Linux内核线程死锁或死循环之后如何让系统宕机重(3)

Based on kernel version 3.8. Page generated on 2013-02-20 22:01 EST.

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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
 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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