FreeBSD/Linux Kernel Cross Reference
sys/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.
    
    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.
    
    On x86 nmi_watchdog is disabled by default so you have to enable it with
    a boot time parameter.
    
    It's possible to disable the NMI watchdog in run-time by writing "0" to
    /proc/sys/kernel/nmi_watchdog. Writing "1" to the same file will re-enable
    the NMI watchdog. Notice that you still need to use "nmi_watchdog=" parameter
    at boot time.
    
    NOTE: In kernels prior to 2.4.2-ac18 the NMI-oopser is enabled unconditionally
    on x86 SMP boxes.
    
    [ feel free to send bug reports, suggestions and patches to
      Ingo Molnar <mingo@redhat.com> or the Linux SMP mailing
      list at <linux-smp@vger.kernel.org> ]
    

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