On Tue, Jul 26, 2022 at 09:08:19AM +0800, Yu Kuai wrote: > Hi, Ming! > > 在 2022/07/25 23:43, Ming Lei 写道: > > On Sat, Jul 23, 2022 at 10:50:03AM +0800, Yu Kuai wrote: > > > Hi, Ming! > > > > > > 在 2022/07/19 17:26, Ming Lei 写道: > > > > On Mon, Jul 18, 2022 at 08:35:28PM +0800, Yufen Yu wrote: > > > > > We do test on a virtio scsi device (/dev/sda) and the default mq > > > > > scheduler is 'none'. We found a IO hung as following: > > > > > > > > > > blk_finish_plug > > > > > blk_mq_plug_issue_direct > > > > > scsi_mq_get_budget > > > > > //get budget_token fail and sdev->restarts=1 > > > > > > > > > > scsi_end_request > > > > > scsi_run_queue_async > > > > > //sdev->restart=0 and run queue > > > > > > > > > > blk_mq_request_bypass_insert > > > > > //add request to hctx->dispatch list > > > > > > > > Here the issue shouldn't be related with scsi's get budget or > > > > scsi_run_queue_async. > > > > > > > > If blk-mq adds request into ->dispatch_list, it is blk-mq core's > > > > responsibility to re-run queue for moving on. Can you investigate a > > > > bit more why blk-mq doesn't run queue after adding request to > > > > hctx dispatch list? > > > > > > I think Yufen is probably thinking about the following Concurrent > > > scenario: > > > > > > blk_mq_flush_plug_list > > > # assume there are three rq > > > blk_mq_plug_issue_direct > > > blk_mq_request_issue_directly > > > # dispatch rq1, succeed > > > blk_mq_request_issue_directly > > > # dispatch rq2 > > > __blk_mq_try_issue_directly > > > blk_mq_get_dispatch_budget > > > scsi_mq_get_budget > > > atomic_inc(&sdev->restarts); > > > # rq2 failed to get budget > > > # restarts is 1 now > > > scsi_end_request > > > # rq1 is completed > > > ┊scsi_run_queue_async > > > ┊ atomic_cmpxchg(&sdev->restarts, > > > old, 0) == old > > > ┊ # set restarts to 0 > > > ┊ blk_mq_run_hw_queues > > > ┊ # hctx->dispatch list is empty > > > blk_mq_request_bypass_insert > > > # insert rq2 to hctx->dispatch list > > > > After rq2 is added to ->dispatch_list in blk_mq_try_issue_list_directly(), > > no matter if list_empty(list) is empty or not, queue will be run either from > > blk_mq_request_bypass_insert() or blk_mq_sched_insert_requests(). > > 1) while inserting rq2 to dispatch list, blk_mq_request_bypass_insert() > is called from blk_mq_try_issue_list_directly(), list_empty() won't > pass, thus thus blk_mq_request_bypass_insert() won't run queue. Yeah, but in blk_mq_try_issue_list_directly() after rq2 is inserted to dispatch list, the loop is broken and blk_mq_try_issue_list_directly() returns to blk_mq_sched_insert_requests() in which list_empty() is false, so blk_mq_insert_requests() and blk_mq_run_hw_queue() are called, queue is still run. Also not sure why you make rq3 involved, since the list is local list on stack, and it can be operated concurrently. Thanks, Ming
