On Wed, Feb 18, 2026 at 10:57:02AM +0100, Fiona Ebner wrote: > 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.
Calling aio_notify() from aio-posix.c:aio_add_sqe() sounds better to me because fdmon-io_uring.c has to be careful about calling aio_*() APIs to avoid loops. Stefan > > [0]: > https://lore.kernel.org/qemu-devel/[email protected]/ > > Best Regards, > Fiona >
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