netdev - Re: [PATCH net-next v2 1/2] udp: msg

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Date:   Thu, 29 Nov 2018 11:17:57 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next v2 1/2] udp: msg_zerocopy

On Thu, Nov 29, 2018 at 3:27 AM Paolo Abeni <pabeni@...hat.com> wrote:
>
> Hi,
>
> Thank you for the update!
>
> On Wed, 2018-11-28 at 18:50 -0500, Willem de Bruijn wrote:
> > I did revert to the basic implementation using an extra ref
> > for the function call, similar to TCP, as you suggested.
> >
> > On top of that as a separate optimization patch I have a
> > variant that uses refcnt zero by replacing refcount_inc with
> > refcount_set(.., refcount_read(..) + 1). Not very pretty.
>
> If the skb/uarg is not shared (no other threads can touch the refcnt)
> before ip*_append_data() completes, how about something like the
> following (incremental diff on top of patch 1/2, untested, uncompiled,
> just to give the idea):
>
> The basic idea is using the same schema currently used for wmem
> accounting: do the book-keeping inside the loop and set the atomic
> reference counter only once at the end of the loop.
>
> WDYT?

Thanks for the suggestion. I think that a variant of this might be the
best option indeed.

The common case that we care about (ip_make_skb without
fragmentation) only builds one skb, so we won't necessarily
amortize many atomic ops. I also really would like to avoid the
atomic op and branch in the non-error return path.

But with uarg_refs we can indeed elide the refcount_inc on the
first skb_zcopy_get and detect at skb_zerocopy_put_abort if the
extra reference went unused and it has to put the ref itself.

Let me code that up. Thanks!