/*
   * IPVS:        Shortest Expected Delay 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:
   *
   */
  
  /*
   * The SED algorithm attempts to minimize each job's expected delay until
   * completion. The expected delay that the job will experience is
   * (Ci + 1) / Ui if sent to the ith server, in which Ci is the number of

   * jobs on the ith server and Ui is the fixed service rate (weight) of

   * the ith server. The SED algorithm adopts a greedy policy that each does
   * what is in its own best interest, i.e. to join the queue which would
   * minimize its expected delay of completion.
   *
   * 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 SED to me.
   *
   * The difference between SED and WLC is that SED includes the incoming
   * job in the cost function (the increment of 1). SED may outperform
   * WLC, while scheduling big jobs under larger heterogeneous systems
   * (the server weight varies a lot).
   *
   */

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

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

  static inline int

  ip_vs_sed_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_sed_schedule(struct ip_vs_service *svc, const struct sk_buff *skb,
  		   struct ip_vs_iphdr *iph)

  {
  	struct ip_vs_dest *dest, *least;

  	int loh, doh;

  	IP_VS_DBG(6, "%s(): Scheduling...
  ", __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) > 0) {
  			least = dest;
  			loh = ip_vs_sed_dest_overhead(least);
  			goto nextstage;
  		}
  	}

  	ip_vs_scheduler_err(svc, "no destination available");

  	return NULL;
  
  	/*
  	 *    Find the destination with the least load.
  	 */
    nextstage:

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

  		if (dest->flags & IP_VS_DEST_F_OVERLOAD)
  			continue;
  		doh = ip_vs_sed_dest_overhead(dest);

  		if ((__s64)loh * atomic_read(&dest->weight) >
  		    (__s64)doh * atomic_read(&least->weight)) {

  			least = dest;
  			loh = doh;
  		}
  	}

  	IP_VS_DBG_BUF(6, "SED: server %s:%u "
  		      "activeconns %d refcnt %d weight %d overhead %d
  ",

  		      IP_VS_DBG_ADDR(least->af, &least->addr),
  		      ntohs(least->port),

  		      atomic_read(&least->activeconns),
  		      atomic_read(&least->refcnt),
  		      atomic_read(&least->weight), loh);

  
  	return least;
  }
  
  
  static struct ip_vs_scheduler ip_vs_sed_scheduler =
  {
  	.name =			"sed",
  	.refcnt =		ATOMIC_INIT(0),
  	.module =		THIS_MODULE,

  	.n_list =		LIST_HEAD_INIT(ip_vs_sed_scheduler.n_list),

  	.schedule =		ip_vs_sed_schedule,
  };
  
  
  static int __init ip_vs_sed_init(void)
  {

  	return register_ip_vs_scheduler(&ip_vs_sed_scheduler);
  }
  
  static void __exit ip_vs_sed_cleanup(void)
  {
  	unregister_ip_vs_scheduler(&ip_vs_sed_scheduler);

  	synchronize_rcu();

  }
  
  module_init(ip_vs_sed_init);
  module_exit(ip_vs_sed_cleanup);
  MODULE_LICENSE("GPL");