Merge branch 'FACK-loss-recovery-remove'

Yuchung Cheng says:

====================
remove FACK loss recovery

This patch set removes the forward-acknowledgment (FACK)
packet-based loss and reordering detection. This simplifies TCP
loss recovery since the SACK scoreboard no longer needs to track
the number of pending packets under highest SACKed sequence. FACK
is subsumed by the time-based RACK loss detection which is more
robust under reordering and second order losses.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>

10 files changed, 80 insertions, 184 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index d8676dd..46c7e10 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -289,8 +289,7 @@ tcp_ecn_fallback - BOOLEAN
 	Default: 1 (fallback enabled)
 
 tcp_fack - BOOLEAN
-	Enable FACK congestion avoidance and fast retransmission.
-	The value is not used, if tcp_sack is not enabled.
+	This is a legacy option, it has no effect anymore.
 
 tcp_fin_timeout - INTEGER
 	The length of time an orphaned (no longer referenced by any
diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 22f40c9..df5d97a 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -85,7 +85,6 @@ struct tcp_sack_block {
 
 /*These are used to set the sack_ok field in struct tcp_options_received */
 #define TCP_SACK_SEEN     (1 << 0)   /*1 = peer is SACK capable, */
-#define TCP_FACK_ENABLED  (1 << 1)   /*1 = FACK is enabled locally*/
 #define TCP_DSACK_SEEN    (1 << 2)   /*1 = DSACK was received from peer*/
 
 struct tcp_options_received {
@@ -294,7 +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;
 
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 2f2c69a..ed71511 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -384,7 +384,6 @@ void tcp_update_metrics(struct sock *sk);
 void tcp_init_metrics(struct sock *sk);
 void tcp_metrics_init(void);
 bool tcp_peer_is_proven(struct request_sock *req, struct dst_entry *dst);
-void tcp_disable_fack(struct tcp_sock *tp);
 void tcp_close(struct sock *sk, long timeout);
 void tcp_init_sock(struct sock *sk);
 void tcp_init_transfer(struct sock *sk, int bpf_op);
@@ -776,7 +775,7 @@ struct tcp_skb_cb {
 	};
 	__u8		tcp_flags;	/* TCP header flags. (tcp[13])	*/
 
-	__u8		sacked;		/* State flags for SACK/FACK.	*/
+	__u8		sacked;		/* State flags for SACK.	*/
 #define TCPCB_SACKED_ACKED	0x01	/* SKB ACK'd by a SACK block	*/
 #define TCPCB_SACKED_RETRANS	0x02	/* SKB retransmitted		*/
 #define TCPCB_LOST		0x04	/* SKB is lost			*/
@@ -1066,7 +1065,6 @@ void tcp_rate_check_app_limited(struct sock *sk);
  *
  * tcp_is_sack - SACK enabled
  * tcp_is_reno - No SACK
- * tcp_is_fack - FACK enabled, implies SACK enabled
  */
 static inline int tcp_is_sack(const struct tcp_sock *tp)
 {
@@ -1078,16 +1076,6 @@ static inline bool tcp_is_reno(const struct tcp_sock *tp)
 	return !tcp_is_sack(tp);
 }
 
