net-tcp_bbr: v2: snapshot packets in flight at transmit time and pass…

… in rate_sample

CC algorithms may want to snapshot the number of packets in flight at
transmit time and pass in rate_sample, to understand the relationship
between inflight and losses or ECN signals, to try to find the highest
inflight value that has acceptable levels of loss/ECN marking.

We split out the code to set an skb's tx.in_flight field into its own
function, so that this code can be used for the TCP_REPAIR "fake send"
code path that inserts skbs into the rtx queue without sending them.

Effort: net-tcp_bbr
Origin-9xx-SHA1: b3eb4f2d20efab4ca001f32c9294739036c493ea
Origin-9xx-SHA1: e880fc907d06ea7354333f60f712748ebce9497b
Origin-9xx-SHA1: 330f825a08a6fe92cef74d799cc468864c479f63
Change-Id: I7314047d0ff14dd261a04b1969a46dc658c8836a
Signed-off-by: Alexandre Frade <kernel@xanmod.org>

include/net/tcp.h

@@ -907,6 +907,10 @@ struct tcp_skb_cb {
 			u32 first_tx_mstamp;
 			/* when we reached the "delivered" count */
 			u32 delivered_mstamp;
+#define TCPCB_IN_FLIGHT_BITS 20
+#define TCPCB_IN_FLIGHT_MAX ((1U << TCPCB_IN_FLIGHT_BITS) - 1)
+			u32 in_flight:20,   /* packets in flight at transmit */
+			    unused2:12;
 		} tx;   /* only used for outgoing skbs */
 		union {
 			struct inet_skb_parm	h4;
@@ -1054,6 +1058,7 @@ struct rate_sample {
 	u64  prior_mstamp; /* starting timestamp for interval */
 	u32  prior_delivered;	/* tp->delivered at "prior_mstamp" */
 	u32  prior_delivered_ce;/* tp->delivered_ce at "prior_mstamp" */
+	u32 tx_in_flight;	/* packets in flight at starting timestamp */
 	s32  delivered;		/* number of packets delivered over interval */
 	s32  delivered_ce;	/* number of packets delivered w/ CE marks*/
 	long interval_us;	/* time for tp->delivered to incr "delivered" */
@@ -1173,6 +1178,7 @@ static inline void tcp_ca_event(struct sock *sk, const enum tcp_ca_event event)
 void tcp_set_ca_state(struct sock *sk, const u8 ca_state);
 
 /* From tcp_rate.c */
+void tcp_set_tx_in_flight(struct sock *sk, struct sk_buff *skb);
 void tcp_rate_skb_sent(struct sock *sk, struct sk_buff *skb);
 void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb,
 			    struct rate_sample *rs);

net/ipv4/tcp_output.c

@@ -2639,6 +2639,7 @@ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
 			skb_set_delivery_time(skb, tp->tcp_wstamp_ns, true);
 			list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
 			tcp_init_tso_segs(skb, mss_now);
+			tcp_set_tx_in_flight(sk, skb);
 			goto repair; /* Skip network transmission */
 		}
 

net/ipv4/tcp_rate.c

@@ -34,6 +34,24 @@
  * ready to send in the write queue.
  */
 
+void tcp_set_tx_in_flight(struct sock *sk, struct sk_buff *skb)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	u32 in_flight;
+
+	/* Check, sanitize, and record packets in flight after skb was sent. */
+	in_flight = tcp_packets_in_flight(tp) + tcp_skb_pcount(skb);
+	if (WARN_ONCE(in_flight > TCPCB_IN_FLIGHT_MAX,
+		      "insane in_flight %u cc %s mss %u "
+		      "cwnd %u pif %u %u %u %u\n",
+		      in_flight, inet_csk(sk)->icsk_ca_ops->name,
+		      tp->mss_cache, tp->snd_cwnd,
+		      tp->packets_out, tp->retrans_out,
+		      tp->sacked_out, tp->lost_out))
+		in_flight = TCPCB_IN_FLIGHT_MAX;
+	TCP_SKB_CB(skb)->tx.in_flight = in_flight;
+}
+
 /* Snapshot the current delivery information in the skb, to generate
  * a rate sample later when the skb is (s)acked in tcp_rate_skb_delivered().
  */
@@ -67,6 +85,7 @@ void tcp_rate_skb_sent(struct sock *sk, struct sk_buff *skb)
 	TCP_SKB_CB(skb)->tx.delivered		= tp->delivered;
 	TCP_SKB_CB(skb)->tx.delivered_ce	= tp->delivered_ce;
 	TCP_SKB_CB(skb)->tx.is_app_limited	= tp->app_limited ? 1 : 0;
+	tcp_set_tx_in_flight(sk, skb);
 }
 
 /* When an skb is sacked or acked, we fill in the rate sample with the (prior)
@@ -96,6 +115,7 @@ void tcp_rate_skb_delivered(struct sock *sk, struct sk_buff *skb,
 		rs->prior_mstamp     = scb->tx.delivered_mstamp;
 		rs->is_app_limited   = scb->tx.is_app_limited;
 		rs->is_retrans	     = scb->sacked & TCPCB_RETRANS;
+		rs->tx_in_flight     = scb->tx.in_flight;
 		rs->last_end_seq     = scb->end_seq;
 
 		/* Record send time of most recently ACKed packet: */