Re: 2.6.14-rc5-rt6 -- False NMI lockup detects

[Ingo Molnar <mingo@elte.hu>]

* Steven Rostedt <rostedt@goodmis.org> wrote:

> Hi Ingo and Thomas,
> 
> On some of my machines, I've been experiencing false NMI lockups.  
> This usually happens on slower machines, and taking a look into this, 
> it seems to be due to a short time where no processes are using 
> timers, and the ktimer interrupts aren't needed. So the APIC timer, 
> which now is used only for the ktimers, has a five second pause, and 
> causes the NMI to go off.  The NMI uses the apic timer to determine 
> lockups.

this would be a bug - the jiffy tick should be processed every 1 msec, 
regardless of whether there are any ktimers pending. (in the future we 
want to use a special ktimer for the jiffy tick, but that's not 
implemented yet.)

> So, I added a more generic method. This only works for x86 for now, 
> but it has a #ifdef to keep other archs working until it implements 
> this as well.  I added a nmi_irq_incr which is called by __do_IRQ in 
> the generic code.  This is what is used in the NMI code to determine 
> if the CPU has locked up.  This way we don't have to worry about what 
> resource we are using for timers.

this will be useful for tickless stuff - but right now 'no APIC timer 
irq for 5 seconds' is a 'must not happen'.

	Ingo