1 files changed, 418 insertions, 0 deletions
diff --git a/net/rxrpc/input_rack.c b/net/rxrpc/input_rack.c
new file mode 100644
index 000000000000..13c371261e0a
--- /dev/null
+++ b/net/rxrpc/input_rack.c
@@ -0,0 +1,418 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* RACK-TLP [RFC8958] Implementation
+ *
+ * Copyright (C) 2024 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include "ar-internal.h"
+
+static bool rxrpc_rack_sent_after(ktime_t t1, rxrpc_seq_t seq1,
+				  ktime_t t2, rxrpc_seq_t seq2)
+{
+	if (ktime_after(t1, t2))
+		return true;
+	return t1 == t2 && after(seq1, seq2);
+}
+
+/*
+ * Mark a packet lost.
+ */
+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 {
+		call->tx_nr_lost++;
+	}
+	tq->segment_xmit_ts[ix] = UINT_MAX;
+}
+
+/*
+ * Get the transmission time of a packet in the Tx queue.
+ */
+static ktime_t rxrpc_get_xmit_ts(const struct rxrpc_txqueue *tq, unsigned int ix)
+{
+	if (tq->segment_xmit_ts[ix] == UINT_MAX)
+		return KTIME_MAX;
+	return ktime_add_us(tq->xmit_ts_base, tq->segment_xmit_ts[ix]);
+}
+
+/*
+ * Get a bitmask of nack bits for a queue segment and mask off any that aren't
+ * yet reported.
+ */
+static unsigned long rxrpc_tq_nacks(const struct rxrpc_txqueue *tq)
+{
+	unsigned long nacks = ~tq->segment_acked;
+
+	if (tq->nr_reported_acks < RXRPC_NR_TXQUEUE)
+		nacks &= (1UL << tq->nr_reported_acks) - 1;
+	return nacks;
+}
+
+/*
+ * Update the RACK state for the most recently sent packet that has been
+ * delivered [RFC8958 6.2 Step 2].
+ */
+static void rxrpc_rack_update(struct rxrpc_call *call,
+			      struct rxrpc_ack_summary *summary,
+			      struct rxrpc_txqueue *tq,
+			      unsigned int ix)
+{
+	rxrpc_seq_t seq = tq->qbase + ix;
+	ktime_t xmit_ts = rxrpc_get_xmit_ts(tq, ix);
+	ktime_t rtt = ktime_sub(call->acks_latest_ts, xmit_ts);
+
+	if (__test_and_clear_bit(ix, &tq->segment_lost))
+		call->tx_nr_lost--;
+
+	if (test_bit(ix, &tq->segment_retransmitted)) {
+		/* Use Rx.serial instead of TCP.ACK.ts_option.echo_reply. */
+		if (before(call->acks_highest_serial, tq->segment_serial[ix]))
+			return;
+		if (rtt < minmax_get(&call->min_rtt))
+			return;
+	}
+
+	/* The RACK algorithm requires the segment ACKs to be traversed in
+	 * order of segment transmission - but the only thing this seems to
+	 * matter for is that RACK.rtt is set to the rtt of the most recently
+	 * transmitted segment.  We should be able to achieve the same by only
+	 * setting RACK.rtt if the xmit time is greater.
+	 */
+	if (ktime_after(xmit_ts, call->rack_rtt_ts)) {
+		call->rack_rtt	  = rtt;
+		call->rack_rtt_ts = xmit_ts;
+	}
+
+	if (rxrpc_rack_sent_after(xmit_ts, seq, call->rack_xmit_ts, call->rack_end_seq)) {
+		call->rack_rtt = rtt;
+		call->rack_xmit_ts = xmit_ts;
+		call->rack_end_seq = seq;
+	}
+}
+
+/*
+ * Detect data segment reordering [RFC8958 6.2 Step 3].
+ */
+static void rxrpc_rack_detect_reordering(struct rxrpc_call *call,
+					 struct rxrpc_ack_summary *summary,
+					 struct rxrpc_txqueue *tq,
+					 unsigned int ix)
+{
+	rxrpc_seq_t seq = tq->qbase + ix;
+
+	/* Track the highest sequence number so far ACK'd.  This is not
+	 * necessarily the same as ack.firstPacket + ack.nAcks - 1 as the peer
+	 * could put a NACK in the last SACK slot.
