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

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Date: Tue, 30 Apr 2024 12:23:52 +0200
From: Felix Fietkau <nbd@....name>
To: Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
 Eric Dumazet <edumazet@...gle.com>, "David S. Miller" <davem@...emloft.net>,
 David Ahern <dsahern@...nel.org>, Jakub Kicinski <kuba@...nel.org>
Cc: willemdebruijn.kernel@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 net-next v4 6/6] net: add heuristic for enabling TCP
 fraglist GRO

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@....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