[PATCH net v2 2/3] rxrpc: Fix counting of in-flight transmitted packets

[David Howells <dhowells@redhat.com>]

The RACK-TLP algorithm [RFC8985] makes use of the count of in-flight
packets in decisions (denoted as FlightSize).  This is currently based on
an approximation derived from the sum of other values:

	tx_nr_sent - (acks_nr_sacks + tx_nr_lost) + tx_nr_resent

This, however, turns out not to be very good here as the component counters
aren't updated together and this may lead to an incorrect estimate.

Fix counting of in-flight transmitted packets by tracking them directly
rather than deriving the value:

 (1) On each rxrpc_txqueue struct, keep a bitmap of which packets (txbuf)
     from the set managed by that txqueue are considered in-flight.

 (2) A new counter, tx_in_flight, is then added to the rxrpc_call struct to
     keep track of the number of packets considered to be in flight.

 (3) Set a packet's bit when the packet is transmitted and if the bit
     wasn't already set, increment tx_in_flight.

 (4) Clear a packet's bit when an ACK is received for the packet or if the
     packet is declared lost - and if the bit wasn't already cleared,
     decrement tx_in_flight.

The value in tx_in_flight is then used instead of the approximation.

The tx_nr_sent and tx_nr_resent counters are then removed and the
rxrpc_rack tracepoint updated.

Fixes: 7c482665931b ("rxrpc: Implement RACK/TLP to deal with transmission stalls [RFC8985]")
Link: https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260925191520.2206700-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeffrey Altman <jaltman@auristor.com>
cc: Eric Dumazet <edumazet@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: stable@vger.kernel.org
---
 include/trace/events/rxrpc.h | 11 ++++-------
 net/rxrpc/ar-internal.h      | 18 ++----------------
 net/rxrpc/input.c            |  8 ++++----
 net/rxrpc/input_rack.c       | 19 +++++++++++--------
 net/rxrpc/output.c           |  5 +++--
 5 files changed, 24 insertions(+), 37 deletions(-)

diff --git a/include/trace/events/rxrpc.h b/include/trace/events/rxrpc.h
index 56dc9b614071..67a5612cdc55 100644
--- a/include/trace/events/rxrpc.h
+++ b/include/trace/events/rxrpc.h
@@ -2584,9 +2584,8 @@ TRACE_EVENT(rxrpc_rack,
 		    __field(rxrpc_serial_t,	ack_serial)
 		    __field(rxrpc_seq_t,	seq)
 		    __field(enum rxrpc_rack_timer_mode, mode)
-		    __field(unsigned short,	nr_sent)
+		    __field(unsigned short,	in_flight)
 		    __field(unsigned short,	nr_lost)
-		    __field(unsigned short,	nr_resent)
 		    __field(unsigned short,	nr_sacked)
 		    __field(ktime_t,		timo)
 			     ),
@@ -2596,18 +2595,16 @@ TRACE_EVENT(rxrpc_rack,
 		    __entry->ack_serial	= call->rx_serial;
 		    __entry->seq	= call->rack_end_seq;
 		    __entry->mode	= call->rack_timer_mode;
-		    __entry->nr_sent	= call->tx_nr_sent;
+		    __entry->in_flight	= call->tx_in_flight;
 		    __entry->nr_lost	= call->tx_nr_lost;
-		    __entry->nr_resent	= call->tx_nr_resent;
 		    __entry->nr_sacked	= call->acks_nr_sacks;
 		    __entry->timo	= timo;
 			   ),
 
-	    TP_printk("c=%08x r=%08x q=%08x %s slrs=%u,%u,%u,%u t=%lld",
+	    TP_printk("c=%08x r=%08x q=%08x %s fls=%u,%u,%u t=%lld",
 		      __entry->call, __entry->ack_serial, __entry->seq,
 		      __print_symbolic(__entry->mode, rxrpc_rack_timer_modes),
-		      __entry->nr_sent, __entry->nr_lost,
-		      __entry->nr_resent, __entry->nr_sacked,
+		      __entry->in_flight, __entry->nr_lost, __entry->nr_sacked,
 		      ktime_to_us(__entry->timo))
 	    );
 
