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? >>> #7 0x0000558712f697e4 in virtio_scsi_handle_cmd_req_prepare >>> (s=0x558716c049c0, req=0x7ff7e400b650) at ../hw/scsi/virtio-scsi.c:788 >>> #8 0x0000558712f699b0 in virtio_scsi_handle_cmd_vq (s=0x558716c049c0, >>> vq=0x558716c0d2a8) at ../hw/scsi/virtio-scsi.c:831 >>> #9 0x0000558712f69bcb in virtio_scsi_handle_cmd (vdev=0x558716c049c0, >>> vq=0x558716c0d2a8) at ../hw/scsi/virtio-scsi.c:867 >>> #10 0x0000558712f96812 in virtio_queue_notify_vq (vq=0x558716c0d2a8) at >>> ../hw/virtio/virtio.c:2263 >>> #11 0x0000558712f99b75 in virtio_queue_host_notifier_read >>> (n=0x558716c0d31c) at ../hw/virtio/virtio.c:3575 >>> #12 0x000055871323d8b5 in aio_dispatch_handler (ctx=0x55871598d5e0, >>> node=0x558716771000) at ../util/aio-posix.c:372 >>> #13 0x000055871323d988 in aio_dispatch_ready_handlers (ctx=0x55871598d5e0, >>> ready_list=0x7ff7f288eeb0) at ../util/aio-posix.c:401 >> >> >>>> As for why it doesn't progress, blk_co_is_available_entry() uses >>>> bdrv_graph_co_rdlock() and can't get it, because the main thread has the >>>> write lock. Should be fixed once the AioContext locks are gone, but not >>>> sure what should be done to avoid it until then. >>> >>> Then the nested event loop in blk_is_available() would probably be >>> enough to make progress, yes. >>> >>> Maybe we could actually drop the lock (and immediately reacquire it) in >>> AIO_WAIT_WHILE() even if we're in the home thread? That should give the >>> main thread a chance to make progress. >> >> Seems to work :) I haven't run into the issue with the following change >> anymore, but I have to say, running into that specific deadlock only >> happened every 10-15 tries or so before. Did 30 tests now. But >> unfortunately, the stuck IO issue is still there. >> >>> diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h >>> index 5449b6d742..da159501ca 100644 >>> --- a/include/block/aio-wait.h >>> +++ b/include/block/aio-wait.h >>> @@ -88,7 +88,13 @@ extern AioWait global_aio_wait; >>> smp_mb__after_rmw(); \ >>> if (ctx_ && in_aio_context_home_thread(ctx_)) { \ >>> while ((cond)) { \ >>> + if (unlock && ctx_) { \ >>> + aio_context_release(ctx_); \ >>> + } \ >>> aio_poll(ctx_, true); \ >>> + if (unlock && ctx_) { \ >>> + aio_context_acquire(ctx_); \ >>> + } \ >>> waited_ = true; \ >>> } \ >>> } else { \ > > For reacquiring the lock, I really meant "immediately". Calling > aio_poll() without the lock is wrong. Unfortunately, then it's not enough, because the call to aio_poll() is blocking and because the lock is held during that call, the very same deadlock can still happen. > > 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. > > 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. >>> But I think we're actually not very far from removing the AioContext >>> lock, so if we can't find an easy fix in the meantime, waiting for that >>> could be a realistic option. >>> >> >> Glad to hear :) Do you think it will be in time for QEMU 8.2? Otherwise, >> I'll go ahead and send what I have for fixing the deadlocks from this >> mail thread in the following days. The stuck guest IO can happen even >> without any of those changes (on current master, i.e. >> ebca80bbdb5c1650e4b753a3d13b43634e7dfe05, at least when starting a >> mirror job). > > Having it in 8.2 is certainly the plan, but plans don't always work out. > If you have fixes that aren't too ugly, we can still apply them. > Okay, I'll give the ones for the deadlocks via block_job_remove_all_bdrv() and bdrv_close() another look/spin and try to send them in the next few days. Best Regards, Fiona