/*
 * IPVS:        Destination Hashing scheduling module
 *
 * Authors:     Wensong Zhang <wensong@gnuchina.org>
 *
 *              Inspired by the consistent hashing scheduler patch from
 *              Thomas Proell <proellt@gmx.de>
 *
 *              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:
 *
 */

/*
 * The dh algorithm is to select server by the hash key of destination IP
 * address. The pseudo code is as follows:
 *
 *       n <- servernode[dest_ip];
 *       if (n is dead) OR
 *          (n is overloaded) OR (n.weight <= 0) then
 *                 return NULL;
 *
 *       return n;
 *
 * Notes that servernode is a 256-bucket hash table that maps the hash
 * index derived from packet destination IP address to the current server
 * array. If the dh scheduler is used in cache cluster, it is good to
 * combine it with cache_bypass feature. When the statically assigned
 * server is dead or overloaded, the load balancer can bypass the cache
 * server and send requests to the original server directly.
 *
 */

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

#include <linux/ip.h>
#include <linux/slab.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/skbuff.h>

#include <net/ip_vs.h>


/*
 *      IPVS DH bucket
 */
struct ip_vs_dh_bucket {
        struct ip_vs_dest __rcu *dest;  /* real server (cache) */
};

/*
 *     for IPVS DH entry hash table
 */
#ifndef CONFIG_IP_VS_DH_TAB_BITS
#define CONFIG_IP_VS_DH_TAB_BITS        8
#endif
#define IP_VS_DH_TAB_BITS               CONFIG_IP_VS_DH_TAB_BITS
#define IP_VS_DH_TAB_SIZE               (1 << IP_VS_DH_TAB_BITS)
#define IP_VS_DH_TAB_MASK               (IP_VS_DH_TAB_SIZE - 1)

struct ip_vs_dh_state {
        struct ip_vs_dh_bucket          buckets[IP_VS_DH_TAB_SIZE];
        struct rcu_head                 rcu_head;
};

/*
 *      Returns hash value for IPVS DH entry
 */
static inline unsigned int ip_vs_dh_hashkey(int af, const union nf_inet_addr *addr)
{
        __be32 addr_fold = addr->ip;

#ifdef CONFIG_IP_VS_IPV6
        if (af == AF_INET6)
                addr_fold = addr->ip6[0]^addr->ip6[1]^
                            addr->ip6[2]^addr->ip6[3];
#endif
        return (ntohl(addr_fold)*2654435761UL) & IP_VS_DH_TAB_MASK;
}


/*
 *      Get ip_vs_dest associated with supplied parameters.
 */
static inline struct ip_vs_dest *
ip_vs_dh_get(int af, struct ip_vs_dh_state *s, const union nf_inet_addr *addr)
{
        return rcu_dereference(s->buckets[ip_vs_dh_hashkey(af, addr)].dest);
}


/*
 *      Assign all the hash buckets of the specified table with the service.
 */
static int
ip_vs_dh_reassign(struct ip_vs_dh_state *s, struct ip_vs_service *svc)
{
        int i;
        struct ip_vs_dh_bucket *b;
        struct list_head *p;
        struct ip_vs_dest *dest;
        bool empty;

        b = &s->buckets[0];
        p = &svc->destinations;
        empty = list_empty(p);
        for (i=0; i<IP_VS_DH_TAB_SIZE; i++) {
                dest = rcu_dereference_protected(b->dest, 1);
                if (dest)
                        ip_vs_dest_put(dest);
                if (empty)
                        RCU_INIT_POINTER(b->dest, NULL);
                else {
                        if (p == &svc->destinations)
                                p = p->next;

                        dest = list_entry(p, struct ip_vs_dest, n_list);
                        ip_vs_dest_hold(dest);
                        RCU_INIT_POINTER(b->dest, dest);

                        p = p->next;
                }
                b++;
        }
        return 0;
}


