diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/lockup-watchdogs.txt | 18 | ||||
| -rw-r--r-- | Documentation/sysctl/kernel.txt | 21 |
2 files changed, 39 insertions, 0 deletions
diff --git a/Documentation/lockup-watchdogs.txt b/Documentation/lockup-watchdogs.txt index ab0baa6..22dd6af 100644 --- a/Documentation/lockup-watchdogs.txt +++ b/Documentation/lockup-watchdogs.txt @@ -61,3 +61,21 @@ 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. diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt index c831001..e5d528e 100644 --- a/Documentation/sysctl/kernel.txt +++ b/Documentation/sysctl/kernel.txt @@ -923,6 +923,27 @@ and nmi_watchdog. ============================================================== +watchdog_cpumask: + +This value can be used to control on which cpus the watchdog may run. +The default cpumask is all possible cores, but if NO_HZ_FULL is +enabled in the kernel config, and cores are specified with the +nohz_full= boot argument, those cores are excluded by default. +Offline cores can be included in this mask, and if the core is later +brought online, the watchdog will be started based on the mask value. + +Typically this value would only be touched in the nohz_full case +to re-enable cores that by default were not running the watchdog, +if a kernel lockup was suspected on those cores. + +The argument value is the standard cpulist format for cpumasks, +so for example to enable the watchdog on cores 0, 2, 3, and 4 you +might say: + + echo 0,2-4 > /proc/sys/kernel/watchdog_cpumask + +============================================================== + watchdog_thresh: This value can be used to control the frequency of hrtimer and NMI |