On 3/19/25 01:54, Jens Axboe wrote:
...
Do we really want it though? What are you trying to achieve, fixing
the iowait stat problem or providing an optimisation option? Because
as I see it, what's good for one is bad for the other, unfortunately.
A sysctl is not a great option as an optimisation, because with that
all apps in the system has either to be storage or net to be optimal
in relation to iowait / power consumption. That one you won't even
be able to use in a good number of server setups while getting
optimal power consumption, even if you own the entire stack.
It sounds to me like the best option is to choose which one we want
to solve at the moment. Global / sysctl option for the stat, but I'm
not sure it's that important atm, people complain less nowadays
as well. Enter flag goes fine for the iowait optimisation, but
messes with the stat. IMHO, that should be fine if we're clear
about it and that the stat part of it can change. That's what
I'd suggest doing.
The third option is to try to solve them both, but seems your
patches got buried in a discussion, and working it around at
io_uring side doesn't sound pretty, like two flags you
mentioned.
I'm not digging into that again. Once those guys figure out what they
want, we can address it on our side.
Another option is to just have v2 and tell that the optimisation
and the accounting is the same, having some mess on the stat
side, and deal with the consequences when the in-kernel semantics
changes.
After thinking about it a bit more, I do think v2 is the best approach.
And the name is probably fine, IORING_ENTER_NO_IOWAIT. If we at some
point end up having the ability to control boost and stats separately,
we could add IORING_ENTER_IOWAIT_BOOST or something. That'll allow you
to control both separately.
After a private discussion, apparently you do care about the
iowait stat, and there are just too many users that complain
about it and tools that misinterpret it. All that "hint" and
"side effect" discussion of mine doesn't solve that.
I think we do agree that it's a poor interface, but there is
no good alternative, not on the io_uring side, and it's pretty
sad that we're forced into a bad design because the patches
actually solving the problem are blocked for a year now by
sched/cpufreq.
> What do you think? I do want to get this sorted for 6.15, I feel like
we've been ignoring or stalling on this issue for way too long.
--
Pavel Begunkov
