CPU hotplug is used to facilitate the modification of the HK_TYPE_KERNEL_NOISE and HK_TYPE_MANAGED_IRQ cpumasks. However, tearing down and bringing up CPUs can impact the cpuset partition states as well. For instance, tearing down the last CPU of a partition can invalidate the partition with active tasks which will not happen if CPU hotplug isn't used.
A workaround of this issue is disable the invalidation by pretending that the partition has no task, and making the tasks within the partition to the effective CPUs of its parent for a short while during the short transition process where the CPUs will be teared down and the brought up again. Signed-off-by: Waiman Long <[email protected]> --- kernel/cgroup/cpuset.c | 18 ++++++++++++++++++ 1 file changed, 18 insertions(+) diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c index a927b9cd4f71..5f6b4e67748f 100644 --- a/kernel/cgroup/cpuset.c +++ b/kernel/cgroup/cpuset.c @@ -434,6 +434,13 @@ static inline bool partition_is_populated(struct cpuset *cs, struct cpuset *cp; struct cgroup_subsys_state *pos_css; + /* + * Hack: In cpuhp_offline_cb_mode, pretend all partitions are empty + * to prevent unnecessary partition invalidation. + */ + if (cpuhp_offline_cb_mode) + return false; + /* * We cannot call cs_is_populated(cs) directly, as * nr_populated_domain_children may include populated @@ -3881,6 +3888,17 @@ hotplug_update_tasks(struct cpuset *cs, cs->effective_mems = *new_mems; spin_unlock_irq(&callback_lock); + /* + * When cpuhp_offline_cb_mode is active, valid isolated partition + * with tasks may have no online CPUs available for a short while. + * In that case, we fall back to parent's effective CPUs temporarily + * which will be reset back to their rightful value once the affected + * CPUs are online again. + */ + if (cpuhp_offline_cb_mode && cpumask_empty(new_cpus) && + (cs->partition_root_state == PRS_ISOLATED)) + cpumask_copy(new_cpus, parent_cs(cs)->effective_cpus); + if (cpus_updated) cpuset_update_tasks_cpumask(cs, new_cpus); if (mems_updated) -- 2.53.0
