[Bug 199727] CPU freezes in KVM guests during high IO load on host

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=199727

--- Comment #16 from Stefan Hajnoczi (stefanha@gmail.com) ---
On Tue, 8 Mar 2022 at 08:01, <bugzilla-daemon@kernel.org> wrote:
>
> https://bugzilla.kernel.org/show_bug.cgi?id=199727
>
> --- Comment #15 from Roland Kletzing (devzero@web.de) ---
> yes, i was using cache=none and io_uring also caused issues.
>
> >aio=threads avoids softlockups because the preadv(2)/pwritev(2)/fdatasync(2)
> > syscalls run in worker threads that don't take the QEMU global mutex.
> >Therefore vcpu threads can execute even when I/O is stuck in the kernel due
> to
> >a lock.
>
> yes, that was a long search/journey to get to this information/params....
>
> regarding io_uring - after proxmox enabled it as default, it was taken back
> again after some issues had been reported.
>
> have look at:
> https://github.com/proxmox/qemu-server/blob/master/debian/changelog
>
> maybe it's not ready for primetime yet !?
>
> -- Proxmox Support Team <support@proxmox.com>  Fri, 30 Jul 2021 16:53:44
> +0200
> qemu-server (7.0-11) bullseye; urgency=medium
> <snip>
>   * lvm: avoid the use of io_uring for now
> <snip>
> -- Proxmox Support Team <support@proxmox.com>  Fri, 23 Jul 2021 11:08:48
> +0200
> qemu-server (7.0-10) bullseye; urgency=medium
> <snip>
>   * avoid using io_uring for drives backed by LVM and configured for
>   write-back
>     or write-through cache
> <snip>
>  -- Proxmox Support Team <support@proxmox.com>  Mon, 05 Jul 2021 20:49:50
>  +0200
> qemu-server (7.0-6) bullseye; urgency=medium
> <snip>
>   * For now do not use io_uring for drives backed by Ceph RBD, with KRBD and
>     write-back or write-through cache enabled, as in that case some
>     polling/IO
>     may hang in QEMU 6.0.
> <snip>

Changelog messages mention cache=writethrough and cache=writeback,
which are both problematic because host memory pressure will interfere
with guest performance. That is probably not an issue with io_uring
per se, just another symptom of using cache=writeback/writethrough in
cases where it's inappropriate.

If you have trace data showing io_uring_enter(2) hanging with
cache=none then Jens Axboe and other io_uring developers may be able
to help resolve that.

Stefan

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