1 files changed, 83 insertions, 0 deletions
diff --git a/Documentation/lockup-watchdogs.txt b/Documentation/lockup-watchdogs.txt
new file mode 100644
index 000000000..290840c16
--- /dev/null
+++ b/Documentation/lockup-watchdogs.txt
@@ -0,0 +1,83 @@
+===============================================================
+Softlockup detector and hardlockup detector (aka nmi_watchdog)
+===============================================================
+
+The Linux kernel can act as a watchdog to detect both soft and hard
+lockups.
+
+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/admin-guide/kernel-parameters.rst" for
+details), and a compile option, "BOOTPARAM_SOFTLOCKUP_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 sysctl,
+'hardlockup_panic', a compile time knob, "BOOTPARAM_HARDLOCKUP_PANIC",
+and a kernel parameter, "nmi_watchdog"
+(see "Documentation/admin-guide/kernel-parameters.rst" 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
+configuration.
+
+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.
+
+By default, the watchdog runs on all online cores.  However, on a
+kernel configured with NO_HZ_FULL, by default the watchdog runs only
+on the housekeeping cores, not the cores specified in the "nohz_full"
+boot argument.  If we allowed the watchdog to run by default on
+the "nohz_full" cores, we would have to run timer ticks to activate
+the scheduler, which would prevent the "nohz_full" functionality
+from protecting the user code on those cores from the kernel.
+Of course, disabling it by default on the nohz_full cores means that
+when those cores do enter the kernel, by default we will not be
+able to detect if they lock up.  However, allowing the watchdog
+to continue to run on the housekeeping (non-tickless) cores means
+that we will continue to detect lockups properly on those cores.
+
+In either case, the set of cores excluded from running the watchdog
+may be adjusted via the kernel.watchdog_cpumask sysctl.  For
+nohz_full cores, this may be useful for debugging a case where the
+kernel seems to be hanging on the nohz_full cores.