+	 */
+	if (after(seq, call->rack_fack))
+		call->rack_fack = seq;
+	else if (before(seq, call->rack_fack) &&
+		 test_bit(ix, &tq->segment_retransmitted))
+		call->rack_reordering_seen = true;
+}
+
+void rxrpc_input_rack_one(struct rxrpc_call *call,
+			  struct rxrpc_ack_summary *summary,
+			  struct rxrpc_txqueue *tq,
+			  unsigned int ix)
+{
+	rxrpc_rack_update(call, summary, tq, ix);
+	rxrpc_rack_detect_reordering(call, summary, tq, ix);
+}
+
+void rxrpc_input_rack(struct rxrpc_call *call,
+		      struct rxrpc_ack_summary *summary,
+		      struct rxrpc_txqueue *tq,
+		      unsigned long new_acks)
+{
+	while (new_acks) {
+		unsigned int ix = __ffs(new_acks);
+
+		__clear_bit(ix, &new_acks);
+		rxrpc_input_rack_one(call, summary, tq, ix);
+	}
+
+	trace_rxrpc_rack_update(call, summary);
+}
+
+/*
+ * Update the reordering window [RFC8958 6.2 Step 4].  Returns the updated
+ * duration of the reordering window.
+ *
+ * Note that the Rx protocol doesn't have a 'DSACK option' per se, but ACKs can
+ * be given a 'DUPLICATE' reason with the serial number referring to the
+ * duplicated DATA packet.  Rx does not inform as to whether this was a
+ * reception of the same packet twice or of a retransmission of a packet we
+ * already received (though this could be determined by the transmitter based
+ * on the serial number).
+ */
+static ktime_t rxrpc_rack_update_reo_wnd(struct rxrpc_call *call,
+					 struct rxrpc_ack_summary *summary)
+{
+	rxrpc_seq_t snd_una = call->acks_lowest_nak; /* Lowest unack'd seq */
+	rxrpc_seq_t snd_nxt = call->tx_transmitted + 1; /* Next seq to be sent */
+	bool have_dsack_option = summary->ack_reason == RXRPC_ACK_DUPLICATE;
+	int dup_thresh = 3;
+
+	/* DSACK-based reordering window adaptation */
+	if (!call->rack_dsack_round_none &&
+	    after_eq(snd_una, call->rack_dsack_round))
+		call->rack_dsack_round_none = true;
+
+	/* Grow the reordering window per round that sees DSACK.  Reset the
+	 * window after 16 DSACK-free recoveries.
+	 */
+	if (call->rack_dsack_round_none && have_dsack_option) {
+		call->rack_dsack_round_none = false;
+		call->rack_dsack_round = snd_nxt;
+		call->rack_reo_wnd_mult++;
+		call->rack_reo_wnd_persist = 16;
+	} else if (summary->exiting_fast_or_rto_recovery) {
+		call->rack_reo_wnd_persist--;
+		if (call->rack_reo_wnd_persist <= 0)
+			call->rack_reo_wnd_mult = 1;
+	}
+
+	if (!call->rack_reordering_seen) {
+		if (summary->in_fast_or_rto_recovery)
+			return 0;
+		if (call->acks_nr_sacks >= dup_thresh)
+			return 0;
+	}
+
+	return us_to_ktime(umin(call->rack_reo_wnd_mult * minmax_get(&call->min_rtt) / 4,
+				call->srtt_us >> 3));
+}
+
+/*
+ * Detect losses [RFC8958 6.2 Step 5].
+ */
+static ktime_t rxrpc_rack_detect_loss(struct rxrpc_call *call,
+				      struct rxrpc_ack_summary *summary)
+{
+	struct rxrpc_txqueue *tq;
+	ktime_t timeout = 0, lost_after, now = ktime_get_real();
+
+	call->rack_reo_wnd = rxrpc_rack_update_reo_wnd(call, summary);
+	lost_after = ktime_add(call->rack_rtt, call->rack_reo_wnd);
+	trace_rxrpc_rack_scan_loss(call);
+
+	for (tq = call->tx_queue; tq; tq = tq->next) {
+		unsigned long nacks = rxrpc_tq_nacks(tq);
+
+		if (after(tq->qbase, call->tx_transmitted))
+			break;
+		trace_rxrpc_rack_scan_loss_tq(call, tq, nacks);
+
+		/* Skip ones marked lost but not yet retransmitted */
+		nacks &= ~tq->segment_lost | tq->segment_retransmitted;
+
+		while (nacks) {
+			unsigned int ix = __ffs(nacks);
+			rxrpc_seq_t seq = tq->qbase + ix;
+			ktime_t remaining;
+			ktime_t xmit_ts = rxrpc_get_xmit_ts(tq, ix);
+
+			__clear_bit(ix, &nacks);
+
+			if (rxrpc_rack_sent_after(call->rack_xmit_ts, call->rack_end_seq,
+						  xmit_ts, seq)) {
+				remaining = ktime_sub(ktime_add(xmit_ts, lost_after), now);
+				if (remaining <= 0) {
+					rxrpc_rack_mark_lost(call, tq, ix);
+					trace_rxrpc_rack_detect_loss(call, summary, seq);
+				} else {
+					timeout = max(remaining, timeout);
+				}
+			}
+		}
+	}
+
+	return timeout;
+}
+
+/*
+ * Detect losses and set a timer to retry the detection [RFC8958 6.2 Step 5].
