path: root/Documentation/lockup-watchdogs.txt
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authorFernando Luis Vázquez Cao <>2012-02-09 17:42:20 -0500
committerIngo Molnar <>2012-02-11 15:11:28 +0100
commit9919cba7ff71147803c988521cc1ceb80e7f0f6d (patch)
tree2e790fe9373225bb72fc74b3f14702bc04252508 /Documentation/lockup-watchdogs.txt
parentc98fdeaa92731308ed80386261fa2589addefa47 (diff)
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<> Acked-by: Randy Dunlap <> Signed-off-by: Don Zickus <> Link: Signed-off-by: Ingo Molnar <>
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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.