/*
 * IPVS:        Weighted Round-Robin Scheduling module
 *
 * Authors:     Wensong Zhang <wensong@linuxvirtualserver.org>
 *
 *              This program is free software; you can redistribute it and/or
 *              modify it under the terms of the GNU General Public License
 *              as published by the Free Software Foundation; either version
 *              2 of the License, or (at your option) any later version.
 *
 * Changes:
 *     Wensong Zhang            :     changed the ip_vs_wrr_schedule to return dest
 *     Wensong Zhang            :     changed some comestics things for debugging
 *     Wensong Zhang            :     changed for the d-linked destination list
 *     Wensong Zhang            :     added the ip_vs_wrr_update_svc
 *     Julian Anastasov         :     fixed the bug of returning destination
 *                                    with weight 0 when all weights are zero
 *
 */

#define KMSG_COMPONENT "IPVS"
#define pr_fmt(fmt) KMSG_COMPONENT ": " fmt

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/slab.h>
#include <linux/net.h>
#include <linux/gcd.h>

#include <net/ip_vs.h>

/* The WRR algorithm depends on some caclulations:
 * - mw: maximum weight
 * - di: weight step, greatest common divisor from all weights
 * - cw: current required weight
 * As result, all weights are in the [di..mw] range with a step=di.
 *
 * First, we start with cw = mw and select dests with weight >= cw.
 * Then cw is reduced with di and all dests are checked again.
 * Last pass should be with cw = di. We have mw/di passes in total:
 *
 * pass 1: cw = max weight
 * pass 2: cw = max weight - di
 * pass 3: cw = max weight - 2 * di
 * ...
 * last pass: cw = di
 *
 * Weights are supposed to be >= di but we run in parallel with
 * weight changes, it is possible some dest weight to be reduced
 * below di, bad if it is the only available dest.
 *
 * So, we modify how mw is calculated, now it is reduced with (di - 1),
 * so that last cw is 1 to catch such dests with weight below di:
 * pass 1: cw = max weight - (di - 1)
 * pass 2: cw = max weight - di - (di - 1)
 * pass 3: cw = max weight - 2 * di - (di - 1)
 * ...
 * last pass: cw = 1
 *
 */

/*
 * current destination pointer for weighted round-robin scheduling
 */
struct ip_vs_wrr_mark {
        struct ip_vs_dest *cl;  /* current dest or head */
        int cw;                 /* current weight */
        int mw;                 /* maximum weight */
        int di;                 /* decreasing interval */
        struct rcu_head         rcu_head;
};


static int ip_vs_wrr_gcd_weight(struct ip_vs_service *svc)
{
        struct ip_vs_dest *dest;
        int weight;
        int g = 0;

        list_for_each_entry(dest, &svc->destinations, n_list) {
                weight = atomic_read(&dest->weight);
                if (weight > 0) {
                        if (g > 0)
                                g = gcd(weight, g);
                        else
                                g = weight;
                }
        }
        return g ? g : 1;
}


/*
 *    Get the maximum weight of the service destinations.
 */
static int ip_vs_wrr_max_weight(struct ip_vs_service *svc)
{
        struct ip_vs_dest *dest;
        int new_weight, weight = 0;

        list_for_each_entry(dest, &svc->destinations, n_list) {
                new_weight = atomic_read(&dest->weight);
                if (new_weight > weight)
                        weight = new_weight;
        }

        return weight;
}


static int ip_vs_wrr_init_svc(struct ip_vs_service *svc)
{
        struct ip_vs_wrr_mark *mark;

        /*
         *    Allocate the mark variable for WRR scheduling
         */
        mark = kmalloc(sizeof(struct ip_vs_wrr_mark), GFP_KERNEL);
        if (mark == NULL)
                return -ENOMEM;

        mark->cl = list_entry(&svc->destinations, struct ip_vs_dest, n_list);
        mark->di = ip_vs_wrr_gcd_weight(svc);
        mark->mw = ip_vs_wrr_max_weight(svc) - (mark->di - 1);
        mark->cw = mark->mw;
        svc->sched_data = mark;

        return 0;
}


static void ip_vs_wrr_done_svc(struct ip_vs_service *svc)
{
        struct ip_vs_wrr_mark *mark = svc->sched_data;

