src/lb_fwlc.c - haproxy - Gitiles

 /*
  * Fast Weighted Least Connection load balancing algorithm.
  *
  * Copyright 2000-2009 Willy Tarreau <w@1wt.eu>
  *
  * 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.
  *
  */

 #include <import/eb32tree.h>
 #include <haproxy/api.h>
 #include <haproxy/backend.h>
 #include <haproxy/queue.h>
 #include <haproxy/server-t.h>


 /* Remove a server from a tree. It must have previously been dequeued. This
  * function is meant to be called when a server is going down or has its
  * weight disabled.
  *
  * The server's lock and the lbprm's lock must be held.
  */
 static inline void fwlc_remove_from_tree(struct server *s)
 {
 	s->lb_tree = NULL;
 }

 /* simply removes a server from a tree.
  *
  * The lbprm's lock must be held.
  */
 static inline void fwlc_dequeue_srv(struct server *s)
 {
 	eb32_delete(&s->lb_node);
 }

 /* Queue a server in its associated tree, assuming the <eweight> is >0.
  * Servers are sorted by (#conns+1)/weight. To ensure maximum accuracy,
  * we use (#conns+1)*SRV_EWGHT_MAX/eweight as the sorting key. The reason
  * for using #conns+1 is to sort by weights in case the server is picked
  * and not before it is picked. This provides a better load accuracy for
  * low connection counts when weights differ and makes sure the round-robin
  * applies between servers of highest weight first. However servers with no
  * connection are always picked first so that under low loads, it's not
  * always the single server with the highest weight that gets picked.
  *
  * NOTE: Depending on the calling context, we use s->next_eweight or
  *       s->cur_eweight. The next value is used when the server state is updated
  *       (because the weight changed for instance). During this step, the server
  *       state is not yet committed. The current value is used to reposition the
  *       server in the tree. This happens when the server is used.
  *
  * The lbprm's lock must be held.
  */
 static inline void fwlc_queue_srv(struct server *s, unsigned int eweight)
 {
 	unsigned int inflight = _HA_ATOMIC_LOAD(&s->served) + _HA_ATOMIC_LOAD(&s->queue.length);

 	s->lb_node.key = inflight ? (inflight + 1) * SRV_EWGHT_MAX / eweight : 0;
 	eb32_insert(s->lb_tree, &s->lb_node);
 }

 /* Re-position the server in the FWLC tree after it has been assigned one
  * connection or after it has released one. Note that it is possible that
  * the server has been moved out of the tree due to failed health-checks.
  * The lbprm's lock will be used.
  */
 static void fwlc_srv_reposition(struct server *s)
 {
 	unsigned int inflight = _HA_ATOMIC_LOAD(&s->served) + _HA_ATOMIC_LOAD(&s->queue.length);
 	unsigned int new_key = inflight ? (inflight + 1) * SRV_EWGHT_MAX / s->cur_eweight : 0;

 	/* some calls will be made for no change (e.g connect_server() after
 	 * assign_server(). Let's check that first.
 	 */
 	if (s->lb_node.node.leaf_p && s->lb_node.key == new_key)
 		return;

 	HA_RWLOCK_WRLOCK(LBPRM_LOCK, &s->proxy->lbprm.lock);
 	if (s->lb_tree) {
 		/* we might have been waiting for a while on the lock above
 		 * so it's worth testing again because other threads are very
 		 * likely to have released a connection or taken one leading
 		 * to our target value (50% of the case in measurements).
 		 */
 		inflight = _HA_ATOMIC_LOAD(&s->served) + _HA_ATOMIC_LOAD(&s->queue.length);
 		new_key = inflight ? (inflight + 1) * SRV_EWGHT_MAX / s->cur_eweight : 0;
 		if (!s->lb_node.node.leaf_p || s->lb_node.key != new_key) {
 			eb32_delete(&s->lb_node);
 			s->lb_node.key = new_key;
 			eb32_insert(s->lb_tree, &s->lb_node);
 		}
 	}
 	HA_RWLOCK_WRUNLOCK(LBPRM_LOCK, &s->proxy->lbprm.lock);
 }

