Re: [PATCH 1/2] fdmon-io_uring: notify main loop when SQEs are queued

[Kevin Wolf <kwolf@redhat.com>]

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?

Kevin