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Subject: Re: [PATCH v2] io_uring: enable toggle of iowait usage when waiting
on CQEs
To: Pavel Begunkov <asml.silence@gmail.com>,
io-uring <io-uring@vger.kernel.org>
References: <f548f142-d6f3-46d8-9c58-6cf595c968fb@kernel.dk>
<c8e9602a-a510-4e7a-b4e9-1234e7e17ca9@gmail.com>
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From: Jens Axboe <axboe@kernel.dk>
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On 3/16/25 12:57 AM, Pavel Begunkov wrote:
> On 3/14/25 18:48, Jens Axboe wrote:
>> By default, io_uring marks a waiting task as being in iowait, if it's
>> sleeping waiting on events and there are pending requests. This isn't
>> necessarily always useful, and may be confusing on non-storage setups
>> where iowait isn't expected. It can also cause extra power usage, by
>
> I think this passage hints on controlling iowait stats, and in my opinion
> we shouldn't conflate stats and optimisations. Global iowait stats
> is there to stay, but ideally we want to never account io_uring as iowait.
> That's while there were talks about removing optimisation toggle at all
> (and do it as internal cpufreq magic, I suppose).
>
> How about posing it as an optimisation option only and that iowait stat
> is a side effect that can change. Explicitly spelling that in the commit
> message and in a comment on top of the flag in an attempt to avoid the
> uapi regression trap. We'd also need it in the option's man when it's
> written. And I'd also add "hint" to the flag name, like
> IORING_ENTER_HINT_NO_IOWAIT, as we might need to nop it if anything
> changes on the cpufreq side.
Having potentially the control of both would be useful, the stat
accounting and the cpufreq boosting. I do think the current name is
better, though, the hint doesn't really add anything. I think we'd want
to have it be clear on one controlling accounting only. Maybe adding
both flagts now would be the better choice, except you'd get -EINVAL if
you set IORING_ENTER_IOWAIT_BOOST. And then you'd need two FEAT flags,
which is pretty damn wasteful.
Hmm...
--
Jens Axboe