Re: [PATCH] blk-mq: run queue after issuing the last request of the plug list

[Ming Lei <ming.lei@redhat.com>]

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