tcp: use sequence distance to detect reordering

Replace the reordering distance measurement in packet unit with
sequence based approach. Previously it trackes the number of "packets"
toward the forward ACK (i.e.  highest sacked sequence)in a state
variable "fackets_out".

Precisely measuring reordering degree on packet distance has not much
benefit, as the degree constantly changes by factors like path, load,
and congestion window. It is also complicated and prone to arcane bugs.
This patch replaces with sequence-based approach that's much simpler.
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: Priyaranjan Jha <priyarjha@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/tcp.h

@@ -293,7 +293,6 @@ struct tcp_sock {
 	u32	pushed_seq;	/* Last pushed seq, required to talk to windows */
 	u32	lost_out;	/* Lost packets			*/
 	u32	sacked_out;	/* SACK'd packets			*/
-	u32	fackets_out;	/* FACK'd packets			*/
 
 	struct hrtimer	pacing_timer;
 

net/ipv4/tcp.c

@@ -2977,7 +2977,6 @@ void tcp_get_info(struct sock *sk, struct tcp_info *info)
 
 	info->tcpi_lost = tp->lost_out;
 	info->tcpi_retrans = tp->retrans_out;
-	info->tcpi_fackets = tp->fackets_out;
 
 	now = tcp_jiffies32;
 	info->tcpi_last_data_sent = jiffies_to_msecs(now - tp->lsndtime);

net/ipv4/tcp_minisocks.c

@@ -475,7 +475,6 @@ struct sock *tcp_create_openreq_child(const struct sock *sk,
 		newtp->packets_out = 0;
 		newtp->retrans_out = 0;
 		newtp->sacked_out = 0;
-		newtp->fackets_out = 0;
 		newtp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
 		newtp->tlp_high_seq = 0;
 		newtp->lsndtime = tcp_jiffies32;

net/ipv4/tcp_output.c

@@ -1218,21 +1218,6 @@ static void tcp_set_skb_tso_segs(struct sk_buff *skb, unsigned int mss_now)
 	}
 }
 
-/* When a modification to fackets out becomes necessary, we need to check
- * skb is counted to fackets_out or not.
- */
-static void tcp_adjust_fackets_out(struct sock *sk, const struct sk_buff *skb,
-				   int decr)
-{
-	struct tcp_sock *tp = tcp_sk(sk);
-
-	if (!tp->sacked_out || tcp_is_reno(tp))
-		return;
-
-	if (after(tcp_highest_sack_seq(tp), TCP_SKB_CB(skb)->seq))
-		tp->fackets_out -= decr;
-}
-
 /* Pcount in the middle of the write queue got changed, we need to do various
  * tweaks to fix counters
  */
@@ -1253,8 +1238,6 @@ static void tcp_adjust_pcount(struct sock *sk, const struct sk_buff *skb, int de
 	if (tcp_is_reno(tp) && decr > 0)
 		tp->sacked_out -= min_t(u32, tp->sacked_out, decr);
 
-	tcp_adjust_fackets_out(sk, skb, decr);
-
 	if (tp->lost_skb_hint &&
 	    before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(tp->lost_skb_hint)->seq) &&
 	    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))