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(). And rq2 should be visible to the run queue, just wondering why rq2 isn't issued finally? Thanks, Ming