-static inline bool tcp_is_fack(const struct tcp_sock *tp)
-{
-	return tp->rx_opt.sack_ok & TCP_FACK_ENABLED;
-}
-
-static inline void tcp_enable_fack(struct tcp_sock *tp)
-{
-	tp->rx_opt.sack_ok |= TCP_FACK_ENABLED;
-}
-
 static inline unsigned int tcp_left_out(const struct tcp_sock *tp)
 {
 	return tp->sacked_out + tp->lost_out;
diff --git a/include/uapi/linux/snmp.h b/include/uapi/linux/snmp.h
index 0d941cd..33a70ec 100644
--- a/include/uapi/linux/snmp.h
+++ b/include/uapi/linux/snmp.h
@@ -191,7 +191,6 @@ enum
 	LINUX_MIB_TCPRENORECOVERY,		/* TCPRenoRecovery */
 	LINUX_MIB_TCPSACKRECOVERY,		/* TCPSackRecovery */
 	LINUX_MIB_TCPSACKRENEGING,		/* TCPSACKReneging */
-	LINUX_MIB_TCPFACKREORDER,		/* TCPFACKReorder */
 	LINUX_MIB_TCPSACKREORDER,		/* TCPSACKReorder */
 	LINUX_MIB_TCPRENOREORDER,		/* TCPRenoReorder */
 	LINUX_MIB_TCPTSREORDER,			/* TCPTSReorder */
diff --git a/net/ipv4/proc.c b/net/ipv4/proc.c
index 127153f..9f37c47 100644
--- a/net/ipv4/proc.c
+++ b/net/ipv4/proc.c
@@ -212,7 +212,6 @@ static const struct snmp_mib snmp4_net_list[] = {
 	SNMP_MIB_ITEM("TCPRenoRecovery", LINUX_MIB_TCPRENORECOVERY),
 	SNMP_MIB_ITEM("TCPSackRecovery", LINUX_MIB_TCPSACKRECOVERY),
 	SNMP_MIB_ITEM("TCPSACKReneging", LINUX_MIB_TCPSACKRENEGING),
-	SNMP_MIB_ITEM("TCPFACKReorder", LINUX_MIB_TCPFACKREORDER),
 	SNMP_MIB_ITEM("TCPSACKReorder", LINUX_MIB_TCPSACKREORDER),
 	SNMP_MIB_ITEM("TCPRenoReorder", LINUX_MIB_TCPRENOREORDER),
 	SNMP_MIB_ITEM("TCPTSReorder", LINUX_MIB_TCPTSREORDER),
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index bc71a27..bf97317 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2509,8 +2509,6 @@ static int tcp_repair_options_est(struct sock *sk,
 				return -EINVAL;
 
 			tp->rx_opt.sack_ok |= TCP_SACK_SEEN;
-			if (sock_net(sk)->ipv4.sysctl_tcp_fack)
-				tcp_enable_fack(tp);
 			break;
 		case TCPOPT_TIMESTAMP:
 			if (opt.opt_val != 0)
@@ -2979,7 +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);
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 9ceaa1f..94d729b 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -842,18 +842,6 @@ __u32 tcp_init_cwnd(const struct tcp_sock *tp, const struct dst_entry *dst)
 	return min_t(__u32, cwnd, tp->snd_cwnd_clamp);
 }
 
-/*
- * Packet counting of FACK is based on in-order assumptions, therefore TCP
- * disables it when reordering is detected
- */
-void tcp_disable_fack(struct tcp_sock *tp)
-{
-	/* RFC3517 uses different metric in lost marker => reset on change */
-	if (tcp_is_fack(tp))
-		tp->lost_skb_hint = NULL;
-	tp->rx_opt.sack_ok &= ~TCP_FACK_ENABLED;
-}
-
 /* Take a notice that peer is sending D-SACKs */
 static void tcp_dsack_seen(struct tcp_sock *tp)
 {
@@ -861,42 +849,39 @@ static void tcp_dsack_seen(struct tcp_sock *tp)
 	tp->rack.dsack_seen = 1;
 }
 
-static void tcp_update_reordering(struct sock *sk, const int metric,
-				  const int ts)
+/* It's reordering when higher sequence was delivered (i.e. sacked) before
+ * some lower never-retransmitted sequence ("low_seq"). The maximum reordering
+ * distance is approximated in full-mss packet distance ("reordering").
+ */
+static void tcp_check_sack_reordering(struct sock *sk, const u32 low_seq,
+				      const int ts)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
-	int mib_idx;
+	const u32 mss = tp->mss_cache;
+	u32 fack, metric;
 
-	if (WARN_ON_ONCE(metric < 0))
+	fack = tcp_highest_sack_seq(tp);
+	if (!before(low_seq, fack))
 		return;
 
