[PATCH v2 net-next v2 5/5] net: add heuristic for enabling TCP fraglist GRO

[Felix Fietkau <nbd@nbd.name>]

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   | 48 +++++++++++++++++++++++++++++++++++++
 net/ipv6/tcpv6_offload.c | 51 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 99 insertions(+)

diff --git a/net/ipv4/tcp_offload.c b/net/ipv4/tcp_offload.c
index 520fd425ab19..3bb96a110402 100644
--- a/net/ipv4/tcp_offload.c
+++ b/net/ipv4/tcp_offload.c
@@ -405,6 +405,52 @@ 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)
+{
+	const struct iphdr *iph = skb_gro_network_header(skb);
+	struct net *net = dev_net(skb->dev);
+	unsigned int off, hlen, thlen;
+	struct sk_buff *p;
+	struct tcphdr *th;
+	struct sock *sk;
+	int iif, sdif;
+
+	if (!(skb->dev->features & NETIF_F_GRO_FRAGLIST))
+		return;
+
+	off = skb_gro_offset(skb);
+	hlen = off + sizeof(*th);
+	th = skb_gro_header(skb, hlen, off);
+	if (unlikely(!th))
+		return;
+
+	thlen = th->doff * 4;
+	if (thlen < sizeof(*th))
+		return;
+
+	hlen = off + thlen;
+	if (!skb_gro_may_pull(skb, hlen)) {
+		th = skb_gro_header_slow(skb, hlen, off);
+		if (unlikely(!th))
+			return;
+	}
+
+	p = tcp_gro_lookup(head, th);
+	if (p) {
+		NAPI_GRO_CB(skb)->is_flist = NAPI_GRO_CB(p)->is_flist;
+		return;
+	}
+
+	inet_get_iif_sdif(skb, &iif, &sdif);
+	sk = __inet_lookup_established(net, net->ipv4.tcp_death_row.hashinfo,
+				       iph->saddr, th->source,
+				       iph->daddr, ntohs(th->dest),
+				       iif, sdif);
+	NAPI_GRO_CB(skb)->is_flist = !sk;
+	if (sk)
+		sock_put(sk);
+}
+
 INDIRECT_CALLABLE_SCOPE
 struct sk_buff *tcp4_gro_receive(struct list_head *head, struct sk_buff *skb)
 {
@@ -416,6 +462,8 @@ struct sk_buff *tcp4_gro_receive(struct list_head *head, struct sk_buff *skb)
 		return NULL;
 	}
 
+	tcp4_check_fraglist_gro(head, skb);
+
 	return tcp_gro_receive(head, skb);
 }
 
diff --git a/net/ipv6/tcpv6_offload.c b/net/ipv6/tcpv6_offload.c
index c97d55cf036f..7948420dcad0 100644
--- a/net/ipv6/tcpv6_offload.c
+++ b/net/ipv6/tcpv6_offload.c
@@ -7,12 +7,61 @@
  */
 #include <linux/indirect_call_wrapper.h>
 #include <linux/skbuff.h>
+#include <net/inet6_hashtables.h>
 #include <net/gro.h>
 #include <net/protocol.h>
 #include <net/tcp.h>
 #include <net/ip6_checksum.h>
 #include "ip6_offload.h"
 
+static void tcp6_check_fraglist_gro(struct list_head *head, struct sk_buff *skb)
+{
+#if IS_ENABLED(CONFIG_IPV6)
+	const struct ipv6hdr *hdr = skb_gro_network_header(skb);
+	struct net *net = dev_net(skb->dev);
+	unsigned int off, hlen, thlen;
+	struct sk_buff *p;
+	struct tcphdr *th;
+	struct sock *sk;
+	int iif, sdif;
+
+	if (!(skb->dev->features & NETIF_F_GRO_FRAGLIST))
+		return;
+
+	off = skb_gro_offset(skb);
+	hlen = off + sizeof(*th);
+	th = skb_gro_header(skb, hlen, off);
+	if (unlikely(!th))
+		return;
+
+	thlen = th->doff * 4;
+	if (thlen < sizeof(*th))
+		return;
+
+	hlen = off + thlen;
+	if (!skb_gro_may_pull(skb, hlen)) {
+		th = skb_gro_header_slow(skb, hlen, off);
+		if (unlikely(!th))
+			return;
+	}
+
+	p = tcp_gro_lookup(head, th);
+	if (p) {
+		NAPI_GRO_CB(skb)->is_flist = NAPI_GRO_CB(p)->is_flist;
+		return;
+	}
+
+	inet6_get_iif_sdif(skb, &iif, &sdif);
+	sk = __inet6_lookup_established(net, net->ipv4.tcp_death_row.hashinfo,
+					&hdr->saddr, th->source,
+					&hdr->daddr, ntohs(th->dest),
+					iif, sdif);
+	NAPI_GRO_CB(skb)->is_flist = !sk;
+	if (sk)
+		sock_put(sk);
+#endif /* IS_ENABLED(CONFIG_IPV6) */
+}
+
 INDIRECT_CALLABLE_SCOPE
 struct sk_buff *tcp6_gro_receive(struct list_head *head, struct sk_buff *skb)
 {
@@ -24,6 +73,8 @@ struct sk_buff *tcp6_gro_receive(struct list_head *head, struct sk_buff *skb)
 		return NULL;
 	}
 
+	tcp6_check_fraglist_gro(head, skb);
+
 	return tcp_gro_receive(head, skb);
 }
 
-- 
2.44.0