Re: [PATCHv2] ipvs: improved SH fallback strategy

[Alexander Frolkin <avf@xxxxxxxxxxxxxx>]

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@xxxxxxxxxxxxxx>

--
diff --git a/net/netfilter/ipvs/ip_vs_sh.c b/net/netfilter/ipvs/ip_vs_sh.c
index 3588fae..cc65b2f 100644
--- a/net/netfilter/ipvs/ip_vs_sh.c
+++ b/net/netfilter/ipvs/ip_vs_sh.c
@@ -115,27 +115,46 @@ 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;
 
+       /* 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))
+               return 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++) {
-               hash = ip_vs_sh_hashkey(svc->af, addr, port, 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 (!dest)
                        break;
-               if (is_unavailable(dest))
-                       IP_VS_DBG_BUF(6, "SH: selected unavailable server "
-                                     "%s:%d (offset %d)",
-                                     IP_VS_DBG_ADDR(svc->af, &dest->addr),
-                                     ntohs(dest->port), offset);
-               else
+               if (!is_unavailable(dest))
                        return 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), roffset);
        }
 
        return NULL;

--
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