Subject: Re: [PATCH net-next] net: airoha: Add TCP LRO support

From: Jakub Kicinski <[email-masked]>
Date: Wed, 11 Jun 2025 17:36:26 -0700

On Tue, 10 Jun 2025 15:39:34 +0200 Lorenzo Bianconi wrote:
> > Tell us more... It seems small LRO packets will consume a lot of
> > space, incurring a small skb->len/skb->truesize ratio, and bad TCP WAN
> > performance.  
> 
> I think the main idea is forward to hw LRO queues (queues 24-31 in this
> case) just specific protocols with mostly big packets but I completely
> agree we have an issue for small packets. One possible approach would be
> to define a threshold (e.g. 256B) and allocate a buffer or page from the
> page allocator for small packets (something similar to what mt7601u driver
> is doing[0]).  What do you think?

I'm not Eric but FWIW 256B is not going to help much. It's best to keep
the len / truesize ratio above 50%, so with 32k buffers we're talking
about copying multiple frames.

> > And order-5 pages are unlikely to be available in the long run anyway.  
> 
> I agree. I guess we can reduce the order to ~ 2 (something similar to
> mtk_eth_soc hw LRO implementation [1]).

Would be good to test. SW GRO can "re-GRO" the partially coalesced
packets, so it's going to be diminishing returns.

> > LRO support would only make sense if the NIC is able to use multiple
> > order-0 pages to store the payload.  
> 
> The hw supports splitting big packets over multiple order-0 pages if we
> increase the MTU over one page size, but according to my understanding
> hw LRO requires contiguous memory to work.

Hm, you're already passing buffers smaller than normal TSO so
presumably having a smaller buffers will break the sessions more 
often but still work?

You mean want to steal some of the code from:
https://lore.kernel.org/all/[email-masked]/
and make the buffer size user-configurable. But not a requirement.
Let's at least get some understanding of the perf benefit of 
32k vs 16k or 8k
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