-	if (metric > tp->reordering) {
-		tp->reordering = min(sock_net(sk)->ipv4.sysctl_tcp_max_reordering, metric);
-
+	metric = fack - low_seq;
+	if ((metric > tp->reordering * mss) && mss) {
 #if FASTRETRANS_DEBUG > 1
 		pr_debug("Disorder%d %d %u f%u s%u rr%d\n",
 			 tp->rx_opt.sack_ok, inet_csk(sk)->icsk_ca_state,
 			 tp->reordering,
-			 tp->fackets_out,
+			 0,
 			 tp->sacked_out,
 			 tp->undo_marker ? tp->undo_retrans : 0);
 #endif
-		tcp_disable_fack(tp);
+		tp->reordering = min_t(u32, (metric + mss - 1) / mss,
+				       sock_net(sk)->ipv4.sysctl_tcp_max_reordering);
 	}
 
 	tp->rack.reord = 1;
-
 	/* This exciting event is worth to be remembered. 8) */
-	if (ts)
-		mib_idx = LINUX_MIB_TCPTSREORDER;
-	else if (tcp_is_reno(tp))
-		mib_idx = LINUX_MIB_TCPRENOREORDER;
-	else if (tcp_is_fack(tp))
-		mib_idx = LINUX_MIB_TCPFACKREORDER;
-	else
-		mib_idx = LINUX_MIB_TCPSACKREORDER;
-
-	NET_INC_STATS(sock_net(sk), mib_idx);
+	NET_INC_STATS(sock_net(sk),
+		      ts ? LINUX_MIB_TCPTSREORDER : LINUX_MIB_TCPSACKREORDER);
 }
 
 /* This must be called before lost_out is incremented */
@@ -970,7 +955,6 @@ void tcp_skb_mark_lost_uncond_verify(struct tcp_sock *tp, struct sk_buff *skb)
  * 3. Loss detection event of two flavors:
  *	A. Scoreboard estimator decided the packet is lost.
  *	   A'. Reno "three dupacks" marks head of queue lost.
- *	   A''. Its FACK modification, head until snd.fack is lost.
  *	B. SACK arrives sacking SND.NXT at the moment, when the
  *	   segment was retransmitted.
  * 4. D-SACK added new rule: D-SACK changes any tag to S.
@@ -1113,8 +1097,7 @@ static bool tcp_check_dsack(struct sock *sk, const struct sk_buff *ack_skb,
 }
 
 struct tcp_sacktag_state {
-	int	reord;
-	int	fack_count;
+	u32	reord;
 	/* Timestamps for earliest and latest never-retransmitted segment
 	 * that was SACKed. RTO needs the earliest RTT to stay conservative,
 	 * but congestion control should still get an accurate delay signal.
@@ -1190,15 +1173,15 @@ static u8 tcp_sacktag_one(struct sock *sk,
 			  u64 xmit_time)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
-	int fack_count = state->fack_count;
 
 	/* Account D-SACK for retransmitted packet. */
 	if (dup_sack && (sacked & TCPCB_RETRANS)) {
 		if (tp->undo_marker && tp->undo_retrans > 0 &&
 		    after(end_seq, tp->undo_marker))
 			tp->undo_retrans--;
-		if (sacked & TCPCB_SACKED_ACKED)
-			state->reord = min(fack_count, state->reord);
+		if ((sacked & TCPCB_SACKED_ACKED) &&
+		    before(start_seq, state->reord))
+				state->reord = start_seq;
 	}
 
 	/* Nothing to do; acked frame is about to be dropped (was ACKed). */
@@ -1224,9 +1207,10 @@ static u8 tcp_sacktag_one(struct sock *sk,
 				 * which was in hole. It is reordering.
 				 */
 				if (before(start_seq,
-					   tcp_highest_sack_seq(tp)))
-					state->reord = min(fack_count,
-							   state->reord);
+					   tcp_highest_sack_seq(tp)) &&
+				    before(start_seq, state->reord))
+					state->reord = start_seq;
+
 				if (!after(end_seq, tp->high_seq))
 					state->flag |= FLAG_ORIG_SACK_ACKED;
 				if (state->first_sackt == 0)
@@ -1245,15 +1229,10 @@ static u8 tcp_sacktag_one(struct sock *sk,
 		tp->sacked_out += pcount;
 		tp->delivered += pcount;  /* Out-of-order packets delivered */
 
