Damien Le Moal <damien.lemoal@xxxxxxxxxxxxxxxxxx> writes: > On 11/17/22 20:59, Andreas Hindborg wrote: >> >> Ming Lei <ming.lei@xxxxxxxxxx> writes: >> >>> On Thu, Nov 17, 2022 at 10:07:14AM +0100, Andreas Hindborg wrote: >>>> >>>> Ming Lei <ming.lei@xxxxxxxxxx> writes: >>>> >>>>> 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. >>>> >>>> We would not need to know when the sequence is over. If in ubdsrv we see >>>> request number 1,2,4, we could hold 4 until 3 shows up. When 3 shows up >>>> we go ahead and submit all requests from 3 and up, until there is >>>> another gap. If ublk_drv is setting the sequence numbers, >>>> cancelled/aborted requests would not be an issue. >>>> >>>> I think this would be less overhead than having per zone data structure. >>> >>> You can only assign it to zoned write request, but you still have to check >>> the sequence inside each zone, right? Then why not just check LBAs in >>> each zone simply? >> >> We would need to know the zone map, which is not otherwise required. >> Then we would need to track the write pointer for each open zone for >> each queue, so that we can stall writes that are not issued at the write >> pointer. This is in effect all zones, because we cannot track when zones >> are implicitly closed. Then, if different queues are issuing writes to >> the same zone, we need to sync across queues. Userspace may have >> synchronization in place to issue writes with multiple threads while >> still hitting the write pointer. >> >> It is not good enough to only impose ordering within a zone if we have >> requests in flight for that zone. For the first write to a zone when there >> are no writes in flight to that zone, we can not know if the write is at >> the write pointer, or if more writes to lower LBA is on the way. Not >> without tracking the write pointer. >> >> With a sequence number, the sequence number can be queue local. It would >> not guarantee that writes always happen at the write pointer, but it >> would guarantee that requests are not reordered by ublk_drv, which is >> all that is required. As long as userspace is issuing at the write >> pointer (as they are required to for zoned storage), this solution would >> work, even across multiple queues issuing writes to the same zone. >> >>> >>>> >>>> But I still think it is an unnecessary hack. We could just fix the driver. >>> >>> But not sure if it is easy to make io_uring_cmd_complete_in_task() or >>> task_work_add to complete request in order efficiently. >>> >>>> >>>>> >>>>>> >>>>>> 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. >>>> >>>> If we queue requests on a separate queue, we are fine. I can make a >>>> patch suggestion. >>> >>> The problem is that io_uring_cmd_complete_in_task() or task_work_add() may >>> re-order the request, can you explain why the issue can be solved by >>> separate queue? >> >> I would suggest to still use task_work_add() to queue a callback, but >> instead of carrying the pdu as the callback argument, use it as a >> notification that one or more work items are queued. Then we add the pdu >> to a hwctx local FIFO queue. >> >> __ublk_rq_task_work() would then check this FIFO queue and submit all >> the requests on the queue to userspace. >> >> Without further optimization __ublk_rq_task_work() would some times be >> called with an empty queue, but this should not be too much overhead. >> Maybe we could decide to not call task_work_add() if the hwctx local >> queue of pdus is not empty. >> >>> >>>> >>>>> >>>>>> 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. >>>> >>>> I could implement all three solutions. 