netdev - Re: EIO on send with UDP

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Date: Wed, 8 Nov 2023 14:11:05 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Jakub Sitnicki <jakub@...udflare.com>
Cc: Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org, kernel-team@...udflare.com
Subject: Re: EIO on send with UDP_SEGMENT

On Wed, Nov 8, 2023 at 1:08 PM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> On Wed, Nov 08, 2023 at 10:10 AM -05, Willem de Bruijn wrote:
> > On Wed, Nov 8, 2023 at 6:03 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> [...]
>
> >> Do you think the restriction in udp_send_skb can be lifted or tweaked?
> >
> > The argument against has been that segmentation offload offers no
> > performance benefit if the stack has to fall back onto software
> > checksumming.
>
> Interesting. Thanks for sharing the context. Must admit, it would have
> not been my first guess that the software GSO+checksum itself is not
> worth it. Despite it happening late on the TX path.

The heuristic is that checksum during copy_from_user is cheap, while
checksum after qdisc dequeue might have to read cold memory.

There will be cases where the data is warm. So YMMV. But that is the
basis for the choice.