// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * IPVS:        Never Queue scheduling module
 *
 * Authors:     Wensong Zhang <wensong@linuxvirtualserver.org>
 *
 * Changes:
 */

/*
 * The NQ algorithm adopts a two-speed model. When there is an idle server
 * available, the job will be sent to the idle server, instead of waiting
 * for a fast one. When there is no idle server available, the job will be
 * sent to the server that minimize its expected delay (The Shortest
 * Expected Delay scheduling algorithm).
 *
 * See the following paper for more information:
 * A. Weinrib and S. Shenker, Greed is not enough: Adaptive load sharing
 * in large heterogeneous systems. In Proceedings IEEE INFOCOM'88,
 * pages 986-994, 1988.
 *
 * Thanks must go to Marko Buuri <marko@buuri.name> for talking NQ to me.
 *
 * The difference between NQ and SED is that NQ can improve overall
 * system utilization.
 *
 */

#define pr_fmt(fmt) "IPVS: " fmt

#include <linux/module.h>
#include <linux/kernel.h>

#include <net/ip_vs.h>


static inline int
ip_vs_nq_dest_overhead(struct ip_vs_dest *dest)
{
        /*
         * We only use the active connection number in the cost
         * calculation here.
         */
        return atomic_read(&dest->activeconns) + 1;
}


/*
 *      Weighted Least Connection scheduling
 */
static struct ip_vs_dest *
ip_vs_nq_schedule(struct ip_vs_service *svc, const struct sk_buff *skb,
                  struct ip_vs_iphdr *iph)
{
        struct ip_vs_dest *dest, *least = NULL;
        int loh = 0, doh;

        IP_VS_DBG(6, "%s(): Scheduling...\n", __func__);

        /*
         * We calculate the load of each dest server as follows:
         *      (server expected overhead) / dest->weight
         *
         * Remember -- no floats in kernel mode!!!
         * The comparison of h1*w2 > h2*w1 is equivalent to that of
         *                h1/w1 > h2/w2
         * if every weight is larger than zero.
         *
         * The server with weight=0 is quiesced and will not receive any
         * new connections.
         */

        list_for_each_entry_rcu(dest, &svc->destinations, n_list) {

                if (dest->flags & IP_VS_DEST_F_OVERLOAD ||
                    !atomic_read(&dest->weight))
                        continue;

                doh = ip_vs_nq_dest_overhead(dest);

                /* return the server directly if it is idle */
                if (atomic_read(&dest->activeconns) == 0) {
                        least = dest;
                        loh = doh;
                        goto out;
                }

                if (!least ||
                    ((__s64)loh * atomic_read(&dest->weight) >
                     (__s64)doh * atomic_read(&least->weight))) {
                        least = dest;
                        loh = doh;
                }
        }

        if (!least) {
                ip_vs_scheduler_err(svc, "no destination available");
                return NULL;
        }

  out:
        IP_VS_DBG_BUF(6, "NQ: server %s:%u "
                      "activeconns %d refcnt %d weight %d overhead %d\n",
                      IP_VS_DBG_ADDR(least->af, &least->addr),
                      ntohs(least->port),
                      atomic_read(&least->activeconns),
                      refcount_read(&least->refcnt),
                      atomic_read(&least->weight), loh);

        return least;
}


static struct ip_vs_scheduler ip_vs_nq_scheduler =
{
        .name =                 "nq",
        .refcnt =               ATOMIC_INIT(0),
        .module =               THIS_MODULE,
        .n_list =               LIST_HEAD_INIT(ip_vs_nq_scheduler.n_list),
        .schedule =             ip_vs_nq_schedule,
};


static int __init ip_vs_nq_init(void)
{
        return register_ip_vs_scheduler(&ip_vs_nq_scheduler);
}

static void __exit ip_vs_nq_cleanup(void)
{
        unregister_ip_vs_scheduler(&ip_vs_nq_scheduler);
        synchronize_rcu();
}

module_init(ip_vs_nq_init);
module_exit(ip_vs_nq_cleanup);
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("ipvs never queue scheduler");