        /*
         *    Release the mark variable
         */
        kfree_rcu(mark, rcu_head);
}


static int ip_vs_wrr_dest_changed(struct ip_vs_service *svc,
                                  struct ip_vs_dest *dest)
{
        struct ip_vs_wrr_mark *mark = svc->sched_data;

        spin_lock_bh(&svc->sched_lock);
        mark->cl = list_entry(&svc->destinations, struct ip_vs_dest, n_list);
        mark->di = ip_vs_wrr_gcd_weight(svc);
        mark->mw = ip_vs_wrr_max_weight(svc) - (mark->di - 1);
        if (mark->cw > mark->mw || !mark->cw) {
                gmb();
                mark->cw = mark->mw;
        } else if (mark->di > 1) {
                gmb();
                mark->cw = (mark->cw / mark->di) * mark->di + 1;
        }
        spin_unlock_bh(&svc->sched_lock);
        return 0;
}


/*
 *    Weighted Round-Robin Scheduling
 */
static struct ip_vs_dest *
ip_vs_wrr_schedule(struct ip_vs_service *svc, const struct sk_buff *skb,
                   struct ip_vs_iphdr *iph)
{
        struct ip_vs_dest *dest, *last, *stop = NULL;
        struct ip_vs_wrr_mark *mark = svc->sched_data;
        bool last_pass = false, restarted = false;

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

        spin_lock_bh(&svc->sched_lock);
        dest = mark->cl;
        /* No available dests? */
        if (mark->mw == 0)
                goto err_noavail;
        last = dest;
        /* Stop only after all dests were checked for weight >= 1 (last pass) */
        while (1) {
                list_for_each_entry_continue_rcu(dest,
                                                 &svc->destinations,
                                                 n_list) {
                        if (!(dest->flags & IP_VS_DEST_F_OVERLOAD) &&
                            atomic_read(&dest->weight) >= mark->cw)
                                goto found;
                        if (dest == stop)
                                goto err_over;
                }
                mark->cw -= mark->di;
                if (mark->cw <= 0) {
                        mark->cw = mark->mw;
                        /* Stop if we tried last pass from first dest:
                         * 1. last_pass: we started checks when cw > di but
                         *      then all dests were checked for w >= 1
                         * 2. last was head: the first and only traversal
                         *      was for weight >= 1, for all dests.
                         */
                        if (last_pass ||
                            &last->n_list == &svc->destinations)
                                goto err_over;
                        restarted = true;
                }
                last_pass = mark->cw <= mark->di;
                if (last_pass && restarted &&
                    &last->n_list != &svc->destinations) {
                        /* First traversal was for w >= 1 but only
                         * for dests after 'last', now do the same
                         * for all dests up to 'last'.
                         */
                        stop = last;
                }
        }

found:
        IP_VS_DBG_BUF(6, "WRR: server %s:%u "
                      "activeconns %d refcnt %d weight %d\n",
                      IP_VS_DBG_ADDR(svc->af, &dest->addr), ntohs(dest->port),
                      atomic_read(&dest->activeconns),
                      atomic_read(&dest->refcnt),
                      atomic_read(&dest->weight));
        mark->cl = dest;

  out:
        spin_unlock_bh(&svc->sched_lock);
        return dest;

err_noavail:
        mark->cl = dest;
        dest = NULL;
        ip_vs_scheduler_err(svc, "no destination available");
        goto out;

err_over:
        mark->cl = dest;
        dest = NULL;
        ip_vs_scheduler_err(svc, "no destination available: "
                            "all destinations are overloaded");
        goto out;
}


static struct ip_vs_scheduler ip_vs_wrr_scheduler = {
        .name =                 "wrr",
        .refcnt =               ATOMIC_INIT(0),
        .module =               THIS_MODULE,
        .n_list =               LIST_HEAD_INIT(ip_vs_wrr_scheduler.n_list),
        .init_service =         ip_vs_wrr_init_svc,
        .done_service =         ip_vs_wrr_done_svc,
        .add_dest =             ip_vs_wrr_dest_changed,
        .del_dest =             ip_vs_wrr_dest_changed,
        .upd_dest =             ip_vs_wrr_dest_changed,
        .schedule =             ip_vs_wrr_schedule,
};

static int __init ip_vs_wrr_init(void)
{
        return register_ip_vs_scheduler(&ip_vs_wrr_scheduler) ;
}

static void __exit ip_vs_wrr_cleanup(void)
{
        unregister_ip_vs_scheduler(&ip_vs_wrr_scheduler);
        synchronize_rcu();
}

module_init(ip_vs_wrr_init);
module_exit(ip_vs_wrr_cleanup);
MODULE_LICENSE("GPL");