-		fack_count += pcount;
-
 		/* Lost marker hint past SACKed? Tweak RFC3517 cnt */
-		if (!tcp_is_fack(tp) && tp->lost_skb_hint &&
+		if (tp->lost_skb_hint &&
 		    before(start_seq, TCP_SKB_CB(tp->lost_skb_hint)->seq))
 			tp->lost_cnt_hint += pcount;
-
-		if (fack_count > tp->fackets_out)
-			tp->fackets_out = fack_count;
 	}
 
 	/* D-SACK. We can detect redundant retransmission in S|R and plain R
@@ -1500,7 +1479,6 @@ static struct sk_buff *tcp_shift_skb_data(struct sock *sk, struct sk_buff *skb,
 	}
 
 out:
-	state->fack_count += pcount;
 	return prev;
 
 noop:
@@ -1579,8 +1557,6 @@ static struct sk_buff *tcp_sacktag_walk(struct sk_buff *skb, struct sock *sk,
 				    tcp_highest_sack_seq(tp)))
 				tcp_advance_highest_sack(sk, skb);
 		}
-
-		state->fack_count += tcp_skb_pcount(skb);
 	}
 	return skb;
 }
@@ -1591,7 +1567,6 @@ static struct sk_buff *tcp_sacktag_bsearch(struct sock *sk,
 {
 	struct rb_node *parent, **p = &sk->tcp_rtx_queue.rb_node;
 	struct sk_buff *skb;
-	int unack_bytes;
 
 	while (*p) {
 		parent = *p;
@@ -1604,12 +1579,6 @@ static struct sk_buff *tcp_sacktag_bsearch(struct sock *sk,
 			p = &parent->rb_right;
 			continue;
 		}
-
-		state->fack_count = 0;
-		unack_bytes = TCP_SKB_CB(skb)->seq - tcp_sk(sk)->snd_una;
-		if (state->mss_now && unack_bytes > 0)
-			state->fack_count = unack_bytes / state->mss_now;
-
 		return skb;
 	}
 	return NULL;
@@ -1667,13 +1636,10 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
 	int first_sack_index;
 
 	state->flag = 0;
-	state->reord = tp->packets_out;
+	state->reord = tp->snd_nxt;
 
-	if (!tp->sacked_out) {
-		if (WARN_ON(tp->fackets_out))
-			tp->fackets_out = 0;
+	if (!tp->sacked_out)
 		tcp_highest_sack_reset(sk);
-	}
 
 	found_dup_sack = tcp_check_dsack(sk, ack_skb, sp_wire,
 					 num_sacks, prior_snd_una);
@@ -1745,7 +1711,6 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
 	}
 
 	state->mss_now = tcp_current_mss(sk);
-	state->fack_count = 0;
 	skb = NULL;
 	i = 0;
 
@@ -1803,7 +1768,6 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
 				skb = tcp_highest_sack(sk);
 				if (!skb)
 					break;
-				state->fack_count = tp->fackets_out;
 				cache++;
 				goto walk;
 			}
@@ -1818,7 +1782,6 @@ tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
 			skb = tcp_highest_sack(sk);
 			if (!skb)
 				break;
-			state->fack_count = tp->fackets_out;
 		}
 		skb = tcp_sacktag_skip(skb, sk, state, start_seq);
 
@@ -1838,9 +1801,8 @@ advance_sp:
 	for (j = 0; j < used_sacks; j++)
 		tp->recv_sack_cache[i++] = sp[j];
 
-	if ((state->reord < tp->fackets_out) &&
-	    ((inet_csk(sk)->icsk_ca_state != TCP_CA_Loss) || tp->undo_marker))
-		tcp_update_reordering(sk, tp->fackets_out - state->reord, 0);
+	if (inet_csk(sk)->icsk_ca_state != TCP_CA_Loss || tp->undo_marker)
+		tcp_check_sack_reordering(sk, state->reord, 0);
 
 	tcp_verify_left_out(tp);
 out:
@@ -1878,8 +1840,13 @@ static bool tcp_limit_reno_sacked(struct tcp_sock *tp)
 static void tcp_check_reno_reordering(struct sock *sk, const int addend)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
-	if (tcp_limit_reno_sacked(tp))
-		tcp_update_reordering(sk, tp->packets_out + addend, 0);
+
+	if (!tcp_limit_reno_sacked(tp))
+		return;
+
+	tp->reordering = min_t(u32, tp->packets_out + addend,
+			       sock_net(sk)->ipv4.sysctl_tcp_max_reordering);
+	NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPRENOREORDER);
 }
 
