Hi Max,

On Tue, Feb 20, 2018 at 11:56:07AM +0200, Max Gurtovoy wrote:

hi all,
is there a way to drain a blk-mq based request queue (similar to
blk_drain_queue for non MQ) ?

Generally speaking, blk_mq_freeze_queue() should be fine to drain blk-mq
based request queue, but it may not work well when the hardware is broken.

I try to fix the following situation:
Running DM-multipath over NVMEoF/RDMA block devices, toggling the switch
ports during traffic using fio and making sure the traffic never fails.

when the switch port goes down the initiator driver start an error recovery

What is the code you are referring to?

process
- blk_mq_quiesce_queue for each namespace request queue

blk_mq_quiesce_queue() only guarantees that no requests can be dispatched to
low level driver, and new requests still can be allocated, but can't be
dispatched until the queue becomes unquiesced.

- cancel all requests of the tagset using blk_mq_tagset_busy_iter

Generally blk_mq_tagset_busy_iter() is used to cancel all in-flight
requests, and it depends on implementation of the busy_tag_iter_fn, and
timed-out request can't be covered by blk_mq_tagset_busy_iter().

So blk_mq_tagset_busy_iter() is often used in error recovery path, such
as nvme_dev_disable(), which is usually used in resetting PCIe NVMe controller.

- destroy the QPs/RDMA connections and MR pools
- blk_mq_unquiesce_queue for each namespace request queue
- reconnect to the target (after creating RDMA resources again)

During the QP destruction, I see a warning that not all the memory regions
were back to the mr_pool. For every request we get from the block layer
(well, almost every request) we get a MR from the MR pool.
So what I see is that, depends on the timing, some requests are
dispatched/completed after we blk_mq_unquiesce_queue and after we destroy
the QP and the MR pool. Probably these request were inserted during
quiescing,

Yes.

and I want to flush/drain them before I destroy the QP.

As mentioned above, you can't do that by blk_mq_quiesce_queue() &
blk_mq_tagset_busy_iter().

The PCIe NVMe driver takes two steps for the error recovery: nvme_dev_disable() &
nvme_reset_work(), and you may consider the similar approach, but the in-flight
requests won't be drained in this case because they can be requeued.

Could you explain a bit what your exact problem is?

Thanks,
Ming