On Thu, Feb 19, 2026 at 04:49:11PM +0100, Kevin Wolf wrote: > 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.
Good idea, I generalized the comment when merging the patch. Stefan
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