diff --git a/net/rxrpc/ar-internal.h b/net/rxrpc/ar-internal.h
index f0c8cbcc5dd5..d0e18eb6301e 100644
--- a/net/rxrpc/ar-internal.h
+++ b/net/rxrpc/ar-internal.h
@@ -782,9 +782,8 @@ struct rxrpc_call {
 	rxrpc_seq_t		tx_top;		/* Highest Tx slot allocated. */
 	rxrpc_serial_t		tx_last_serial;	/* Serial of last DATA transmitted */
 	u16			tx_backoff;	/* Delay to insert due to Tx failure (ms) */
-	u16			tx_nr_sent;	/* Number of packets sent, but unacked */
+	u16			tx_in_flight;	/* Number of packets considered "in flight" */
 	u16			tx_nr_lost;	/* Number of packets marked lost */
-	u16			tx_nr_resent;	/* Number of packets resent, but unacked */
 	u16			tx_winsize;	/* Maximum size of Tx window */
 #define RXRPC_TX_MAX_WINDOW	128
 	u8			tx_jumbo_max;	/* Maximum subpkts peer will accept */
@@ -980,6 +979,7 @@ struct rxrpc_txqueue {
 	unsigned long		segment_acked;	/* Bit-per-buf: Set if ACK'd */
 	unsigned long		segment_lost;	/* Bit-per-buf: Set if declared lost */
 	unsigned long		segment_retransmitted; /* Bit-per-buf: Set if retransmitted */
+	unsigned long		in_flight;	/* Bit-per-buf: Set if "in flight" */
 	unsigned long		rtt_samples;	/* Bit-per-buf: Set if available for RTT */
 	unsigned long		ever_retransmitted; /* Bit-per-buf: Set if ever retransmitted */
 
@@ -1601,20 +1601,6 @@ static inline unsigned int rxrpc_tx_window_space(const struct rxrpc_call *call)
 	return max(winsize - transmitted, 0);
 }
 
-static inline unsigned int rxrpc_left_out(const struct rxrpc_call *call)
-{
-	return call->acks_nr_sacks + call->tx_nr_lost;
-}
-
-/*
- * Calculate the number of transmitted DATA packets assumed to be in flight
- * [approx RFC6675].
- */
-static inline unsigned int rxrpc_tx_in_flight(const struct rxrpc_call *call)
-{
-	return call->tx_nr_sent - rxrpc_left_out(call) + call->tx_nr_resent;
-}
-
 /*
  * debug tracing
  */
diff --git a/net/rxrpc/input.c b/net/rxrpc/input.c
index 73cafe6bfa9f..f497eba59cf6 100644
--- a/net/rxrpc/input.c
+++ b/net/rxrpc/input.c
@@ -33,7 +33,7 @@ static void rxrpc_congestion_management(struct rxrpc_call *call,
 					struct rxrpc_ack_summary *summary)
 {
 	summary->change = rxrpc_cong_no_change;
-	summary->in_flight = rxrpc_tx_in_flight(call);
+	summary->in_flight = call->tx_in_flight;
 
 	if (test_and_clear_bit(RXRPC_CALL_RETRANS_TIMEOUT, &call->flags)) {
 		summary->retrans_timeo = true;
@@ -288,11 +288,11 @@ static bool rxrpc_rotate_tx_window(struct rxrpc_call *call, rxrpc_seq_t to,
 			trace_rxrpc_rotate(call, tq, summary, seq, rxrpc_rotate_trace_snak);
 		}
 
-		call->tx_nr_sent--;
+		if (__test_and_clear_bit(ix, &tq->in_flight))
+			call->tx_in_flight--;
 		if (__test_and_clear_bit(ix, &tq->segment_lost))
 			call->tx_nr_lost--;
-		if (__test_and_clear_bit(ix, &tq->segment_retransmitted))
-			call->tx_nr_resent--;
+		__clear_bit(ix, &tq->segment_retransmitted);
 		__clear_bit(ix, &tq->ever_retransmitted);
 
