tcp: add bytes not sent to SCM_TIMESTAMPING_OPT_STATS

Add TCP_NLA_BYTES_NOTSENT to SCM_TIMESTAMPING_OPT_STATS that reports
bytes in the write queue but not sent. This is the same metric as
what is exported with tcp_info.tcpi_notsent_bytes.
Signed-off-by: Yousuk Seung <ysseung@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/tcp.h

@@ -312,6 +312,7 @@ enum {
 	TCP_NLA_REORD_SEEN,	/* reordering events seen */
 	TCP_NLA_SRTT,		/* smoothed RTT in usecs */
 	TCP_NLA_TIMEOUT_REHASH, /* Timeout-triggered rehash attempts */
+	TCP_NLA_BYTES_NOTSENT,	/* Bytes in write queue not yet sent */
 };
 
 /* for TCP_MD5SIG socket option */

net/ipv4/tcp.c

@@ -3344,6 +3344,7 @@ static size_t tcp_opt_stats_get_size(void)
 		nla_total_size(sizeof(u32)) + /* TCP_NLA_REORD_SEEN */
 		nla_total_size(sizeof(u32)) + /* TCP_NLA_SRTT */
 		nla_total_size(sizeof(u16)) + /* TCP_NLA_TIMEOUT_REHASH */
+		nla_total_size(sizeof(u32)) + /* TCP_NLA_BYTES_NOTSENT */
 		0;
 }
 
@@ -3399,6 +3400,8 @@ struct sk_buff *tcp_get_timestamping_opt_stats(const struct sock *sk)
 	nla_put_u32(stats, TCP_NLA_REORD_SEEN, tp->reord_seen);
 	nla_put_u32(stats, TCP_NLA_SRTT, tp->srtt_us >> 3);
 	nla_put_u16(stats, TCP_NLA_TIMEOUT_REHASH, tp->timeout_rehash);
+	nla_put_u32(stats, TCP_NLA_BYTES_NOTSENT,
+		    max_t(int, 0, tp->write_seq - tp->snd_nxt));
 
 	return stats;
 }