this patch adds a new kernel debug feature: CONFIG_DETECT_SOFTLOCKUP.
When enabled then per-CPU watchdog threads are started, which try to run
once per second. If they get delayed for more than 10 seconds then a
callback from the timer interrupt detects this condition and prints out
a warning message and a stack dump (once per lockup incident). The
feature is otherwise non-intrusive, it doesnt try to unlock the box in
any way, it only gets the debug info out, automatically, and on all CPUs
affected by the lockup.