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Date:   Wed, 21 Sep 2022 13:04:57 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Stefan Metzmacher <metze@...ba.org>, io-uring@...r.kernel.org,
        axboe@...nel.dk
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org
Subject: Re: [PATCH 5/5] io_uring/notif: let userspace know how effective the
 zero copy usage was

On 9/17/22 11:24, Stefan Metzmacher wrote:
> Am 17.09.22 um 11:22 schrieb Pavel Begunkov:
>> On 9/16/22 22:36, Stefan Metzmacher wrote:
>>> The 2nd cqe for IORING_OP_SEND_ZC has IORING_CQE_F_NOTIF set in cqe->flags
>>> and it will now have the number of successful completed
>>> io_uring_tx_zerocopy_callback() callbacks in the lower 31-bits
>>> of cqe->res, the high bit (0x80000000) is set when
>>> io_uring_tx_zerocopy_callback() was called with success=false.
>>
>> It has a couple of problems, and because that "simplify uapi"
>> patch is transitional it doesn't go well with what I'm queuing
>> for 6.1, let's hold it for a while.
> 
> Once the current behavior gets released stable, we're no
> longer able to change the meaning of cqe.res.
> 
> As cqe.res == 0 would mean zero copy wasn't used at all,
> which would be the indication for userspace to avoid using SEND_ZC.
> 
> But if 6.0 would always return cqe.res = 0, there's no chance for
> userspace to have a detection strategy.
> 
> And I don't think it will cause a lot of trouble for your 6.1 stuff (assuming
> you mean your SENDMSG_ZC code), I was already having that on top
> of my test branches, the current one is:
> https://git.samba.org/?p=metze/linux/wip.git;a=shortlog;h=refs/heads/io_uring-6.0.0-rc5-metze.08

Not that one though, 1) I want to shrink ubuf_info as we're a bit out
of space on the io_uring side and it prevents some embedding, so there
won't be additional fields you used. And 2) we want to have a feature
merging completion + notif CQEs into one (useful for UDP and some TCP
cases), but that would mean we'll be using cqe->res for the normal
return value.

We can disable the success counting in this case, but it's not nice,
and we can't actually count in io_uring_tx_zerocopy_callback() as in
the patch (racy). Also, the callback will be called multiple times for
a number of different reasons like io_uring flush or net splitting skbs.
The number won't be much useful and unnecessary exposes internal details,
so I think F_COPIED in cqe->flags is a better option.

It's a good question though whether we need more versatile reporting and
how to do it right. Probably should be optional and not a part of IO path,
e.g. send(MSG_PROBE) / ioctl / proc stats / etc.

> I plan to test SENDMSG_ZC with Samba next week.

Awesome

-- 
Pavel Begunkov

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