Re: [Fwd: Re: [patch] Real-Time Preemption, -RT-2.6.9-mm1-V0.4]

Ingo Molnar
2004-11-01T18:59:14Z

* Lee Revell <rlrevell@joe-job.com> wrote:

> This was my conclusion as well. I have a patch sitting around to add
> this to the emu10k1 ALSA driver, it's quite useful. It would be nice
> if there were a facility in the kernel to easily identify missed
> interrupts like this or (even better) unbalanced irq disable/enable -
> AFAICT userspace alone cannot reliably distinguish lost interrupts
> from scheduling problems (though you can get a lot of hints). Paul
> mentioned trying to debug the unbalanced irq disable in his talk at
> ZKM 2003, and said it's hard because the hardware will enable/disable
> interrupts on its own and he could not identify all those places.
> Ingo, is there an easy way to trace this like we do for unbalanced
> preempt count?

i wrote a cli/sti latency tracer a couple of years ago so it's possible.
Note that an irqs-off condition is near impossible to 'leak' into
userspace code though, since the x86 iret path restores flags to the
previous value. Worst-case the irqs-off condition may leak into
kernelspace, and that can still cause bad effects. X startup/shutdown
can disable interrupts for a long time, was that excluded from your
testing?

Ingo