netdev - Re: [RFC] net: add TCP fraglist GRO support

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Date: Tue, 23 Apr 2024 21:24:51 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Felix Fietkau <nbd@....name>, 
 Paolo Abeni <pabeni@...hat.com>, 
 Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, 
 "David S. Miller" <davem@...emloft.net>, 
 Jakub Kicinski <kuba@...nel.org>, 
 David Ahern <dsahern@...nel.org>, 
 linux-kernel@...r.kernel.org
Subject: Re: [RFC] net: add TCP fraglist GRO support

Felix Fietkau wrote:
> On 23.04.24 16:34, Paolo Abeni wrote:
> > On Tue, 2024-04-23 at 14:23 +0200, Felix Fietkau wrote:
> >> On 23.04.24 14:11, Eric Dumazet wrote:
> >> > On Tue, Apr 23, 2024 at 1:55 PM Felix Fietkau <nbd@....name> wrote:
> >> > > 
> >> > > In the world of consumer-grade WiFi devices, there are a lot of chipsets
> >> > > with limited or nonexistent SG support, and very limited checksum
> >> > > offload capabilities on Ethernet. The WiFi side of these devices is
> >> > > often even worse. I think fraglist GRO is a decent fallback for the
> >> > > inevitable corner cases.
> >> > 
> >> > What about netfilter and NAT ? Are they okay with NETIF_F_FRAGLIST_GRO already ?
> >> > 
> >> > Many of these devices are probably using NAT.
> >> 
> >> In my tests, nftables NAT works just fine, both with and without 
> >> flowtable offloading. I didn't see anything in netfilter that would have 
> >> a problem with this.
> > 
> > I see you handle explicitly NAT changes in __tcpv4_gso_segment_csum(),
> > like the current UDP code.
> > 
> > The TCP header has many other fields that could be updated affecting
> > the TCP csum.
> > Handling every possible mutation looks cumbersome and will likely
> > reduce the performance benefits.
> > 
> > What is your plan WRT other TCP header fields update?
> 
> I think that should be easy enough to handle. My patch already only 
> combines packets where tcp_flag_word(th) is identical. So when 
> segmenting, I could handle all flags changes with a single 
> inet_proto_csum_replace4 call.
> 
> > Strictly WRT the patch, I guess it deserves to be split in series,
> > moving UDP helpers in common code and possibly factoring out more
> > helpers with separate patches.
> Will do.

A significant chunk of the complexity is in the
tcp[46]_check_fraglist_gro sk match. Is this heuristic worth the
complexity?

It seems that the platforms that will enable NETIF_F_FRAGLIST will
be mainly forwarding planes.

If keeping, this refinement can probably a separate follow-on patch in
the series too:

- refactor existing udp code
- add segmentation support to handle such packets on tx
- add coalescing support that starts building such packets on rx
- refine coalescing choice

> > e.g. in __tcpv4_gso_segment_csum() is quite similar
> > __udpv4_gso_segment_csum() - even too much, as the tcp csum should be
> > always be updated when the ports or addresses change ;)
> 
> Will fix that.
> 
> Thanks,
> 
> - Felix