mirror of
https://github.com/torvalds/linux.git
synced 2024-12-30 14:52:05 +00:00
82 lines
4.2 KiB
Plaintext
82 lines
4.2 KiB
Plaintext
|
|
||
|
|
[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 <mingo@redhat.com> or the Linux SMP mailing
|
||
|
|
list at <linux-smp@vger.kernel.org> ]
|
||
|
|
|