On 09/07/2017 11:15 PM, Peter Zijlstra wrote: > On Thu, Sep 07, 2017 at 07:26:23PM +0530, Prateek Sood wrote: >> Remove circular dependency deadlock in a scenario where hotplug of CPU is >> being done while there is updation in cgroup and cpuset triggered from >> userspace. >> >> Process A => kthreadd => Process B => Process C => Process A > >> Process A >> cpu_subsys_offline(); >> cpu_down(); >> _cpu_down(); >> percpu_down_write(&cpu_hotplug_lock); //held >> cpuhp_invoke_callback(); >> workqueue_offline_cpu(); >> wq_update_unbound_numa(); >> kthread_create_on_node(); >> wake_up_process(); //wakeup kthreadd > > TJ, I'm puzzled, why would we need to spawn new threads to update NUMA > affinity when taking a CPU out? That doesn't make sense to me, we can > either shrink the affinity of an existing thread or completely kill of a > thread if the mask becomes empty. But why spawn a new thread? > >> flush_work(); >> wait_for_completion(); >> > > Yes, inverting cpuset and hotplug would break that chain, but I'm still > wondering why workqueue needs to spawn threads on CPU down. > Thanks for the comments Peter You rightly mentioned that a new thread will not be spawn while updating NUMA affinity when taking a CPU out. While a CPU is made offline, attempt is made to unbind per-cpu worker for CPU going down. This is done by queuing unbind work to system_highpri_wq. It results in an attempt to create one bounded worker thread as there is none. wait_for_completion() in flush_work() waits for unbinding to finish for CPU going down. Process A cpu_subsys_offline(); cpu_down(); _cpu_down(); percpu_down_write(&cpu_hotplug_lock); //held cpuhp_invoke_callback(); workqueue_offline_cpu(); queue_work_on(system_highpri_wq); __queue_work(); insert_work(); wake_up_worker(); //pool->nr_running = 0 flush_work(); wait_for_completion(); worker_thread(); need_more_worker(); // returns true manage_workers(); maybe_create_worker(); create_worker(); kthread_create_on_node(); wake_up_process(kthreadd_task); -- Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc., is a member of Code Aurora Forum, a Linux Foundation Collaborative Project -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html