tcp_cong.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Pluggable TCP congestion control support and newReno
 * congestion control.
 * Based on ideas from I/O scheduler support and Web100.
 *
 * Copyright (C) 2005 Stephen Hemminger <shemminger@osdl.org>
 */

#define pr_fmt(fmt) "TCP: " fmt

#include <linux/module.h>
#include <linux/mm.h>
#include <linux/types.h>
#include <linux/list.h>
#include <linux/gfp.h>
#include <linux/jhash.h>
#include <net/tcp.h>

static DEFINE_SPINLOCK(tcp_cong_list_lock);
static LIST_HEAD(tcp_cong_list);

/* Simple linear search, don't expect many entries! */
static struct tcp_congestion_ops *tcp_ca_find(const char *name)
{
	struct tcp_congestion_ops *e;

	list_for_each_entry_rcu(e, &tcp_cong_list, list) {
		if (strcmp(e->name, name) == 0)
			return e;
	}

	return NULL;
}

/* Must be called with rcu lock held */
static struct tcp_congestion_ops *tcp_ca_find_autoload(struct net *net,
						       const char *name)
{
	struct tcp_congestion_ops *ca = tcp_ca_find(name);

#ifdef CONFIG_MODULES
	if (!ca && capable(CAP_NET_ADMIN)) {
		rcu_read_unlock();
		request_module("tcp_%s", name);
		rcu_read_lock();
		ca = tcp_ca_find(name);
	}
#endif
	return ca;
}

/* Simple linear search, not much in here. */
struct tcp_congestion_ops *tcp_ca_find_key(u32 key)
{
	struct tcp_congestion_ops *e;

	list_for_each_entry_rcu(e, &tcp_cong_list, list) {
		if (e->key == key)
			return e;
	}

	return NULL;
}

/*
 * Attach new congestion control algorithm to the list
 * of available options.
 */
int tcp_register_congestion_control(struct tcp_congestion_ops *ca)
{
	int ret = 0;

	/* all algorithms must implement these */
	if (!ca->ssthresh || !ca->undo_cwnd ||
	    !(ca->cong_avoid || ca->cong_control)) {
		pr_err("%s does not implement required ops\n", ca->name);
		return -EINVAL;
	}

	ca->key = jhash(ca->name, sizeof(ca->name), strlen(ca->name));

	spin_lock(&tcp_cong_list_lock);
	if (ca->key == TCP_CA_UNSPEC || tcp_ca_find_key(ca->key)) {
		pr_notice("%s already registered or non-unique key\n",
			  ca->name);
		ret = -EEXIST;
	} else {
		list_add_tail_rcu(&ca->list, &tcp_cong_list);
		pr_debug("%s registered\n", ca->name);
	}
	spin_unlock(&tcp_cong_list_lock);

	return ret;
}
EXPORT_SYMBOL_GPL(tcp_register_congestion_control);

/*
 * Remove congestion control algorithm, called from
 * the module's remove function.  Module ref counts are used
 * to ensure that this can't be done till all sockets using
 * that method are closed.
 */
void tcp_unregister_congestion_control(struct tcp_congestion_ops *ca)
{
	spin_lock(&tcp_cong_list_lock);
	list_del_rcu(&ca->list);
	spin_unlock(&tcp_cong_list_lock);

	/* Wait for outstanding readers to complete before the
	 * module gets removed entirely.
	 *
	 * A try_module_get() should fail by now as our module is
	 * in "going" state since no refs are held anymore and
	 * module_exit() handler being called.
	 */
	synchronize_rcu();
}
EXPORT_SYMBOL_GPL(tcp_unregister_congestion_control);

u32 tcp_ca_get_key_by_name(struct net *net, const char *name, bool *ecn_ca)
{
	const struct tcp_congestion_ops *ca;
	u32 key = TCP_CA_UNSPEC;

	might_sleep();

	rcu_read_lock();
	ca = tcp_ca_find_autoload(net, name);
	if (ca) {
		key = ca->key;
		*ecn_ca = ca->flags & TCP_CONG_NEEDS_ECN;
	}
	rcu_read_unlock();

	return key;
}
EXPORT_SYMBOL_GPL(tcp_ca_get_key_by_name);

char *tcp_ca_get_name_by_key(u32 key, char *buffer)
{
	const struct tcp_congestion_ops *ca;
	char *ret = NULL;

	rcu_read_lock();
	ca = tcp_ca_find_key(key);
	if (ca)
		ret = strncpy(buffer, ca->name,
			      TCP_CA_NAME_MAX);
	rcu_read_unlock();

	return ret;
}
EXPORT_SYMBOL_GPL(tcp_ca_get_name_by_key);

