linux-kernel - Re: [PATCH net-next 4/4] net: add heuristic for enabling TCP fraglist GRO

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Date: Wed, 24 Apr 2024 20:48:47 +0200
From: Felix Fietkau <nbd@....name>
To: Eric Dumazet <edumazet@...gle.com>
Cc: netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
 David Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, willemdebruijn.kernel@...il.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 4/4] net: add heuristic for enabling TCP fraglist
 GRO

On 24.04.24 20:23, Eric Dumazet wrote:
> On Wed, Apr 24, 2024 at 8:05 PM 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.
>>
>> When NETIF_F_GRO_FRAGLIST is enabled, perform a lookup for an established
>> socket in the same netns as the receiving device. While this may not
>> cover all relevant use cases in multi-netns configurations, it should be
>> good enough for most configurations that need this.
>>
>> 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
>>
>> Signe-off-by: Felix Fietkau <nbd@....name>
>> ---
>>  net/ipv4/tcp_offload.c   | 45 ++++++++++++++++++++++++++++++++++++++-
>>  net/ipv6/tcpv6_offload.c | 46 +++++++++++++++++++++++++++++++++++++++-
>>  2 files changed, 89 insertions(+), 2 deletions(-)
>>
>> diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
>> index 6294e7a5c099..f987e2d8423a 100644
>> --- a/net/ipv4/tcp_offload.c
>> +++ b/net/ipv4/tcp_offload.c
>> @@ -404,6 +404,49 @@ void tcp_gro_complete(struct sk_buff *skb)
>>  }
>>  EXPORT_SYMBOL(tcp_gro_complete);
>>
>> +static bool tcp4_check_fraglist_gro(struct sk_buff *skb)
>> +{
>> +       const struct iphdr *iph = skb_gro_network_header(skb);
>> +       struct net *net = dev_net(skb->dev);
>> +       unsigned int off, hlen, thlen;
>> +       struct tcphdr *th;
>> +       struct sock *sk;
>> +       int iif, sdif;
>> +
>> +       if (!(skb->dev->features & NETIF_F_GRO_FRAGLIST))
>> +               return false;
>> +
>> +       inet_get_iif_sdif(skb, &iif, &sdif);
>> +
>> +       off = skb_gro_offset(skb);
>> +       hlen = off + sizeof(*th);
>> +       th = skb_gro_header(skb, hlen, off);
>> +       if (unlikely(!th))
>> +               return false;
>> +
>> +       thlen = th->doff * 4;
>> +       if (thlen < sizeof(*th))
>> +               return false;
>> +
>> +       hlen = off + thlen;
>> +       if (!skb_gro_may_pull(skb, hlen)) {
>> +               th = skb_gro_header_slow(skb, hlen, off);
>> +               if (unlikely(!th))
>> +                       return false;
>> +       }
>> +
>> +       sk = __inet_lookup_established(net, net->ipv4.tcp_death_row.hashinfo,
>> +                                      iph->saddr, th->source,
>> +                                      iph->daddr, ntohs(th->dest),
>> +                                      iif, sdif);
> 
> Presumably all this could be done only for the first skb/segment of a GRO train.
> 
> We could store the fraglist in a single bit in NAPI_GRO_CB(skb) ?
> 
> GRO does a full tuple evaluation, we can trust it.

I will look into that, thanks.

- Felix