ip_vs_twos.c

  1// SPDX-License-Identifier: GPL-2.0-or-later
  2/* IPVS:        Power of Twos Choice Scheduling module
  3 *
  4 * Authors:     Darby Payne <darby.payne@applovin.com>
  5 */
  6
  7#define pr_fmt(fmt) "IPVS: " fmt
  8
  9#include <linux/kernel.h>
 10#include <linux/module.h>
 11#include <linux/random.h>
 12
 13#include <net/ip_vs.h>
 14
 15/*    Power of Twos Choice scheduling, algorithm originally described by
 16 *    Michael Mitzenmacher.
 17 *
 18 *    Randomly picks two destinations and picks the one with the least
 19 *    amount of connections
 20 *
 21 *    The algorithm calculates a few variables
 22 *    - total_weight = sum of all weights
 23 *    - rweight1 = random number between [0,total_weight]
 24 *    - rweight2 = random number between [0,total_weight]
 25 *
 26 *    For each destination
 27 *      decrement rweight1 and rweight2 by the destination weight
 28 *      pick choice1 when rweight1 is <= 0
 29 *      pick choice2 when rweight2 is <= 0
 30 *
 31 *    Return choice2 if choice2 has less connections than choice 1 normalized
 32 *    by weight
 33 *
 34 * References
 35 * ----------
 36 *
 37 * [Mitzenmacher 2016]
 38 *    The Power of Two Random Choices: A Survey of Techniques and Results
 39 *    Michael Mitzenmacher, Andrea W. Richa y, Ramesh Sitaraman
 40 *    http://www.eecs.harvard.edu/~michaelm/NEWWORK/postscripts/twosurvey.pdf
 41 *
 42 */
 43static struct ip_vs_dest *ip_vs_twos_schedule(struct ip_vs_service *svc,
 44					      const struct sk_buff *skb,
 45					      struct ip_vs_iphdr *iph)
 46{
 47	struct ip_vs_dest *dest, *choice1 = NULL, *choice2 = NULL;
 48	int rweight1, rweight2, weight1 = -1, weight2 = -1, overhead1 = 0;
 49	int overhead2, total_weight = 0, weight;
 50
 51	IP_VS_DBG(6, "%s(): Scheduling...\n", __func__);
 52
 53	/* Generate a random weight between [0,sum of all weights) */
 54	list_for_each_entry_rcu(dest, &svc->destinations, n_list) {
 55		if (!(dest->flags & IP_VS_DEST_F_OVERLOAD)) {
 56			weight = atomic_read(&dest->weight);
 57			if (weight > 0) {
 58				total_weight += weight;
 59				choice1 = dest;
 60			}
 61		}
 62	}
 63
 64	if (!choice1) {
 65		ip_vs_scheduler_err(svc, "no destination available");
 66		return NULL;
 67	}
 68
 69	/* Add 1 to total_weight so that the random weights are inclusive
 70	 * from 0 to total_weight
 71	 */
 72	total_weight += 1;
 73	rweight1 = get_random_u32_below(total_weight);
 74	rweight2 = get_random_u32_below(total_weight);
 75
 76	/* Pick two weighted servers */
 77	list_for_each_entry_rcu(dest, &svc->destinations, n_list) {
 78		if (dest->flags & IP_VS_DEST_F_OVERLOAD)
 79			continue;
 80
 81		weight = atomic_read(&dest->weight);
 82		if (weight <= 0)
 83			continue;
 84
 85		rweight1 -= weight;
 86		rweight2 -= weight;
 87
 88		if (rweight1 <= 0 && weight1 == -1) {
 89			choice1 = dest;
 90			weight1 = weight;
 91			overhead1 = ip_vs_dest_conn_overhead(dest);
 92		}
 93
 94		if (rweight2 <= 0 && weight2 == -1) {
 95			choice2 = dest;
 96			weight2 = weight;
 97			overhead2 = ip_vs_dest_conn_overhead(dest);
 98		}
 99
100		if (weight1 != -1 && weight2 != -1)
101			goto nextstage;
102	}
103
104nextstage:
105	if (choice2 && (weight2 * overhead1) > (weight1 * overhead2))
106		choice1 = choice2;
107
108	IP_VS_DBG_BUF(6, "twos: server %s:%u conns %d refcnt %d weight %d\n",
109		      IP_VS_DBG_ADDR(choice1->af, &choice1->addr),
110		      ntohs(choice1->port), atomic_read(&choice1->activeconns),
111		      refcount_read(&choice1->refcnt),
112		      atomic_read(&choice1->weight));
113
114	return choice1;
115}
116
117static struct ip_vs_scheduler ip_vs_twos_scheduler = {
118	.name = "twos",
119	.refcnt = ATOMIC_INIT(0),
120	.module = THIS_MODULE,
121	.n_list = LIST_HEAD_INIT(ip_vs_twos_scheduler.n_list),
122	.schedule = ip_vs_twos_schedule,
123};
124
125static int __init ip_vs_twos_init(void)
126{
127	return register_ip_vs_scheduler(&ip_vs_twos_scheduler);
128}
129
130static void __exit ip_vs_twos_cleanup(void)
131{
132	unregister_ip_vs_scheduler(&ip_vs_twos_scheduler);
133	synchronize_rcu();
134}
135
136module_init(ip_vs_twos_init);
137module_exit(ip_vs_twos_cleanup);
138MODULE_LICENSE("GPL");
139MODULE_DESCRIPTION("ipvs power of twos choice scheduler");