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/admin-guide/kernel-parameters.rst" 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/admin-guide/kernel-parameters.rst" 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 33Implementation 34============== 35 36The soft and hard lockup detectors are built on top of the hrtimer and 37perf subsystems, respectively. A direct consequence of this is that, 38in principle, they should work in any architecture where these 39subsystems are present. 40 41A periodic hrtimer runs to generate interrupts and kick the watchdog 42task. An NMI perf event is generated every "watchdog_thresh" 43(compile-time initialized to 10 and configurable through sysctl of the 44same name) seconds to check for hardlockups. If any CPU in the system 45does not receive any hrtimer interrupt during that time the 46'hardlockup detector' (the handler for the NMI perf event) will 47generate a kernel warning or call panic, depending on the 48configuration. 49 50The watchdog task is a high priority kernel thread that updates a 51timestamp every time it is scheduled. If that timestamp is not updated 52for 2*watchdog_thresh seconds (the softlockup threshold) the 53'softlockup detector' (coded inside the hrtimer callback function) 54will dump useful debug information to the system log, after which it 55will call panic if it was instructed to do so or resume execution of 56other kernel code. 57 58The period of the hrtimer is 2*watchdog_thresh/5, which means it has 59two or three chances to generate an interrupt before the hardlockup 60detector kicks in. 61 62As explained above, a kernel knob is provided that allows 63administrators to configure the period of the hrtimer and the perf 64event. The right value for a particular environment is a trade-off 65between fast response to lockups and detection overhead. 66 67By default, the watchdog runs on all online cores. However, on a 68kernel configured with NO_HZ_FULL, by default the watchdog runs only 69on the housekeeping cores, not the cores specified in the "nohz_full" 70boot argument. If we allowed the watchdog to run by default on 71the "nohz_full" cores, we would have to run timer ticks to activate 72the scheduler, which would prevent the "nohz_full" functionality 73from protecting the user code on those cores from the kernel. 74Of course, disabling it by default on the nohz_full cores means that 75when those cores do enter the kernel, by default we will not be 76able to detect if they lock up. However, allowing the watchdog 77to continue to run on the housekeeping (non-tickless) cores means 78that we will continue to detect lockups properly on those cores. 79 80In either case, the set of cores excluded from running the watchdog 81may be adjusted via the kernel.watchdog_cpumask sysctl. For 82nohz_full cores, this may be useful for debugging a case where the 83kernel seems to be hanging on the nohz_full cores. 84
