tcp_cong.c

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/*
 * Plugable TCP congestion control support and newReno
 * congestion control.
 * Based on ideas from I/O scheduler suport and Web100.
 *
 * Copyright (C) 2005 Stephen Hemminger <[email protected]>
 */

#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 <net/tcp.h>

int sysctl_tcp_max_ssthresh = 0;

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;
}

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

    spin_lock(&tcp_cong_list_lock);
    if (tcp_ca_find(ca->name)) {
        pr_notice("%s already registered\n", ca->name);
        ret = -EEXIST;
    } else {
        list_add_tail_rcu(&ca->list, &tcp_cong_list);
        pr_info("%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);
}
EXPORT_SYMBOL_GPL(tcp_unregister_congestion_control);

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

    /* if no choice made yet assign the current value set as default */
    if (icsk->icsk_ca_ops == &tcp_init_congestion_ops) {
        rcu_read_lock();
        list_for_each_entry_rcu(ca, &tcp_cong_list, list) {
            if (try_module_get(ca->owner)) {
                icsk->icsk_ca_ops = ca;
                break;
            }

            /* fallback to next available */
        }
        rcu_read_unlock();
    }

    if (icsk->icsk_ca_ops->init)
        icsk->icsk_ca_ops->init(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(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();
}

/* 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 */
int tcp_set_congestion_control(struct sock *sk, const char *name)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct tcp_congestion_ops *ca;
    int err = 0;

    rcu_read_lock();
    ca = tcp_ca_find(name);

    /* no change asking for existing value */
    if (ca == icsk->icsk_ca_ops)
        goto out;

#ifdef CONFIG_MODULES
    /* not found attempt to autoload module */
    if (!ca && capable(CAP_NET_ADMIN)) {
        rcu_read_unlock();
        request_module("tcp_%s", name);
        rcu_read_lock();
        ca = tcp_ca_find(name);
    }
#endif
    if (!ca)
        err = -ENOENT;

    else if (!((ca->flags & TCP_CONG_NON_RESTRICTED) || capable(CAP_NET_ADMIN)))
        err = -EPERM;

    else if (!try_module_get(ca->owner))
        err = -EBUSY;

    else {
        tcp_cleanup_congestion_control(sk);
        icsk->icsk_ca_ops = ca;

        if (sk->sk_state != TCP_CLOSE && icsk->icsk_ca_ops->init)
            icsk->icsk_ca_ops->init(sk);
    }
 out:
    rcu_read_unlock();
    return err;
}

/* RFC2861 Check whether we are limited by application or congestion window
 * This is the inverse of cwnd check in tcp_tso_should_defer
 */
bool tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    u32 left;

    if (in_flight >= tp->snd_cwnd)
        return true;

    left = tp->snd_cwnd - in_flight;
    if (sk_can_gso(sk) &&
        left * sysctl_tcp_tso_win_divisor < tp->snd_cwnd &&
        left * tp->mss_cache < sk->sk_gso_max_size &&
        left < sk->sk_gso_max_segs)
        return true;
    return left <= tcp_max_tso_deferred_mss(tp);
}
EXPORT_SYMBOL_GPL(tcp_is_cwnd_limited);

/*
 * Slow start is used when congestion window is less than slow start
 * threshold. This version implements the basic RFC2581 version
 * and optionally supports:
 *  RFC3742 Limited Slow Start        - growth limited to max_ssthresh
 *  RFC3465 Appropriate Byte Counting - growth limited by bytes acknowledged
 */
void tcp_slow_start(struct tcp_sock *tp)
{
    int cnt; /* increase in packets */
    unsigned int delta = 0;

    /* RFC3465: ABC Slow start
     * Increase only after a full MSS of bytes is acked
     *
     * TCP sender SHOULD increase cwnd by the number of
     * previously unacknowledged bytes ACKed by each incoming
     * acknowledgment, provided the increase is not more than L
     */
    if (sysctl_tcp_abc && tp->bytes_acked < tp->mss_cache)
        return;

    if (sysctl_tcp_max_ssthresh > 0 && tp->snd_cwnd > sysctl_tcp_max_ssthresh)
        cnt = sysctl_tcp_max_ssthresh >> 1; /* limited slow start */
    else
        cnt = tp->snd_cwnd;         /* exponential increase */

    /* RFC3465: ABC
     * We MAY increase by 2 if discovered delayed ack
     */
    if (sysctl_tcp_abc > 1 && tp->bytes_acked >= 2*tp->mss_cache)
        cnt <<= 1;
    tp->bytes_acked = 0;

    tp->snd_cwnd_cnt += cnt;
    while (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
        tp->snd_cwnd_cnt -= tp->snd_cwnd;
        delta++;
    }
    tp->snd_cwnd = min(tp->snd_cwnd + delta, tp->snd_cwnd_clamp);
}
EXPORT_SYMBOL_GPL(tcp_slow_start);

/* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w) */
void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w)
{
    if (tp->snd_cwnd_cnt >= w) {
        if (tp->snd_cwnd < tp->snd_cwnd_clamp)
            tp->snd_cwnd++;
        tp->snd_cwnd_cnt = 0;
    } else {
        tp->snd_cwnd_cnt++;
    }
}
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 in_flight)
{
    struct tcp_sock *tp = tcp_sk(sk);

    if (!tcp_is_cwnd_limited(sk, in_flight))
        return;

    /* In "safe" area, increase. */
    if (tp->snd_cwnd <= tp->snd_ssthresh)
        tcp_slow_start(tp);

    /* In dangerous area, increase slowly. */
    else if (sysctl_tcp_abc) {
        /* RFC3465: Appropriate Byte Count
         * increase once for each full cwnd acked
         */
        if (tp->bytes_acked >= tp->snd_cwnd*tp->mss_cache) {
            tp->bytes_acked -= tp->snd_cwnd*tp->mss_cache;
            if (tp->snd_cwnd < tp->snd_cwnd_clamp)
                tp->snd_cwnd++;
        }
    } else {
        tcp_cong_avoid_ai(tp, tp->snd_cwnd);
    }
}
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);

/* Lower bound on congestion window with halving. */
u32 tcp_reno_min_cwnd(const struct sock *sk)
{
    const struct tcp_sock *tp = tcp_sk(sk);
    return tp->snd_ssthresh/2;
}
EXPORT_SYMBOL_GPL(tcp_reno_min_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,
    .min_cwnd   = tcp_reno_min_cwnd,
};

/* Initial congestion control used (until SYN)
 * really reno under another name so we can tell difference
 * during tcp_set_default_congestion_control
 */
struct tcp_congestion_ops tcp_init_congestion_ops  = {
    .name       = "",
    .owner      = THIS_MODULE,
    .ssthresh   = tcp_reno_ssthresh,
    .cong_avoid = tcp_reno_cong_avoid,
    .min_cwnd   = tcp_reno_min_cwnd,
};
EXPORT_SYMBOL_GPL(tcp_init_congestion_ops);