Am 19.10.23 um 15:53 schrieb Fiona Ebner: > 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 :) >
Unfortunately, scsi_dma_command() also has a blk_is_available() call and I ran into a similar deadlock with that once. > (...) > >>>> >>>> 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. > I should also mention that I used drive-mirror and drive-backup. I can also trigger the issue with blockdev-mirror when canceling, but didn't run into it when starting with blockdev-mirror yet. And I found that with drive-mirror, the issue during starting seems to manifest with the bdrv_open() call. Adding a return before it, the guest IO didn't get stuck in my testing, but adding a return after it, it can get stuck. I'll try to see if I can further narrow it down next week, but maybe that's already a useful hint? > diff --git a/blockdev.c b/blockdev.c > index a01c62596b..156915d15d 100644 > --- a/blockdev.c > +++ b/blockdev.c > @@ -3126,6 +3126,8 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp) > } > aio_context_release(aio_context); > > + // return; // GOOD > + > /* Mirroring takes care of copy-on-write using the source's backing > * file. > */ > @@ -3133,6 +3135,8 @@ void qmp_drive_mirror(DriveMirror *arg, Error **errp) > target_bs = bdrv_open(arg->target, NULL, options, flags, errp); > aio_context_release(qemu_get_aio_context()); > > + // return; // BAD > + > if (!target_bs) { > return; > } Best Regards, Fiona