codel.h

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#ifndef __NET_SCHED_CODEL_H
#define __NET_SCHED_CODEL_H

/*
 * Codel - The Controlled-Delay Active Queue Management algorithm
 *
 *  Copyright (C) 2011-2012 Kathleen Nichols <[email protected]>
 *  Copyright (C) 2011-2012 Van Jacobson <[email protected]>
 *  Copyright (C) 2012 Michael D. Taht <[email protected]>
 *  Copyright (C) 2012 Eric Dumazet <[email protected]>
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions, and the following disclaimer,
 *    without modification.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The names of the authors may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * Alternatively, provided that this notice is retained in full, this
 * software may be distributed under the terms of the GNU General
 * Public License ("GPL") version 2, in which case the provisions of the
 * GPL apply INSTEAD OF those given above.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
 * DAMAGE.
 *
 */

#include <linux/types.h>
#include <linux/ktime.h>
#include <linux/skbuff.h>
#include <net/pkt_sched.h>
#include <net/inet_ecn.h>
#include <linux/reciprocal_div.h>

/* Controlling Queue Delay (CoDel) algorithm
 * =========================================
 * Source : Kathleen Nichols and Van Jacobson
 * http://queue.acm.org/detail.cfm?id=2209336
 *
 * Implemented on linux by Dave Taht and Eric Dumazet
 */


/* CoDel uses a 1024 nsec clock, encoded in u32
 * This gives a range of 2199 seconds, because of signed compares
 */
typedef u32 codel_time_t;
typedef s32 codel_tdiff_t;
#define CODEL_SHIFT 10
#define MS2TIME(a) ((a * NSEC_PER_MSEC) >> CODEL_SHIFT)

static inline codel_time_t codel_get_time(void)
{
    u64 ns = ktime_to_ns(ktime_get());

    return ns >> CODEL_SHIFT;
}

#define codel_time_after(a, b)      ((s32)(a) - (s32)(b) > 0)
#define codel_time_after_eq(a, b)   ((s32)(a) - (s32)(b) >= 0)
#define codel_time_before(a, b)     ((s32)(a) - (s32)(b) < 0)
#define codel_time_before_eq(a, b)  ((s32)(a) - (s32)(b) <= 0)

/* Qdiscs using codel plugin must use codel_skb_cb in their own cb[] */
struct codel_skb_cb {
    codel_time_t enqueue_time;
};

static struct codel_skb_cb *get_codel_cb(const struct sk_buff *skb)
{
    qdisc_cb_private_validate(skb, sizeof(struct codel_skb_cb));
    return (struct codel_skb_cb *)qdisc_skb_cb(skb)->data;
}

static codel_time_t codel_get_enqueue_time(const struct sk_buff *skb)
{
    return get_codel_cb(skb)->enqueue_time;
}

static void codel_set_enqueue_time(struct sk_buff *skb)
{
    get_codel_cb(skb)->enqueue_time = codel_get_time();
}

static inline u32 codel_time_to_us(codel_time_t val)
{
    u64 valns = ((u64)val << CODEL_SHIFT);

    do_div(valns, NSEC_PER_USEC);
    return (u32)valns;
}

struct codel_params {
    codel_time_t    target;
    codel_time_t    interval;
    bool        ecn;
};

struct codel_vars {
    u32     count;
    u32     lastcount;
    bool        dropping;
    u16     rec_inv_sqrt;
    codel_time_t    first_above_time;
    codel_time_t    drop_next;
    codel_time_t    ldelay;
};

#define REC_INV_SQRT_BITS (8 * sizeof(u16)) /* or sizeof_in_bits(rec_inv_sqrt) */
/* needed shift to get a Q0.32 number from rec_inv_sqrt */
#define REC_INV_SQRT_SHIFT (32 - REC_INV_SQRT_BITS)

struct codel_stats {
    u32     maxpacket;
    u32     drop_count;
    u32     ecn_mark;
};

static void codel_params_init(struct codel_params *params)
{
    params->interval = MS2TIME(100);
    params->target = MS2TIME(5);
    params->ecn = false;
}

static void codel_vars_init(struct codel_vars *vars)
{
    memset(vars, 0, sizeof(*vars));
}

static void codel_stats_init(struct codel_stats *stats)
{
    stats->maxpacket = 256;
}

