Am 13.02.26 um 5:05 PM schrieb Kevin Wolf: > Am 13.02.2026 um 15:26 hat Jens Axboe geschrieben: >> When a vCPU thread handles MMIO (holding BQL), aio_co_enter() runs the >> block I/O coroutine inline on the vCPU thread because >> qemu_get_current_aio_context() returns the main AioContext when BQL is >> held. The coroutine calls luring_co_submit() which queues an SQE via >> fdmon_io_uring_add_sqe(), but the actual io_uring_submit() only happens >> in gsource_prepare() on the main loop thread. > > Ouch! Yes, looks like we completely missed I/O submitted in vCPU threads > in the recent changes (or I guess worker threads in theory, but I don't > think there any that actually make use of aio_add_sqe()). > >> Since the coroutine ran inline (not via aio_co_schedule()), no BH is >> scheduled and aio_notify() is never called. The main loop remains asleep >> in ppoll() with up to a 499ms timeout, leaving the SQE unsubmitted until >> the next timer fires. >> >> Fix this by calling aio_notify() after queuing the SQE. This wakes the >> main loop via the eventfd so it can run gsource_prepare() and submit the >> pending SQE promptly. >> >> This is a generic fix that benefits all devices using aio=io_uring. >> Without it, AHCI/SATA devices see MUCH worse I/O latency since they use >> MMIO (not ioeventfd like virtio) and have no other mechanism to wake the >> main loop after queuing block I/O. >> >> This is usually a bit hard to detect, as it also relies on the ppoll >> loop not waking up for other activity, and micro benchmarks tend not to >> see it because they don't have any real processing time. With a >> synthetic test case that has a few usleep() to simulate processing of >> read data, it's very noticeable. The below example reads 128MB with >> O_DIRECT in 128KB chunks in batches of 16, and has a 1ms delay before >> each batch submit, and a 1ms delay after processing each completion. >> Running it on /dev/sda yields: >> >> time sudo ./iotest /dev/sda >> >> ________________________________________________________ >> Executed in 25.76 secs fish external >> usr time 6.19 millis 783.00 micros 5.41 millis >> sys time 12.43 millis 642.00 micros 11.79 millis >> >> while on a virtio-blk or NVMe device we get: >> >> time sudo ./iotest /dev/vdb >> >> ________________________________________________________ >> Executed in 1.25 secs fish external >> usr time 1.40 millis 0.30 millis 1.10 millis >> sys time 17.61 millis 1.43 millis 16.18 millis >> >> time sudo ./iotest /dev/nvme0n1 >> >> ________________________________________________________ >> Executed in 1.26 secs fish external >> usr time 6.11 millis 0.52 millis 5.59 millis >> sys time 13.94 millis 1.50 millis 12.43 millis >> >> where the latter are consistent. If we run the same test but keep the >> socket for the ssh connection active by having activity there, then >> the sda test looks as follows: >> >> time sudo ./iotest /dev/sda >> >> ________________________________________________________ >> Executed in 1.23 secs fish external >> usr time 2.70 millis 39.00 micros 2.66 millis >> sys time 4.97 millis 977.00 micros 3.99 millis >> >> as now the ppoll loop is woken all the time anyway. >> >> After this fix, on an idle system: >> >> time sudo ./iotest /dev/sda >> >> ________________________________________________________ >> Executed in 1.30 secs fish external >> usr time 2.14 millis 0.14 millis 2.00 millis >> sys time 16.93 millis 1.16 millis 15.76 millis >> >> Signed-off-by: Jens Axboe <[email protected]> >> --- >> util/fdmon-io_uring.c | 8 ++++++++ >> 1 file changed, 8 insertions(+) >> >> diff --git a/util/fdmon-io_uring.c b/util/fdmon-io_uring.c >> index d0b56127c670..96392876b490 100644 >> --- a/util/fdmon-io_uring.c >> +++ b/util/fdmon-io_uring.c >> @@ -181,6 +181,14 @@ static void fdmon_io_uring_add_sqe(AioContext *ctx, >> >> trace_fdmon_io_uring_add_sqe(ctx, opaque, sqe->opcode, sqe->fd, >> sqe->off, >> cqe_handler); >> + >> + /* >> + * Wake the main loop if it is sleeping in ppoll(). When a vCPU thread >> + * runs a coroutine inline (holding BQL), it queues SQEs here but the >> + * actual io_uring_submit() only happens in gsource_prepare(). Without >> + * this notify, ppoll() can sleep up to 499ms before submitting. >> + */ >> + aio_notify(ctx); >> } > > Makes sense to me. > > At first I wondered if we should use defer_call() for the aio_notify() > to batch the submission, but of course holding the BQL will already take > care of that. And in iothreads where there is no BQL, the aio_notify() > shouldn't make a difference anyway because we're already in the right > thread. > > I suppose the other variation could be have another io_uring_enter() > call here (but then probably really through defer_call()) to avoid > waiting for another CPU to submit the request in its main loop. But I > don't really have an intuition if that would make things better or worse > in the common case. > > Fiona, does this fix your case, too?
Yes, it does fix my issue [0] and the second patch gives another small improvement :) Would it be slightly cleaner to have aio_add_sqe() call aio_notify() itself? Since aio-posix.c calls downwards into fdmon-io_uring.c, it would feel nicer to me to not have fdmon-io_uring.c call "back up". I guess it also depends on whether we expect another future fdmon implementation with .add_sqe() to also benefit from it. [0]: https://lore.kernel.org/qemu-devel/[email protected]/ Best Regards, Fiona