 /* Emulate SACKs for SACKless connection: account for a new dupack. */
@@ -1925,7 +1892,6 @@ void tcp_clear_retrans(struct tcp_sock *tp)
 	tp->lost_out = 0;
 	tp->undo_marker = 0;
 	tp->undo_retrans = -1;
-	tp->fackets_out = 0;
 	tp->sacked_out = 0;
 }
 
@@ -1975,7 +1941,6 @@ void tcp_enter_loss(struct sock *sk)
 	if (is_reneg) {
 		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSACKRENEGING);
 		tp->sacked_out = 0;
-		tp->fackets_out = 0;
 	}
 	tcp_clear_all_retrans_hints(tp);
 
@@ -2042,19 +2007,10 @@ static bool tcp_check_sack_reneging(struct sock *sk, int flag)
 	return false;
 }
 
-static inline int tcp_fackets_out(const struct tcp_sock *tp)
-{
-	return tcp_is_reno(tp) ? tp->sacked_out + 1 : tp->fackets_out;
-}
-
 /* Heurestics to calculate number of duplicate ACKs. There's no dupACKs
  * counter when SACK is enabled (without SACK, sacked_out is used for
  * that purpose).
  *
- * Instead, with FACK TCP uses fackets_out that includes both SACKed
- * segments up to the highest received SACK block so far and holes in
- * between them.
- *
  * With reordering, holes may still be in flight, so RFC3517 recovery
  * uses pure sacked_out (total number of SACKed segments) even though
  * it violates the RFC that uses duplicate ACKs, often these are equal
@@ -2064,10 +2020,10 @@ static inline int tcp_fackets_out(const struct tcp_sock *tp)
  */
 static inline int tcp_dupack_heuristics(const struct tcp_sock *tp)
 {
-	return tcp_is_fack(tp) ? tp->fackets_out : tp->sacked_out + 1;
+	return tp->sacked_out + 1;
 }
 
-/* Linux NewReno/SACK/FACK/ECN state machine.
+/* Linux NewReno/SACK/ECN state machine.
  * --------------------------------------
  *
  * "Open"	Normal state, no dubious events, fast path.
@@ -2132,16 +2088,6 @@ static inline int tcp_dupack_heuristics(const struct tcp_sock *tp)
  *		dynamically measured and adjusted. This is implemented in
  *		tcp_rack_mark_lost.
  *
- *		FACK (Disabled by default. Subsumbed by RACK):
- *		It is the simplest heuristics. As soon as we decided
- *		that something is lost, we decide that _all_ not SACKed
- *		packets until the most forward SACK are lost. I.e.
- *		lost_out = fackets_out - sacked_out and left_out = fackets_out.
- *		It is absolutely correct estimate, if network does not reorder
- *		packets. And it loses any connection to reality when reordering
- *		takes place. We use FACK by default until reordering
- *		is suspected on the path to this destination.
- *
  *		If the receiver does not support SACK:
  *
  *		NewReno (RFC6582): in Recovery we assume that one segment
@@ -2190,7 +2136,7 @@ static bool tcp_time_to_recover(struct sock *sk, int flag)
 }
 
 /* Detect loss in event "A" above by marking head of queue up as lost.
- * For FACK or non-SACK(Reno) senders, the first "packets" number of segments
+ * For non-SACK(Reno) senders, the first "packets" number of segments
  * are considered lost. For RFC3517 SACK, a segment is considered lost if it
  * has at least tp->reordering SACKed seqments above it; "packets" refers to
  * the maximum SACKed segments to pass before reaching this limit.
@@ -2226,12 +2172,12 @@ static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head)
 			break;
 
 		oldcnt = cnt;
-		if (tcp_is_fack(tp) || tcp_is_reno(tp) ||
+		if (tcp_is_reno(tp) ||
 		    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
 			cnt += tcp_skb_pcount(skb);
 
