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? Kevin
