[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 #18 from Gergely Kovacs (gkovacs@gmail.com) ---
Thank you Roland Kletzing for your exhaustive investigation and Stefan Hajnoczi
for your insightful comments. This is a problem that has been affecting us (and
many many users of Proxmox and likely vanilla KVM) for more than a decade, yet
the Proxmox developers were unable to solve it or even reproduce it (despite
the large number of forum threads and bugs filed), hence the reason for me
creating this bugreport 4 years ago.

It looks like we are closing in: the KVM global mutex could be the real
culprit, as in our case the problems were only mostly gone by moving all our VM
storage to NVMe (increasing IO bandwidth by a LOT), but fully gone after
setting VirtIO SCSI Single / iothread=1 / aio=threads on all our KVM guests.
For many years VM migrations or restores could render other VMs on the same
host practically unusable for the duration of the heavy IO, now these
operations can be safely done.

I will experiment with io_uring in the near future and report back my findings,
will leave the status NEW since I reckon attention should be given to the ring
io code to achieve the same stability as threaded io.

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