Re: [PATCH 1/5] block: don't call blk_mq_delay_run_hw_queue() in case of BLK_STS_RESOURCE

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

On Tue, Sep 19, 2017 at 04:49:15PM +0000, Bart Van Assche wrote:
> On Wed, 2017-09-20 at 00:04 +0800, Ming Lei wrote:
> > Run queue at end_io is definitely wrong, because blk-mq has SCHED_RESTART
> > to do that already.
> 
> Sorry but I disagree. If SCHED_RESTART is set that causes the blk-mq core to
> reexamine the software queues and the hctx dispatch list but not the requeue
> list. If a block driver returns BLK_STS_RESOURCE then requests end up on the
> requeue list. Hence the following code in scsi_end_request():
> 
> 	if (scsi_target(sdev)->single_lun || !list_empty(&sdev->host->starved_list))
> 		kblockd_schedule_work(&sdev->requeue_work);
> 	else
> 		blk_mq_run_hw_queues(q, true);

Let's focus on dm-rq.

I have explained before, dm-rq is different with SCSI's, we don't need
to requeue request any more in dm-rq if blk_get_request() returns NULL
in multipath_clone_and_map(), since SCHED_RESTART can cover that
definitely.

Not mention dm_mq_delay_requeue_request() will run the queue explicitly
if it has to be done. That needn't SCHED_RESTART to cover, right?

-- 
Ming