[TFRC]: Ringbuffer to track loss interval history

A ringbuffer-based implementation of loss interval history is easier to
maintain, allocate, and update.

The `swap' routine to keep the RX history sorted is due to and was written
by Arnaldo Carvalho de Melo, simplifying an earlier macro-based variant.

Details:
 * access to the Loss Interval Records via macro wrappers (with safety checks);
 * simplified, on-demand allocation of entries (no extra memory consumption on
   lossless links); cache allocation is local to the module / exported as service;
 * provision of RFC-compliant algorithm to re-compute average loss interval;
 * provision of comprehensive, new loss detection algorithm
 	- support for all cases of loss, including re-ordered/duplicate packets;
 	- waiting for NDUPACK=3 packets to fill the hole;
	- updating loss records when a late-arriving packet fills a hole.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dccp/ccids/lib/loss_interval.c

@@ -1,6 +1,7 @@
 /*
  *  net/dccp/ccids/lib/loss_interval.c
  *
+ *  Copyright (c) 2007   The University of Aberdeen, Scotland, UK
  *  Copyright (c) 2005-7 The University of Waikato, Hamilton, New Zealand.
  *  Copyright (c) 2005-7 Ian McDonald <ian.mcdonald@jandi.co.nz>
  *  Copyright (c) 2005 Arnaldo Carvalho de Melo <acme@conectiva.com.br>
@@ -10,12 +11,7 @@
  *  the Free Software Foundation; either version 2 of the License, or
  *  (at your option) any later version.
  */
-
-#include <linux/module.h>
 #include <net/sock.h>
-#include "../../dccp.h"
-#include "loss_interval.h"
-#include "packet_history.h"
 #include "tfrc.h"
 
 #define DCCP_LI_HIST_IVAL_F_LENGTH  8
@@ -27,6 +23,54 @@ struct dccp_li_hist_entry {
 	u32		 dccplih_interval;
 };
 
+static struct kmem_cache  *tfrc_lh_slab  __read_mostly;
+/* Loss Interval weights from [RFC 3448, 5.4], scaled by 10 */
+static const int tfrc_lh_weights[NINTERVAL] = { 10, 10, 10, 10, 8, 6, 4, 2 };
+
+/* implements LIFO semantics on the array */
+static inline u8 LIH_INDEX(const u8 ctr)
+{
+	return (LIH_SIZE - 1 - (ctr % LIH_SIZE));
+}
+
+/* the `counter' index always points at the next entry to be populated */
+static inline struct tfrc_loss_interval *tfrc_lh_peek(struct tfrc_loss_hist *lh)
+{
+	return lh->counter ? lh->ring[LIH_INDEX(lh->counter - 1)] : NULL;
+}
+
+/* given i with 0 <= i <= k, return I_i as per the rfc3448bis notation */
+static inline u32 tfrc_lh_get_interval(struct tfrc_loss_hist *lh, const u8 i)
+{
+	BUG_ON(i >= lh->counter);
+	return lh->ring[LIH_INDEX(lh->counter - i - 1)]->li_length;
+}
+
+/*
+ *	On-demand allocation and de-allocation of entries
+ */
+static struct tfrc_loss_interval *tfrc_lh_demand_next(struct tfrc_loss_hist *lh)
+{
+	if (lh->ring[LIH_INDEX(lh->counter)] == NULL)
+		lh->ring[LIH_INDEX(lh->counter)] = kmem_cache_alloc(tfrc_lh_slab,
+								    GFP_ATOMIC);
+	return lh->ring[LIH_INDEX(lh->counter)];
+}
+
+void tfrc_lh_cleanup(struct tfrc_loss_hist *lh)
+{
+	if (!tfrc_lh_is_initialised(lh))
+		return;
+
+	for (lh->counter = 0; lh->counter < LIH_SIZE; lh->counter++)
+		if (lh->ring[LIH_INDEX(lh->counter)] != NULL) {
+			kmem_cache_free(tfrc_lh_slab,
+					lh->ring[LIH_INDEX(lh->counter)]);
+			lh->ring[LIH_INDEX(lh->counter)] = NULL;
+		}
+}
+EXPORT_SYMBOL_GPL(tfrc_lh_cleanup);
+
 static struct kmem_cache *dccp_li_cachep __read_mostly;
 
 static inline struct dccp_li_hist_entry *dccp_li_hist_entry_new(const gfp_t prio)
@@ -98,6 +142,65 @@ u32 dccp_li_hist_calc_i_mean(struct list_head *list)
 