 /* This function updates the server trees according to server <srv>'s new
  * state. It should be called when server <srv>'s status changes to down.
  * It is not important whether the server was already down or not. It is not
  * important either that the new state is completely down (the caller may not
  * know all the variables of a server's state).
  *
  * The server's lock must be held. The lbprm's lock will be used.
  */
 static void fwlc_set_server_status_down(struct server *srv)
 {
 	struct proxy *p = srv->proxy;

 	if (!srv_lb_status_changed(srv))
 		return;

 	if (srv_willbe_usable(srv))
 		goto out_update_state;
 	HA_RWLOCK_WRLOCK(LBPRM_LOCK, &p->lbprm.lock);


 	if (!srv_currently_usable(srv))
 		/* server was already down */
 		goto out_update_backend;

 	if (srv->flags & SRV_F_BACKUP) {
 		p->lbprm.tot_wbck -= srv->cur_eweight;
 		p->srv_bck--;

 		if (srv == p->lbprm.fbck) {
 			/* we lost the first backup server in a single-backup
 			 * configuration, we must search another one.
 			 */
 			struct server *srv2 = p->lbprm.fbck;
 			do {
 				srv2 = srv2->next;
 			} while (srv2 &&
 				 !((srv2->flags & SRV_F_BACKUP) &&
 				   srv_willbe_usable(srv2)));
 			p->lbprm.fbck = srv2;
 		}
 	} else {
 		p->lbprm.tot_wact -= srv->cur_eweight;
 		p->srv_act--;
 	}

 	fwlc_dequeue_srv(srv);
 	fwlc_remove_from_tree(srv);

 out_update_backend:
 	/* check/update tot_used, tot_weight */
 	update_backend_weight(p);
 	HA_RWLOCK_WRUNLOCK(LBPRM_LOCK, &p->lbprm.lock);

  out_update_state:
 	srv_lb_commit_status(srv);
 }

 /* This function updates the server trees according to server <srv>'s new
  * state. It should be called when server <srv>'s status changes to up.
  * It is not important whether the server was already down or not. It is not
  * important either that the new state is completely UP (the caller may not
  * know all the variables of a server's state). This function will not change
  * the weight of a server which was already up.
  *
  * The server's lock must be held. The lbprm's lock will be used.
  */
 static void fwlc_set_server_status_up(struct server *srv)
 {
 	struct proxy *p = srv->proxy;

 	if (!srv_lb_status_changed(srv))
 		return;

 	if (!srv_willbe_usable(srv))
 		goto out_update_state;

 	HA_RWLOCK_WRLOCK(LBPRM_LOCK, &p->lbprm.lock);

 	if (srv_currently_usable(srv))
 		/* server was already up */
 		goto out_update_backend;

 	if (srv->flags & SRV_F_BACKUP) {
 		srv->lb_tree = &p->lbprm.fwlc.bck;
 		p->lbprm.tot_wbck += srv->next_eweight;
 		p->srv_bck++;

 		if (!(p->options & PR_O_USE_ALL_BK)) {
 			if (!p->lbprm.fbck) {
 				/* there was no backup server anymore */
 				p->lbprm.fbck = srv;
 			} else {
 				/* we may have restored a backup server prior to fbck,
 				 * in which case it should replace it.
 				 */
 				struct server *srv2 = srv;
 				do {
 					srv2 = srv2->next;
 				} while (srv2 && (srv2 != p->lbprm.fbck));
 				if (srv2)
 					p->lbprm.fbck = srv;
 			}
 		}
 	} else {
 		srv->lb_tree = &p->lbprm.fwlc.act;
 		p->lbprm.tot_wact += srv->next_eweight;
 		p->srv_act++;
 	}

 	/* note that eweight cannot be 0 here */
 	fwlc_queue_srv(srv, srv->next_eweight);

  out_update_backend:
 	/* check/update tot_used, tot_weight */
 	update_backend_weight(p);
 	HA_RWLOCK_WRUNLOCK(LBPRM_LOCK, &p->lbprm.lock);

  out_update_state:
 	srv_lb_commit_status(srv);
 }

 /* This function must be called after an update to server <srv>'s effective
  * weight. It may be called after a state change too.
  *
  * The server's lock must be held. The lbprm's lock will be used.
  */
 static void fwlc_update_server_weight(struct server *srv)
 {
 	int old_state, new_state;
 	struct proxy *p = srv->proxy;

 	if (!srv_lb_status_changed(srv))
 		return;

