Re: [PATCH] ipvs: improved SH fallback strategy

[Alexander Frolkin <avf@eldamar.org.uk>]

Improve the SH fallback realserver selection strategy.

With sh and sh-fallback, if a realserver is down, this attempts to
distribute the traffic that would have gone to that server evenly
among the remaining servers.
 
Signed-off-by: Alexander Frolkin <avf@eldamar.org.uk>

--
diff --git a/net/netfilter/ipvs/ip_vs_sh.c b/net/netfilter/ipvs/ip_vs_sh.c
index 3588fae..3d5ab7c 100644
--- a/net/netfilter/ipvs/ip_vs_sh.c
+++ b/net/netfilter/ipvs/ip_vs_sh.c
@@ -115,27 +115,47 @@ ip_vs_sh_get(struct ip_vs_service *svc, struct ip_vs_sh_state *s,
 }
 
 
-/* As ip_vs_sh_get, but with fallback if selected server is unavailable */
+/* As ip_vs_sh_get, but with fallback if selected server is unavailable
+ *
+ * The fallback strategy loops around the table starting from a "random"
+ * point (in fact, it is chosen to be the original hash value to make the
+ * algorithm deterministic) to find a new server.
+ */
 static inline struct ip_vs_dest *
 ip_vs_sh_get_fallback(struct ip_vs_service *svc, struct ip_vs_sh_state *s,
 		      const union nf_inet_addr *addr, __be16 port)
 {
-	unsigned int offset;
-	unsigned int hash;
+	unsigned int offset, roffset;
+	unsigned int hash, ihash;
 	struct ip_vs_dest *dest;
 
-	for (offset = 0; offset < IP_VS_SH_TAB_SIZE; offset++) {
-		hash = ip_vs_sh_hashkey(svc->af, addr, port, offset);
-		dest = rcu_dereference(s->buckets[hash].dest);
-		if (!dest)
-			break;
-		if (is_unavailable(dest))
-			IP_VS_DBG_BUF(6, "SH: selected unavailable server "
-				      "%s:%d (offset %d)",
+	/* first try the dest it's supposed to go to */
+	ihash = ip_vs_sh_hashkey(svc->af, addr, port, 0);
+	dest = rcu_dereference(s->buckets[ihash].dest);
+	if (!dest)
+		return NULL;
+	if (is_unavailable(dest)) {
+		IP_VS_DBG_BUF(6, "SH: selected unavailable server "
+		      "%s:%d, reselecting",
+		      IP_VS_DBG_ADDR(svc->af, &dest->addr),
+		      ntohs(dest->port));
+		/* if the original dest is unavailable, loop around the table
+		 * starting from ihash to find a new dest
+		 */
+		for (offset = 0; offset < IP_VS_SH_TAB_SIZE; offset++) {
+			roffset = (offset + ihash) % IP_VS_SH_TAB_SIZE;
+			hash = ip_vs_sh_hashkey(svc->af, addr, port, roffset);
+			dest = rcu_dereference(s->buckets[hash].dest);
+			if (is_unavailable(dest))
+				IP_VS_DBG_BUF(6, "SH: selected unavailable "
+				      "server %s:%d (offset %d), reselecting",
 				      IP_VS_DBG_ADDR(svc->af, &dest->addr),
-				      ntohs(dest->port), offset);
-		else
-			return dest;
+				      ntohs(dest->port), roffset);
+			else
+				return dest;
+		}
+	} else {
+		return dest;
 	}
 
 	return NULL;