Hi Ming On 4/1/19 6:03 PM, Ming Lei wrote: > On Mon, Apr 01, 2019 at 05:19:01PM +0800, jianchao.wang wrote: >> 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. > > If percpu_ref_tryget() is used, percpu_ref_exit() has to be moved into > queue's release handler. > > Then we still have to move freeing hctx's resource into hctx or queue's > release handler, that is exactly what this patch is doing. Then > percpu_ref_tryget() becomes unnecessary again, right? I'm not sure about the percpu_ref_exit. Perhaps I have some misunderstanding about it. >From the code of it, it frees the percpu_count and set ref->percpu_count_ptr to __PERCPU_REF_ATOMIC_DEAD. The comment says 'the caller is responsible for ensuring that @ref is no longer in active use' But if we use it after kill, does it count a active use ? Based on the code, the __ref_is_percpu is always false during this, and percpu_ref_tryget will not touch the freed percpu counter but just the atomic ref->count. It looks safe. > >> >> 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. > > All are canceled in blk_cleanup_queue(), but not enough, given queue can > be run in sync mode(such as via plug, direct issue, ...), or driver's > requeue, such as SCSI's requeue. SCSI's requeue may run other LUN's queue > just by holding queue's kobject refcount. Yes, so we need a checkpoint here to ensure the request_queue to enter into a certain state. We provide a guarantee that all of the activities are stopped after this checkpoint. It will be convenient for us to do other things following, for example release request_queue's resource. Thanks Jianchao > >> >> Perhaps, this will be a good change to do this ；） > > However, I don't see it is necessary if we simply move freeing hctx's > resource into its release handler, just like V2. > > > Thanks, > Ming >
