tcp: add send queue size stat in SCM_TIMESTAMPING_OPT_STATS

This patch adds TCP_NLA_SENDQ_SIZE stat into SCM_TIMESTAMPING_OPT_STATS.
It reports no. of bytes present in send queue, when timestamp is
generated.

Signed-off-by: Priyaranjan Jha <[e-mail]>
Signed-off-by: Neal Cardwell <[e-mail]>
Signed-off-by: Yuchung Cheng <[e-mail]>
Signed-off-by: Soheil Hassas Yeganeh <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/uapi/linux/tcp.h

···
 	TCP_NLA_MIN_RTT,        /* minimum RTT */
 	TCP_NLA_RECUR_RETRANS,  /* Recurring retransmits for the current pkt */
 	TCP_NLA_DELIVERY_RATE_APP_LMT, /* delivery rate application limited ? */
+	TCP_NLA_SNDQ_SIZE,	/* Data (bytes) pending in send queue */
 
 };
 

net/ipv4/tcp.c

···
 	u32 rate;
 
 	stats = alloc_skb(7 * nla_total_size_64bit(sizeof(u64)) +
-			  3 * nla_total_size(sizeof(u32)) +
+			  4 * nla_total_size(sizeof(u32)) +
 			  2 * nla_total_size(sizeof(u8)), GFP_ATOMIC);
 	if (!stats)
 		return NULL;
···
 
 	nla_put_u8(stats, TCP_NLA_RECUR_RETRANS, inet_csk(sk)->icsk_retransmits);
 	nla_put_u8(stats, TCP_NLA_DELIVERY_RATE_APP_LMT, !!tp->rate_app_limited);
+
+	nla_put_u32(stats, TCP_NLA_SNDQ_SIZE, tp->write_seq - tp->snd_una);
 	return stats;
 }