1// SPDX-License-Identifier: GPL-2.0-or-later
2/*
3 * IPVS:        Shortest Expected Delay scheduling module
4 *
5 * Authors:     Wensong Zhang <wensong@linuxvirtualserver.org>
6 *
7 * Changes:
8 */
9
10/*
11 * The SED algorithm attempts to minimize each job's expected delay until
12 * completion. The expected delay that the job will experience is
13 * (Ci + 1) / Ui if sent to the ith server, in which Ci is the number of
14 * jobs on the ith server and Ui is the fixed service rate (weight) of
15 * the ith server. The SED algorithm adopts a greedy policy that each does
16 * what is in its own best interest, i.e. to join the queue which would
17 * minimize its expected delay of completion.
18 *
19 * See the following paper for more information:
20 * A. Weinrib and S. Shenker, Greed is not enough: Adaptive load sharing
21 * in large heterogeneous systems. In Proceedings IEEE INFOCOM'88,
22 * pages 986-994, 1988.
23 *
24 * Thanks must go to Marko Buuri <marko@buuri.name> for talking SED to me.
25 *
26 * The difference between SED and WLC is that SED includes the incoming
27 * job in the cost function (the increment of 1). SED may outperform
28 * WLC, while scheduling big jobs under larger heterogeneous systems
29 * (the server weight varies a lot).
30 *
31 */
32
33#define KMSG_COMPONENT "IPVS"
34#define pr_fmt(fmt) KMSG_COMPONENT ": " fmt
35
36#include <linux/module.h>
37#include <linux/kernel.h>
38
39#include <net/ip_vs.h>
40
41
42static inline int
43ip_vs_sed_dest_overhead(struct ip_vs_dest *dest)
44{
45	/*
46	 * We only use the active connection number in the cost
47	 * calculation here.
48	 */
49	return atomic_read(&dest->activeconns) + 1;
50}
51
52
53/*
54 *	Weighted Least Connection scheduling
55 */
56static struct ip_vs_dest *
57ip_vs_sed_schedule(struct ip_vs_service *svc, const struct sk_buff *skb,
58		   struct ip_vs_iphdr *iph)
59{
60	struct ip_vs_dest *dest, *least;
61	int loh, doh;
62
63	IP_VS_DBG(6, "%s(): Scheduling...\n", __func__);
64
65	/*
66	 * We calculate the load of each dest server as follows:
67	 *	(server expected overhead) / dest->weight
68	 *
69	 * Remember -- no floats in kernel mode!!!
70	 * The comparison of h1*w2 > h2*w1 is equivalent to that of
71	 *		  h1/w1 > h2/w2
72	 * if every weight is larger than zero.
73	 *
74	 * The server with weight=0 is quiesced and will not receive any
75	 * new connections.
76	 */
77
78	list_for_each_entry_rcu(dest, &svc->destinations, n_list) {
79		if (!(dest->flags & IP_VS_DEST_F_OVERLOAD) &&
80		    atomic_read(&dest->weight) > 0) {
81			least = dest;
82			loh = ip_vs_sed_dest_overhead(least);
83			goto nextstage;
84		}
85	}
86	ip_vs_scheduler_err(svc, "no destination available");
87	return NULL;
88
89	/*
90	 *    Find the destination with the least load.
91	 */
92  nextstage:
93	list_for_each_entry_continue_rcu(dest, &svc->destinations, n_list) {
94		if (dest->flags & IP_VS_DEST_F_OVERLOAD)
95			continue;
96		doh = ip_vs_sed_dest_overhead(dest);
97		if ((__s64)loh * atomic_read(&dest->weight) >
98		    (__s64)doh * atomic_read(&least->weight)) {
99			least = dest;
100			loh = doh;
101		}
102	}
103
104	IP_VS_DBG_BUF(6, "SED: server %s:%u "
105		      "activeconns %d refcnt %d weight %d overhead %d\n",
106		      IP_VS_DBG_ADDR(least->af, &least->addr),
107		      ntohs(least->port),
108		      atomic_read(&least->activeconns),
109		      refcount_read(&least->refcnt),
110		      atomic_read(&least->weight), loh);
111
112	return least;
113}
114
115
116static struct ip_vs_scheduler ip_vs_sed_scheduler =
117{
118	.name =			"sed",
119	.refcnt =		ATOMIC_INIT(0),
120	.module =		THIS_MODULE,
121	.n_list =		LIST_HEAD_INIT(ip_vs_sed_scheduler.n_list),
122	.schedule =		ip_vs_sed_schedule,
123};
124
125
126static int __init ip_vs_sed_init(void)
127{
128	return register_ip_vs_scheduler(&ip_vs_sed_scheduler);
129}
130
131static void __exit ip_vs_sed_cleanup(void)
132{
133	unregister_ip_vs_scheduler(&ip_vs_sed_scheduler);
134	synchronize_rcu();
135}
136
137module_init(ip_vs_sed_init);
138module_exit(ip_vs_sed_cleanup);
139MODULE_LICENSE("GPL");
140MODULE_DESCRIPTION("ipvs shortest expected delay scheduler");
141