/*
 * http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
 * new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
 *
 * Here, invsqrt is a fixed point number (< 1.0), 32bit mantissa, aka Q0.32
 */
static void codel_Newton_step(struct codel_vars *vars)
{
    u32 invsqrt = ((u32)vars->rec_inv_sqrt) << REC_INV_SQRT_SHIFT;
    u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 32;
    u64 val = (3LL << 32) - ((u64)vars->count * invsqrt2);

    val >>= 2; /* avoid overflow in following multiply */
    val = (val * invsqrt) >> (32 - 2 + 1);

    vars->rec_inv_sqrt = val >> REC_INV_SQRT_SHIFT;
}

/*
 * CoDel control_law is t + interval/sqrt(count)
 * We maintain in rec_inv_sqrt the reciprocal value of sqrt(count) to avoid
 * both sqrt() and divide operation.
 */
static codel_time_t codel_control_law(codel_time_t t,
                      codel_time_t interval,
                      u32 rec_inv_sqrt)
{
    return t + reciprocal_divide(interval, rec_inv_sqrt << REC_INV_SQRT_SHIFT);
}


static bool codel_should_drop(const struct sk_buff *skb,
                  struct Qdisc *sch,
                  struct codel_vars *vars,
                  struct codel_params *params,
                  struct codel_stats *stats,
                  codel_time_t now)
{
    bool ok_to_drop;

    if (!skb) {
        vars->first_above_time = 0;
        return false;
    }

    vars->ldelay = now - codel_get_enqueue_time(skb);
    sch->qstats.backlog -= qdisc_pkt_len(skb);

    if (unlikely(qdisc_pkt_len(skb) > stats->maxpacket))
        stats->maxpacket = qdisc_pkt_len(skb);

    if (codel_time_before(vars->ldelay, params->target) ||
        sch->qstats.backlog <= stats->maxpacket) {
        /* went below - stay below for at least interval */
        vars->first_above_time = 0;
        return false;
    }
    ok_to_drop = false;
    if (vars->first_above_time == 0) {
        /* just went above from below. If we stay above
         * for at least interval we'll say it's ok to drop
         */
        vars->first_above_time = now + params->interval;
    } else if (codel_time_after(now, vars->first_above_time)) {
        ok_to_drop = true;
    }
    return ok_to_drop;
}

typedef struct sk_buff * (*codel_skb_dequeue_t)(struct codel_vars *vars,
                        struct Qdisc *sch);

static struct sk_buff *codel_dequeue(struct Qdisc *sch,
                     struct codel_params *params,
                     struct codel_vars *vars,
                     struct codel_stats *stats,
                     codel_skb_dequeue_t dequeue_func)
{
    struct sk_buff *skb = dequeue_func(vars, sch);
    codel_time_t now;
    bool drop;

    if (!skb) {
        vars->dropping = false;
        return skb;
    }
    now = codel_get_time();
    drop = codel_should_drop(skb, sch, vars, params, stats, now);
    if (vars->dropping) {
        if (!drop) {
            /* sojourn time below target - leave dropping state */
            vars->dropping = false;
        } else if (codel_time_after_eq(now, vars->drop_next)) {
            /* It's time for the next drop. Drop the current
             * packet and dequeue the next. The dequeue might
             * take us out of dropping state.
             * If not, schedule the next drop.
             * A large backlog might result in drop rates so high
             * that the next drop should happen now,
             * hence the while loop.
             */
            while (vars->dropping &&
                   codel_time_after_eq(now, vars->drop_next)) {
                vars->count++; /* dont care of possible wrap
                        * since there is no more divide
                        */
                codel_Newton_step(vars);
                if (params->ecn && INET_ECN_set_ce(skb)) {
                    stats->ecn_mark++;
                    vars->drop_next =
                        codel_control_law(vars->drop_next,
                                  params->interval,
                                  vars->rec_inv_sqrt);
                    goto end;
                }
                qdisc_drop(skb, sch);
                stats->drop_count++;
                skb = dequeue_func(vars, sch);
                if (!codel_should_drop(skb, sch,
                               vars, params, stats, now)) {
                    /* leave dropping state */
                    vars->dropping = false;
                } else {
                    /* and schedule the next drop */
                    vars->drop_next =
                        codel_control_law(vars->drop_next,
                                  params->interval,
                                  vars->rec_inv_sqrt);
                }
            }
        }
    } else if (drop) {
        u32 delta;

        if (params->ecn && INET_ECN_set_ce(skb)) {
            stats->ecn_mark++;
        } else {
            qdisc_drop(skb, sch);
            stats->drop_count++;

            skb = dequeue_func(vars, sch);
            drop = codel_should_drop(skb, sch, vars, params,
                         stats, now);
        }
        vars->dropping = true;
        /* if min went above target close to when we last went below it
         * assume that the drop rate that controlled the queue on the
         * last cycle is a good starting point to control it now.
         */
        delta = vars->count - vars->lastcount;
        if (delta > 1 &&
            codel_time_before(now - vars->drop_next,
                      16 * params->interval)) {
            vars->count = delta;
            /* we dont care if rec_inv_sqrt approximation
             * is not very precise :
             * Next Newton steps will correct it quadratically.
             */
            codel_Newton_step(vars);
        } else {
            vars->count = 1;
            vars->rec_inv_sqrt = ~0U >> REC_INV_SQRT_SHIFT;
        }
        vars->lastcount = vars->count;
        vars->drop_next = codel_control_law(now, params->interval,
                            vars->rec_inv_sqrt);
    }
end:
    return skb;
}
#endif