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x86: nmi_watchdog - documentation fix
nmi_watchdog is set to NMI_NONE by default (ie disabled) on _any_
mode so lets fix documentation too.
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: "Maciej W. Rozycki" <macro@linux-mips.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
authored by
Cyrill Gorcunov
and committed by
Ingo Molnar
afda335d
914bebfa
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Documentation/nmi_watchdog.txt
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Documentation/nmi_watchdog.txt
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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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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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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).
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For x86-64, the needed APIC is always compiled in.
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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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"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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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 nmi_watchdog is disabled by default so you have to enable it with
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a boot time parameter.
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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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NOTE: Prior to 2.4.2-ac18 the NMI-oopser is enabled unconditionally
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on x86 SMP boxes.
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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