Is your SMP system locking up unpredictably? No keyboard activity, just a frustrating complete hard lockup? Do you want to help us debugging such lockups? If all yes then this document is definitely for you. on Intel SMP hardware there is a feature that enables us to generate 'watchdog NMI interrupts'. (NMI: Non Maskable Interrupt - these get executed even if the system is otherwise locked up hard) This can be used to debug hard kernel lockups. By executing periodic NMI interrupts, the kernel can monitor wether any CPU has locked up, and print out debugging messages if so. You can enable/disable the NMI watchdog at boot time with the 'nmi_watchdog=1' boot parameter. Eg. the relevant lilo.conf entry: append="nmi_watchdog=1" A 'lockup' is the following scenario: if any CPU in the system does not execute the period local timer interrupt for more than 5 seconds, then the NMI handler generates an oops and kills the process. This 'controlled crash' (and the resulting kernel messages) can be used to debug the lockup. Thus whenever the lockup happens, wait 5 seconds and the oops will show up automatically. If the kernel produces no messages then the system has crashed so hard (eg. hardware-wise) that either it cannot even accept NMI interrupts, or the crash has made the kernel unable to print messages. You can find NMI watchdog patch against Linux 2.2.x at http://people.redhat.com/mingo/NMI-watchdog-patches/ [ feel free to send bug reports, suggestions and patches to Ingo Molnar or the Linux SMP mailing list at ]