From 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sat, 16 Apr 2005 15:20:36 -0700 Subject: Linux-2.6.12-rc2 Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip! --- Documentation/nmi_watchdog.txt | 81 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 81 insertions(+) create mode 100644 Documentation/nmi_watchdog.txt (limited to 'Documentation/nmi_watchdog.txt') diff --git a/Documentation/nmi_watchdog.txt b/Documentation/nmi_watchdog.txt new file mode 100644 index 000000000000..c025a4561c10 --- /dev/null +++ b/Documentation/nmi_watchdog.txt @@ -0,0 +1,81 @@ + +[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, and the NMI watchdog is +always enabled with I/O-APIC mode (nmi_watchdog=1). Currently, local APIC +mode (nmi_watchdog=2) does not work on x86-64. + +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. + +NOTE: starting with 2.4.2-ac18 the NMI-oopser is disabled by default, +you have to enable it with a boot time parameter. Prior to 2.4.2-ac18 +the NMI-oopser is enabled unconditionally on x86 SMP boxes. + +On x86-64 the NMI oopser is on by default. On 64bit Intel CPUs +it uses IO-APIC by default and on AMD it uses local APIC. + +[ feel free to send bug reports, suggestions and patches to + Ingo Molnar or the Linux SMP mailing + list at ] + -- cgit v1.2.3