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

Willem de Bruijn
2024-04-15T19:01:37Z

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.