watchdog: Update documentation
The soft and hard lockup detectors are now built on top of the hrtimer and perf subsystems. Update the documentation accordingly. Signed-off-by: Fernando Luis Vazquez Cao<fernando@oss.ntt.co.jp> Acked-by: Randy Dunlap <rdunlap@xenotime.net> Signed-off-by: Don Zickus <dzickus@redhat.com> Link: http://lkml.kernel.org/r/1328827342-6253-1-git-send-email-dzickus@redhat.com Signed-off-by: Ingo Molnar <mingo@elte.hu>
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Fernando Luis Vázquez Cao
Ingo Molnar
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c98fdeaa92
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===============================================================
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Softlockup detector and hardlockup detector (aka nmi_watchdog)
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===============================================================
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The Linux kernel can act as a watchdog to detect both soft and hard
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lockups.
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A 'softlockup' is defined as a bug that causes the kernel to loop in
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kernel mode for more than 20 seconds (see "Implementation" below for
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details), without giving other tasks a chance to run. The current
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stack trace is displayed upon detection and, by default, the system
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will stay locked up. Alternatively, the kernel can be configured to
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panic; a sysctl, "kernel.softlockup_panic", a kernel parameter,
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"softlockup_panic" (see "Documentation/kernel-parameters.txt" for
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details), and a compile option, "BOOTPARAM_HARDLOCKUP_PANIC", are
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provided for this.
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A 'hardlockup' is defined as a bug that causes the CPU to loop in
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kernel mode for more than 10 seconds (see "Implementation" below for
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details), without letting other interrupts have a chance to run.
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Similarly to the softlockup case, the current stack trace is displayed
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upon detection and the system will stay locked up unless the default
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behavior is changed, which can be done through a compile time knob,
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"BOOTPARAM_HARDLOCKUP_PANIC", and a kernel parameter, "nmi_watchdog"
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(see "Documentation/kernel-parameters.txt" for details).
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The panic option can be used in combination with panic_timeout (this
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timeout is set through the confusingly named "kernel.panic" sysctl),
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to cause the system to reboot automatically after a specified amount
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of time.
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=== Implementation ===
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The soft and hard lockup detectors are built on top of the hrtimer and
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perf subsystems, respectively. A direct consequence of this is that,
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in principle, they should work in any architecture where these
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subsystems are present.
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A periodic hrtimer runs to generate interrupts and kick the watchdog
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task. An NMI perf event is generated every "watchdog_thresh"
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(compile-time initialized to 10 and configurable through sysctl of the
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same name) seconds to check for hardlockups. If any CPU in the system
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does not receive any hrtimer interrupt during that time the
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'hardlockup detector' (the handler for the NMI perf event) will
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generate a kernel warning or call panic, depending on the
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configuration.
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The watchdog task is a high priority kernel thread that updates a
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timestamp every time it is scheduled. If that timestamp is not updated
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for 2*watchdog_thresh seconds (the softlockup threshold) the
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'softlockup detector' (coded inside the hrtimer callback function)
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will dump useful debug information to the system log, after which it
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will call panic if it was instructed to do so or resume execution of
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other kernel code.
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The period of the hrtimer is 2*watchdog_thresh/5, which means it has
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two or three chances to generate an interrupt before the hardlockup
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detector kicks in.
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As explained above, a kernel knob is provided that allows
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administrators to configure the period of the hrtimer and the perf
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event. The right value for a particular environment is a trade-off
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between fast response to lockups and detection overhead.
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[NMI watchdog is available for x86 and x86-64 architectures]
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Is your system locking up unpredictably? No keyboard activity, just
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a frustrating complete hard lockup? Do you want to help us debugging
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such lockups? If all yes then this document is definitely for you.
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On many x86/x86-64 type hardware there is a feature that enables
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us to generate 'watchdog NMI interrupts'. (NMI: Non Maskable Interrupt
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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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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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UP, enable either CONFIG_X86_UP_APIC (Processor type and features -> Local
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APIC support on uniprocessors) or CONFIG_X86_UP_IOAPIC (Processor type and
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features -> IO-APIC support on uniprocessors) in your kernel config.
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CONFIG_X86_UP_APIC is for uniprocessor machines without an IO-APIC.
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CONFIG_X86_UP_IOAPIC is for uniprocessor with an IO-APIC. [Note: certain
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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.
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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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profiling.) However, at least oprofile and the perfctr driver disable the
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local APIC NMI watchdog automatically.
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To actually enable the NMI watchdog, use the 'nmi_watchdog=N' boot
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parameter. Eg. the relevant lilo.conf entry:
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append="nmi_watchdog=1"
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For SMP machines and UP machines with an IO-APIC use nmi_watchdog=1.
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For UP machines without an IO-APIC use nmi_watchdog=2, this only works
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for some processor types. If in doubt, boot with nmi_watchdog=1 and
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check the NMI count in /proc/interrupts; if the count is zero then
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reboot with nmi_watchdog=2 and check the NMI count. If it is still
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zero then log a problem, you probably have a processor that needs to be
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added to the nmi code.
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A 'lockup' is the following scenario: if any CPU in the system does not
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execute the period local timer interrupt for more than 5 seconds, then
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the NMI handler generates an oops and kills the process. This
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'controlled crash' (and the resulting kernel messages) can be used to
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debug the lockup. Thus whenever the lockup happens, wait 5 seconds and
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the oops will show up automatically. If the kernel produces no messages
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then the system has crashed so hard (eg. hardware-wise) that either it
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cannot even accept NMI interrupts, or the crash has made the kernel
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unable to print messages.
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Be aware that when using local APIC, the frequency of NMI interrupts
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it generates, depends on the system load. The local APIC NMI watchdog,
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lacking a better source, uses the "cycles unhalted" event. As you may
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guess it doesn't tick when the CPU is in the halted state (which happens
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when the system is idle), but if your system locks up on anything but the
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"hlt" processor instruction, the watchdog will trigger very soon as the
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"cycles unhalted" event will happen every clock tick. If it locks up on
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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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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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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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It's possible to disable the NMI watchdog in run-time by writing "0" to
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/proc/sys/kernel/nmi_watchdog. Writing "1" to the same file will re-enable
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the NMI watchdog. Notice that you still need to use "nmi_watchdog=" parameter
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at boot time.
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NOTE: In kernels 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
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list at <linux-smp@vger.kernel.org> ]
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