Re: [PATCH] sched/fair: add !se->on_rq check before dequeue entity

[chenqiwu]

> 
> Hmm i have been too quick in my reply. I wanted to say:
> AFAICT, there is no other way to dequeue a task from a cfs_rq for
> which the group entity is not enqueued

But we should notice the potential racy pathes called by deactivate_task().
For example:
One path is dequeue a task from its cfs_rq called by schedule():
__schedule
	deactivate_task
		dequeue_task
			dequeue_task_fair

Another path is trying to migrate the same task to a CPU on the preferred node:
numa_migrate_preferred
	task_numa_migrate	
		migrate_swap
			stop_two_cpus
				migrate_swap_stop
					__migrate_swap_task
						deactivate_task
							dequeue_task_fair

There could be a racy if the task is dequeued form its cfs_rq in parallel.