 EXPORT_SYMBOL_GPL(dccp_li_hist_calc_i_mean);
 
+static void tfrc_lh_calc_i_mean(struct tfrc_loss_hist *lh)
+{
+	u32 i_i, i_tot0 = 0, i_tot1 = 0, w_tot = 0;
+	int i, k = tfrc_lh_length(lh) - 1; /* k is as in rfc3448bis, 5.4 */
+
+	for (i=0; i <= k; i++) {
+		i_i = tfrc_lh_get_interval(lh, i);
+
+		if (i < k) {
+			i_tot0 += i_i * tfrc_lh_weights[i];
+			w_tot  += tfrc_lh_weights[i];
+		}
+		if (i > 0)
+			i_tot1 += i_i * tfrc_lh_weights[i-1];
+	}
+
+	BUG_ON(w_tot == 0);
+	lh->i_mean = max(i_tot0, i_tot1) / w_tot;
+}
+
+/**
+ * tfrc_lh_update_i_mean  -  Update the `open' loss interval I_0
+ * For recomputing p: returns `true' if p > p_prev  <=>  1/p < 1/p_prev
+ */
+u8 tfrc_lh_update_i_mean(struct tfrc_loss_hist *lh, struct sk_buff *skb)
+{
+	struct tfrc_loss_interval *cur = tfrc_lh_peek(lh);
+	u32 old_i_mean = lh->i_mean;
+	s64 length;
+
+	if (cur == NULL)			/* not initialised */
+		return 0;
+
+	length = dccp_delta_seqno(cur->li_seqno, DCCP_SKB_CB(skb)->dccpd_seq);
+
+	if (length - cur->li_length <= 0)	/* duplicate or reordered */
+		return 0;
+
+	if (SUB16(dccp_hdr(skb)->dccph_ccval, cur->li_ccval) > 4)
+		/*
+		 * Implements RFC 4342, 10.2:
+		 * If a packet S (skb) exists whose seqno comes `after' the one
+		 * starting the current loss interval (cur) and if the modulo-16
+		 * distance from C(cur) to C(S) is greater than 4, consider all
+		 * subsequent packets as belonging to a new loss interval. This
+		 * test is necessary since CCVal may wrap between intervals.
+		 */
+		cur->li_is_closed = 1;
+
+	if (tfrc_lh_length(lh) == 1)		/* due to RFC 3448, 6.3.1 */
+		return 0;
+
+	cur->li_length = length;
+	tfrc_lh_calc_i_mean(lh);
+
+	return (lh->i_mean < old_i_mean);
+}
+EXPORT_SYMBOL_GPL(tfrc_lh_update_i_mean);
+
 static int dccp_li_hist_interval_new(struct list_head *list,
 				     const u64 seq_loss, const u8 win_loss)
 {
@@ -284,6 +387,54 @@ void dccp_li_update_li(struct sock *sk,
 
 EXPORT_SYMBOL_GPL(dccp_li_update_li);
 
+/* Determine if `new_loss' does begin a new loss interval [RFC 4342, 10.2] */
+static inline u8 tfrc_lh_is_new_loss(struct tfrc_loss_interval *cur,
+				     struct tfrc_rx_hist_entry *new_loss)
+{
+	return	dccp_delta_seqno(cur->li_seqno, new_loss->tfrchrx_seqno) > 0 &&
+		(cur->li_is_closed || SUB16(new_loss->tfrchrx_ccval, cur->li_ccval) > 4);
+}
+
+/** tfrc_lh_interval_add  -  Insert new record into the Loss Interval database
+ * @lh:		   Loss Interval database
+ * @rh:		   Receive history containing a fresh loss event
+ * @calc_first_li: Caller-dependent routine to compute length of first interval
+ * @sk:		   Used by @calc_first_li in caller-specific way (subtyping)
+ * Updates I_mean and returns 1 if a new interval has in fact been added to @lh.
+ */
+int tfrc_lh_interval_add(struct tfrc_loss_hist *lh, struct tfrc_rx_hist *rh,
+			 u32 (*calc_first_li)(struct sock *), struct sock *sk)
+{
+	struct tfrc_loss_interval *cur = tfrc_lh_peek(lh), *new;
+
+	if (cur != NULL && !tfrc_lh_is_new_loss(cur, tfrc_rx_hist_loss_prev(rh)))
+		return 0;
+
+	new = tfrc_lh_demand_next(lh);
+	if (unlikely(new == NULL)) {
+		DCCP_CRIT("Cannot allocate/add loss record.");
+		return 0;
+	}
+
+	new->li_seqno	  = tfrc_rx_hist_loss_prev(rh)->tfrchrx_seqno;
+	new->li_ccval	  = tfrc_rx_hist_loss_prev(rh)->tfrchrx_ccval;
+	new->li_is_closed = 0;
+
+	if (++lh->counter == 1)
+		lh->i_mean = new->li_length = (*calc_first_li)(sk);
+	else {
+		cur->li_length = dccp_delta_seqno(cur->li_seqno, new->li_seqno);
+		new->li_length = dccp_delta_seqno(new->li_seqno,
+				  tfrc_rx_hist_last_rcv(rh)->tfrchrx_seqno);
+		if (lh->counter > (2*LIH_SIZE))
+			lh->counter -= LIH_SIZE;
+
+		tfrc_lh_calc_i_mean(lh);
+	}
+	return 1;
+}
+EXPORT_SYMBOL_GPL(tfrc_lh_interval_add);
+
 int __init dccp_li_init(void)
 {
 	dccp_li_cachep = kmem_cache_create("dccp_li_hist",

