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

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Date: Tue, 23 Apr 2024 12:25:26 +0200
From: Felix Fietkau <nbd@....name>
To: Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 David Ahern <dsahern@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [RFC] net: add TCP fraglist GRO support

On 23.04.24 12:15, Eric Dumazet wrote:
> On Tue, Apr 23, 2024 at 11:41 AM Felix Fietkau <nbd@....name> wrote:
>>
>> When forwarding TCP after GRO, software segmentation is very expensive,
>> especially when the checksum needs to be recalculated.
>> One case where that's currently unavoidable is when routing packets over
>> PPPoE. Performance improves significantly when using fraglist GRO
>> implemented in the same way as for UDP.
>>
>> Here's a measurement of running 2 TCP streams through a MediaTek MT7622
>> device (2-core Cortex-A53), which runs NAT with flow offload enabled from
>> one ethernet port to PPPoE on another ethernet port + cake qdisc set to
>> 1Gbps.
>>
>> rx-gro-list off: 630 Mbit/s, CPU 35% idle
>> rx-gro-list on:  770 Mbit/s, CPU 40% idle
> 
> Hi Felix
> 
> changelog is a bit terse, and patch complex.
> 
> Could you elaborate why this issue
> seems to be related to a specific driver ?
> 
> I think we should push hard to not use frag_list in drivers :/
> 
> And GRO itself could avoid building frag_list skbs
> in hosts where forwarding is enabled.
> 
> (Note that we also can increase MAX_SKB_FRAGS to 45 these days)

The issue is not related to a specific driver at all. Here's how traffic 
flows: TCP packets are received on the SoC ethernet driver, the network 
stack performs regular GRO. The packet gets forwarded by flow offloading 
until it reaches the PPPoE device. PPPoE does not support GSO packets, 
so the packets need to be segmented again.
This is *very* expensive, since data needs to be copied and checksummed.

So in my patch, I changed the code to build fraglist GRO instead of 
regular GRO packets, whenever there is no local socket to receive the 
packets. This makes segmenting very cheap, since the original skbs are 
preserved on the trip through the stack. The only cost is an extra 
socket lookup whenever NETIF_F_FRAGLIST_GRO is enabled.

PPPoE in this case is only an example. The same issue appears when 
forwarding to any netdev which does not support TSO, which in my case 
affects most wifi drivers as well.

- Felix