Am 01.06.26 um 7:20 PM schrieb Stefan Hajnoczi: > 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?
Hi Stefan, I did not proceed with the above, since I did not get an ack from Jens regarding the suggested approach. We needed to go ahead with a release downstream, so for the meantime, we applied a workaround by Thomas with setting the IORING_ENTER_NO_IOWAIT flag when there is no actual IO in-flight [0]. Should it be submitted to qemu-devel too? [0]: https://git.proxmox.com/?p=pve-qemu.git;a=commitdiff;h=775e41b890a645db75119233fe2b21f139bf8e4f Best Regards, Fiona
