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-[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 <> or the Linux SMP mailing
- list at <> ]