/* Assign choice of congestion control. */
void tcp_assign_congestion_control(struct sock *sk)
{
	struct net *net = sock_net(sk);
	struct inet_connection_sock *icsk = inet_csk(sk);
	const struct tcp_congestion_ops *ca;

	rcu_read_lock();
	ca = rcu_dereference(net->ipv4.tcp_congestion_control);
	if (unlikely(!try_module_get(ca->owner)))
		ca = &tcp_reno;
	icsk->icsk_ca_ops = ca;
	rcu_read_unlock();

	memset(icsk->icsk_ca_priv, 0, sizeof(icsk->icsk_ca_priv));
	if (ca->flags & TCP_CONG_NEEDS_ECN)
		INET_ECN_xmit(sk);
	else
		INET_ECN_dontxmit(sk);
}

void tcp_init_congestion_control(struct sock *sk)
{
	const struct inet_connection_sock *icsk = inet_csk(sk);

	tcp_sk(sk)->prior_ssthresh = 0;
	if (icsk->icsk_ca_ops->init)
		icsk->icsk_ca_ops->init(sk);
	if (tcp_ca_needs_ecn(sk))
		INET_ECN_xmit(sk);
	else
		INET_ECN_dontxmit(sk);
}

static void tcp_reinit_congestion_control(struct sock *sk,
					  const struct tcp_congestion_ops *ca)
{
	struct inet_connection_sock *icsk = inet_csk(sk);

	tcp_cleanup_congestion_control(sk);
	icsk->icsk_ca_ops = ca;
	icsk->icsk_ca_setsockopt = 1;
	memset(icsk->icsk_ca_priv, 0, sizeof(icsk->icsk_ca_priv));

	if (sk->sk_state != TCP_CLOSE)
		tcp_init_congestion_control(sk);
}

/* Manage refcounts on socket close. */
void tcp_cleanup_congestion_control(struct sock *sk)
{
	struct inet_connection_sock *icsk = inet_csk(sk);

	if (icsk->icsk_ca_ops->release)
		icsk->icsk_ca_ops->release(sk);
	module_put(icsk->icsk_ca_ops->owner);
}

/* Used by sysctl to change default congestion control */
int tcp_set_default_congestion_control(struct net *net, const char *name)
{
	struct tcp_congestion_ops *ca;
	const struct tcp_congestion_ops *prev;
	int ret;

	rcu_read_lock();
	ca = tcp_ca_find_autoload(net, name);
	if (!ca) {
		ret = -ENOENT;
	} else if (!try_module_get(ca->owner)) {
		ret = -EBUSY;
	} else {
		prev = xchg(&net->ipv4.tcp_congestion_control, ca);
		if (prev)
			module_put(prev->owner);

		ca->flags |= TCP_CONG_NON_RESTRICTED;
		ret = 0;
	}
	rcu_read_unlock();

	return ret;
}

/* Set default value from kernel configuration at bootup */
static int __init tcp_congestion_default(void)
{
	return tcp_set_default_congestion_control(&init_net,
						  CONFIG_DEFAULT_TCP_CONG);
}
late_initcall(tcp_congestion_default);

/* Build string with list of available congestion control values */
void tcp_get_available_congestion_control(char *buf, size_t maxlen)
{
	struct tcp_congestion_ops *ca;
	size_t offs = 0;

	rcu_read_lock();
	list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
		offs += snprintf(buf + offs, maxlen - offs,
				 "%s%s",
				 offs == 0 ? "" : " ", ca->name);
	}
	rcu_read_unlock();
}

/* Get current default congestion control */
void tcp_get_default_congestion_control(struct net *net, char *name)
{
	const struct tcp_congestion_ops *ca;

	rcu_read_lock();
	ca = rcu_dereference(net->ipv4.tcp_congestion_control);
	strncpy(name, ca->name, TCP_CA_NAME_MAX);
	rcu_read_unlock();
}

/* Built list of non-restricted congestion control values */
void tcp_get_allowed_congestion_control(char *buf, size_t maxlen)
{
	struct tcp_congestion_ops *ca;
	size_t offs = 0;

	*buf = '\0';
	rcu_read_lock();
	list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
		if (!(ca->flags & TCP_CONG_NON_RESTRICTED))
			continue;
		offs += snprintf(buf + offs, maxlen - offs,
				 "%s%s",
				 offs == 0 ? "" : " ", ca->name);
	}
	rcu_read_unlock();
}

/* Change list of non-restricted congestion control */
int tcp_set_allowed_congestion_control(char *val)
{
	struct tcp_congestion_ops *ca;
	char *saved_clone, *clone, *name;
	int ret = 0;

	saved_clone = clone = kstrdup(val, GFP_USER);
	if (!clone)
		return -ENOMEM;

	spin_lock(&tcp_cong_list_lock);
	/* pass 1 check for bad entries */
	while ((name = strsep(&clone, " ")) && *name) {
		ca = tcp_ca_find(name);
		if (!ca) {
			ret = -ENOENT;
			goto out;
		}
	}

	/* pass 2 clear old values */
	list_for_each_entry_rcu(ca, &tcp_cong_list, list)
		ca->flags &= ~TCP_CONG_NON_RESTRICTED;

