Am 19.10.23 um 14:14 schrieb Kevin Wolf: > Am 18.10.2023 um 11:42 hat Fiona Ebner geschrieben: >> Am 17.10.23 um 16:20 schrieb Kevin Wolf: >>> Am 17.10.2023 um 15:37 hat Fiona Ebner geschrieben: >>>> Am 17.10.23 um 14:12 schrieb Kevin Wolf: >>>>> Am 17.10.2023 um 12:18 hat Fiona Ebner geschrieben: >>>>>> I ran into similar issues now with mirror, (both deadlocks and stuck >>>>>> guest IO at other times), and interestingly, also during job start. >>>>>> >>>>>> Also had a backtrace similar to [0] once, so I took a closer look. >>>>>> Probably was obvious to others already, but for the record: >>>>>> >>>>>> 1. the graph is locked by the main thread >>>>>> 2. the iothread holds the AioContext lock >>>>>> 3. the main thread waits on the AioContext lock >>>>>> 4. the iothread waits for coroutine spawned by blk_is_available() >>>>> >>>>> Where does this blk_is_available() in the iothread come from? Having it >>>>> wait without dropping the AioContext lock sounds like something that >>>>> we'd want to avoid. Ideally, devices using iothreads shouldn't use >>>>> synchronous requests at all, but I think scsi-disk might have some of >>>>> them. >>>>> >>>> >>>> It's part of the request handling in virtio-scsi: >>>> >>>>> #0 0x00007ff7f5f55136 in __ppoll (fds=0x7ff7e40030c0, nfds=8, >>>>> timeout=<optimized out>, sigmask=0x0) at >>>>> ../sysdeps/unix/sysv/linux/ppoll.c:42 >>>>> #1 0x00005587132615ab in qemu_poll_ns (fds=0x7ff7e40030c0, nfds=8, >>>>> timeout=-1) at ../util/qemu-timer.c:339 >>>>> #2 0x000055871323e8b1 in fdmon_poll_wait (ctx=0x55871598d5e0, >>>>> ready_list=0x7ff7f288ebe0, timeout=-1) at ../util/fdmon-poll.c:79 >>>>> #3 0x000055871323e1ed in aio_poll (ctx=0x55871598d5e0, blocking=true) at >>>>> ../util/aio-posix.c:670 >>>>> #4 0x0000558713089efa in bdrv_poll_co (s=0x7ff7f288ec90) at >>>>> /home/febner/repos/qemu/block/block-gen.h:43 >>>>> #5 0x000055871308c362 in blk_is_available (blk=0x55871599e2f0) at >>>>> block/block-gen.c:1426 >>>>> #6 0x0000558712f6843b in virtio_scsi_ctx_check (s=0x558716c049c0, >>>>> d=0x55871581cd30) at ../hw/scsi/virtio-scsi.c:290 >>> >>> Oh... So essentially for an assertion. >>> >>> I wonder if the blk_is_available() check introduced in 2a2d69f490c is >>> even necessary any more, because BlockBackend has its own AioContext >>> now. And if blk_bs(blk) != NULL isn't what we actually want to check if >>> the check is necessary, because calling bdrv_is_inserted() doesn't seem >>> to have been intended. blk_bs() wouldn't have to poll. >>> >> >> Could virtio_scsi_hotunplug() be an issue with removing or modifying >> the check? There's a call there which sets the blk's AioContext to >> qemu_get_aio_context(). Or are we sure that the assert in >> virtio_scsi_ctx_check() can't be reached after that? > > I think that would be the kind of bug that the assertion tries to > catch, because then we would be sending requests to blk from a thread > that doesn't match its AioContext (which will be allowed soon, but not > quite yet). > > Before resetting the AioContext, virtio_scsi_hotunplug() calls > qdev_simple_device_unplug_cb(), which unrealizes the SCSI device. This > calls scsi_qdev_unrealize() -> scsi_device_purge_requests(), which in > turn drains blk and cancels all pending requests. So there should be > nothing left that could call into virtio_scsi_ctx_check() any more. > > The other argument is that after unrealize, virtio_scsi_device_get() > would return NULL anyway, so even if a request were still pending, it > would just fail instead of accessing the unplugged device. >
Okay, sounds like a way to get around that deadlock issue then :) (...) >>> >>> What does the stuck I/O look like? Is it stuck in the backend, i.e. the >>> device started requests that never complete? Or stuck from the guest >>> perspective, i.e. the device never checks for new requests? >>> >> >> AFAICT, from the guest perspective. >> >>> I don't really have an idea immediately, we'd have to find out where the >>> stuck I/O stops being processed. >>> >> >> I've described it in an earlier mail in this thread: >> https://lists.nongnu.org/archive/html/qemu-devel/2023-10/msg01900.html >> >> Quoting from there: >> >>> After the IO was stuck in the guest, I used bdrv_next_all_states() to >>> iterate over the states and there's only the bdrv_raw and the >>> bdrv_host_device. For both, tracked_requests was empty. > > And bs->in_flight and blk->in_flight are 0, too? > Yes. And queued_requests in the BlockBackend is also empty. > Is anything quiesced? No. quiesce_counter is 0 for both BlockDriverState instances as well as for the BlockBackend. quiesced_parent is false for both parents (i.e. child_root for the bdrv_raw and child_of_bds for the bdrv_file (this time I used VirtIO SCSI, in the quote it was VirtIO block)). >>> What is also very interesting is that the IO isn't always dead >>> immediately. It can be that the fio command still runs with lower speed >>> for a while (sometimes even up to about a minute, but most often about >>> 10-15 seconds or so). During that time, I still can see calls to >>> virtio_scsi_handle_cmd() and blk_aio_write_entry(). Then they suddenly stop. >> >> Noting again that (at least for backup) it happens with both virtio-blk >> and virtio-scsi and with both aio=io_uring and aio=threads. I also tried >> different host kernels 5.15, 6.2 and 6.5 and guest kernels 5.10 and 6.1. > > You say "at least for backup". Did you see the bug for other job types, > too? > Yes, sorry. I meant to say that I only tested it for the backup canceling with all those configurations. I did run into the stuck guest IO issue in three different scenarios: canceling backup, canceling mirror and starting mirror: >>>>> Am 17.10.2023 um 12:18 hat Fiona Ebner geschrieben: >>>>>> I ran into similar issues now with mirror, (both deadlocks and stuck >>>>>> guest IO at other times), and interestingly, also during job start. But for mirror, I only used VirtIO SCSI and aio=io_uring and didn't test other configurations. Best Regards, Fiona