 	/* If changing the server's weight changes its state, we simply apply
 	 * the procedures we already have for status change. If the state
 	 * remains down, the server is not in any tree, so it's as easy as
 	 * updating its values. If the state remains up with different weights,
 	 * there are some computations to perform to find a new place and
 	 * possibly a new tree for this server.
 	 */

 	old_state = srv_currently_usable(srv);
 	new_state = srv_willbe_usable(srv);

 	if (!old_state && !new_state) {
 		srv_lb_commit_status(srv);
 		return;
 	}
 	else if (!old_state && new_state) {
 		fwlc_set_server_status_up(srv);
 		return;
 	}
 	else if (old_state && !new_state) {
 		fwlc_set_server_status_down(srv);
 		return;
 	}

 	HA_RWLOCK_WRLOCK(LBPRM_LOCK, &p->lbprm.lock);

 	if (srv->lb_tree)
 		fwlc_dequeue_srv(srv);

 	if (srv->flags & SRV_F_BACKUP) {
 		p->lbprm.tot_wbck += srv->next_eweight - srv->cur_eweight;
 		srv->lb_tree = &p->lbprm.fwlc.bck;
 	} else {
 		p->lbprm.tot_wact += srv->next_eweight - srv->cur_eweight;
 		srv->lb_tree = &p->lbprm.fwlc.act;
 	}

 	fwlc_queue_srv(srv, srv->next_eweight);

 	update_backend_weight(p);
 	HA_RWLOCK_WRUNLOCK(LBPRM_LOCK, &p->lbprm.lock);

 	srv_lb_commit_status(srv);
 }

 /* This function is responsible for building the trees in case of fast
  * weighted least-conns. It also sets p->lbprm.wdiv to the eweight to
  * uweight ratio. Both active and backup groups are initialized.
  */
 void fwlc_init_server_tree(struct proxy *p)
 {
 	struct server *srv;
 	struct eb_root init_head = EB_ROOT;

 	p->lbprm.set_server_status_up   = fwlc_set_server_status_up;
 	p->lbprm.set_server_status_down = fwlc_set_server_status_down;
 	p->lbprm.update_server_eweight  = fwlc_update_server_weight;
 	p->lbprm.server_take_conn = fwlc_srv_reposition;
 	p->lbprm.server_drop_conn = fwlc_srv_reposition;

 	p->lbprm.wdiv = BE_WEIGHT_SCALE;
 	for (srv = p->srv; srv; srv = srv->next) {
 		srv->next_eweight = (srv->uweight * p->lbprm.wdiv + p->lbprm.wmult - 1) / p->lbprm.wmult;
 		srv_lb_commit_status(srv);
 	}

 	recount_servers(p);
 	update_backend_weight(p);

 	p->lbprm.fwlc.act = init_head;
 	p->lbprm.fwlc.bck = init_head;

 	/* queue active and backup servers in two distinct groups */
 	for (srv = p->srv; srv; srv = srv->next) {
 		if (!srv_currently_usable(srv))
 			continue;
 		srv->lb_tree = (srv->flags & SRV_F_BACKUP) ? &p->lbprm.fwlc.bck : &p->lbprm.fwlc.act;
 		fwlc_queue_srv(srv, srv->next_eweight);
 	}
 }

 /* Return next server from the FWLC tree in backend <p>. If the tree is empty,
  * return NULL. Saturated servers are skipped.
  *
  * The lbprm's lock will be used in R/O mode. The server's lock is not used.
  */
 struct server *fwlc_get_next_server(struct proxy *p, struct server *srvtoavoid)
 {
 	struct server *srv, *avoided;
 	struct eb32_node *node;

 	srv = avoided = NULL;

 	HA_RWLOCK_RDLOCK(LBPRM_LOCK, &p->lbprm.lock);
 	if (p->srv_act)
 		node = eb32_first(&p->lbprm.fwlc.act);
 	else if (p->lbprm.fbck) {
 		srv = p->lbprm.fbck;
 		goto out;
 	}
 	else if (p->srv_bck)
 		node = eb32_first(&p->lbprm.fwlc.bck);
 	else {
 		srv = NULL;
 		goto out;
 	}

 	while (node) {
 		/* OK, we have a server. However, it may be saturated, in which
 		 * case we don't want to reconsider it for now, so we'll simply
 		 * skip it. Same if it's the server we try to avoid, in which
 		 * case we simply remember it for later use if needed.
 		 */
 		struct server *s;

 		s = eb32_entry(node, struct server, lb_node);
 		if (!s->maxconn || s->served + s->queue.length < srv_dynamic_maxconn(s) + s->maxqueue) {
 			if (s != srvtoavoid) {
 				srv = s;
 				break;
 			}
 			avoided = s;
 		}
 		node = eb32_next(node);
 	}

 	if (!srv)
 		srv = avoided;
  out:
 	HA_RWLOCK_RDUNLOCK(LBPRM_LOCK, &p->lbprm.lock);
 	return srv;
 }


