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SubjectNMI is broken!
It appears that the NMI generation set up code is NOT correctly setting
up the system to generate NMIs if the interrupt system is off! Kernel
version 2.4.2 NMI worked correctly, 2.4.7 and later do not. I have not
tested kernels 2.4.3 thru 2.4.6. If the interrupt system is on, NMIs
are being correctly generated.

IMHO the check_nmi_watchdog() test is flawed in that it explicitly
enables interrupts prior to the test. Below is a patch to fix this
issue. When this patch is applied to the 2.4.2 kernel, the test works
correctly, however, 2.4.7 and above all report "NMI appears to be
stuck". (Note, the check_nmi_watchdog() code is in io_apic.c in the
2.4.2 kernel and is called "nmi_irq_works()".)

Note that this is not just a testing issue, NMI is no longer pulling the
system out of deadlocks with any of the irq spinlocks.

I am afraid that I don't know enough or have the right documentation to
figure out what changed, but it appears that the change was about the
same time that the nmi.c module was introduced.

Here is the recommended patch for nmi.c:

--- linux-2.4.13-org/arch/i386/kernel/nmi.c Tue Sep 25 00:34:57 2001
+++ linux/arch/i386/kernel/nmi.c Fri Nov 30 00:31:03 2001
@@ -46,28 +54,30 @@
int __init check_nmi_watchdog (void)
irq_cpustat_t tmp[NR_CPUS];
- int j, cpu;
+ int j, cpu, err = 0;

printk(KERN_INFO "testing NMI watchdog ... ");

memcpy(tmp, irq_stat, sizeof(tmp));
- sti();
+ cli();
mdelay((10*1000)/nmi_hz); // wait 10 ticks

for (j = 0; j < smp_num_cpus; j++) {
cpu = cpu_logical_map(j);
if (nmi_count(cpu) - tmp[cpu].__nmi_count <= 5) {
printk("CPU#%d: NMI appears to be stuck!\n", cpu);
- return -1;
+ err = -1;
- printk("OK.\n");
+ if (! err ){
+ printk("OK.\n");
+ }

/* now that we know it works we can reduce NMI frequency to
something more reasonable; makes a difference in some configs */
if (nmi_watchdog == NMI_LOCAL_APIC)
nmi_hz = 1;
+ sti();
return 0;

Real time sched:
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