Hi Andreas, On Wed, Nov 16, 2022 at 04:00:15PM +0100, Andreas Hindborg wrote: > > Hi Ming, > > I have a question regarding ublk. For context, I am working on adding > zoned storage support to ublk, and you have been very kind to help me on > Github [1]. > > I have a problem with ordering of requests after they are issued to the > ublk driver. Basically ublk will reverse the ordering of requests that are > batched. The behavior can be observed with the following fio workload: > > > fio --name=test --ioengine=io_uring --rw=read --bs=4m --direct=1 > > --size=4m --filename=/dev/ublkb0 > > For a loopback ublk target I get the following from blktrace: > > > 259,2 0 3469 286.337681303 724 D R 0 + 1024 [fio] > > 259,2 0 3470 286.337691313 724 D R 1024 + 1024 [fio] > > 259,2 0 3471 286.337694423 724 D R 2048 + 1024 [fio] > > 259,2 0 3472 286.337696583 724 D R 3072 + 1024 [fio] > > 259,2 0 3473 286.337698433 724 D R 4096 + 1024 [fio] > > 259,2 0 3474 286.337700213 724 D R 5120 + 1024 [fio] > > 259,2 0 3475 286.337702723 724 D R 6144 + 1024 [fio] > > 259,2 0 3476 286.337704323 724 D R 7168 + 1024 [fio] > > 259,1 0 1794 286.337794934 390 D R 6144 + 2048 [ublk] > > 259,1 0 1795 286.337805504 390 D R 4096 + 2048 [ublk] > > 259,1 0 1796 286.337816274 390 D R 2048 + 2048 [ublk] > > 259,1 0 1797 286.337821744 390 D R 0 + 2048 [ublk] > > And enabling debug prints in ublk shows that the reversal happens when > ublk defers work to the io_uring context: > > > kernel: ublk_queue_rq: qid 0, tag 180, sect 0 > > kernel: ublk_queue_rq: qid 0, tag 181, sect 1024 > > kernel: ublk_queue_rq: qid 0, tag 182, sect 2048 > > kernel: ublk_queue_rq: qid 0, tag 183, sect 3072 > > kernel: ublk_queue_rq: qid 0, tag 184, sect 4096 > > kernel: ublk_queue_rq: qid 0, tag 185, sect 5120 > > kernel: ublk_queue_rq: qid 0, tag 186, sect 6144 > > kernel: ublk_queue_rq: qid 0, tag 187, sect 7168 > > kernel: __ublk_rq_task_work: complete: op 33, qid 0 tag 187 io_flags 1 addr 7f245d359000, sect 7168 > > kernel: __ublk_rq_task_work: complete: op 33, qid 0 tag 186 io_flags 1 addr 7f245fcfd000, sect 6144 > > kernel: __ublk_rq_task_work: complete: op 33, qid 0 tag 185 io_flags 1 addr 7f245fd7f000, sect 5120 > > kernel: __ublk_rq_task_work: complete: op 33, qid 0 tag 184 io_flags 1 addr 7f245fe01000, sect 4096 > > kernel: __ublk_rq_task_work: complete: op 33, qid 0 tag 183 io_flags 1 addr 7f245fe83000, sect 3072 > > kernel: __ublk_rq_task_work: complete: op 33, qid 0 tag 182 io_flags 1 addr 7f245ff05000, sect 2048 > > kernel: __ublk_rq_task_work: complete: op 33, qid 0 tag 181 io_flags 1 addr 7f245ff87000, sect 1024 > > kernel: __ublk_rq_task_work: complete: op 33, qid 0 tag 180 io_flags 1 addr 7f2460009000, sect 0 > > The problem seems to be that the method used to defer work to the > io_uring context, task_work_add(), is using a stack to queue the > callbacks. Is the observation done on zoned ublk or plain ublk-loop? If it is plain ublk-loop, the reverse order can be started from blk_add_rq_to_plug(), but it won't happen for zoned write request which isn't queued to plug list. Otherwise, can you observe zoned write req reorder from ublksrv side? > > As you probably are aware, writes to zoned storage must > happen at the write pointer, so when order is lost there is a problem. > But I would assume that this behavior is also undesirable in other > circumstances. > > I am not sure what is the best approach to change this behavor. Maybe > having a separate queue for the requests and then only scheduling work > if a worker is not already processing the queue? > > If you like, I can try to implement a fix? Yeah, I think zoned write requests could be forwarded to ublk server out of order. Is it possible for re-order the writes in ublksrv side? I guess it is be doable: 1) zoned write request is sent to ublk_queue_rq() in order always 2) when ublksrv zone target/backend code gets zoned write request in each zone, it can wait until the next expected write comes, then handle the write and advance write pointer, then repeat the whole process. 3) because of 1), the next expected zoned write req is guaranteed to come without much delay, and the per-zone queue won't be long. Thanks, Ming
