FreeBSD/Linux Kernel Cross Reference
sys/Documentation/lockup-watchdogs.txt

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     Softlockup detector and hardlockup detector (aka nmi_watchdog)
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     The Linux kernel can act as a watchdog to detect both soft and hard
     lockups.
     
     A 'softlockup' is defined as a bug that causes the kernel to loop in
     kernel mode for more than 20 seconds (see "Implementation" below for
    details), without giving other tasks a chance to run. The current
    stack trace is displayed upon detection and, by default, the system
    will stay locked up. Alternatively, the kernel can be configured to
    panic; a sysctl, "kernel.softlockup_panic", a kernel parameter,
    "softlockup_panic" (see "Documentation/kernel-parameters.txt" for
    details), and a compile option, "BOOTPARAM_HARDLOCKUP_PANIC", are
    provided for this.
    
    A 'hardlockup' is defined as a bug that causes the CPU to loop in
    kernel mode for more than 10 seconds (see "Implementation" below for
    details), without letting other interrupts have a chance to run.
    Similarly to the softlockup case, the current stack trace is displayed
    upon detection and the system will stay locked up unless the default
    behavior is changed, which can be done through a compile time knob,
    "BOOTPARAM_HARDLOCKUP_PANIC", and a kernel parameter, "nmi_watchdog"
    (see "Documentation/kernel-parameters.txt" for details).
    
    The panic option can be used in combination with panic_timeout (this
    timeout is set through the confusingly named "kernel.panic" sysctl),
    to cause the system to reboot automatically after a specified amount
    of time.
    
    === Implementation ===
    
    The soft and hard lockup detectors are built on top of the hrtimer and
    perf subsystems, respectively. A direct consequence of this is that,
    in principle, they should work in any architecture where these
    subsystems are present.
    
    A periodic hrtimer runs to generate interrupts and kick the watchdog
    task. An NMI perf event is generated every "watchdog_thresh"
    (compile-time initialized to 10 and configurable through sysctl of the
    same name) seconds to check for hardlockups. If any CPU in the system
    does not receive any hrtimer interrupt during that time the
    'hardlockup detector' (the handler for the NMI perf event) will
    generate a kernel warning or call panic, depending on the
    configuration.
    
    The watchdog task is a high priority kernel thread that updates a
    timestamp every time it is scheduled. If that timestamp is not updated
    for 2*watchdog_thresh seconds (the softlockup threshold) the
    'softlockup detector' (coded inside the hrtimer callback function)
    will dump useful debug information to the system log, after which it
    will call panic if it was instructed to do so or resume execution of
    other kernel code.
    
    The period of the hrtimer is 2*watchdog_thresh/5, which means it has
    two or three chances to generate an interrupt before the hardlockup
    detector kicks in.
    
    As explained above, a kernel knob is provided that allows
    administrators to configure the period of the hrtimer and the perf
    event. The right value for a particular environment is a trade-off
    between fast response to lockups and detection overhead.

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