1) fix the dirver, 2) per zone >>>> structure and 3) sequence numbers. Then I benchmark them and we will >>>> know what works. It's a lot of work though. >>> >>> Let's prove if the theory for each solution is correct first. >> >> Alright. >> >>> >>>> >>>>> >>>>>> >>>>>> 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. >>>> >>>> When mq-deadline scheduler is set for a queue, requests are not >>>> reordered, right? >>> >>> In case of batching submission, mq-deadline will order requests by >>> LBA in this whole batch. >> >> That is not what I am seeing in practice. I tried looking through >> mq-deadline.c, but I could not find code that would order by LBA. Could >> you point me to the code that implements this policy? > > see the use of the rb tree, deadline_add_rq_rb() and friends. The rb tree > maintains the requests in LBA order. The fifo list, maintains an arrival > order for starvation control. Thanks! > >> >>> >>>> >>>> I am no block layer expert, so I cannot argue about the implementation. >>>> But I think that both spinning rust and flash would benefit from having >>>> sequential requests delivered in order? Would it not hurt performance to >>>> reverse order for sequential requests all the time? I have a hard time >>>> understanding why the block layer would do this by default. >>>> >>>> One thing is to offer no guarantees, but to _always_ reverse the >>>> ordering of sequential requests seem a little counter productive to me. >>> >>> If io_uring_cmd_complete_in_task()/task_work_add() can complete io >>> commands to ublksrv in order without extra cost, that is better. >> >> I agree :) >> >>> >>> But I don't think it is a big deal for HDD. because when these requests >>> are re-issued from ublksrv to block layer, deadline or bfq will order >>> them too since all are submitted via io_uring in batch. >> >> As I wrote, that is not what I am seeing in my experiment. Repeating the >> output of blktrace from the top of this email: >> >> 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] >> >> Here the read of 6144: >> 259,1 0 1794 286.337794934 390 D R 6144 + 2048 [ublk] >> >> is clearly issued before the read of 4096: >> 259,1 0 1795 286.337805504 390 D R 4096 + 2048 [ublk] >> >> It is not because there are no IO in flight for the target device. Here >> is he trace with completions included: >> >> 259,2 10 1 0.000000000 468 D R 0 + 1024 [fio] >> 259,2 10 2 0.000005680 468 D R 1024 + 1024 [fio] >> 259,2 10 3 0.000007760 468 D R 2048 + 1024 [fio] >> 259,2 10 4 0.000009140 468 D R 3072 + 1024 [fio] >> 259,2 10 5 0.000010420 468 D R 4096 + 1024 [fio] >> 259,2 10 6 0.000011640 468 D R 5120 + 1024 [fio] >> 259,2 10 7 0.000013350 468 D R 6144 + 1024 [fio] >> 259,2 10 8 0.000014350 468 D R 7168 + 1024 [fio] >> 259,1 10 1 0.000280540 412 D R 6144 + 2048 [ublk] >> 259,1 10 2 0.000433260 412 D R 4096 + 2048 [ublk] >> 259,1 10 3 0.000603920 412 D R 2048 + 2048 [ublk] >> 259,1 10 4 0.000698070 412 D R 0 + 2048 [ublk] >> 259,1 10 5 0.000839250 0 C R 6144 + 2048 [0] >> 259,2 10 9 0.000891270 412 C R 7168 + 1024 [0] >> 259,2 10 10 0.000919780 412 C R 6144 + 1024 [0] >> 259,1 10 6 0.001258791 0 C R 4096 + 2048 [0] >> 259,2 10 11 0.001306541 412 C R 5120 + 1024 [0] >> 259,2 10 12 0.001335291 412 C R 4096 + 1024 [0] >> 259,1 10 7 0.001469911 0 C R 2048 + 2048 [0] >> 259,2 10 13 0.001518281 412 C R 3072 + 1024 [0] >> 259,2 10 14 0.001547041 412 C R 2048 + 1024 [0] >> 259,1 10 8 0.001600801 0 C R 0 + 2048 [0] >> 259,2 10 15 0.001641871 412 C R 1024 + 1024 [0] >> 259,2 10 16 0.001671921 412 C R 0 + 1024 [0] >> >> This last trace is vanilla Linux 6.0 with mq-deadline enabled for ublkb0(259,2) >> and the loopback target nvme0n1(259,1). > > queue at head that should be queue at tail ? if nvme0n1 is a multi queue > device and does not have a scheduler, there may be a lot of "issue > directly" that can really destroy the order of requests. It is a regular nvme drive handled by the nvme driver. So multi queue with mq-deadline set. Unless I am reading the trace wrong there are 3 reads issued to 259,1 sectors 6144, 4096 and 2048 in that order. This is the order userspace is issuing the reads, so the trace matches what I would expect. But now you tell me that the mq-deadline scheduler should reorder the requests based on LBA? But maybe the ordering by LBA is only for writes? I'll rerun the test with writes. Best regards, Andreas