/*
 *      Flush all the hash buckets of the specified table.
 */
static void ip_vs_dh_flush(struct ip_vs_dh_state *s)
{
        int i;
        struct ip_vs_dh_bucket *b;
        struct ip_vs_dest *dest;

        b = &s->buckets[0];
        for (i=0; i<IP_VS_DH_TAB_SIZE; i++) {
                dest = rcu_dereference_protected(b->dest, 1);
                if (dest) {
                        ip_vs_dest_put(dest);
                        RCU_INIT_POINTER(b->dest, NULL);
                }
                b++;
        }
}


static int ip_vs_dh_init_svc(struct ip_vs_service *svc)
{
        struct ip_vs_dh_state *s;

        /* allocate the DH table for this service */
        s = kzalloc(sizeof(struct ip_vs_dh_state), GFP_KERNEL);
        if (s == NULL)
                return -ENOMEM;

        svc->sched_data = s;
        IP_VS_DBG(6, "DH hash table (memory=%Zdbytes) allocated for "
                  "current service\n",
                  sizeof(struct ip_vs_dh_bucket)*IP_VS_DH_TAB_SIZE);

        /* assign the hash buckets with current dests */
        ip_vs_dh_reassign(s, svc);

        return 0;
}


static void ip_vs_dh_done_svc(struct ip_vs_service *svc)
{
        struct ip_vs_dh_state *s = svc->sched_data;

        /* got to clean up hash buckets here */
        ip_vs_dh_flush(s);

        /* release the table itself */
        kfree_rcu(s, rcu_head);
        IP_VS_DBG(6, "DH hash table (memory=%Zdbytes) released\n",
                  sizeof(struct ip_vs_dh_bucket)*IP_VS_DH_TAB_SIZE);
}


static int ip_vs_dh_dest_changed(struct ip_vs_service *svc,
                                 struct ip_vs_dest *dest)
{
        struct ip_vs_dh_state *s = svc->sched_data;

        /* assign the hash buckets with the updated service */
        ip_vs_dh_reassign(s, svc);

        return 0;
}


/*
 *      If the dest flags is set with IP_VS_DEST_F_OVERLOAD,
 *      consider that the server is overloaded here.
 */
static inline int is_overloaded(struct ip_vs_dest *dest)
{
        return dest->flags & IP_VS_DEST_F_OVERLOAD;
}


/*
 *      Destination hashing scheduling
 */
static struct ip_vs_dest *
ip_vs_dh_schedule(struct ip_vs_service *svc, const struct sk_buff *skb,
                  struct ip_vs_iphdr *iph)
{
        struct ip_vs_dest *dest;
        struct ip_vs_dh_state *s;

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

        s = (struct ip_vs_dh_state *) svc->sched_data;
        dest = ip_vs_dh_get(svc->af, s, &iph->daddr);
        if (!dest
            || !(dest->flags & IP_VS_DEST_F_AVAILABLE)
            || atomic_read(&dest->weight) <= 0
            || is_overloaded(dest)) {
                ip_vs_scheduler_err(svc, "no destination available");
                return NULL;
        }

        IP_VS_DBG_BUF(6, "DH: destination IP address %s --> server %s:%d\n",
                      IP_VS_DBG_ADDR(svc->af, &iph->daddr),
                      IP_VS_DBG_ADDR(svc->af, &dest->addr),
                      ntohs(dest->port));

        return dest;
}


/*
 *      IPVS DH Scheduler structure
 */
static struct ip_vs_scheduler ip_vs_dh_scheduler =
{
        .name =                 "dh",
        .refcnt =               ATOMIC_INIT(0),
        .module =               THIS_MODULE,
        .n_list =               LIST_HEAD_INIT(ip_vs_dh_scheduler.n_list),
        .init_service =         ip_vs_dh_init_svc,
        .done_service =         ip_vs_dh_done_svc,
        .add_dest =             ip_vs_dh_dest_changed,
        .del_dest =             ip_vs_dh_dest_changed,
        .schedule =             ip_vs_dh_schedule,
};


static int __init ip_vs_dh_init(void)
{
        return register_ip_vs_scheduler(&ip_vs_dh_scheduler);
}


static void __exit ip_vs_dh_cleanup(void)
{
        unregister_ip_vs_scheduler(&ip_vs_dh_scheduler);
        synchronize_rcu();
}


module_init(ip_vs_dh_init);
module_exit(ip_vs_dh_cleanup);
MODULE_LICENSE("GPL");