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phoenix |
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[NMI watchdog is available for x86 and x86-64 architectures]
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Is your system locking up unpredictably? No keyboard activity, just
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a frustrating complete hard lockup? Do you want to help us debugging
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such lockups? If all yes then this document is definitely for you.
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On many x86/x86-64 type hardware there is a feature that enables
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us to generate 'watchdog NMI interrupts'. (NMI: Non Maskable Interrupt
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which get executed even if the system is otherwise locked up hard).
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This can be used to debug hard kernel lockups. By executing periodic
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NMI interrupts, the kernel can monitor whether any CPU has locked up,
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and print out debugging messages if so.
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In order to use the NMI watchdoc, you need to have APIC support in your
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kernel. For SMP kernels, APIC support gets compiled in automatically. For
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UP, enable either CONFIG_X86_UP_APIC (Processor type and features -> Local
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APIC support on uniprocessors) or CONFIG_X86_UP_IOAPIC (Processor type and
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features -> IO-APIC support on uniprocessors) in your kernel config.
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CONFIG_X86_UP_APIC is for uniprocessor machines without an IO-APIC.
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CONFIG_X86_UP_IOAPIC is for uniprocessor with an IO-APIC. [Note: certain
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kernel debugging options, such as Kernel Stack Meter or Kernel Tracer,
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may implicitly disable the NMI watchdog.]
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For x86-64, the needed APIC is always compiled in, and the NMI watchdog is
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always enabled with I/O-APIC mode (nmi_watchdog=1). Currently, local APIC
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mode (nmi_watchdog=2) does not work on x86-64.
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Using local APIC (nmi_watchdog=2) needs the first performance register, so
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you can't use it for other purposes (such as high precision performance
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profiling.) However, at least oprofile and the perfctr driver disable the
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local APIC NMI watchdog automatically.
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To actually enable the NMI watchdog, use the 'nmi_watchdog=N' boot
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parameter. Eg. the relevant lilo.conf entry:
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append="nmi_watchdog=1"
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For SMP machines and UP machines with an IO-APIC use nmi_watchdog=1.
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For UP machines without an IO-APIC use nmi_watchdog=2, this only works
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for some processor types. If in doubt, boot with nmi_watchdog=1 and
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check the NMI count in /proc/interrupts; if the count is zero then
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reboot with nmi_watchdog=2 and check the NMI count. If it is still
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zero then log a problem, you probably have a processor that needs to be
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added to the nmi code.
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A 'lockup' is the following scenario: if any CPU in the system does not
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execute the period local timer interrupt for more than 5 seconds, then
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the NMI handler generates an oops and kills the process. This
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'controlled crash' (and the resulting kernel messages) can be used to
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debug the lockup. Thus whenever the lockup happens, wait 5 seconds and
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the oops will show up automatically. If the kernel produces no messages
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then the system has crashed so hard (eg. hardware-wise) that either it
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cannot even accept NMI interrupts, or the crash has made the kernel
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unable to print messages.
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NOTE: starting with 2.4.2-ac18 the NMI-oopser is disabled by default,
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you have to enable it with a boot time parameter. Prior to 2.4.2-ac18
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the NMI-oopser is enabled unconditionally on x86 SMP boxes.
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[ feel free to send bug reports, suggestions and patches to
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Ingo Molnar or the Linux SMP mailing
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list at ]
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