 /*
  * Local variables:
  *  c-indent-level: 8
  *  c-basic-offset: 8
  * End:
  */
	/*
	* Fast Weighted Least Connection load balancing algorithm.
	*
	* Copyright 2000-2009 Willy Tarreau <w@1wt.eu>
	*
	* 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.
	*
	*/

	#include <import/eb32tree.h>
	#include <haproxy/api.h>
	#include <haproxy/backend.h>
	#include <haproxy/queue.h>
	#include <haproxy/server-t.h>


	/* Remove a server from a tree. It must have previously been dequeued. This
	* function is meant to be called when a server is going down or has its
	* weight disabled.
	*
	* The server's lock and the lbprm's lock must be held.
	*/
	static inline void fwlc_remove_from_tree(struct server *s)
	{
	s->lb_tree = NULL;
	}

	/* simply removes a server from a tree.
	*
	* The lbprm's lock must be held.
	*/
	static inline void fwlc_dequeue_srv(struct server *s)
	{
	eb32_delete(&s->lb_node);
	}

	/* Queue a server in its associated tree, assuming the <eweight> is >0.
	* Servers are sorted by (#conns+1)/weight. To ensure maximum accuracy,
	* we use (#conns+1)*SRV_EWGHT_MAX/eweight as the sorting key. The reason
	* for using #conns+1 is to sort by weights in case the server is picked
	* and not before it is picked. This provides a better load accuracy for
	* low connection counts when weights differ and makes sure the round-robin
	* applies between servers of highest weight first. However servers with no
	* connection are always picked first so that under low loads, it's not
	* always the single server with the highest weight that gets picked.
	*
	* NOTE: Depending on the calling context, we use s->next_eweight or
	* s->cur_eweight. The next value is used when the server state is updated
	* (because the weight changed for instance). During this step, the server
	* state is not yet committed. The current value is used to reposition the
	* server in the tree. This happens when the server is used.
	*
	* The lbprm's lock must be held.
	*/
	static inline void fwlc_queue_srv(struct server *s, unsigned int eweight)
	{
	unsigned int inflight = _HA_ATOMIC_LOAD(&s->served) + _HA_ATOMIC_LOAD(&s->queue.length);

	s->lb_node.key = inflight ? (inflight + 1) * SRV_EWGHT_MAX / eweight : 0;
	eb32_insert(s->lb_tree, &s->lb_node);
	}

	/* Re-position the server in the FWLC tree after it has been assigned one
	* connection or after it has released one. Note that it is possible that
	* the server has been moved out of the tree due to failed health-checks.
	* The lbprm's lock will be used.
	*/
	static void fwlc_srv_reposition(struct server *s)
	{
	unsigned int inflight = _HA_ATOMIC_LOAD(&s->served) + _HA_ATOMIC_LOAD(&s->queue.length);
	unsigned int new_key = inflight ? (inflight + 1) * SRV_EWGHT_MAX / s->cur_eweight : 0;

	/* some calls will be made for no change (e.g connect_server() after
	* assign_server(). Let's check that first.
	*/
	if (s->lb_node.node.leaf_p && s->lb_node.key == new_key)
	return;

	HA_RWLOCK_WRLOCK(LBPRM_LOCK, &s->proxy->lbprm.lock);
	if (s->lb_tree) {
	/* we might have been waiting for a while on the lock above
	* so it's worth testing again because other threads are very
	* likely to have released a connection or taken one leading
	* to our target value (50% of the case in measurements).
	*/
	inflight = _HA_ATOMIC_LOAD(&s->served) + _HA_ATOMIC_LOAD(&s->queue.length);
	new_key = inflight ? (inflight + 1) * SRV_EWGHT_MAX / s->cur_eweight : 0;
	if (!s->lb_node.node.leaf_p \|\| s->lb_node.key != new_key) {
	eb32_delete(&s->lb_node);
	s->lb_node.key = new_key;
	eb32_insert(s->lb_tree, &s->lb_node);
	}
	}
	HA_RWLOCK_WRUNLOCK(LBPRM_LOCK, &s->proxy->lbprm.lock);
	}

