Re: [PATCH v4 6/6] dm rq: Avoid that request processing stalls sporadically

[Bart Van Assche <Bart.VanAssche@sandisk.com>]

On Tue, 2017-04-11 at 14:03 -0400, Mike Snitzer wrote:
> Rather than working so hard to use DM code against me, your argument
> should be: "blk-mq drivers X, Y and Z rerun the hw queue; this is a well
> established pattern"
> 
> I see drivers/nvme/host/fc.c:nvme_fc_start_fcp_op() does.  But that is
> only one other driver out of ~20 BLK_MQ_RQ_QUEUE_BUSY returns
> tree-wide.
> 
> Could be there are some others, but hardly a well-established pattern.

Hello Mike,

Several blk-mq drivers that can return BLK_MQ_RQ_QUEUE_BUSY from their
.queue_rq() implementation stop the request queue (blk_mq_stop_hw_queue())
before returning "busy" and restart the queue after the busy condition has
been cleared (blk_mq_start_stopped_hw_queues()). Examples are virtio_blk and
xen-blkfront. However, this approach is not appropriate for the dm-mq core
nor for the scsi core since both drivers already use the "stopped" state for
another purpose than tracking whether or not a hardware queue is busy. Hence
the blk_mq_delay_run_hw_queue() and blk_mq_run_hw_queue() calls in these last
two drivers to rerun a hardware queue after the busy state has been cleared.

Bart.