net: add heuristic for enabling TCP fraglist GRO

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
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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 (likely(!(skb->dev->features & NETIF_F_GRO_FRAGLIST)))
+		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);
+	iph = skb_gro_network_header(skb);
+	net = dev_net(skb->dev);
+	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)
 {
@@ -422,6 +452,8 @@ struct sk_buff *tcp4_gro_receive(struct list_head *head, struct sk_buff *skb)
 	if (!th)
 		goto flush;
 
+	tcp4_check_fraglist_gro(head, skb, th);
+
 	return tcp_gro_receive(head, skb, th);
 
 flush:

net/ipv6/tcpv6_offload.c

@@ -7,12 +7,45 @@
  */
 #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,
+				    struct tcphdr *th)
+{
+#if IS_ENABLED(CONFIG_IPV6)
+	const struct ipv6hdr *hdr;
+	struct sk_buff *p;
+	struct sock *sk;
+	struct net *net;
+	int iif, sdif;
+
+	if (likely(!(skb->dev->features & NETIF_F_GRO_FRAGLIST)))
+		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);
+	hdr = skb_gro_network_header(skb);
+	net = dev_net(skb->dev);
+	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)
 {
@@ -28,6 +61,8 @@ struct sk_buff *tcp6_gro_receive(struct list_head *head, struct sk_buff *skb)
 	if (!th)
 		goto flush;
 
+	tcp6_check_fraglist_gro(head, skb, th);
+
 	return tcp_gro_receive(head, skb, th);
 
 flush: