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 12:12:56 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Felix Fietkau <nbd@....name>, 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 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?

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

(Also waiting for explicit ack from Eric)

Thank,

Paolo