Linux Kernel TCP Congestion Control Algorithms – tcp_cong.c – APIs and data-structures

Refer:
Linux Kernel main TCP congestion control support implementation /net/ipv4/tcp_cong.c ↗

APIs:
tcp_register_congestion_control(), tcp_unregister_congestion_control() ↗
tcp_ca_find() ↗
tcp_set_congestion_control() ↗
tcp_init_congestion_control(), tcp_reinit_congestion_control() ↗
tcp_get_allowed_congestion_control(), tcp_set_allowed_congestion_control() ↗
tcp_set_default_congestion_control() ↗
tcp_get_available_congestion_control() ↗

Data-structures:
struct tcp_congestion_ops data-structure ↗
static LIST_HEAD(tcp_cong_list); – Linked List ↗

Here is the LIST_HEAD(tcp_cong_list) linked list, tcp_ca_find() API (/net/ipv4/tcp_cong.c) from the Kernel-source version 4.14 for quick reference:

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;
}

Here is the tcp_init_congestion_control(), tcp_reinit_congestion_control() APIs (/net/ipv4/tcp_cong.c) from the Kernel-source version 4.14 for quick reference:

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);
}

Here is the tcp_set_default_congestion_control(), tcp_get_available_congestion_control(), tcp_set_allowed_congestion_control() APIs (/net/ipv4/tcp_cong.c) from the Kernel-source version 4.14 for quick reference:

/* Used by sysctl to change default congestion control */
int tcp_set_default_congestion_control(const char *name)
{
	struct tcp_congestion_ops *ca;
	int ret = -ENOENT;

	spin_lock(&tcp_cong_list_lock);
	ca = tcp_ca_find(name);
#ifdef CONFIG_MODULES
	if (!ca && capable(CAP_NET_ADMIN)) {
		spin_unlock(&tcp_cong_list_lock);

		request_module("tcp_%s", name);
		spin_lock(&tcp_cong_list_lock);
		ca = tcp_ca_find(name);
	}
#endif

	if (ca) {
		ca->flags |= TCP_CONG_NON_RESTRICTED;	/* default is always allowed */
		list_move(&ca->list, &tcp_cong_list);
		ret = 0;
	}
	spin_unlock(&tcp_cong_list_lock);

	return ret;
}

/* Set default value from kernel configuration at bootup */
static int __init tcp_congestion_default(void)
{
	return tcp_set_default_congestion_control(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(char *name)
{
	struct tcp_congestion_ops *ca;
	/* We will always have reno... */
	BUG_ON(list_empty(&tcp_cong_list));

	rcu_read_lock();
	ca = list_entry(tcp_cong_list.next, struct tcp_congestion_ops, list);
	strncpy(name, ca->name, TCP_CA_NAME_MAX);
	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;
}