Re: [PATCH v2] aio-posix: notify main loop when SQEs are queued

[Kevin Wolf <kwolf@redhat.com>]

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 <axboe@kernel.dk>
> >>>>> ---
> >>>>>  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 <axboe@kernel.dk>
> 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 <axboe@kernel.dk>
> 
> 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 <kwolf@redhat.com>