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

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

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