net/dccp/ccids/lib/loss_interval.h

@@ -3,6 +3,7 @@
 /*
  *  net/dccp/ccids/lib/loss_interval.h
  *
+ *  Copyright (c) 2007   The University of Aberdeen, Scotland, UK
  *  Copyright (c) 2005-7 The University of Waikato, Hamilton, New Zealand.
  *  Copyright (c) 2005-7 Ian McDonald <ian.mcdonald@jandi.co.nz>
  *  Copyright (c) 2005 Arnaldo Carvalho de Melo <acme@conectiva.com.br>
@@ -12,11 +13,64 @@
  *  Software Foundation; either version 2 of the License, or (at your option)
  *  any later version.
  */
-
 #include <linux/ktime.h>
 #include <linux/list.h>
+#include <linux/slab.h>
+
+/*
+ * Number of loss intervals (RFC 4342, 8.6.1). The history size is one more than
+ * NINTERVAL, since the `open' interval I_0 is always stored as the first entry.
+ */
+#define NINTERVAL	8
+#define LIH_SIZE	(NINTERVAL + 1)
+
+/**
+ *  tfrc_loss_interval  -  Loss history record for TFRC-based protocols
+ *  @li_seqno:		Highest received seqno before the start of loss
+ *  @li_ccval:		The CCVal belonging to @li_seqno
+ *  @li_is_closed:	Whether @li_seqno is older than 1 RTT
+ *  @li_length:		Loss interval sequence length
+ */
+struct tfrc_loss_interval {
+	u64		 li_seqno:48,
+			 li_ccval:4,
+			 li_is_closed:1;
+	u32		 li_length;
+};
+
+/**
+ *  tfrc_loss_hist  -  Loss record database
+ *  @ring:	Circular queue managed in LIFO manner
+ *  @counter:	Current count of entries (can be more than %LIH_SIZE)
+ *  @i_mean:	Current Average Loss Interval [RFC 3448, 5.4]
+ */
+struct tfrc_loss_hist {
+	struct tfrc_loss_interval	*ring[LIH_SIZE];
+	u8				counter;
+	u32				i_mean;
+};
+
+static inline void tfrc_lh_init(struct tfrc_loss_hist *lh)
+{
+	memset(lh, 0, sizeof(struct tfrc_loss_hist));
+}
+
+static inline u8 tfrc_lh_is_initialised(struct tfrc_loss_hist *lh)
+{
+	return lh->counter > 0;
+}
+
+static inline u8 tfrc_lh_length(struct tfrc_loss_hist *lh)
+{
+	return min(lh->counter, (u8)LIH_SIZE);
+}
 
 extern void dccp_li_hist_purge(struct list_head *list);
+struct tfrc_rx_hist;
+extern int  tfrc_lh_interval_add(struct tfrc_loss_hist *, struct tfrc_rx_hist *,
+				 u32 (*first_li)(struct sock *), struct sock *);
+extern u8   tfrc_lh_update_i_mean(struct tfrc_loss_hist *lh, struct sk_buff *);
+extern void tfrc_lh_cleanup(struct tfrc_loss_hist *lh);
 
 extern u32 dccp_li_hist_calc_i_mean(struct list_head *list);