+ */
+void rxrpc_rack_detect_loss_and_arm_timer(struct rxrpc_call *call,
+					  struct rxrpc_ack_summary *summary)
+{
+	ktime_t timeout = rxrpc_rack_detect_loss(call, summary);
+
+	if (timeout) {
+		call->rack_timer_mode = RXRPC_CALL_RACKTIMER_RACK_REORDER;
+		call->rack_timo_at = ktime_add(ktime_get_real(), timeout);
+		trace_rxrpc_rack_timer(call, timeout, false);
+		trace_rxrpc_timer_set(call, timeout, rxrpc_timer_trace_rack_reo);
+	}
+}
+
+/*
+ * Handle RACK-TLP RTO expiration [RFC8958 6.3].
+ */
+static void rxrpc_rack_mark_losses_on_rto(struct rxrpc_call *call)
+{
+	struct rxrpc_txqueue *tq;
+	rxrpc_seq_t snd_una = call->acks_lowest_nak; /* Lowest unack'd seq */
+	ktime_t lost_after = ktime_add(call->rack_rtt, call->rack_reo_wnd);
+	ktime_t deadline = ktime_sub(ktime_get_real(), lost_after);
+
+	for (tq = call->tx_queue; tq; tq = tq->next) {
+		unsigned long unacked = ~tq->segment_acked;
+
+		trace_rxrpc_rack_mark_loss_tq(call, tq);
+		while (unacked) {
+			unsigned int ix = __ffs(unacked);
+			rxrpc_seq_t seq = tq->qbase + ix;
+			ktime_t xmit_ts = rxrpc_get_xmit_ts(tq, ix);
+
+			if (after(seq, call->tx_transmitted))
+				return;
+			__clear_bit(ix, &unacked);
+
+			if (seq == snd_una ||
+			    ktime_before(xmit_ts, deadline))
+				rxrpc_rack_mark_lost(call, tq, ix);
+		}
+	}
+}
+
+/*
+ * Calculate the TLP loss probe timeout (PTO) [RFC8958 7.2].
+ */
+ktime_t rxrpc_tlp_calc_pto(struct rxrpc_call *call, ktime_t now)
+{
+	unsigned int flight_size = rxrpc_tx_in_flight(call);
+	ktime_t rto_at = ktime_add(call->tx_last_sent,
+				   rxrpc_get_rto_backoff(call, false));
+	ktime_t pto;
+
+	if (call->rtt_count > 0) {
+		/* Use 2*SRTT as the timeout. */
+		pto = ns_to_ktime(call->srtt_us * NSEC_PER_USEC / 4);
+		if (flight_size)
+			pto = ktime_add(pto, call->tlp_max_ack_delay);
+	} else {
+		pto = NSEC_PER_SEC;
+	}
+
+	if (ktime_after(ktime_add(now, pto), rto_at))
+		pto = ktime_sub(rto_at, now);
+	return pto;
+}
+
+/*
+ * Send a TLP loss probe on PTO expiration [RFC8958 7.3].
+ */
+void rxrpc_tlp_send_probe(struct rxrpc_call *call)
+{
+	unsigned int in_flight = rxrpc_tx_in_flight(call);
+
+	if (after_eq(call->acks_hard_ack, call->tx_transmitted))
+		return; /* Everything we transmitted has been acked. */
+
+	/* There must be no other loss probe still in flight and we need to
+	 * have taken a new RTT sample since last probe or the start of
+	 * connection.
