file: /proc/sys/kernel/unknown_nmi_panic
variable: kernel.unknown_nmi_panic
Official reference

The value in this file affects behavior of handling NMI. When the value is non-zero, unknown NMI is trapped and then panic occurs. At that time, kernel debugging information is displayed on console.

NMI switch that most IA32 servers have fires unknown NMI up, for example. If a system hangs up, try pressing the NMI switch.