Oops on Power8 (was Re: [PATCH v2 1/7] workqueue: make workqueue available early during boot)

[Balbir Singh]

On 10/10/16 23:53, Tejun Heo wrote:
> On Mon, Oct 10, 2016 at 10:17:16PM +1100, Balbir Singh wrote:
>> rest_init()
>> {
>> 	...
>> 	kernel_thread(kernel_init, NULL, CLONE_FS);
>> 	numa_default_policy();
>> 	pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
>> 	rcu_read_lock();
>> 	kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
>> 	...
>>
>> }
>>
>> create_worker() needs kthreadd, it wakes up kthreadd in kthread_create_on_node,
>> workqueue_init() is called from kernel_init() , but kthreadd is created after
>> the call to kernel_init(), so its touch and go
> 
> But the first thing kernel_init_freeable() does is
> wait_for_completion(&kthreadd_done).
> 

Yes, Of course, looking at the stack trace again, it was not the wake_up itself,
but the absence of cfs_rq of p->se that caused the issue. Will try and chase it
down. Quick look shows cgroup_init() has occurred before workqueue_init(), so
ideally p->se.cfs_rq should be allocated.

Sorry for the noise,

Balbir