Based on kernel version 3.3. Page generated on 2012-03-23 21:36 EST.
1 2 [NMI watchdog is available for x86 and x86-64 architectures] 3 4 Is your system locking up unpredictably? No keyboard activity, just 5 a frustrating complete hard lockup? Do you want to help us debugging 6 such lockups? If all yes then this document is definitely for you. 7 8 On many x86/x86-64 type hardware there is a feature that enables 9 us to generate 'watchdog NMI interrupts'. (NMI: Non Maskable Interrupt 10 which get executed even if the system is otherwise locked up hard). 11 This can be used to debug hard kernel lockups. By executing periodic 12 NMI interrupts, the kernel can monitor whether any CPU has locked up, 13 and print out debugging messages if so. 14 15 In order to use the NMI watchdog, you need to have APIC support in your 16 kernel. For SMP kernels, APIC support gets compiled in automatically. For 17 UP, enable either CONFIG_X86_UP_APIC (Processor type and features -> Local 18 APIC support on uniprocessors) or CONFIG_X86_UP_IOAPIC (Processor type and 19 features -> IO-APIC support on uniprocessors) in your kernel config. 20 CONFIG_X86_UP_APIC is for uniprocessor machines without an IO-APIC. 21 CONFIG_X86_UP_IOAPIC is for uniprocessor with an IO-APIC. [Note: certain 22 kernel debugging options, such as Kernel Stack Meter or Kernel Tracer, 23 may implicitly disable the NMI watchdog.] 24 25 For x86-64, the needed APIC is always compiled in. 26 27 Using local APIC (nmi_watchdog=2) needs the first performance register, so 28 you can't use it for other purposes (such as high precision performance 29 profiling.) However, at least oprofile and the perfctr driver disable the 30 local APIC NMI watchdog automatically. 31 32 To actually enable the NMI watchdog, use the 'nmi_watchdog=N' boot 33 parameter. Eg. the relevant lilo.conf entry: 34 35 append="nmi_watchdog=1" 36 37 For SMP machines and UP machines with an IO-APIC use nmi_watchdog=1. 38 For UP machines without an IO-APIC use nmi_watchdog=2, this only works 39 for some processor types. If in doubt, boot with nmi_watchdog=1 and 40 check the NMI count in /proc/interrupts; if the count is zero then 41 reboot with nmi_watchdog=2 and check the NMI count. If it is still 42 zero then log a problem, you probably have a processor that needs to be 43 added to the nmi code. 44 45 A 'lockup' is the following scenario: if any CPU in the system does not 46 execute the period local timer interrupt for more than 5 seconds, then 47 the NMI handler generates an oops and kills the process. This 48 'controlled crash' (and the resulting kernel messages) can be used to 49 debug the lockup. Thus whenever the lockup happens, wait 5 seconds and 50 the oops will show up automatically. If the kernel produces no messages 51 then the system has crashed so hard (eg. hardware-wise) that either it 52 cannot even accept NMI interrupts, or the crash has made the kernel 53 unable to print messages. 54 55 Be aware that when using local APIC, the frequency of NMI interrupts 56 it generates, depends on the system load. The local APIC NMI watchdog, 57 lacking a better source, uses the "cycles unhalted" event. As you may 58 guess it doesn't tick when the CPU is in the halted state (which happens 59 when the system is idle), but if your system locks up on anything but the 60 "hlt" processor instruction, the watchdog will trigger very soon as the 61 "cycles unhalted" event will happen every clock tick. If it locks up on 62 "hlt", then you are out of luck -- the event will not happen at all and the 63 watchdog won't trigger. This is a shortcoming of the local APIC watchdog 64 -- unfortunately there is no "clock ticks" event that would work all the 65 time. The I/O APIC watchdog is driven externally and has no such shortcoming. 66 But its NMI frequency is much higher, resulting in a more significant hit 67 to the overall system performance. 68 69 On x86 nmi_watchdog is disabled by default so you have to enable it with 70 a boot time parameter. 71 72 It's possible to disable the NMI watchdog in run-time by writing "0" to 73 /proc/sys/kernel/nmi_watchdog. Writing "1" to the same file will re-enable 74 the NMI watchdog. Notice that you still need to use "nmi_watchdog=" parameter 75 at boot time. 76 77 NOTE: In kernels prior to 2.4.2-ac18 the NMI-oopser is enabled unconditionally 78 on x86 SMP boxes. 79 80 [ feel free to send bug reports, suggestions and patches to 81 Ingo Molnar <firstname.lastname@example.org> or the Linux SMP mailing 82 list at <email@example.com> ]