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.
