1=============================================================== 2Softlockup detector and hardlockup detector (aka nmi_watchdog) 3=============================================================== 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