For the record, this was the closest I got to an answer:
Cgroups developers tell me that BFQ "can't currently implement
comprehensive IO" isolation, doesn't handle 'writeback / forced IO
backcharging", and has scalability issues.
Resource control is a significant effort in Fedora, in particular
GNOME and KDE, and one of the reasons for switching to Btrfs by
default in Fedora 33. I think the presumption should be that we
revert to mq-deadline by default in Fedora 34.
(Chris Murphy,
https://bugzilla.redhat.com/show_bug.cgi?id=1851783#c21 )
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