[linux-audio-dev] kernel preemption patch and 2.4.19

[Michael Ost]

On Wed, 2004-06-02 at 14:26, Roger Larsson wrote:
> > We found that a nasty system crash was fixed by turning off preemption.
> > The crash would happen fairly reliably by switching between virtual
> > terminals a number of times. It locked up the system hard. So hard that
> > we can't really find the problem in the kernel; we just found a
> > work-around basically by trial and error.
> 
> Do you use a SMP computer? If not then you might have run into something
> that could hurt a SMP computer running without preemption...
> 
> (Running SMP and preemption is kind of weird. And you may run into additional
> problems - per CPU data has also to be protected by spinlocks, etc... )

Nope. No SMP.
 
> >
> > We turned on kernel preemption basically because "everyone was doing
> > it". We haven't noticed any obvious, serious problems or differences
> > since turning preemption off.
> >
> > Does anyone have some suggestions about what differences to expect and
> > what we might have a closer look at?
> 
> You won't notice much difference unless you run real time processes. But they
> will need to be suid root (or some wrapper/helper) to become RT processes,
> and many kernels do not ship with multimedia stuff suid...

We have a couple of SCHED_RR r/t threads running; and the app does run
as root. This is what I guess you would call an embedded Linux system: a
vst player on x86 hardware in a rack mount box (called Receptor; sketchy
info at http://www.museresearch.com). It's using Linux and Wine and a
bunch of our own code.
 
> >
> > Also, we are newbies about reporting this kind of crash. Any clues about
> > where to report it or ask about it?
> 
> First - you mention virtual terminals
> 	What video driver are you using? Nvidia?
> 	If that is the case retest with xfree86 (nv)

Hmmm. It's onboard video. I'll have to look into that. I'll see if there
is any xfree86 more specific to our hardware.

> Second - try to what is happening (Magic SysRq)

Yes. We'll give that go.
 
> Third - 2.4.19 is kind of old now (not that many will be interested...), 
> upgrade and try to reproduce.

We're stuck with it for the time being. It's too close to shipping to
switch over.

Thanks, Roger! ... mo