Alexey Klimov <aklimov@xxxxxxxxxx> writes: > When a CPU offlined and onlined via device_offline() and device_online() > the userspace gets uevent notification. If, after receiving "online" uevent, > userspace executes sched_setaffinity() on some task trying to move it > to a recently onlined CPU, then it sometimes fails with -EINVAL. Userspace > needs to wait around 5..30 ms before sched_setaffinity() will succeed for > recently onlined CPU after receiving uevent. > > If in_mask argument for sched_setaffinity() has only recently onlined CPU, > it could fail with such flow: > > sched_setaffinity() > cpuset_cpus_allowed() > guarantee_online_cpus() <-- cs->effective_cpus mask does not > contain recently onlined cpu > cpumask_and() <-- final new_mask is empty > __set_cpus_allowed_ptr() > cpumask_any_and_distribute() <-- returns dest_cpu equal to nr_cpu_ids > returns -EINVAL > > Cpusets used in guarantee_online_cpus() are updated using workqueue from > cpuset_update_active_cpus() which in its turn is called from cpu hotplug callback > sched_cpu_activate() hence it may not be observable by sched_setaffinity() if > it is called immediately after uevent. > > Out of line uevent can be avoided if we will ensure that cpuset_hotplug_work > has run to completion using cpuset_wait_for_hotplug() after onlining the > cpu in cpu_device_up() and in cpuhp_smt_enable(). > > Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx> > Reviewed-by: Qais Yousef <qais.yousef@xxxxxxx> > Co-analyzed-by: Joshua Baker <jobaker@xxxxxxxxxx> > Signed-off-by: Alexey Klimov <aklimov@xxxxxxxxxx> Looks good to me. Reviewed-by: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
