[linuxkernelnewbies] LXR linux/Documentation/nmi_watchdog.txt

[Peter Teoh]

http://lxr.linux.no/linux/Documentation/nmi_watchdog.txt [NMI watchdog is available for x86 and x86-64 architectures] Is your 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 many x86/x86-64 type hardware there is a feature that enables us to generate 'watchdog NMI interrupts'. (NMI: Non Maskable Interrupt which 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 whether any CPU has locked up, and print out debugging messages if so. In order to use the NMI watchdog, you need to have APIC support in your kernel. For SMP kernels, APIC support gets compiled in automatically. For UP, enable either CONFIG_X86_UP_APIC (Processor type and features -> Local APIC support on uniprocessors) or CONFIG_X86_UP_IOAPIC (Processor type and features -> IO-APIC support on uniprocessors) in your kernel config. CONFIG_X86_UP_APIC is for uniprocessor machines without an IO-APIC. CONFIG_X86_UP_IOAPIC is for uniprocessor with an IO-APIC. [Note: certain kernel debugging options, such as Kernel Stack Meter or Kernel Tracer, may implicitly disable the NMI watchdog.] For x86-64, the needed APIC is always compiled in, and the NMI watchdog is always enabled with I/O-APIC mode (nmi_watchdog=1). Using local APIC (nmi_watchdog=2) needs the first performance register, so you can't use it for other purposes (such as high precision performance profiling.) However, at least oprofile and the perfctr driver disable the local APIC NMI watchdog automatically. To actually enable the NMI watchdog, use the 'nmi_watchdog=N' boot parameter. Eg. the relevant lilo.conf entry: append="nmi_watchdog=1" For SMP machines and UP machines with an IO-APIC use nmi_watchdog=1. For UP machines without an IO-APIC use nmi_watchdog=2, this only works for some processor types. If in doubt, boot with nmi_watchdog=1 and check the NMI count in /proc/interrupts; if the count is zero then reboot with nmi_watchdog=2 and check the NMI count. If it is still zero then log a problem, you probably have a processor that needs to be added to the nmi code. 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. Be aware that when using local APIC, the frequency of NMI interrupts it generates, depends on the system load. The local APIC NMI watchdog, lacking a better source, uses the "cycles unhalted" event. As you may guess it doesn't tick when the CPU is in the halted state (which happens when the system is idle), but if your system locks up on anything but the "hlt" processor instruction, the watchdog will trigger very soon as the "cycles unhalted" event will happen every clock tick. If it locks up on "hlt", then you are out of luck -- the event will not happen at all and the watchdog won't trigger. This is a shortcoming of the local APIC watchdog -- unfortunately there is no "clock ticks" event that would work all the time. The I/O APIC watchdog is driven externally and has no such shortcoming. But its NMI frequency is much higher, resulting in a more significant hit to the overall system performance. NOTE: starting with 2.4.2-ac18 the NMI-oopser is disabled by default, you have to enable it with a boot time parameter. Prior to 2.4.2-ac18 the NMI-oopser is enabled unconditionally on x86 SMP boxes. On x86-64 the NMI oopser is on by default. On 64bit Intel CPUs it uses IO-APIC by default and on AMD it uses local APIC. [ feel free to send bug reports, suggestions and patches to Ingo Molnar <[EMAIL PROTECTED]> or the Linux SMP mailing list at <[EMAIL PROTECTED]> ]