netdev - Re: [RFC 1/6] net: extend ubuf_info callback to ops structure

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Message-ID: <bc8eb305-84e0-46ab-86b1-907dcf864452@gmail.com>
Date: Mon, 15 Apr 2024 19:55:13 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 io-uring@...r.kernel.org, netdev@...r.kernel.org
Cc: Jens Axboe <axboe@...nel.dk>, "David S . Miller" <davem@...emloft.net>,
 Jakub Kicinski <kuba@...nel.org>, David Ahern <dsahern@...nel.org>,
 Eric Dumazet <edumazet@...gle.com>
Subject: Re: [RFC 1/6] net: extend ubuf_info callback to ops structure

On 4/15/24 16:06, Willem de Bruijn wrote:
> Pavel Begunkov wrote:
>> On 4/14/24 18:07, Willem de Bruijn wrote:
>>> Pavel Begunkov wrote:
>>>> We'll need to associate additional callbacks with ubuf_info, introduce
>>>> a structure holding ubuf_info callbacks. Apart from a more smarter
>>>> io_uring notification management introduced in next patches, it can be
>>>> used to generalise msg_zerocopy_put_abort() and also store
>>>> ->sg_from_iter, which is currently passed in struct msghdr.
>>>
>>> This adds an extra indirection for all other ubuf implementations.
>>> Can that be avoided?
>>
>> It could be fitted directly into ubuf_info, but that doesn't feel
>> right. It should be hot, so does it even matter?
> 
> That depends on the workload (working set size)?
>>> On the bright side,
>> with the patch I'll also ->sg_from_iter from msghdr into it, so it
>> doesn't have to be in the generic path.
> 
> I don't follow this: is this suggested future work?

Right, a small change I will add later. Without ops though
having 3 callback fields in uargs would be out of hands.

>> I think it's the right approach, but if you have a strong opinion
>> I can fit it as a new field in ubuf_info.
> 
> If there is a significant cost, I suppose we could use
> INDIRECT_CALL or go one step further and demultiplex
> based on the new ops
> 
>      if (uarg->ops == &msg_zerocopy_ubuf_ops)
>          msg_zerocopy_callback(..);

Let me note that the patch doesn't change the number of indirect
calls but only adds one extra deref to get the callback, i.e.
uarg->ops->callback() instead of uarg->callback(). Your snippet
goes an extra mile and removes the indirect call.

Can I take it as that you're fine with the direction of the
patch? Or do you want me to change anything?

-- 
Pavel Begunkov