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

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> 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.

Hi Jakub,

what I mean here is reallocate the skb if the true size is small (e.g. below
256B) in order to avoid consuming the high order page from the page_pool. Maybe
we can avoid it if reducing the page order to 2 for LRO queues provide
comparable results.

> 
> > > 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.

ack, I will do.

> 
> > > 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?

I will test it.

> 
> You mean want to steal some of the code from:
> https://lore.kernel.org/all/20250421222827.283737-1-kuba@kernel.org/

ack, I will take a look.

> 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

ack, I will do.

Regards,
Lorenzo

> -- 
> pw-bot: cr

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