tcp: Implement hash table with indices rather than pointers

We implement our hash table with pointers to the entry for each bucket (or
NULL).  However, the entries are always allocated within the flow table,
meaning that a flow index will suffice, halving the size of the hash table.

For TCP, just a flow index would be enough, but future uses will want to
expand the hash table to cover indexing either side of a flow, so use a
flow_sidx_t as the type for each hash bucket.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

2 files changed, 33 insertions, 11 deletions

diff --git a/flow.h b/flow.h
index c2a5190..959b461 100644
--- a/flow.h
+++ b/flow.h
@@ -53,6 +53,17 @@ static_assert(sizeof(flow_sidx_t) <= sizeof(uint32_t),
 
 #define FLOW_SIDX_NONE ((flow_sidx_t){ .flow = FLOW_MAX })
 
+/**
+ * flow_sidx_eq() - Test if two sidx values are equal
+ * @a, @b:	sidx values
+ *
+ * Return: true iff @a and @b refer to the same side of the same flow
+ */
+static inline bool flow_sidx_eq(flow_sidx_t a, flow_sidx_t b)
+{
+	return (a.flow == b.flow) && (a.side == b.side);
+}
+
 union flow;
 
 void flow_table_compact(struct ctx *c, union flow *hole);
diff --git a/tcp.c b/tcp.c
index 4e3a134..cdaf2c2 100644
--- a/tcp.c
+++ b/tcp.c
@@ -574,7 +574,7 @@ static unsigned int tcp6_l2_flags_buf_used;
 #define CONN(idx)		(&(FLOW(idx)->tcp))
 
 /* Table for lookup from remote address, local port, remote port */
-static struct tcp_tap_conn *tc_hash[TCP_HASH_TABLE_SIZE];
+static flow_sidx_t tc_hash[TCP_HASH_TABLE_SIZE];
 
 static_assert(ARRAY_SIZE(tc_hash) >= FLOW_MAX,
 	"Safe linear probing requires hash table larger than connection table");
@@ -1198,10 +1198,12 @@ static unsigned int tcp_conn_hash(const struct ctx *c,
 static inline unsigned tcp_hash_probe(const struct ctx *c,
 				      const struct tcp_tap_conn *conn)
 {
+	flow_sidx_t sidx = FLOW_SIDX(conn, TAPSIDE);
 	unsigned b = tcp_conn_hash(c, conn);
 
 	/* Linear probing */
-	while (tc_hash[b] && tc_hash[b] != conn)
+	while (!flow_sidx_eq(tc_hash[b], FLOW_SIDX_NONE) &&
+	       !flow_sidx_eq(tc_hash[b], sidx))
 		b = mod_sub(b, 1, TCP_HASH_TABLE_SIZE);
 
 	return b;
@@ -1216,7 +1218,7 @@ static void tcp_hash_insert(const struct ctx *c, struct tcp_tap_conn *conn)
 {
 	unsigned b = tcp_hash_probe(c, conn);
 
-	tc_hash[b] = conn;
+	tc_hash[b] = FLOW_SIDX(conn, TAPSIDE);
 	flow_dbg(conn, "hash table insert: sock %i, bucket: %u", conn->sock, b);
 }
 
@@ -1229,16 +1231,18 @@ static void tcp_hash_remove(const struct ctx *c,
 			    const struct tcp_tap_conn *conn)
 {
 	unsigned b = tcp_hash_probe(c, conn), s;
+	union flow *flow = flow_at_sidx(tc_hash[b]);
 
-	if (!tc_hash[b])
+	if (!flow)
 		return; /* Redundant remove */
 
 	flow_dbg(conn, "hash table remove: sock %i, bucket: %u", conn->sock, b);
 
 	/* Scan the remainder of the cluster */
-	for (s = mod_sub(b, 1, TCP_HASH_TABLE_SIZE); tc_hash[s];
+	for (s = mod_sub(b, 1, TCP_HASH_TABLE_SIZE);
+	     (flow = flow_at_sidx(tc_hash[s]));
 	     s = mod_sub(s, 1, TCP_HASH_TABLE_SIZE)) {
-		unsigned h = tcp_conn_hash(c, tc_hash[s]);
+		unsigned h = tcp_conn_hash(c, &flow->tcp);
 
 		if (!mod_between(h, s, b, TCP_HASH_TABLE_SIZE)) {
 			/* tc_hash[s] can live in tc_hash[b]'s slot */
@@ -1248,7 +1252,7 @@ static void tcp_hash_remove(const struct ctx *c,
 		}
 	}
 
-	tc_hash[b] = NULL;
+	tc_hash[b] = FLOW_SIDX_NONE;
 }
 
 /**
@@ -1263,10 +1267,10 @@ void tcp_tap_conn_update(const struct ctx *c, struct tcp_tap_conn *old,
 {
 	unsigned b = tcp_hash_probe(c, old);
 
-	if (!tc_hash[b])
+	if (!flow_at_sidx(tc_hash[b]))
 		return; /* Not in hash table, nothing to update */
 
-	tc_hash[b] = new;
+	tc_hash[b] = FLOW_SIDX(new, TAPSIDE);
 
 	debug("TCP: hash table update: old index %u, new index %u, sock %i, "
 	      "bucket: %u", FLOW_IDX(old), FLOW_IDX(new), new->sock, b);
@@ -1289,15 +1293,17 @@ static struct tcp_tap_conn *tcp_hash_lookup(const struct ctx *c,
 					    in_port_t eport, in_port_t fport)
 {
 	union inany_addr aany;
+	union flow *flow;
 	unsigned b;
 
 	inany_from_af(&aany, af, faddr);
 
 	b = tcp_hash(c, &aany, eport, fport);
-	while (tc_hash[b] && !tcp_hash_match(tc_hash[b], &aany, eport, fport))
+	while ((flow = flow_at_sidx(tc_hash[b])) &&
+	       !tcp_hash_match(&flow->tcp, &aany, eport, fport))
 		b = mod_sub(b, 1, TCP_HASH_TABLE_SIZE);
 
-	return tc_hash[b];
+	return &flow->tcp;
 }
 
 /**
@@ -3088,6 +3094,11 @@ static void tcp_sock_refill_init(const struct ctx *c)
  */
 int tcp_init(struct ctx *c)
 {
+	unsigned b;
+
+	for (b = 0; b < TCP_HASH_TABLE_SIZE; b++)
+		tc_hash[b] = FLOW_SIDX_NONE;
+
 	if (c->ifi4)
 		tcp_sock4_iov_init(c);