	/* This function updates the server trees according to server <srv>'s new
	* state. It should be called when server <srv>'s status changes to down.
	* It is not important whether the server was already down or not. It is not
	* important either that the new state is completely down (the caller may not
	* know all the variables of a server's state).
	*
	* The server's lock must be held. The lbprm's lock will be used.
	*/
	static void fwlc_set_server_status_down(struct server *srv)
	{
	struct proxy *p = srv->proxy;

	if (!srv_lb_status_changed(srv))
	return;

	if (srv_willbe_usable(srv))
	goto out_update_state;
	HA_RWLOCK_WRLOCK(LBPRM_LOCK, &p->lbprm.lock);


	if (!srv_currently_usable(srv))
	/* server was already down */
	goto out_update_backend;

	if (srv->flags & SRV_F_BACKUP) {
	p->lbprm.tot_wbck -= srv->cur_eweight;
	p->srv_bck--;

	if (srv == p->lbprm.fbck) {
	/* we lost the first backup server in a single-backup
	* configuration, we must search another one.
	*/
	struct server *srv2 = p->lbprm.fbck;
	do {
	srv2 = srv2->next;
	} while (srv2 &&
	!((srv2->flags & SRV_F_BACKUP) &&
	srv_willbe_usable(srv2)));
	p->lbprm.fbck = srv2;
	}
	} else {
	p->lbprm.tot_wact -= srv->cur_eweight;
	p->srv_act--;
	}

	fwlc_dequeue_srv(srv);
	fwlc_remove_from_tree(srv);

	out_update_backend:
	/* check/update tot_used, tot_weight */
	update_backend_weight(p);
	HA_RWLOCK_WRUNLOCK(LBPRM_LOCK, &p->lbprm.lock);

	out_update_state:
	srv_lb_commit_status(srv);
	}

	/* This function updates the server trees according to server <srv>'s new
	* state. It should be called when server <srv>'s status changes to up.
	* It is not important whether the server was already down or not. It is not
	* important either that the new state is completely UP (the caller may not
	* know all the variables of a server's state). This function will not change
	* the weight of a server which was already up.
	*
	* The server's lock must be held. The lbprm's lock will be used.
	*/
	static void fwlc_set_server_status_up(struct server *srv)
	{
	struct proxy *p = srv->proxy;

	if (!srv_lb_status_changed(srv))
	return;

	if (!srv_willbe_usable(srv))
	goto out_update_state;

	HA_RWLOCK_WRLOCK(LBPRM_LOCK, &p->lbprm.lock);

	if (srv_currently_usable(srv))
	/* server was already up */
	goto out_update_backend;

	if (srv->flags & SRV_F_BACKUP) {
	srv->lb_tree = &p->lbprm.fwlc.bck;
	p->lbprm.tot_wbck += srv->next_eweight;
	p->srv_bck++;

	if (!(p->options & PR_O_USE_ALL_BK)) {
	if (!p->lbprm.fbck) {
	/* there was no backup server anymore */
	p->lbprm.fbck = srv;
	} else {
	/* we may have restored a backup server prior to fbck,
	* in which case it should replace it.
	*/
	struct server *srv2 = srv;
	do {
	srv2 = srv2->next;
	} while (srv2 && (srv2 != p->lbprm.fbck));
	if (srv2)
	p->lbprm.fbck = srv;
	}
	}
	} else {
	srv->lb_tree = &p->lbprm.fwlc.act;
	p->lbprm.tot_wact += srv->next_eweight;
	p->srv_act++;
	}

	/* note that eweight cannot be 0 here */
	fwlc_queue_srv(srv, srv->next_eweight);

	out_update_backend:
	/* check/update tot_used, tot_weight */
	update_backend_weight(p);
	HA_RWLOCK_WRUNLOCK(LBPRM_LOCK, &p->lbprm.lock);

	out_update_state:
	srv_lb_commit_status(srv);
	}

	/* This function must be called after an update to server <srv>'s effective
	* weight. It may be called after a state change too.
	*
	* The server's lock must be held. The lbprm's lock will be used.
	*/
	static void fwlc_update_server_weight(struct server *srv)
	{
	int old_state, new_state;
	struct proxy *p = srv->proxy;

	if (!srv_lb_status_changed(srv))
	return;

	/* If changing the server's weight changes its state, we simply apply
	* the procedures we already have for status change. If the state
	* remains down, the server is not in any tree, so it's as easy as
	* updating its values. If the state remains up with different weights,
	* there are some computations to perform to find a new place and
	* possibly a new tree for this server.
	*/

	old_state = srv_currently_usable(srv);
	new_state = srv_willbe_usable(srv);

	if (!old_state && !new_state) {
	srv_lb_commit_status(srv);
	return;
	}
	else if (!old_state && new_state) {
	fwlc_set_server_status_up(srv);
	return;
	}
	else if (old_state && !new_state) {
	fwlc_set_server_status_down(srv);
	return;
	}

