tcp: provide earliest departure time in skb->tstamp

Switch internal TCP skb->skb_mstamp to skb->skb_mstamp_ns,
from usec units to nsec units.

Do not clear skb->tstamp before entering IP stacks in TX,
so that qdisc or devices can implement pacing based on the
earliest departure time instead of socket sk->sk_pacing_rate

Packets are fed with tcp_wstamp_ns, and following patch
will update tcp_wstamp_ns when both TCP and sch_fq switch to
the earliest departure time mechanism.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -689,7 +689,7 @@ struct sk_buff {
 
 	union {
 		ktime_t		tstamp;
-		u64		skb_mstamp;
+		u64		skb_mstamp_ns; /* earliest departure time */
 	};
 	/*
 	 * This is the control buffer. It is free to use for every

include/net/tcp.h

@@ -761,13 +761,13 @@ static inline u32 tcp_stamp_us_delta(u64 t1, u64 t0)
 
 static inline u32 tcp_skb_timestamp(const struct sk_buff *skb)
 {
-	return div_u64(skb->skb_mstamp, USEC_PER_SEC / TCP_TS_HZ);
+	return div_u64(skb->skb_mstamp_ns, NSEC_PER_SEC / TCP_TS_HZ);
 }
 
 /* provide the departure time in us unit */
 static inline u64 tcp_skb_timestamp_us(const struct sk_buff *skb)
 {
-	return skb->skb_mstamp;
+	return div_u64(skb->skb_mstamp_ns, NSEC_PER_USEC);
 }
 
 
@@ -813,7 +813,7 @@ struct tcp_skb_cb {
 #define TCPCB_SACKED_RETRANS	0x02	/* SKB retransmitted		*/
 #define TCPCB_LOST		0x04	/* SKB is lost			*/
 #define TCPCB_TAGBITS		0x07	/* All tag bits			*/
-#define TCPCB_REPAIRED		0x10	/* SKB repaired (no skb_mstamp)	*/
+#define TCPCB_REPAIRED		0x10	/* SKB repaired (no skb_mstamp_ns)	*/
 #define TCPCB_EVER_RETRANS	0x80	/* Ever retransmitted frame	*/
 #define TCPCB_RETRANS		(TCPCB_SACKED_RETRANS|TCPCB_EVER_RETRANS| \
 				TCPCB_REPAIRED)

net/ipv4/syncookies.c

@@ -88,7 +88,7 @@ u64 cookie_init_timestamp(struct request_sock *req)
 		ts <<= TSBITS;
 		ts |= options;
 	}
-	return (u64)ts * (USEC_PER_SEC / TCP_TS_HZ);
+	return (u64)ts * (NSEC_PER_SEC / TCP_TS_HZ);
 }
 
 

net/ipv4/tcp.c

@@ -1295,7 +1295,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 			copy = size_goal;
 
 			/* All packets are restored as if they have
-			 * already been sent. skb_mstamp isn't set to
+			 * already been sent. skb_mstamp_ns isn't set to
 			 * avoid wrong rtt estimation.
 			 */
 			if (tp->repair)

net/ipv4/tcp_output.c

@@ -1014,7 +1014,7 @@ static void tcp_internal_pacing(struct sock *sk, const struct sk_buff *skb)
 
 static void tcp_update_skb_after_send(struct tcp_sock *tp, struct sk_buff *skb)
 {
-	skb->skb_mstamp = tp->tcp_mstamp;
+	skb->skb_mstamp_ns = tp->tcp_wstamp_ns;
 	list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
 }
 
@@ -1061,7 +1061,7 @@ static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
 		if (unlikely(!skb))
 			return -ENOBUFS;
 	}
-	skb->skb_mstamp = tp->tcp_mstamp;
+	skb->skb_mstamp_ns = tp->tcp_wstamp_ns;
 
 	inet = inet_sk(sk);
 	tcb = TCP_SKB_CB(skb);
@@ -1165,8 +1165,7 @@ static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
 	skb_shinfo(skb)->gso_segs = tcp_skb_pcount(skb);
 	skb_shinfo(skb)->gso_size = tcp_skb_mss(skb);
 
-	/* Our usage of tstamp should remain private */
-	skb->tstamp = 0;
+	/* Leave earliest departure time in skb->tstamp (skb->skb_mstamp_ns) */
 
 	/* Cleanup our debris for IP stacks */
 	memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
@@ -3221,10 +3220,10 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
 	memset(&opts, 0, sizeof(opts));
 #ifdef CONFIG_SYN_COOKIES
 	if (unlikely(req->cookie_ts))
-		skb->skb_mstamp = cookie_init_timestamp(req);
+		skb->skb_mstamp_ns = cookie_init_timestamp(req);
 	else
 #endif
-		skb->skb_mstamp = tcp_clock_us();
+		skb->skb_mstamp_ns = tcp_clock_ns();
 
 #ifdef CONFIG_TCP_MD5SIG
 	rcu_read_lock();
@@ -3440,7 +3439,7 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
 
 	err = tcp_transmit_skb(sk, syn_data, 1, sk->sk_allocation);
 
-	syn->skb_mstamp = syn_data->skb_mstamp;
+	syn->skb_mstamp_ns = syn_data->skb_mstamp_ns;
 
 	/* Now full SYN+DATA was cloned and sent (or not),
 	 * remove the SYN from the original skb (syn_data)

net/ipv4/tcp_timer.c

@@ -360,7 +360,7 @@ static void tcp_probe_timer(struct sock *sk)
 	 */
 	start_ts = tcp_skb_timestamp(skb);
 	if (!start_ts)
-		skb->skb_mstamp = tp->tcp_mstamp;
+		skb->skb_mstamp_ns = tp->tcp_wstamp_ns;
 	else if (icsk->icsk_user_timeout &&
 		 (s32)(tcp_time_stamp(tp) - start_ts) > icsk->icsk_user_timeout)
 		goto abort;