Am 18.02.2026 um 17:41 hat Jens Axboe geschrieben: > On 2/18/26 9:19 AM, Jens Axboe wrote: > > On 2/18/26 9:11 AM, Stefan Hajnoczi wrote: > >> 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. > > > > Would anyone care to make that edit? I'm on a plane and gone for a bit, > > so won't get back to this for the next week. But I would love to see a > > fix go in, as this issue has been plaguing me with test timeouts for > > quite a while on the CI front. And seems like I'm not alone, if the > > patches fix Fiona's issues as well. > > Still on a plane but tested this one and it works for me too. Does seem > like a better approach, rather than stuff it in the fdmon part. > > Feel free to run with this one and also to update the commit message if > you want. Thanks! > > > commit a8a94e7a05964d470b8fba50c9d4769489c21752 > Author: Jens Axboe <[email protected]> > Date: Fri Feb 13 06:52:14 2026 -0700 > > aio-posix: notify main loop when SQEs are queued > > 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. > > 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]> > > diff --git a/util/aio-posix.c b/util/aio-posix.c > index e24b955fd91a..8c7b3795c82d 100644 > --- a/util/aio-posix.c > +++ b/util/aio-posix.c > @@ -813,5 +813,13 @@ void aio_add_sqe(void (*prep_sqe)(struct io_uring_sqe > *sqe, void *opaque), > { > AioContext *ctx = qemu_get_current_aio_context(); > ctx->fdmon_ops->add_sqe(ctx, prep_sqe, opaque, 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
I think the comment could even be more generic here. This is not specific to coroutines, but the scenario is just that a vCPU thread holding the BQL performs I/O. > + * actual io_uring_submit() only happens in gsource_prepare(). Without > + * this notify, ppoll() can sleep up to 499ms before submitting. > + */ > + aio_notify(ctx); > } > #endif /* CONFIG_LINUX_IO_URING */ With or without a changed comment to that effect: Reviewed-by: Kevin Wolf <[email protected]>