 		rxrpc_put_txbuf(tq->bufs[ix], rxrpc_txbuf_put_rotated);
diff --git a/net/rxrpc/input_rack.c b/net/rxrpc/input_rack.c
index 072e1d9ecb2e..e43ce032a261 100644
--- a/net/rxrpc/input_rack.c
+++ b/net/rxrpc/input_rack.c
@@ -23,12 +23,13 @@ static bool rxrpc_rack_sent_after(ktime_t t1, rxrpc_seq_t seq1,
 static void rxrpc_rack_mark_lost(struct rxrpc_call *call,
 				 struct rxrpc_txqueue *tq, unsigned int ix)
 {
-	if (__test_and_set_bit(ix, &tq->segment_lost)) {
-		if (__test_and_clear_bit(ix, &tq->segment_retransmitted))
-			call->tx_nr_resent--;
-	} else {
+	if (__test_and_set_bit(ix, &tq->segment_lost))
+		__clear_bit(ix, &tq->segment_retransmitted);
+	else
 		call->tx_nr_lost++;
-	}
+
+	if (__test_and_clear_bit(ix, &tq->in_flight))
+		call->tx_in_flight--;
 	tq->segment_xmit_ts[ix] = UINT_MAX;
 }
 
@@ -136,6 +137,8 @@ void rxrpc_input_rack(struct rxrpc_call *call,
 		unsigned int ix = __ffs(new_acks);
 
 		__clear_bit(ix, &new_acks);
+		if (__test_and_clear_bit(ix, &tq->in_flight))
+			call->tx_in_flight--;
 		rxrpc_input_rack_one(call, summary, tq, ix);
 	}
 
@@ -289,7 +292,7 @@ static void rxrpc_rack_mark_losses_on_rto(struct rxrpc_call *call)
  */
 ktime_t rxrpc_tlp_calc_pto(struct rxrpc_call *call, ktime_t now)
 {
-	unsigned int flight_size = rxrpc_tx_in_flight(call);
+	unsigned int flight_size = call->tx_in_flight;
 	ktime_t rto_at = ktime_add(call->tx_last_sent,
 				   rxrpc_get_rto_backoff(call, false));
 	ktime_t pto;
@@ -313,7 +316,7 @@ ktime_t rxrpc_tlp_calc_pto(struct rxrpc_call *call, ktime_t now)
  */
 void rxrpc_tlp_send_probe(struct rxrpc_call *call)
 {
-	unsigned int in_flight = rxrpc_tx_in_flight(call);
+	unsigned int in_flight = call->tx_in_flight;
 
 	if (after_eq(call->acks_hard_ack, call->tx_transmitted))
 		return; /* Everything we transmitted has been acked. */
@@ -333,7 +336,7 @@ void rxrpc_tlp_send_probe(struct rxrpc_call *call)
 			call->tlp_serial = call->tx_last_serial;
 			call->tlp_seq = call->tx_transmitted;
 			trace_rxrpc_tlp_probe(call, rxrpc_tlp_probe_trace_transmit_new);
-			in_flight = rxrpc_tx_in_flight(call);
+			in_flight = call->tx_in_flight;
 		} else {
 			/* Retransmit the highest-sequence DATA sent */
 			call->tx_last_serial = 0;
diff --git a/net/rxrpc/output.c b/net/rxrpc/output.c
index 88cad087f13b..3ccd9a27643a 100644
--- a/net/rxrpc/output.c
+++ b/net/rxrpc/output.c
@@ -571,15 +571,16 @@ static size_t rxrpc_prepare_data_packet(struct rxrpc_call *call,
 
 		_debug("prep[%u] tq=%x q=%x", i, tq->qbase, seq);
 
+		if (!__test_and_set_bit(ix, &tq->in_flight))
+			call->tx_in_flight++;
+
 		/* Record (re-)transmission for RACK [RFC8985 6.1]. */
 		if (__test_and_clear_bit(ix, &tq->segment_lost))
 			call->tx_nr_lost--;
 		if (req->retrans) {
 			__set_bit(ix, &tq->ever_retransmitted);
 			__set_bit(ix, &tq->segment_retransmitted);
-			call->tx_nr_resent++;
 		} else {
-			call->tx_nr_sent++;
 			start_tlp = true;
 		}
 		tq->segment_xmit_ts[ix] = xmit_ts;