Re: [PATCH v4 net-next v4 6/6] net: add heuristic for enabling TCP fraglist GRO

Felix Fietkau
2024-04-30T10:24:05Z

On 30.04.24 12:12, Paolo Abeni wrote:
> On Sat, 2024-04-27 at 20:23 +0200, Felix Fietkau 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@nbd.name>
>> ---
>> net/ipv4/tcp_offload.c | 32 ++++++++++++++++++++++++++++++++
>> net/ipv6/tcpv6_offload.c | 35 +++++++++++++++++++++++++++++++++++
>> 2 files changed, 67 insertions(+)
>>
>> diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
>> index 87ae9808e260..3e9b8c6f9c8c 100644
>> --- a/net/ipv4/tcp_offload.c
>> +++ b/net/ipv4/tcp_offload.c
>> @@ -407,6 +407,36 @@ void tcp_gro_complete(struct sk_buff *skb)
>> }
>> EXPORT_SYMBOL(tcp_gro_complete);
>>
>> +static void tcp4_check_fraglist_gro(struct list_head *head, struct sk_buff *skb,
>> + struct tcphdr *th)
>> +{
>> + const struct iphdr *iph;
>> + struct sk_buff *p;
>> + struct sock *sk;
>> + struct net *net;
>> + int iif, sdif;
>> +
>> + if (!(skb->dev->features & NETIF_F_GRO_FRAGLIST))
>
> Should we add an 'unlikely()' here to pair with unlikely(is_flist) in
> *gro_receive / *gro_complete?
Not sure if unlikely() will make any difference here. I think it makes
more sense in the other places than here.

> Should this test be moved into the caller, to avoid an unconditional
> function call in the ipv6 code?

The function is already called from tcp4_gro_receive, which is only
called by IPv4 code. Also, since it's a static function called in only
one place, it gets inlined by the compiler (at least in my builds).
Not sure what unconditional function call you're referring to.

- Felix