Re: dm-rq: don't call blk_mq_queue_stopped in dm_stop_queue()

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

On Sat, Jun 20, 2020 at 06:37:44AM +0800, Ming Lei wrote:
> On Fri, Jun 19, 2020 at 01:40:41PM -0400, Mike Snitzer wrote:
> > On Fri, Jun 19 2020 at 12:06pm -0400,
> > Mike Snitzer <snitzer@redhat.com> wrote:
> > 
> > > On Fri, Jun 19 2020 at  6:11am -0400,
> > > Ming Lei <ming.lei@redhat.com> wrote:
> > > 
> > > > Hi Mike,
> > > > 
> > > > On Fri, Jun 19, 2020 at 05:42:50AM -0400, Mike Snitzer wrote:
> > > > > Hi Ming,
> > > > > 
> > > > > Thanks for the patch!  But I'm having a hard time understanding what
> > > > > you've written in the patch header,
> > > > > 
> > > > > On Fri, Jun 19 2020 at  4:42am -0400,
> > > > > Ming Lei <ming.lei@redhat.com> wrote:
> > > > > 
> > > > > > dm-rq won't stop queue, meantime blk-mq won't stop one queue too, so
> > > > > > remove the check.
> > > > > 
> > > > > It'd be helpful if you could unpack this with more detail before going on
> > > > > to explain why using blk_queue_quiesced, despite dm-rq using
> > > > > blk_mq_queue_stopped, would also be ineffective.
> > > > > 
> > > > > SO:
> > > > > 
> > > > > > dm-rq won't stop queue
> > > > > 
> > > > > 1) why won't dm-rq stop the queue?  Do you mean it won't reliably
> > > > >    _always_ stop the queue because of the blk_mq_queue_stopped() check?
> > > > 
> > > > device mapper doesn't call blk_mq_stop_hw_queue or blk_mq_stop_hw_queues.
> > > > 
> > > > > 
> > > > > > meantime blk-mq won't stop one queue too, so remove the check.
> > > > > 
> > > > > 2) Meaning?: blk_mq_queue_stopped() will return true even if only one hw
> > > > > queue is stopped, given blk-mq must stop all hw queues a positive return
> > > > > from this blk_mq_queue_stopped() check is incorrectly assuming it meanss
> > > > > all hw queues are stopped.
> > > > 
> > > > blk-mq won't call blk_mq_stop_hw_queue or blk_mq_stop_hw_queues for
> > > > dm-rq's queue too, so dm-rq's hw queue won't be stopped.
> > > > 
> > > > BTW blk_mq_stop_hw_queue or blk_mq_stop_hw_queues are supposed to be
> > > > used for throttling queue.
> > > 
> > > I'm going to look at actually stopping the queue (using one of these
> > > interfaces).  I didn't realize I wasn't actually stopping the queue.
> > > The intent was to do so.
> > > 
> > > In speaking with Jens yesterday about freeze vs stop: it is clear that
> > > dm-rq needs to still be able to allocate new requests, but _not_ call
> > > the queue_rq to issue the requests, while "stopped" (due to dm-mpath
> > > potentially deferring retries of failed requests because of path failure
> > > while quiescing the queue during DM device suspend).  But that freezing
> > > the queue goes too far because it won't allow such request allocation.
> > 
> > Seems I'm damned if I do (stop) or damned if I don't (new reports of
> > requests completing after DM device suspend's
> > blk_mq_quiesce_queue()+dm_wait_for_completion()).
> 
> request(but not new) completing is possible after blk_mq_quiesce_queue()+
> dm_wait_for_completion, because blk_mq_rq_inflight() only checks INFLIGHT
> request. If all requests are marked as MQ_RQ_COMPLETE, blk_mq_rq_inflight()
> still may return false. However, MQ_RQ_COMPLETE is one transient state.
> 
> So what does dm-rq expect from dm_wait_for_completion()?
> 
> If it is just no new request entering dm_queue_rq(), there shouldn't be
> issue.
> 
> If dm-rq hopes there aren't any real inflight request(MQ_RQ_COMPLETE &
> MQ_RQ_INFLIGHT), we can change blk_mq_rq_inflight to support that.

Hi Mike,

Please test the following patch and see if the issue can be fixed:

From faf0f9f15627446e8c35db518e37a4a2e4323eb2 Mon Sep 17 00:00:00 2001
From: Ming Lei <ming.lei@redhat.com>
Date: Sat, 20 Jun 2020 06:45:49 +0800
Subject: [PATCH] blk-mq: cover request of MQ_RQ_COMPLETE as inflight in
 blk_mq_rq_inflight

When request is marked as MQ_RQ_COMPLETE, ->complete isn't called & done
yet, and driver may expect that there isn't any driver related activity since
blk_mq_queue_inflight() returns.

Fixes it by covering request of MQ_RQ_COMPLETE as inflight in blk_mq_rq_inflight().

Cc: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 4f57d27bfa73..7bc084b5bc37 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -831,7 +831,7 @@ static bool blk_mq_rq_inflight(struct blk_mq_hw_ctx *hctx, struct request *rq,
 	 * If we find a request that is inflight and the queue matches,
 	 * we know the queue is busy. Return false to stop the iteration.
 	 */
-	if (rq->state == MQ_RQ_IN_FLIGHT && rq->q == hctx->queue) {
+	if (rq->state != MQ_RQ_IDLE && rq->q == hctx->queue) {
 		bool *busy = priv;
 
 		*busy = true;
-- 
2.25.2



Thanks, 
Ming