82 lines
4.2 KiB
Text
82 lines
4.2 KiB
Text
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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 watchdog, 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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Be aware that when using local APIC, the frequency of NMI interrupts
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it generates, depends on the system load. The local APIC NMI watchdog,
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lacking a better source, uses the "cycles unhalted" event. As you may
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guess it doesn't tick when the CPU is in the halted state (which happens
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when the system is idle), but if your system locks up on anything but the
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"hlt" processor instruction, the watchdog will trigger very soon as the
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"cycles unhalted" event will happen every clock tick. If it locks up on
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"hlt", then you are out of luck -- the event will not happen at all and the
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watchdog won't trigger. This is a shortcoming of the local APIC watchdog
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-- unfortunately there is no "clock ticks" event that would work all the
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time. The I/O APIC watchdog is driven externally and has no such shortcoming.
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But its NMI frequency is much higher, resulting in a more significant hit
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to the overall system performance.
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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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On x86-64 the NMI oopser is on by default. On 64bit Intel CPUs
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it uses IO-APIC by default and on AMD it uses local APIC.
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[ feel free to send bug reports, suggestions and patches to
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Ingo Molnar <mingo@redhat.com> or the Linux SMP mailing
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list at <linux-smp@vger.kernel.org> ]
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