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

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Feb 18, 2026 at 09:17:57AM -0700, Jens Axboe wrote:
> On 2/18/26 9:06 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.
> > 
> > It's possible to call io_uring_enter(). QEMU currently doesn't use
> > IORING_SETUP_SINGLE_ISSUER, so it's okay for multiple threads to call
> > io_uring_enter() on the same io_uring fd.
> 
> I would not recommend that, see below.
> 
> > I experimented with IORING_SETUP_SINGLE_ISSUER (as well as
> > IORING_SETUP_COOP_TASKRUN and IORING_SETUP_TASKRUN_FLAG) in the past and
> > didn't measure a performance improvement:
> > https://lore.kernel.org/qemu-devel/20250724204702.576637-1-stefanha@redhat.com/
> > 
> > Jens, any advice regarding these flags?
> 
> None other than "yes you should use them" - it's an expanding area of
> "let's make that faster", so if you tested something older, then that
> may be why as we didn't have a lot earlier. We're toying with getting
> rid of the uring_lock for SINGLE_ISSUER, for example.
> 
> Hence I think having multiple threads do enter is a design mistake, and
> one that might snowball down the line and make it harder to step back
> and make SINGLE_ISSUER work for you. Certain features also end up being
> gated behing DEFER_TASKRUN, which requires SINGLE_ISSUER as well.
> 
> tldr - don't have multiple threads do enter on the same ring, ever, if
> it can be avoided. It's a design mistake.

That's useful information, thanks. I will resurrect the patches to add
modern io_uring_setup() flags and we'll document the assumption that
only one thread invokes io_uring_enter().

Stefan

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