennart Poettering wrote:
> > Heya,
> >
> > Just a quick announcement:
> >
> > I just moved into Fedora Rawhide a little daemon called "RealtimeKit"
> > which will be enabled by default, and since it is now a dependency of
> > PulseAudio and things work how they work this will then not only be
> > available in Fedora 12 but also sooner or later in the other
> > distributions as well, installed by default.
>
> What other distros are considering its use? (or will use it?, or use it
> currently?)

This is a dep of PA now. Since all major distros adopted PA I'd be
surprised if they wouldn't adopt rtkit too. If things run like they
usually run then this will enter the other distros one cycle past F12.

> > So what does RealtimeKit do that previous solutions didn't do? rtkit
> > relies on a new kernel feature SCHED_RESET_ON_FORK that got recently
> > merged into Ingo's tree and will hence shortly appear in 2.6.31.
>
> So this is, at this point, kernel version dependent, right?
> What happens if you run a 2.6.29.x kernel?

Nothing. Then the call into rtkit will simply fail. Since the rtkit
call is intended to be used as fallback only things will work as well
or badly as they did before rtkit was introduced.

> The question is relevant, I think, as the kernels that I use (Planet
> CCRMA) are the rt patched kernels, currently limited to 2.6.29.5 (I
> think Thomas and the rt gang are working on 2.6.30, I imagine 2.6.31
> support is still far in the future).

Dunno. I disagree. The primary objective for rtkit is to be able to
run media applications as RT by default, out-of-the-box. I doubt that
this feature matters for legacy distros/kernels.

> > can set that flag when entering SCHED_RR scheduling and this will
> > then make sure that after forking a child will be reset to
> > SCHED_OTHER. RT fork bombs can thus be made impossible: if we hand
> > out RT to a process we can be sure it won't "leak", and if we
> > decide to take it away again we can be sure we can do that without
> > having to be afraid of races around forking.
>
> If I understand correctly then the mechanism would not be useful for
> jack (leaving aside the issue of SCHED_RR vs. SCHED_FIFO), as jack
> actually gives rt priority to threads in other processes (the clients of
> jack). But maybe things have changed in the way jack works internally
> these days, or, possibly I'm not completely understanding the
> implications... hmmmmm... jack does not do any forking right?

Dunno. All processes that want to have rt need to ask rtkit
themselves. (which is the only safe thing to do, only then we get the
credentials properly defined)

> > rtkit enforces limits on the the number of threads/processes/users
> > that get RT sched. It also does rate limiting, and calls into
> > PolicyKit before handing out RT. Finally, as extra a-posteriori
> > protection it also includes a canary watchdog.
>
> How are all those limits set up and/or configured?

Right now you can edit the service startup file and pass them as
command line parameters. Just read the README I linked.

Lennart

-- 
Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net
http://0pointer.net/lennart/           GnuPG 0x1A015CC4
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