8c27ceff36
The previous patch renamed several files that are cross-referenced along the Kernel documentation. Adjust the links to point to the right places. Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
83 lines
4.1 KiB
Plaintext
83 lines
4.1 KiB
Plaintext
===============================================================
|
|
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.
|