net-tcp_bbr: v2: shrink delivered_mstamp, first_tx_mstamp to u32 to f…

…ree up 8 bytes

Free up some space for tracking inflight and losses for each
bw sample, in upcoming commits.

These timestamps are in microseconds, and are now stored in 32
bits. So they can only hold time intervals up to roughly 2^12 = 4096
seconds.  But Linux TCP RTT and RTO tracking has the same 32-bit
microsecond implementation approach and resulting deployment
limitations. So this is not introducing a new limit. And these should
not be a limitation for the foreseeable future.

Effort: net-tcp_bbr
Origin-9xx-SHA1: 238a7e6b5d51625fef1ce7769826a7b21b02ae55
Change-Id: I3b779603797263b52a61ad57c565eb91fe42680c
Signed-off-by: Alexandre Frade <kernel@xanmod.org>

include/net/tcp.h

@@ -824,6 +824,11 @@ static inline u32 tcp_stamp_us_delta(u64 t1, u64 t0)
 	return max_t(s64, t1 - t0, 0);
 }
 
+static inline u32 tcp_stamp32_us_delta(u32 t1, u32 t0)
+{
+	return max_t(s32, t1 - t0, 0);
+}
+
 static inline u32 tcp_skb_timestamp(const struct sk_buff *skb)
 {
 	return tcp_ns_to_ts(skb->skb_mstamp_ns);
@@ -899,9 +904,9 @@ struct tcp_skb_cb {
 			/* pkts S/ACKed so far upon tx of skb, incl retrans: */
 			__u32 delivered;
 			/* start of send pipeline phase */
-			u64 first_tx_mstamp;
+			u32 first_tx_mstamp;
 			/* when we reached the "delivered" count */
-			u64 delivered_mstamp;
+			u32 delivered_mstamp;
 		} tx;   /* only used for outgoing skbs */
 		union {
 			struct inet_skb_parm	h4;

net/ipv4/tcp_rate.c

@@ -101,8 +101,9 @@ void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb,
 		/* Record send time of most recently ACKed packet: */
 		tp->first_tx_mstamp  = tx_tstamp;
 		/* Find the duration of the "send phase" of this window: */
-		rs->interval_us = tcp_stamp_us_delta(tp->first_tx_mstamp,
-						     scb->tx.first_tx_mstamp);
+		rs->interval_us      = tcp_stamp32_us_delta(
+						tp->first_tx_mstamp,
+						scb->tx.first_tx_mstamp);
 
 	}
 	/* Mark off the skb delivered once it's sacked to avoid being
@@ -155,7 +156,7 @@ void tcp_rate_gen(struct sock *sk, u32 delivered, u32 lost,
 	 * longer phase.
 	 */
 	snd_us = rs->interval_us;				/* send phase */
-	ack_us = tcp_stamp_us_delta(tp->tcp_mstamp,
+	ack_us = tcp_stamp32_us_delta(tp->tcp_mstamp,
 				    rs->prior_mstamp); /* ack phase */
 	rs->interval_us = max(snd_us, ack_us);