+	 */
+	if (!call->tlp_serial &&
+	    call->tlp_rtt_taken != call->rtt_taken) {
+		call->tlp_is_retrans = false;
+		if (after(call->send_top, call->tx_transmitted) &&
+		    rxrpc_tx_window_space(call) > 0) {
+			/* Transmit the lowest-sequence unsent DATA */
+			call->tx_last_serial = 0;
+			rxrpc_transmit_some_data(call, 1, rxrpc_txdata_tlp_new_data);
+			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);
+		} else {
+			/* Retransmit the highest-sequence DATA sent */
+			call->tx_last_serial = 0;
+			rxrpc_resend_tlp(call);
+			call->tlp_is_retrans = true;
+			trace_rxrpc_tlp_probe(call, rxrpc_tlp_probe_trace_retransmit);
+		}
+	} else {
+		trace_rxrpc_tlp_probe(call, rxrpc_tlp_probe_trace_busy);
+	}
+
+	if (in_flight != 0) {
+		ktime_t rto = rxrpc_get_rto_backoff(call, false);
+
+		call->rack_timer_mode = RXRPC_CALL_RACKTIMER_RTO;
+		call->rack_timo_at = ktime_add(ktime_get_real(), rto);
+		trace_rxrpc_rack_timer(call, rto, false);
+		trace_rxrpc_timer_set(call, rto, rxrpc_timer_trace_rack_rto);
+	}
+}
+
+/*
+ * Detect losses using the ACK of a TLP loss probe [RFC8958 7.4].
+ */
+void rxrpc_tlp_process_ack(struct rxrpc_call *call, struct rxrpc_ack_summary *summary)
+{
+	if (!call->tlp_serial || after(call->tlp_seq, call->acks_hard_ack))
+		return;
+
+	if (!call->tlp_is_retrans) {
+		/* TLP of new data delivered */
+		trace_rxrpc_tlp_ack(call, summary, rxrpc_tlp_ack_trace_new_data);
+		call->tlp_serial = 0;
+	} else if (summary->ack_reason == RXRPC_ACK_DUPLICATE &&
+		   summary->acked_serial == call->tlp_serial) {
+		/* General Case: Detected packet losses using RACK [7.4.1] */
+		trace_rxrpc_tlp_ack(call, summary, rxrpc_tlp_ack_trace_dup_acked);
+		call->tlp_serial = 0;
+	} else if (after(call->acks_hard_ack, call->tlp_seq)) {
+		/* Repaired the single loss */
+		trace_rxrpc_tlp_ack(call, summary, rxrpc_tlp_ack_trace_hard_beyond);
+		call->tlp_serial = 0;
+		// TODO: Invoke congestion control to react to the loss
+		// event the probe has repaired
+	} else if (summary->tlp_probe_acked) {
+		trace_rxrpc_tlp_ack(call, summary, rxrpc_tlp_ack_trace_acked);
+		/* Special Case: Detected a single loss repaired by the loss
+		 * probe [7.4.2]
+		 */
+		call->tlp_serial = 0;
+	} else {
+		trace_rxrpc_tlp_ack(call, summary, rxrpc_tlp_ack_trace_incomplete);
+	}
+}
+
+/*
+ * Handle RACK timer expiration; returns true to request a resend.
+ */
+void rxrpc_rack_timer_expired(struct rxrpc_call *call, ktime_t overran_by)
+{
+	struct rxrpc_ack_summary summary = {};
+	enum rxrpc_rack_timer_mode mode = call->rack_timer_mode;
+
+	trace_rxrpc_rack_timer(call, overran_by, true);
+	call->rack_timer_mode = RXRPC_CALL_RACKTIMER_OFF;
+
+	switch (mode) {
+	case RXRPC_CALL_RACKTIMER_RACK_REORDER:
+		rxrpc_rack_detect_loss_and_arm_timer(call, &summary);
+		break;
+	case RXRPC_CALL_RACKTIMER_TLP_PTO:
+		rxrpc_tlp_send_probe(call);
+		break;
+	case RXRPC_CALL_RACKTIMER_RTO:
+		// Might need to poke the congestion algo in some way
+		rxrpc_rack_mark_losses_on_rto(call);
+		break;
+	//case RXRPC_CALL_RACKTIMER_ZEROWIN:
+	default:
+		pr_warn("Unexpected rack timer %u", call->rack_timer_mode);
+	}
+}