On Wed, Apr 29, 2026 at 10:00:34AM +0200, Fiona Ebner wrote: > Am 28.04.26 um 6:18 PM schrieb Stefan Hajnoczi: > > 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? > > Apparently yes. See the C program below [3]. > > > 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. > > @Jens Axboe: should there be a separate internal counter for > poll/timeout SQEs and have them not count towards IO wait by default?
Hi Fiona, Any update on this issue? Was it resolved in io_uring or is a QEMU patch still needed? Thanks, Stefan
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