linux-kernel - Re: [RFC net-next 1/9] skb: introduce gro

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Date: Fri, 21 Jun 2024 11:57:04 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Yan Zhai <yan@...udflare.com>, netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>,  Jakub Kicinski <kuba@...nel.org>, Alexei
 Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend
 <john.fastabend@...il.com>, Willem de Bruijn <willemb@...gle.com>, Simon
 Horman <horms@...nel.org>, Florian Westphal <fw@...len.de>, Mina Almasry
 <almasrymina@...gle.com>, Abhishek Chauhan <quic_abchauha@...cinc.com>, 
 David Howells <dhowells@...hat.com>, Alexander Lobakin
 <aleksander.lobakin@...el.com>, David Ahern <dsahern@...nel.org>, Richard
 Gobert <richardbgobert@...il.com>, Antoine Tenart <atenart@...nel.org>,
 Felix Fietkau <nbd@....name>, Soheil Hassas Yeganeh <soheil@...gle.com>,
 Pavel Begunkov <asml.silence@...il.com>, Lorenzo Bianconi
 <lorenzo@...nel.org>, Thomas Weißschuh
 <linux@...ssschuh.net>,  linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [RFC net-next 1/9] skb: introduce gro_disabled bit

On Thu, 2024-06-20 at 15:19 -0700, Yan Zhai wrote:
> Software GRO is currently controlled by a single switch, i.e.
> 
>   ethtool -K dev gro on|off
> 
> However, this is not always desired. When GRO is enabled, even if the
> kernel cannot GRO certain traffic, it has to run through the GRO receive
> handlers with no benefit.
> 
> There are also scenarios that turning off GRO is a requirement. For
> example, our production environment has a scenario that a TC egress hook
> may add multiple encapsulation headers to forwarded skbs for load
> balancing and isolation purpose. The encapsulation is implemented via
> BPF. But the problem arises then: there is no way to properly offload a
> double-encapsulated packet, since skb only has network_header and
> inner_network_header to track one layer of encapsulation, but not two.
> On the other hand, not all the traffic through this device needs double
> encapsulation. But we have to turn off GRO completely for any ingress
> device as a result.

Could you please add more details WRT this last statement? I'm unsure
if I understand your problem. My guess is as follow:

Your device receive some traffic, GRO and forward it, and the multiple
encapsulation can happen on such forwarded traffic (since I can't find
almost none of the above your message is mainly a wild guess).

Assuming I guessed correctly, I think you could solve the problem with
no kernel changes: redirect the to-be-tunneled traffic to some virtual
device and all TX offload on top of it and let the encap happen there.

Cheers,

Paolo