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. -- Jens Axboe