	/* pass 3 mark as allowed */
	while ((name = strsep(&val, " ")) && *name) {
		ca = tcp_ca_find(name);
		WARN_ON(!ca);
		if (ca)
			ca->flags |= TCP_CONG_NON_RESTRICTED;
	}
out:
	spin_unlock(&tcp_cong_list_lock);
	kfree(saved_clone);

	return ret;
}

/* Change congestion control for socket. If load is false, then it is the
 * responsibility of the caller to call tcp_init_congestion_control or
 * tcp_reinit_congestion_control (if the current congestion control was
 * already initialized.
 */
int tcp_set_congestion_control(struct sock *sk, const char *name, bool load,
			       bool reinit, bool cap_net_admin)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	const struct tcp_congestion_ops *ca;
	int err = 0;

	if (icsk->icsk_ca_dst_locked)
		return -EPERM;

	rcu_read_lock();
	if (!load)
		ca = tcp_ca_find(name);
	else
		ca = tcp_ca_find_autoload(sock_net(sk), name);

	/* No change asking for existing value */
	if (ca == icsk->icsk_ca_ops) {
		icsk->icsk_ca_setsockopt = 1;
		goto out;
	}

	if (!ca) {
		err = -ENOENT;
	} else if (!load) {
		const struct tcp_congestion_ops *old_ca = icsk->icsk_ca_ops;

		if (try_module_get(ca->owner)) {
			if (reinit) {
				tcp_reinit_congestion_control(sk, ca);
			} else {
				icsk->icsk_ca_ops = ca;
				module_put(old_ca->owner);
			}
		} else {
			err = -EBUSY;
		}
	} else if (!((ca->flags & TCP_CONG_NON_RESTRICTED) || cap_net_admin)) {
		err = -EPERM;
	} else if (!try_module_get(ca->owner)) {
		err = -EBUSY;
	} else {
		tcp_reinit_congestion_control(sk, ca);
	}
 out:
	rcu_read_unlock();
	return err;
}

/* Slow start is used when congestion window is no greater than the slow start
 * threshold. We base on RFC2581 and also handle stretch ACKs properly.
 * We do not implement RFC3465 Appropriate Byte Counting (ABC) per se but
 * something better;) a packet is only considered (s)acked in its entirety to
 * defend the ACK attacks described in the RFC. Slow start processes a stretch
 * ACK of degree N as if N acks of degree 1 are received back to back except
 * ABC caps N to 2. Slow start exits when cwnd grows over ssthresh and
 * returns the leftover acks to adjust cwnd in congestion avoidance mode.
 */
u32 tcp_slow_start(struct tcp_sock *tp, u32 acked)
{
	u32 cwnd = min(tp->snd_cwnd + acked, tp->snd_ssthresh);

	acked -= cwnd - tp->snd_cwnd;
	tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);

	return acked;
}
EXPORT_SYMBOL_GPL(tcp_slow_start);

/* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w),
 * for every packet that was ACKed.
 */
void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked)
{
	/* If credits accumulated at a higher w, apply them gently now. */
	if (tp->snd_cwnd_cnt >= w) {
		tp->snd_cwnd_cnt = 0;
		tp->snd_cwnd++;
	}

	tp->snd_cwnd_cnt += acked;
	if (tp->snd_cwnd_cnt >= w) {
		u32 delta = tp->snd_cwnd_cnt / w;

		tp->snd_cwnd_cnt -= delta * w;
		tp->snd_cwnd += delta;
	}
	tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_cwnd_clamp);
}
EXPORT_SYMBOL_GPL(tcp_cong_avoid_ai);

/*
 * TCP Reno congestion control
 * This is special case used for fallback as well.
 */
/* This is Jacobson's slow start and congestion avoidance.
 * SIGCOMM '88, p. 328.
 */
void tcp_reno_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
	struct tcp_sock *tp = tcp_sk(sk);

	if (!tcp_is_cwnd_limited(sk))
		return;

	/* In "safe" area, increase. */
	if (tcp_in_slow_start(tp)) {
		acked = tcp_slow_start(tp, acked);
		if (!acked)
			return;
	}
	/* In dangerous area, increase slowly. */
	tcp_cong_avoid_ai(tp, tp->snd_cwnd, acked);
}
EXPORT_SYMBOL_GPL(tcp_reno_cong_avoid);

/* Slow start threshold is half the congestion window (min 2) */
u32 tcp_reno_ssthresh(struct sock *sk)
{
	const struct tcp_sock *tp = tcp_sk(sk);

	return max(tp->snd_cwnd >> 1U, 2U);
}
EXPORT_SYMBOL_GPL(tcp_reno_ssthresh);

u32 tcp_reno_undo_cwnd(struct sock *sk)
{
	const struct tcp_sock *tp = tcp_sk(sk);

	return max(tp->snd_cwnd, tp->prior_cwnd);
}
EXPORT_SYMBOL_GPL(tcp_reno_undo_cwnd);

struct tcp_congestion_ops tcp_reno = {
	.flags		= TCP_CONG_NON_RESTRICTED,
	.name		= "reno",
	.owner		= THIS_MODULE,
	.ssthresh	= tcp_reno_ssthresh,
	.cong_avoid	= tcp_reno_cong_avoid,
	.undo_cwnd	= tcp_reno_undo_cwnd,
};