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  1 ===============================================================
  2 Softlockup detector and hardlockup detector (aka nmi_watchdog)
  3 ===============================================================
  5 The Linux kernel can act as a watchdog to detect both soft and hard
  6 lockups.
  8 A 'softlockup' is defined as a bug that causes the kernel to loop in
  9 kernel mode for more than 20 seconds (see "Implementation" below for
 10 details), without giving other tasks a chance to run. The current
 11 stack trace is displayed upon detection and, by default, the system
 12 will stay locked up. Alternatively, the kernel can be configured to
 13 panic; a sysctl, "kernel.softlockup_panic", a kernel parameter,
 14 "softlockup_panic" (see "Documentation/admin-guide/kernel-parameters.rst" for
 15 details), and a compile option, "BOOTPARAM_SOFTLOCKUP_PANIC", are
 16 provided for this.
 18 A 'hardlockup' is defined as a bug that causes the CPU to loop in
 19 kernel mode for more than 10 seconds (see "Implementation" below for
 20 details), without letting other interrupts have a chance to run.
 21 Similarly to the softlockup case, the current stack trace is displayed
 22 upon detection and the system will stay locked up unless the default
 23 behavior is changed, which can be done through a sysctl,
 24 'hardlockup_panic', a compile time knob, "BOOTPARAM_HARDLOCKUP_PANIC",
 25 and a kernel parameter, "nmi_watchdog"
 26 (see "Documentation/admin-guide/kernel-parameters.rst" for details).
 28 The panic option can be used in combination with panic_timeout (this
 29 timeout is set through the confusingly named "kernel.panic" sysctl),
 30 to cause the system to reboot automatically after a specified amount
 31 of time.
 33 === Implementation ===
 35 The soft and hard lockup detectors are built on top of the hrtimer and
 36 perf subsystems, respectively. A direct consequence of this is that,
 37 in principle, they should work in any architecture where these
 38 subsystems are present.
 40 A periodic hrtimer runs to generate interrupts and kick the watchdog
 41 task. An NMI perf event is generated every "watchdog_thresh"
 42 (compile-time initialized to 10 and configurable through sysctl of the
 43 same name) seconds to check for hardlockups. If any CPU in the system
 44 does not receive any hrtimer interrupt during that time the
 45 'hardlockup detector' (the handler for the NMI perf event) will
 46 generate a kernel warning or call panic, depending on the
 47 configuration.
 49 The watchdog task is a high priority kernel thread that updates a
 50 timestamp every time it is scheduled. If that timestamp is not updated
 51 for 2*watchdog_thresh seconds (the softlockup threshold) the
 52 'softlockup detector' (coded inside the hrtimer callback function)
 53 will dump useful debug information to the system log, after which it
 54 will call panic if it was instructed to do so or resume execution of
 55 other kernel code.
 57 The period of the hrtimer is 2*watchdog_thresh/5, which means it has
 58 two or three chances to generate an interrupt before the hardlockup
 59 detector kicks in.
 61 As explained above, a kernel knob is provided that allows
 62 administrators to configure the period of the hrtimer and the perf
 63 event. The right value for a particular environment is a trade-off
 64 between fast response to lockups and detection overhead.
 66 By default, the watchdog runs on all online cores.  However, on a
 67 kernel configured with NO_HZ_FULL, by default the watchdog runs only
 68 on the housekeeping cores, not the cores specified in the "nohz_full"
 69 boot argument.  If we allowed the watchdog to run by default on
 70 the "nohz_full" cores, we would have to run timer ticks to activate
 71 the scheduler, which would prevent the "nohz_full" functionality
 72 from protecting the user code on those cores from the kernel.
 73 Of course, disabling it by default on the nohz_full cores means that
 74 when those cores do enter the kernel, by default we will not be
 75 able to detect if they lock up.  However, allowing the watchdog
 76 to continue to run on the housekeeping (non-tickless) cores means
 77 that we will continue to detect lockups properly on those cores.
 79 In either case, the set of cores excluded from running the watchdog
 80 may be adjusted via the kernel.watchdog_cpumask sysctl.  For
 81 nohz_full cores, this may be useful for debugging a case where the
 82 kernel seems to be hanging on the nohz_full cores.

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