Re: [patch] Real-Time Preemption, -RT-2.6.10-rc2-mm3-V0.7.31-19

[Florian Schmidt <mista.tapas@gmx.net>]

On Thu, 2 Dec 2004 14:10:02 +0100
Ingo Molnar <mingo@elte.hu> wrote:

> 
> * Florian Schmidt <mista.tapas@gmx.net> wrote:
> 
> > Hmm, i wonder if there's a way to detect non RT behaviour in jackd
> > clients. I mean AFAIK the only thing allowed for the process callback
> > of on is the FIFO it waits on to be woken, right? Every other sleeping
> > is to be considered a bug. 
> 
> there's such a feature in -RT kernels. If a user process calls:
> 
> 	gettimeofday(1,1);
> 
> then the kernel turns 'atomic mode' on. To turn it off, call:
> 
> 	gettimeofday(1,0);
> 
> while in atomic-mode, any non-atomic activity (scheduling) will produce
> a kernel message and a SIGUSR2 sent to the offending process (once,
> atomic mode has to be re-enabled again for the next message). Preemption
> by a higher-prio task does not trigger a message/signal.
> 
> If you run the client under gdb you should be able to catch the SIGUSR2
> signal and then you can see the offending code's backtrace via 'bt'.

Ok, so if i want to find out whether a client violates the RT
constraints for its process callback i would have to add a call to
gettimeofday(1,1) at the start of the process callback and
gettimeofday(1,0) at the end.

Everything which causes a reschedule inbetween will then cause SIGUSR2
to be sent to the client for which i could either add a signal handler
in the client or just use gdb to get notified of it. 

Cool!

Flo