ipvs: add weighted random twos choice algorithm

Adds the random twos choice load-balancing algorithm. The algorithm will
pick two random servers based on weights. Then select the server with
the least amount of connections normalized by weight. The algorithm
avoids the "herd behavior" problem. The algorithm comes from a paper
by Michael Mitzenmacher available here
http://www.eecs.harvard.edu/~michaelm/NEWWORK/postscripts/twosurvey.pdf

Signed-off-by: Darby Payne <darby.payne@gmail.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/ipvs/Kconfig

@@ -271,6 +271,17 @@ config	IP_VS_NQ
 	  If you want to compile it in kernel, say Y. To compile it as a
 	  module, choose M here. If unsure, say N.
 
+config	IP_VS_TWOS
+	tristate "weighted random twos choice least-connection scheduling"
+	help
+	  The weighted random twos choice least-connection scheduling
+	  algorithm picks two random real servers and directs network
+	  connections to the server with the least active connections
+	  normalized by the server weight.
+
+	  If you want to compile it in kernel, say Y. To compile it as a
+	  module, choose M here. If unsure, say N.
+
 comment 'IPVS SH scheduler'
 
 config IP_VS_SH_TAB_BITS

net/netfilter/ipvs/Makefile

@@ -36,6 +36,7 @@ obj-$(CONFIG_IP_VS_SH) += ip_vs_sh.o
 obj-$(CONFIG_IP_VS_MH) += ip_vs_mh.o
 obj-$(CONFIG_IP_VS_SED) += ip_vs_sed.o
 obj-$(CONFIG_IP_VS_NQ) += ip_vs_nq.o
+obj-$(CONFIG_IP_VS_TWOS) += ip_vs_twos.o
 
 # IPVS application helpers
 obj-$(CONFIG_IP_VS_FTP) += ip_vs_ftp.o

net/netfilter/ipvs/ip_vs_twos.c

@@ -0,0 +1,139 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* IPVS:        Power of Twos Choice Scheduling module
+ *
+ * Authors:     Darby Payne <darby.payne@applovin.com>
+ */
+
+#define KMSG_COMPONENT "IPVS"
+#define pr_fmt(fmt) KMSG_COMPONENT ": " fmt
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/random.h>
+
+#include <net/ip_vs.h>
+
+/*    Power of Twos Choice scheduling, algorithm originally described by
+ *    Michael Mitzenmacher.
+ *
+ *    Randomly picks two destinations and picks the one with the least
+ *    amount of connections
+ *
+ *    The algorithm calculates a few variables
+ *    - total_weight = sum of all weights
+ *    - rweight1 = random number between [0,total_weight]
+ *    - rweight2 = random number between [0,total_weight]
+ *
+ *    For each destination
+ *      decrement rweight1 and rweight2 by the destination weight
+ *      pick choice1 when rweight1 is <= 0
+ *      pick choice2 when rweight2 is <= 0
+ *
+ *    Return choice2 if choice2 has less connections than choice 1 normalized
+ *    by weight
+ *
+ * References
+ * ----------
+ *
+ * [Mitzenmacher 2016]
+ *    The Power of Two Random Choices: A Survey of Techniques and Results
+ *    Michael Mitzenmacher, Andrea W. Richa y, Ramesh Sitaraman
+ *    http://www.eecs.harvard.edu/~michaelm/NEWWORK/postscripts/twosurvey.pdf
+ *
+ */
+static struct ip_vs_dest *ip_vs_twos_schedule(struct ip_vs_service *svc,
+					      const struct sk_buff *skb,
+					      struct ip_vs_iphdr *iph)
+{
+	struct ip_vs_dest *dest, *choice1 = NULL, *choice2 = NULL;
+	int rweight1, rweight2, weight1 = -1, weight2 = -1, overhead1 = 0;
+	int overhead2, total_weight = 0, weight;
+
+	IP_VS_DBG(6, "%s(): Scheduling...\n", __func__);
+
+	/* Generate a random weight between [0,sum of all weights) */
+	list_for_each_entry_rcu(dest, &svc->destinations, n_list) {
+		if (!(dest->flags & IP_VS_DEST_F_OVERLOAD)) {
+			weight = atomic_read(&dest->weight);
+			if (weight > 0) {
+				total_weight += weight;
+				choice1 = dest;
+			}
+		}
+	}
+
+	if (!choice1) {
+		ip_vs_scheduler_err(svc, "no destination available");
+		return NULL;
+	}
+
+	/* Add 1 to total_weight so that the random weights are inclusive
+	 * from 0 to total_weight
+	 */
+	total_weight += 1;
+	rweight1 = prandom_u32() % total_weight;
+	rweight2 = prandom_u32() % total_weight;
+
+	/* Pick two weighted servers */
+	list_for_each_entry_rcu(dest, &svc->destinations, n_list) {
+		if (dest->flags & IP_VS_DEST_F_OVERLOAD)
+			continue;
+
+		weight = atomic_read(&dest->weight);
+		if (weight <= 0)
+			continue;
+
+		rweight1 -= weight;
+		rweight2 -= weight;
+
+		if (rweight1 <= 0 && weight1 == -1) {
+			choice1 = dest;
+			weight1 = weight;
+			overhead1 = ip_vs_dest_conn_overhead(dest);
+		}
+
+		if (rweight2 <= 0 && weight2 == -1) {
+			choice2 = dest;
+			weight2 = weight;
+			overhead2 = ip_vs_dest_conn_overhead(dest);
+		}
+
+		if (weight1 != -1 && weight2 != -1)
+			goto nextstage;
+	}
+
+nextstage:
+	if (choice2 && (weight2 * overhead1) > (weight1 * overhead2))
+		choice1 = choice2;
+
+	IP_VS_DBG_BUF(6, "twos: server %s:%u conns %d refcnt %d weight %d\n",
+		      IP_VS_DBG_ADDR(choice1->af, &choice1->addr),
+		      ntohs(choice1->port), atomic_read(&choice1->activeconns),
+		      refcount_read(&choice1->refcnt),
+		      atomic_read(&choice1->weight));
+
+	return choice1;
+}
+
+static struct ip_vs_scheduler ip_vs_twos_scheduler = {
+	.name = "twos",
+	.refcnt = ATOMIC_INIT(0),
+	.module = THIS_MODULE,
+	.n_list = LIST_HEAD_INIT(ip_vs_twos_scheduler.n_list),
+	.schedule = ip_vs_twos_schedule,
+};
+
+static int __init ip_vs_twos_init(void)
+{
+	return register_ip_vs_scheduler(&ip_vs_twos_scheduler);
+}
+
+static void __exit ip_vs_twos_cleanup(void)
+{
+	unregister_ip_vs_scheduler(&ip_vs_twos_scheduler);
+	synchronize_rcu();
+}
+
+module_init(ip_vs_twos_init);
+module_exit(ip_vs_twos_cleanup);
+MODULE_LICENSE("GPL");