 		if (cnt > packets) {
-			if ((tcp_is_sack(tp) && !tcp_is_fack(tp)) ||
+			if (tcp_is_sack(tp) ||
 			    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) ||
 			    (oldcnt >= packets))
 				break;
@@ -2262,11 +2208,6 @@ static void tcp_update_scoreboard(struct sock *sk, int fast_rexmit)
 
 	if (tcp_is_reno(tp)) {
 		tcp_mark_head_lost(sk, 1, 1);
-	} else if (tcp_is_fack(tp)) {
-		int lost = tp->fackets_out - tp->reordering;
-		if (lost <= 0)
-			lost = 1;
-		tcp_mark_head_lost(sk, lost, 0);
 	} else {
 		int sacked_upto = tp->sacked_out - tp->reordering;
 		if (sacked_upto >= 0)
@@ -2736,15 +2677,15 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack,
 }
 
 /* Undo during fast recovery after partial ACK. */
-static bool tcp_try_undo_partial(struct sock *sk, const int acked)
+static bool tcp_try_undo_partial(struct sock *sk, u32 prior_snd_una)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
 	if (tp->undo_marker && tcp_packet_delayed(tp)) {
 		/* Plain luck! Hole if filled with delayed
-		 * packet, rather than with a retransmit.
+		 * packet, rather than with a retransmit. Check reordering.
 		 */
-		tcp_update_reordering(sk, tcp_fackets_out(tp) + acked, 1);
+		tcp_check_sack_reordering(sk, prior_snd_una, 1);
 
 		/* We are getting evidence that the reordering degree is higher
 		 * than we realized. If there are no retransmits out then we
@@ -2780,6 +2721,14 @@ static void tcp_rack_identify_loss(struct sock *sk, int *ack_flag)
 	}
 }
 
+static bool tcp_force_fast_retransmit(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	return after(tcp_highest_sack_seq(tp),
+		     tp->snd_una + tp->reordering * tp->mss_cache);
+}
+
 /* Process an event, which can update packets-in-flight not trivially.
  * Main goal of this function is to calculate new estimate for left_out,
  * taking into account both packets sitting in receiver's buffer and
@@ -2792,19 +2741,17 @@ static void tcp_rack_identify_loss(struct sock *sk, int *ack_flag)
  * It does _not_ decide what to send, it is made in function
  * tcp_xmit_retransmit_queue().
  */
-static void tcp_fastretrans_alert(struct sock *sk, const int acked,
+static void tcp_fastretrans_alert(struct sock *sk, const u32 prior_snd_una,
 				  bool is_dupack, int *ack_flag, int *rexmit)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	int fast_rexmit = 0, flag = *ack_flag;
 	bool do_lost = is_dupack || ((flag & FLAG_DATA_SACKED) &&
-				    (tcp_fackets_out(tp) > tp->reordering));
+				     tcp_force_fast_retransmit(sk));
 
 	if (!tp->packets_out && tp->sacked_out)
 		tp->sacked_out = 0;
-	if (!tp->sacked_out && tp->fackets_out)
-		tp->fackets_out = 0;
 
 	/* Now state machine starts.
 	 * A. ECE, hence prohibit cwnd undoing, the reduction is required. */
@@ -2851,11 +2798,11 @@ static void tcp_fastretrans_alert(struct sock *sk, const int acked,
 			if (tcp_is_reno(tp) && is_dupack)
 				tcp_add_reno_sack(sk);
 		} else {
-			if (tcp_try_undo_partial(sk, acked))
+			if (tcp_try_undo_partial(sk, prior_snd_una))
 				return;
 			/* Partial ACK arrived. Force fast retransmit. */
 			do_lost = tcp_is_reno(tp) ||
-				  tcp_fackets_out(tp) > tp->reordering;
+				  tcp_force_fast_retransmit(sk);
 		}
 		if (tcp_try_undo_dsack(sk)) {
 			tcp_try_keep_open(sk);
@@ -3065,15 +3012,15 @@ static void tcp_ack_tstamp(struct sock *sk, struct sk_buff *skb,
  * is before the ack sequence we can discard it as it's confirmed to have
  * arrived at the other end.
  */
-static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
-			       u32 prior_snd_una, int *acked,
+static int tcp_clean_rtx_queue(struct sock *sk, u32 prior_fack,
+			       u32 prior_snd_una,
 			       struct tcp_sacktag_state *sack)
 {
 	const struct inet_connection_sock *icsk = inet_csk(sk);
 	u64 first_ackt, last_ackt;
 	struct tcp_sock *tp = tcp_sk(sk);
 	u32 prior_sacked = tp->sacked_out;
-	u32 reord = tp->packets_out;
+	u32 reord = tp->snd_nxt; /* lowest acked un-retx un-sacked seq */
 	struct sk_buff *skb, *next;
 	bool fully_acked = true;
 	long sack_rtt_us = -1L;
@@ -3088,6 +3035,7 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
 
