Re: Excessive IO PSI for iothread when using io_uring since QEMU 10.2

[Stefan Hajnoczi <stefanha@redhat.com>]

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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/14bc6266-5bc9-4454-9518-d1016bfe417b@proxmox.com/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.

Stefan

> 
> [1]:
> https://lore.proxmox.com/pve-devel/525c4dad-6d04-41f0-8a21-9302b0c6baa4@proxmox.com/T/
> 
> Best Regards,
> Fiona
> 

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