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Diffstat (limited to 'Documentation/lockup-watchdogs.txt')
| -rw-r--r-- | Documentation/lockup-watchdogs.txt | 18 |
1 files changed, 18 insertions, 0 deletions
diff --git a/Documentation/lockup-watchdogs.txt b/Documentation/lockup-watchdogs.txt index ab0baa692c13..22dd6af2e4bd 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. |