logical CPUs on a system to be used as a dedicated watchdog to cause a
drop to the debugger and/or generate an NMI to the boot processor if
the kernel ceases to respond. A sysctl enables the watchdog running
out of the processor's idle thread; a callout is launched to reset a
timer in the watchdog. If the callout fails to reset the timer for ten
seconds, the watchdog will fire. The sysctl allows you to select which
CPU will run the watchdog.
A sample "debug.leak_schedlock" is included, which causes a sysctl to
spin holding sched_lock in order to trigger the watchdog. On my Xeons,
the watchdog is able to detect this failure mode and break into the
debugger, which cannot otherwise be done without an NMI button.
This option does not currently work with sched_ule due to ule's push
notion of scheduling, similar to machdep.hlt_logical_cpus failing to
work with that scheduler.
On face value, this might seem somewhat inefficient, but there are a
lot of dual-processor Xeons with HTT around, so using one as a watchdog
for testing is not as inefficient as one might fear.
