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-Softlockup detector and hardlockup detector (aka nmi_watchdog)
-The Linux kernel can act as a watchdog to detect both soft and hard
-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
-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.