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

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

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.


Thanks, 
Ming