On Tue, Apr 28, 2026 at 02:10:02PM +0200, Fiona Ebner wrote: > Hi Stefan, > > Am 27.04.26 um 9:12 PM schrieb Stefan Hajnoczi: > > On Fri, Apr 24, 2026 at 12:25:41PM +0200, Fiona Ebner wrote: > >> Dear maintainers, > >> > >> since QEMU 10.2, if io_uring is enabled, it will be used for the event > >> loop of iothreads and this causes an IO pressure stall value of nearly > >> 100 when idle. > >> > >> The issue was also reported on the kernel mailing list [0]. The > >> suggestion from Jens Axboe was to just turn off the iowait accounting > >> completely. But since (for block/file-posix.c), there is actual IO > >> submitted via the same ring, I wasn't sure if that is the right approach. > >> > >> So the idea was to keep track of whether the event loop is otherwise > >> idle and only use the IORING_ENTER_NO_IOWAIT flag in that case [1]. > >> > >> However, doing so would only help for block/file-posix.c, which submits > >> IO via luring_co_submit() -> fdmon_io_uring_add_sqe(). For example, for > >> block/rbd.c, only a poll SQE for the AioHandler node's fd is used. When > >> submitting that poll SQE in the iothread, we would need to be able to > >> know if IO for RBD is currently in-flight or not to be able to decide > >> whether to use the IORING_ENTER_NO_IOWAIT flag or not. Is there a good > >> way to do this (in a general way)? > >> > >> Or should the flag really always be used (if supported by the kernel)? > >> Is there a way to tell io_uring/kernel that we are an event loop and our > >> waiting should only be accounted for when there is actual IO in-flight? > >> > >> Happy to hear your opinions and suggestions! > >> > >> [0]: > >> https://lore.kernel.org/io-uring/[email protected]/T/ > > > > Hi Fiona, > > Jens replied yesterday confirmed your suspicion that the number of > > inflight requests is not being tracked correctly. > > > > Is there still a problem after fixing the kernel's inflight counting? If > > not, then no QEMU change is necessary and that seems like the cleanest > > solution anyway. The kernel should know whether there is I/O in flight > > and so it doesn't seem right that userspace needs to hint this. > > > unfortunately, yes. Even with the kernel fix [2], the real problem with > poll SQEs described above remains. I'm still seeing high IO pressure > stall values when using QEMU. In add_poll_add_sqe(), QEMU submits poll > SQEs for the AioHandler node fd, and that does count as pending IO. A > small reproducer modeling this [3].
Does the kernel account POLL_ADD SQEs as blocking I/O activity? That behavior is inconsistent if select(2)/poll(2)/epoll_wait(2) syscalls do not count as blocking I/O activity. The kernel io_uring code should account them correctly and not rely on a userspace hint. Stefan > > So the question from above, how to deal with this for block drivers not > going through file-posix.c remains. > > Best Regards, > Fiona > > [2]: > https://lore.kernel.org/io-uring/[email protected]/T/ > > [3]: > > #include <assert.h> > #include <errno.h> > #include <stdio.h> > #include <unistd.h> > #include <liburing.h> > #include <sys/eventfd.h> > > int main(void) { > int fd; > int ret; > struct io_uring ring; > struct io_uring_sqe *sqe; > > fd = eventfd(0, 0); > assert(fd >= 0); > > ret = io_uring_queue_init(128, &ring, 0); > assert(ret == 0); > > sqe = io_uring_get_sqe(&ring); > assert(sqe); > > io_uring_prep_poll_add(sqe, fd, 1); > > ret = io_uring_submit_and_wait(&ring, 1); > printf("got ret %d\n", ret); > > io_uring_queue_exit(&ring); > > return 0; > } > >
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