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

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: Pavel Begunkov <asml.silence@gmail.com>,
	 Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
	 io-uring@vger.kernel.org,  netdev@vger.kernel.org
Cc: Jens Axboe <axboe@kernel.dk>,
	 "David S . Miller" <davem@davemloft.net>,
	 Jakub Kicinski <kuba@kernel.org>,
	 David Ahern <dsahern@kernel.org>,
	 Eric Dumazet <edumazet@google.com>
Subject: Re: [RFC 1/6] net: extend ubuf_info callback to ops structure
Date: Mon, 15 Apr 2024 15:01:36 -0400	[thread overview]
Message-ID: <661d79901c9e_2ce362948f@willemb.c.googlers.com.notmuch> (raw)
In-Reply-To: <bc8eb305-84e0-46ab-86b1-907dcf864452@gmail.com>

Pavel Begunkov wrote:
> 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().

Of course. Didn't mean to imply otherwise.

> 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?

It's fine. I want to avoid new paths slowing down existing code where
possible. But if this extra deref would prove significant we have a
workaround.