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 <[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. > > > > 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/[email protected]/ > > > > 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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