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

[Bart Van Assche]

On Wed, 2017-04-12 at 11:42 +0800, Ming Lei wrote:
> On Tue, Apr 11, 2017 at 06:18:36PM +0000, Bart Van Assche wrote:
> > 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.
> 
> But looks this patch just reruns the hw queue after 100ms, which isn't
> that after the busy state has been cleared, right?

Hello Ming,

That patch can be considered as a first step that can be refined further, namely
by modifying the dm-rq code further such that dm-rq queues are only rerun after
the busy condition has been cleared. The patch at the start of this thread is
easier to review and easier to test than any patch that would only rerun dm-rq
queues after the busy condition has been cleared.

> Actually if BLK_MQ_RQ_QUEUE_BUSY is returned from .queue_rq(), blk-mq
> will buffer this request into hctx->dispatch and run the hw queue again,
> so looks blk_mq_delay_run_hw_queue() in this situation shouldn't have been
> needed at my 1st impression.

If the blk-mq core would always rerun a hardware queue if a block driver
returns BLK_MQ_RQ_QUEUE_BUSY then that would cause 100% of a single CPU core
to be busy with polling a hardware queue until the "busy" condition has been
cleared. One can see easily that that's not what the blk-mq core does. From
blk_mq_sched_dispatch_requests():

	if (!list_empty(&rq_list)) {
		blk_mq_sched_mark_restart_hctx(hctx);
		did_work = blk_mq_dispatch_rq_list(q, &rq_list);
	}

>From the end of blk_mq_dispatch_rq_list():

	if (!list_empty(list)) {
		[ ... ]
		if (!blk_mq_sched_needs_restart(hctx) &&
		    !test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state))
			blk_mq_run_hw_queue(hctx, true);
	}

In other words, the BLK_MQ_S_SCHED_RESTART flag is set before the dispatch list
is examined and only if that flag gets cleared while blk_mq_dispatch_rq_list()
is in progress by a concurrent blk_mq_sched_restart_hctx() call then the
dispatch list will be rerun after a block driver returned BLK_MQ_RQ_QUEUE_BUSY.

Bart.