tcp: remove some holes in struct tcp_sock

By moving some fields around, this patch shrinks
holes size from 56 to 32, saving 24 bytes on 64bit arches.

After the patch pahole gives the following for 'struct tcp_sock':

	/* size: 2304, cachelines: 36, members: 162 */
	/* sum members: 2234, holes: 6, sum holes: 32 */
	/* sum bitfield members: 34 bits, bit holes: 5, sum bit holes: 14 bits */
	/* padding: 32 */
	/* paddings: 3, sum paddings: 10 */
	/* forced alignments: 1, forced holes: 1, sum forced holes: 12 */

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20240227192721.3558982-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

include/linux/tcp.h

@@ -264,10 +264,10 @@ struct tcp_sock {
 	u32	pushed_seq;	/* Last pushed seq, required to talk to windows */
 	u32	lsndtime;
 	u32	mdev_us;	/* medium deviation			*/
+	u32	rtt_seq;	/* sequence number to update rttvar	*/
 	u64	tcp_wstamp_ns;	/* departure time for next sent data packet */
 	u64	tcp_clock_cache; /* cache last tcp_clock_ns() (see tcp_mstamp_refresh()) */
 	u64	tcp_mstamp;	/* most recent packet received/sent */
-	u32	rtt_seq;	/* sequence number to update rttvar	*/
 	struct list_head tsorted_sent_queue; /* time-sorted sent but un-SACKed skbs */
 	struct sk_buff *highest_sack;   /* skb just after the highest
 					 * skb with SACKed bit set
@@ -350,7 +350,6 @@ struct tcp_sock {
 	u32	dsack_dups;	/* RFC4898 tcpEStatsStackDSACKDups
 				 * total number of DSACK blocks received
 				 */
-	u32	last_oow_ack_time;  /* timestamp of last out-of-window ACK */
 	u32	compressed_ack_rcv_nxt;
 	struct list_head tsq_node; /* anchor in tsq_tasklet.head list */
 
@@ -384,12 +383,12 @@ struct tcp_sock {
 		syn_fastopen_ch:1, /* Active TFO re-enabling probe */
 		syn_data_acked:1;/* data in SYN is acked by SYN-ACK */
 
+	u8	keepalive_probes; /* num of allowed keep alive probes	*/
 	u32	tcp_tx_delay;	/* delay (in usec) added to TX packets */
 
 /* RTT measurement */
 	u32	mdev_max_us;	/* maximal mdev for the last rtt period	*/
 
-	u8	keepalive_probes; /* num of allowed keep alive probes	*/
 	u32	reord_seen;	/* number of data packet reordering events */
 
 /*
@@ -402,6 +401,7 @@ struct tcp_sock {
 	u32	prior_cwnd;	/* cwnd right before starting loss recovery */
 	u32	prr_delivered;	/* Number of newly delivered packets to
 				 * receiver in Recovery. */
+	u32	last_oow_ack_time;  /* timestamp of last out-of-window ACK */
 
 	struct hrtimer	pacing_timer;
 	struct hrtimer	compressed_ack_timer;
@@ -477,8 +477,8 @@ struct tcp_sock {
 	bool	is_mptcp;
 #endif
 #if IS_ENABLED(CONFIG_SMC)
-	bool	(*smc_hs_congested)(const struct sock *sk);
 	bool	syn_smc;	/* SYN includes SMC */
+	bool	(*smc_hs_congested)(const struct sock *sk);
 #endif
 
 #if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)