netdev - Re: IORING_CQE_F

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Date:   Tue, 18 Oct 2022 10:43:17 +0200
From:   Stefan Metzmacher <metze@...ba.org>
To:     Pavel Begunkov <asml.silence@...il.com>,
        io-uring <io-uring@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev <netdev@...r.kernel.org>,
        Dylan Yudaken <dylany@...com>
Subject: Re: IORING_CQE_F_COPIED

Hi Pavel,

> On 10/14/22 12:06, Stefan Metzmacher wrote:
>> Hi Pavel,
>>
>> In the tests I made I used this version of IORING_CQE_F_COPIED:
>> https://git.samba.org/?p=metze/linux/wip.git;a=commitdiff;h=645d3b584c417a247d92d71baa6266a5f3d0d17d
>> (also inlined at the end)
>>
>> Would that something we want for 6.1? (Should I post that with a useful commit message, after doing some more tests)
> 
> I was thinking, can it be delivered separately but not in the same cqe?
> The intention is to keep it off the IO path. For example, it can emit a
> zc status CQE or maybe keep a "zc failed" counter inside the ring. Other
> options? And we can add a separate callback for that, will make a couple
> of things better.
> 
> What do you think? Especially from the userspace usability perspective.

So far I can't think of any other way that would be useful yet,
but that doesn't mean something else might exist...

IORING_CQE_F_COPIED is available per request and makes it possible
to judge why the related SENDMSG_ZC was fast or not.
It's also available in trace-cmd report.

Everything else would likely re-introduce similar complexity like we
had with the notif slots.

Instead of a new IORING_CQE_F_COPIED flag we could also set
cqe.res = SO_EE_CODE_ZEROCOPY_COPIED, but that isn't really different.

As I basically use the same logic that's used to generate SO_EE_CODE_ZEROCOPY_COPIED
for the native MSG_ZEROCOPY, I don't see the problem with IORING_CQE_F_COPIED.
Can you be more verbose why you're thinking about something different?

metze