Hi Ming On 4/1/19 11:28 AM, Ming Lei wrote: > On Mon, Apr 01, 2019 at 11:25:50AM +0800, jianchao.wang wrote: >> Hi Ming >> >> On 4/1/19 10:52 AM, Ming Lei wrote: >>>> percpu_ref_tryget_live() fails if a per-cpu counter is in the "dead" state. >>>> percpu_ref_kill() changes the state of a per-cpu counter to the "dead" >>>> state. blk_freeze_queue_start() calls percpu_ref_kill(). blk_cleanup_queue() >>>> already calls blk_set_queue_dying() and that last function calls >>>> blk_freeze_queue_start(). So I think that what you wrote is not correct and >>>> that inserting a percpu_ref_tryget_live()/percpu_ref_put() pair in >>>> blk_mq_run_hw_queues() or blk_mq_run_hw_queue() would make a difference and >>>> also that moving the percpu_ref_exit() call into blk_release_queue() makes >>>> sense. >>> If percpu_ref_exit() is moved to blk_release_queue(), we still need to >>> move freeing of hw queue's resource into blk_release_queue() like what >>> the patchset is doing. >>> >>> Then we don't need to get/put q_usage_counter in blk_mq_run_hw_queues() any more, >>> do we? >> >> IMO, if we could get a way to prevent any attempt to run queue, it would be >> better and clearer. > > It is hard to do that way, and not necessary. > > I will post V2 soon for review. > Put percpu_ref_tryget/put pair into blk_mq_run_hw_queues could stop run queue after requet_queue is frozen and drained (run queue is also unnecessary because there is no entered requests). And also percpu_ref_tryget could avoid the io hung issue you mentioned. We have similar one in blk_mq_timeout_work. freeze and drain queue to stop new attempt to run queue, blk_sync_queue syncs and stops the started ones, then hctx->run_queue is cleaned totally. IMO, it would be better to have a checkpoint after which there will be no any in-flight asynchronous activities of the request_queue (hctx->run_work, q->requeue_work, q-> timeout_work) and any attempt to start them will fail. Perhaps, this will be a good change to do this ；） Thanks Jianchao
