net: Adjust sk_gso_max_size once when set

sk_gso_max_size is set based on the dst dev. Both users of it
adjust the value by the same offset - (MAX_TCP_HEADER + 1). Rather
than compute the same adjusted value on each call do the adjustment
once when set.

Signed-off-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20220125024511.27480-1-dsahern@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/sock.c

@@ -2266,6 +2266,7 @@ void sk_setup_caps(struct sock *sk, struct dst_entry *dst)
 			sk->sk_route_caps |= NETIF_F_SG | NETIF_F_HW_CSUM;
 			/* pairs with the WRITE_ONCE() in netif_set_gso_max_size() */
 			sk->sk_gso_max_size = READ_ONCE(dst->dev->gso_max_size);
+			sk->sk_gso_max_size -= (MAX_TCP_HEADER + 1);
 			/* pairs with the WRITE_ONCE() in netif_set_gso_max_segs() */
 			max_segs = max_t(u32, READ_ONCE(dst->dev->gso_max_segs), 1);
 		}

net/ipv4/tcp.c

@@ -894,8 +894,7 @@ static unsigned int tcp_xmit_size_goal(struct sock *sk, u32 mss_now,
 		return mss_now;
 
 	/* Note : tcp_tso_autosize() will eventually split this later */
-	new_size_goal = sk->sk_gso_max_size - 1 - MAX_TCP_HEADER;
-	new_size_goal = tcp_bound_to_half_wnd(tp, new_size_goal);
+	new_size_goal = tcp_bound_to_half_wnd(tp, sk->sk_gso_max_size);
 
 	/* We try hard to avoid divides here */
 	size_goal = tp->gso_segs * mss_now;

net/ipv4/tcp_output.c

@@ -1960,7 +1960,7 @@ static u32 tcp_tso_autosize(const struct sock *sk, unsigned int mss_now,
 
 	bytes = min_t(unsigned long,
 		      sk->sk_pacing_rate >> READ_ONCE(sk->sk_pacing_shift),
-		      sk->sk_gso_max_size - 1 - MAX_TCP_HEADER);
+		      sk->sk_gso_max_size);
 
 	/* Goal is to send at least one packet per ms,
 	 * not one big TSO packet every 100 ms.