On Thu, Nov 17, 2022 at 09:05:48AM +0100, Andreas Hindborg wrote: > > Ming Lei <ming.lei@xxxxxxxxxx> writes: > > > 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? > > I collected this trace on my fork, but since the behavior comes from > task_work.c using a stack to queue work, it would be present on upstream > ublk as well. For completeness, I have verified that this is the case. > > > 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. > > I forgot to mention that I set the deadline scheduler for both ublkb0 > and the loopback target. No reordering should happen in mq with the > deadline scheduler, as far as I understand. I meant you need to setup one zoned ublk-loop for observing write request order, block layer never guarantees request order for other devices. > > > > > Otherwise, can you observe zoned write req reorder from ublksrv side? > > > > Yes, the reverse order is observable in ublk server. Reordering happens > in ublk kernel driver. > > >> > >> 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. > > In reverse order for requests that are issued without a context switch, > as far as I can tell. > > > > > 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. > > I think it is not feasible to have per zone data structures. Instead, I If one mapping data structure is used for ordering per-zone write request, it could be pretty easy, such as C++'s map or hash table, and it won't take much memory given the max in-flight IOs are very limited and the zone mapping entry can be reused among different zone, and quite easy to implement. Also most of times, the per-zone ordering can be completed in current batch(requests completed from single io_uring_enter()), then the extra cost could be close to zero, you can simply run the per-zone ordering in ->handle_io_background() callback, when all requests could come for each zone most of times. > considered adding a sequence number to each request, and then queue up > requests if there is a gap in the numbering. IMO, that won't be doable, given you don't know when the sequence will be over. > > But really, the issue should be resolved by changing the way > ublk_queue_rq() is sending work to uring context with task_add_work(). Not sure it can be solved easily given llist is implemented in this way. > Fixing in ublksrv is a bit of a hack and will have performance penalty. As I mentioned, ordering zoned write request in each zone just takes a little memory(mapping, or hashing) and the max size of the mapping table is queue depth, and basically zero cpu cost, not see extra performance penalty is involved. > > Also, it can not be good for any storage device to have sequential > requests delivered in reverse order? Fixing this would benefit all targets. Only zoned target has strict ordering requirement which does take cost, block layer never guarantees request order. As I mentioned, blk_add_rq_to_plug() can re-order requests in reverse order too. Thanks, Ming
