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_SOFTLOCKUP_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 sysctl, 24'hardlockup_panic', a compile time knob, "BOOTPARAM_HARDLOCKUP_PANIC", 25and a kernel parameter, "nmi_watchdog" 26(see "Documentation/kernel-parameters.txt" for details). 27 28The panic option can be used in combination with panic_timeout (this 29timeout is set through the confusingly named "kernel.panic" sysctl), 30to cause the system to reboot automatically after a specified amount 31of time. 32 33=== Implementation === 34 35The soft and hard lockup detectors are built on top of the hrtimer and 36perf subsystems, respectively. A direct consequence of this is that, 37in principle, they should work in any architecture where these 38subsystems are present. 39 40A periodic hrtimer runs to generate interrupts and kick the watchdog 41task. An NMI perf event is generated every "watchdog_thresh" 42(compile-time initialized to 10 and configurable through sysctl of the 43same name) seconds to check for hardlockups. If any CPU in the system 44does not receive any hrtimer interrupt during that time the 45'hardlockup detector' (the handler for the NMI perf event) will 46generate a kernel warning or call panic, depending on the 47configuration. 48 49The watchdog task is a high priority kernel thread that updates a 50timestamp every time it is scheduled. If that timestamp is not updated 51for 2*watchdog_thresh seconds (the softlockup threshold) the 52'softlockup detector' (coded inside the hrtimer callback function) 53will dump useful debug information to the system log, after which it 54will call panic if it was instructed to do so or resume execution of 55other kernel code. 56 57The period of the hrtimer is 2*watchdog_thresh/5, which means it has 58two or three chances to generate an interrupt before the hardlockup 59detector kicks in. 60 61As explained above, a kernel knob is provided that allows 62administrators to configure the period of the hrtimer and the perf 63event. The right value for a particular environment is a trade-off 64between fast response to lockups and detection overhead. 65 66By default, the watchdog runs on all online cores. However, on a 67kernel configured with NO_HZ_FULL, by default the watchdog runs only 68on the housekeeping cores, not the cores specified in the "nohz_full" 69boot argument. If we allowed the watchdog to run by default on 70the "nohz_full" cores, we would have to run timer ticks to activate 71the scheduler, which would prevent the "nohz_full" functionality 72from protecting the user code on those cores from the kernel. 73Of course, disabling it by default on the nohz_full cores means that 74when those cores do enter the kernel, by default we will not be 75able to detect if they lock up. However, allowing the watchdog 76to continue to run on the housekeeping (non-tickless) cores means 77that we will continue to detect lockups properly on those cores. 78 79In either case, the set of cores excluded from running the watchdog 80may be adjusted via the kernel.watchdog_cpumask sysctl. For 81nohz_full cores, this may be useful for debugging a case where the 82kernel seems to be hanging on the nohz_full cores. 83