 	for (skb = skb_rb_first(&sk->tcp_rtx_queue); skb; skb = next) {
 		struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
+		const u32 start_seq = scb->seq;
 		u8 sacked = scb->sacked;
 		u32 acked_pcount;
 
@@ -3118,7 +3066,8 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
 				first_ackt = last_ackt;
 
 			last_in_flight = TCP_SKB_CB(skb)->tx.in_flight;
-			reord = min(pkts_acked, reord);
+			if (before(start_seq, reord))
+				reord = start_seq;
 			if (!after(scb->end_seq, tp->high_seq))
 				flag |= FLAG_ORIG_SACK_ACKED;
 		}
@@ -3196,16 +3145,12 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
 			int delta;
 
 			/* Non-retransmitted hole got filled? That's reordering */
-			if (reord < prior_fackets && reord <= tp->fackets_out)
-				tcp_update_reordering(sk, tp->fackets_out - reord, 0);
+			if (before(reord, prior_fack))
+				tcp_check_sack_reordering(sk, reord, 0);
 
-			delta = tcp_is_fack(tp) ? pkts_acked :
-						  prior_sacked - tp->sacked_out;
+			delta = prior_sacked - tp->sacked_out;
 			tp->lost_cnt_hint -= min(tp->lost_cnt_hint, delta);
 		}
-
-		tp->fackets_out -= min(pkts_acked, tp->fackets_out);
-
 	} else if (skb && rtt_update && sack_rtt_us >= 0 &&
 		   sack_rtt_us > tcp_stamp_us_delta(tp->tcp_mstamp, skb->skb_mstamp)) {
 		/* Do not re-arm RTO if the sack RTT is measured from data sent
@@ -3246,7 +3191,6 @@ static int tcp_clean_rtx_queue(struct sock *sk, int prior_fackets,
 		}
 	}
 #endif
-	*acked = pkts_acked;
 	return flag;
 }
 
@@ -3555,12 +3499,11 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 	u32 ack_seq = TCP_SKB_CB(skb)->seq;
 	u32 ack = TCP_SKB_CB(skb)->ack_seq;
 	bool is_dupack = false;
-	u32 prior_fackets;
 	int prior_packets = tp->packets_out;
 	u32 delivered = tp->delivered;
 	u32 lost = tp->lost;
-	int acked = 0; /* Number of packets newly acked */
 	int rexmit = REXMIT_NONE; /* Flag to (re)transmit to recover losses */
+	u32 prior_fack;
 
 	sack_state.first_sackt = 0;
 	sack_state.rate = &rs;
@@ -3592,7 +3535,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 		icsk->icsk_retransmits = 0;
 	}
 
-	prior_fackets = tp->fackets_out;
+	prior_fack = tcp_highest_sack_seq(tp);
 	rs.prior_in_flight = tcp_packets_in_flight(tp);
 
 	/* ts_recent update must be made after we are sure that the packet
@@ -3648,8 +3591,7 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 		goto no_queue;
 
 	/* See if we can take anything off of the retransmit queue. */
-	flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una, &acked,
-				    &sack_state);
+	flag |= tcp_clean_rtx_queue(sk, prior_fack, prior_snd_una, &sack_state);
 
 	tcp_rack_update_reo_wnd(sk, &rs);
 