	HA_RWLOCK_WRLOCK(LBPRM_LOCK, &p->lbprm.lock);

	if (srv->lb_tree)
	fwlc_dequeue_srv(srv);

	if (srv->flags & SRV_F_BACKUP) {
	p->lbprm.tot_wbck += srv->next_eweight - srv->cur_eweight;
	srv->lb_tree = &p->lbprm.fwlc.bck;
	} else {
	p->lbprm.tot_wact += srv->next_eweight - srv->cur_eweight;
	srv->lb_tree = &p->lbprm.fwlc.act;
	}

	fwlc_queue_srv(srv, srv->next_eweight);

	update_backend_weight(p);
	HA_RWLOCK_WRUNLOCK(LBPRM_LOCK, &p->lbprm.lock);

	srv_lb_commit_status(srv);
	}

	/* This function is responsible for building the trees in case of fast
	* weighted least-conns. It also sets p->lbprm.wdiv to the eweight to
	* uweight ratio. Both active and backup groups are initialized.
	*/
	void fwlc_init_server_tree(struct proxy *p)
	{
	struct server *srv;
	struct eb_root init_head = EB_ROOT;

	p->lbprm.set_server_status_up = fwlc_set_server_status_up;
	p->lbprm.set_server_status_down = fwlc_set_server_status_down;
	p->lbprm.update_server_eweight = fwlc_update_server_weight;
	p->lbprm.server_take_conn = fwlc_srv_reposition;
	p->lbprm.server_drop_conn = fwlc_srv_reposition;

	p->lbprm.wdiv = BE_WEIGHT_SCALE;
	for (srv = p->srv; srv; srv = srv->next) {
	srv->next_eweight = (srv->uweight * p->lbprm.wdiv + p->lbprm.wmult - 1) / p->lbprm.wmult;
	srv_lb_commit_status(srv);
	}

	recount_servers(p);
	update_backend_weight(p);

	p->lbprm.fwlc.act = init_head;
	p->lbprm.fwlc.bck = init_head;

	/* queue active and backup servers in two distinct groups */
	for (srv = p->srv; srv; srv = srv->next) {
	if (!srv_currently_usable(srv))
	continue;
	srv->lb_tree = (srv->flags & SRV_F_BACKUP) ? &p->lbprm.fwlc.bck : &p->lbprm.fwlc.act;
	fwlc_queue_srv(srv, srv->next_eweight);
	}
	}

	/* Return next server from the FWLC tree in backend <p>. If the tree is empty,
	* return NULL. Saturated servers are skipped.
	*
	* The lbprm's lock will be used in R/O mode. The server's lock is not used.
	*/
	struct server fwlc_get_next_server(struct proxy p, struct server *srvtoavoid)
	{
	struct server srv, avoided;
	struct eb32_node *node;

	srv = avoided = NULL;

	HA_RWLOCK_RDLOCK(LBPRM_LOCK, &p->lbprm.lock);
	if (p->srv_act)
	node = eb32_first(&p->lbprm.fwlc.act);
	else if (p->lbprm.fbck) {
	srv = p->lbprm.fbck;
	goto out;
	}
	else if (p->srv_bck)
	node = eb32_first(&p->lbprm.fwlc.bck);
	else {
	srv = NULL;
	goto out;
	}

	while (node) {
	/* OK, we have a server. However, it may be saturated, in which
	* case we don't want to reconsider it for now, so we'll simply
	* skip it. Same if it's the server we try to avoid, in which
	* case we simply remember it for later use if needed.
	*/
	struct server *s;

	s = eb32_entry(node, struct server, lb_node);
	if (!s->maxconn \|\| s->served + s->queue.length < srv_dynamic_maxconn(s) + s->maxqueue) {
	if (s != srvtoavoid) {
	srv = s;
	break;
	}
	avoided = s;
	}
	node = eb32_next(node);
	}

	if (!srv)
	srv = avoided;
	out:
	HA_RWLOCK_RDUNLOCK(LBPRM_LOCK, &p->lbprm.lock);
	return srv;
	}


	/*
	* Local variables:
	* c-indent-level: 8
	* c-basic-offset: 8
	* End:
	*/