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SubjectRe: [PATCHv2] ipvs: improved SH fallback strategy
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..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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