Re: NMI watchdog

[John Sigler <linux.kernel@free.fr>]

Björn Steinbrink wrote:

> John Sigler wrote:
>
> > I'm experiencing a full system lockup. I'm using an out-of-tree driver 
> > which I suspect is responsible. I'm trying to enable the NMI watchdog.
> >
> > # cat /proc/version
> > Linux version 2.6.22.1-rt9 (gcc version 3.4.6) #1 PREEMPT RT Tue Oct 9 
> > 12:25:47 CEST 2007
> >
> > # cat /proc/cmdline
> > ro root=/dev/hdc1 console=ttyS0,57600n8 console=tty0 panic=3 apic=debug 
> > nmi_watchdog=2
> >
> > However, after boot, the NMI count does not change.
> >
> > # cat /proc/interrupts ; sleep 10 ; cat /proc/interrupts
>
> Try running some cpu hog in the background. The performance counters get
> increased only when the CPU is actually doing something. On a mostly
> idle system, it can take quite a while for the next NMI to show up.

You are right. In another shell, I ran while true; do : ; done

# cat /proc/interrupts ; sleep 10 ; cat /proc/interrupts
           CPU0
  0:        100   IO-APIC-edge      timer
  4:         82   IO-APIC-edge      serial
  8:          1   IO-APIC-edge      rtc
  9:          0   IO-APIC-fasteoi   acpi
 15:      13648   IO-APIC-edge      ide1
 16:       1303   IO-APIC-fasteoi   eth0
 17:        575   IO-APIC-fasteoi   eth1
 18:        575   IO-APIC-fasteoi   eth2
 19:        575   IO-APIC-fasteoi   eth3
NMI:       2889
LOC:     115768
ERR:          0
MIS:          0

           CPU0
  0:        100   IO-APIC-edge      timer
  4:         82   IO-APIC-edge      serial
  8:          1   IO-APIC-edge      rtc
  9:          0   IO-APIC-fasteoi   acpi
 15:      13672   IO-APIC-edge      ide1
 16:       1310   IO-APIC-fasteoi   eth0
 17:        580   IO-APIC-fasteoi   eth1
 18:        580   IO-APIC-fasteoi   eth2
 19:        580   IO-APIC-fasteoi   eth3
NMI:       2899
LOC:     116770
ERR:          0
MIS:          0

The performance counter appears to be configured to fire when the event 
count for CPU_CLK_UNHALTED reaches 2,400,000,000 (I have a 2.4 GHz CPU) 
i.e. one NMI per second when the CPU is 100% busy. Is that correct?

On a related note, I have a Pentium 3 which counts CPU_CLK_UNHALTED 
cycles even when the CPU is halted. I was told this is a bug, but it 
actually sounds like a a nice feature!

Is there really no way to have the event counter increment with every 
tick (even when the CPU is halted) on a P4?

Regards.
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