tcp: add tcp_rx_skb_cache sysctl

Instead of relying on rps_needed, it is safer to use a separate
static key, since we do not want to enable TCP rx_skb_cache
by default. This feature can cause huge increase of memory
usage on hosts with millions of sockets.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -772,6 +772,14 @@ tcp_challenge_ack_limit - INTEGER
 	in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
 	Default: 100
 
+tcp_rx_skb_cache - BOOLEAN
+	Controls a per TCP socket cache of one skb, that might help
+	performance of some workloads. This might be dangerous
+	on systems with a lot of TCP sockets, since it increases
+	memory usage.
+
+	Default: 0 (disabled)
+
 UDP variables:
 
 udp_l3mdev_accept - BOOLEAN

include/net/sock.h

@@ -2433,13 +2433,11 @@ static inline void skb_setup_tx_timestamp(struct sk_buff *skb, __u16 tsflags)
  * This routine must be called with interrupts disabled or with the socket
  * locked so that the sk_buff queue operation is ok.
 */
+DECLARE_STATIC_KEY_FALSE(tcp_rx_skb_cache_key);
 static inline void sk_eat_skb(struct sock *sk, struct sk_buff *skb)
 {
 	__skb_unlink(skb, &sk->sk_receive_queue);
-	if (
-#ifdef CONFIG_RPS
-	    !static_branch_unlikely(&rps_needed) &&
-#endif
+	if (static_branch_unlikely(&tcp_rx_skb_cache_key) &&
 	    !sk->sk_rx_skb_cache) {
 		sk->sk_rx_skb_cache = skb;
 		skb_orphan(skb);

net/ipv4/sysctl_net_ipv4.c

@@ -51,6 +51,9 @@ static int comp_sack_nr_max = 255;
 static u32 u32_max_div_HZ = UINT_MAX / HZ;
 static int one_day_secs = 24 * 3600;
 
+DEFINE_STATIC_KEY_FALSE(tcp_rx_skb_cache_key);
+EXPORT_SYMBOL(tcp_rx_skb_cache_key);
+
 /* obsolete */
 static int sysctl_tcp_low_latency __read_mostly;
 
@@ -559,6 +562,12 @@ static struct ctl_table ipv4_table[] = {
 		.extra1		= &sysctl_fib_sync_mem_min,
 		.extra2		= &sysctl_fib_sync_mem_max,
 	},
+	{
+		.procname	= "tcp_rx_skb_cache",
+		.data		= &tcp_rx_skb_cache_key.key,
+		.mode		= 0644,
+		.proc_handler	= proc_do_static_key,
+	},
 	{ }
 };