linux-kernel - Re: [PATCH RFC net-next 05/34] idpf: convert header split mode to libie + napi_build

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Date: Tue, 9 Jan 2024 15:43:30 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: "David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Maciej Fijalkowski <maciej.fijalkowski@...el.com>, 
	Michal Kubiak <michal.kubiak@...el.com>, Larysa Zaremba <larysa.zaremba@...el.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Willem de Bruijn <willemdebruijn.kernel@...il.com>, intel-wired-lan@...ts.osuosl.org, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC net-next 05/34] idpf: convert header split mode to
 libie + napi_build_skb()

On Sat, Dec 23, 2023 at 3:58 AM Alexander Lobakin
<aleksander.lobakin@...el.com> wrote:
>
> Currently, idpf uses the following model for the header buffers:
>
> * buffers are allocated via dma_alloc_coherent();
> * when receiving, napi_alloc_skb() is called and then the header is
>   copied to the newly allocated linear part.
>
> This is far from optimal as DMA coherent zone is slow on many systems
> and memcpy() neutralizes the idea and benefits of the header split.
> Instead, use libie to create page_pools for the header buffers, allocate
> them dynamically and then build an skb via napi_build_skb() around them
> with no memory copy. With one exception...
> When you enable header split, you except you'll always have a separate
> header buffer, so that you could reserve headroom and tailroom only
> there and then use full buffers for the data. For example, this is how
> TCP zerocopy works -- you have to have the payload aligned to PAGE_SIZE.
> The current hardware running idpf does *not* guarantee that you'll
> always have headers placed separately. For example, on my setup, even
> ICMP packets are written as one piece to the data buffers. You can't
> build a valid skb around a data buffer in this case.
> To not complicate things and not lose TCP zerocopy etc., when such thing
> happens, use the empty header buffer and pull either full frame (if it's
> short) or the Ethernet header there and build an skb around it. GRO
> layer will pull more from the data buffer later. This W/A will hopefully
> be removed one day.

We definitely want performance numbers here, for systems that truly matter.

We spent a lot of time trying to make idpf slightly better than it
was, we do not want regressions.

Thank you.