@@ -3661,7 +3603,8 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 
 	if (tcp_ack_is_dubious(sk, flag)) {
 		is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED | FLAG_NOT_DUP));
-		tcp_fastretrans_alert(sk, acked, is_dupack, &flag, &rexmit);
+		tcp_fastretrans_alert(sk, prior_snd_una, is_dupack, &flag,
+				      &rexmit);
 	}
 
 	if ((flag & FLAG_FORWARD_PROGRESS) || !(flag & FLAG_NOT_DUP))
@@ -3677,7 +3620,8 @@ static int tcp_ack(struct sock *sk, const struct sk_buff *skb, int flag)
 no_queue:
 	/* If data was DSACKed, see if we can undo a cwnd reduction. */
 	if (flag & FLAG_DSACKING_ACK)
-		tcp_fastretrans_alert(sk, acked, is_dupack, &flag, &rexmit);
+		tcp_fastretrans_alert(sk, prior_snd_una, is_dupack, &flag,
+				      &rexmit);
 	/* If this ack opens up a zero window, clear backoff.  It was
 	 * being used to time the probes, and is probably far higher than
 	 * it needs to be for normal retransmission.
@@ -3699,7 +3643,8 @@ old_ack:
 	if (TCP_SKB_CB(skb)->sacked) {
 		flag |= tcp_sacktag_write_queue(sk, skb, prior_snd_una,
 						&sack_state);
-		tcp_fastretrans_alert(sk, acked, is_dupack, &flag, &rexmit);
+		tcp_fastretrans_alert(sk, prior_snd_una, is_dupack, &flag,
+				      &rexmit);
 		tcp_xmit_recovery(sk, rexmit);
 	}
 
@@ -5708,9 +5653,6 @@ static int tcp_rcv_synsent_state_process(struct sock *sk, struct sk_buff *skb,
 			tp->tcp_header_len = sizeof(struct tcphdr);
 		}
 
-		if (tcp_is_sack(tp) && sock_net(sk)->ipv4.sysctl_tcp_fack)
-			tcp_enable_fack(tp);
-
 		tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
 		tcp_initialize_rcv_mss(sk);
 
diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c
index 9d5ddeb..7097f92 100644
--- a/net/ipv4/tcp_metrics.c
+++ b/net/ipv4/tcp_metrics.c
@@ -470,10 +470,8 @@ void tcp_init_metrics(struct sock *sk)
 		tp->snd_ssthresh = TCP_INFINITE_SSTHRESH;
 	}
 	val = tcp_metric_get(tm, TCP_METRIC_REORDERING);
-	if (val && tp->reordering != val) {
-		tcp_disable_fack(tp);
+	if (val && tp->reordering != val)
 		tp->reordering = val;
-	}
 
 	crtt = tcp_metric_get(tm, TCP_METRIC_RTT);
 	rcu_read_unlock();
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 4bb8658..e36eff0 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/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;
@@ -509,10 +508,7 @@ struct sock *tcp_create_openreq_child(const struct sock *sk,
 						       keepalive_time_when(newtp));
 
 		newtp->rx_opt.tstamp_ok = ireq->tstamp_ok;
-		if ((newtp->rx_opt.sack_ok = ireq->sack_ok) != 0) {
-			if (sock_net(sk)->ipv4.sysctl_tcp_fack)
-				tcp_enable_fack(newtp);
-		}
+		newtp->rx_opt.sack_ok = ireq->sack_ok;
 		newtp->window_clamp = req->rsk_window_clamp;
 		newtp->rcv_ssthresh = req->rsk_rcv_wnd;
 		newtp->rcv_wnd = req->rsk_rcv_wnd;
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 9b98d35..0256f7a 100644
--- a/net/ipv4/tcp_output.c
+++ b/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
  */
@@ -1253,11 +1238,9 @@ 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_is_fack(tp) || (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)))
+	    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
 		tp->lost_cnt_hint -= decr;
 
 	tcp_verify_left_out(tp);
@@ -2961,9 +2944,6 @@ int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
  * retransmitted data is acknowledged.  It tries to continue
  * resending the rest of the retransmit queue, until either
  * we've sent it all or the congestion window limit is reached.
- * If doing SACK, the first ACK which comes back for a timeout
- * based retransmit packet might feed us FACK information again.
- * If so, we use it to avoid unnecessarily retransmissions.
  */
 void tcp_xmit_retransmit_queue(struct sock *sk)
 {