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@xxxxxxxxxx> wrote: > > > On Fri, Jun 19 2020 at 6:11am -0400, > > Ming Lei <ming.lei@xxxxxxxxxx> 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@xxxxxxxxxx> 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
