linux/Documentation/lockup-watchdogs.txt
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   2Softlockup detector and hardlockup detector (aka nmi_watchdog)
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   4
   5The Linux kernel can act as a watchdog to detect both soft and hard
   6lockups.
   7
   8A 'softlockup' is defined as a bug that causes the kernel to loop in
   9kernel mode for more than 20 seconds (see "Implementation" below for
  10details), without giving other tasks a chance to run. The current
  11stack trace is displayed upon detection and, by default, the system
  12will stay locked up. Alternatively, the kernel can be configured to
  13panic; a sysctl, "kernel.softlockup_panic", a kernel parameter,
  14"softlockup_panic" (see "Documentation/kernel-parameters.txt" for
  15details), and a compile option, "BOOTPARAM_HARDLOCKUP_PANIC", are
  16provided for this.
  17
  18A 'hardlockup' is defined as a bug that causes the CPU to loop in
  19kernel mode for more than 10 seconds (see "Implementation" below for
  20details), without letting other interrupts have a chance to run.
  21Similarly to the softlockup case, the current stack trace is displayed
  22upon detection and the system will stay locked up unless the default
  23behavior is changed, which can be done through a compile time knob,
  24"BOOTPARAM_HARDLOCKUP_PANIC", and a kernel parameter, "nmi_watchdog"
  25(see "Documentation/kernel-parameters.txt" for details).
  26
  27The panic option can be used in combination with panic_timeout (this
  28timeout is set through the confusingly named "kernel.panic" sysctl),
  29to cause the system to reboot automatically after a specified amount
  30of time.
  31
  32=== Implementation ===
  33
  34The soft and hard lockup detectors are built on top of the hrtimer and
  35perf subsystems, respectively. A direct consequence of this is that,
  36in principle, they should work in any architecture where these
  37subsystems are present.
  38
  39A periodic hrtimer runs to generate interrupts and kick the watchdog
  40task. An NMI perf event is generated every "watchdog_thresh"
  41(compile-time initialized to 10 and configurable through sysctl of the
  42same name) seconds to check for hardlockups. If any CPU in the system
  43does not receive any hrtimer interrupt during that time the
  44'hardlockup detector' (the handler for the NMI perf event) will
  45generate a kernel warning or call panic, depending on the
  46configuration.
  47
  48The watchdog task is a high priority kernel thread that updates a
  49timestamp every time it is scheduled. If that timestamp is not updated
  50for 2*watchdog_thresh seconds (the softlockup threshold) the
  51'softlockup detector' (coded inside the hrtimer callback function)
  52will dump useful debug information to the system log, after which it
  53will call panic if it was instructed to do so or resume execution of
  54other kernel code.
  55
  56The period of the hrtimer is 2*watchdog_thresh/5, which means it has
  57two or three chances to generate an interrupt before the hardlockup
  58detector kicks in.
  59
  60As explained above, a kernel knob is provided that allows
  61administrators to configure the period of the hrtimer and the perf
  62event. The right value for a particular environment is a trade-off
  63between fast response to lockups and detection overhead.
  64
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