red.h

Go to the documentation of this file.

#ifndef __NET_SCHED_RED_H
#define __NET_SCHED_RED_H

#include <linux/types.h>
#include <linux/bug.h>
#include <net/pkt_sched.h>
#include <net/inet_ecn.h>
#include <net/dsfield.h>
#include <linux/reciprocal_div.h>

/*  Random Early Detection (RED) algorithm.
    =======================================

    Source: Sally Floyd and Van Jacobson, "Random Early Detection Gateways
    for Congestion Avoidance", 1993, IEEE/ACM Transactions on Networking.

    This file codes a "divisionless" version of RED algorithm
    as written down in Fig.17 of the paper.

    Short description.
    ------------------

    When a new packet arrives we calculate the average queue length:

    avg = (1-W)*avg + W*current_queue_len,

    W is the filter time constant (chosen as 2^(-Wlog)), it controls
    the inertia of the algorithm. To allow larger bursts, W should be
    decreased.

    if (avg > th_max) -> packet marked (dropped).
    if (avg < th_min) -> packet passes.
    if (th_min < avg < th_max) we calculate probability:

    Pb = max_P * (avg - th_min)/(th_max-th_min)

    and mark (drop) packet with this probability.
    Pb changes from 0 (at avg==th_min) to max_P (avg==th_max).
    max_P should be small (not 1), usually 0.01..0.02 is good value.

    max_P is chosen as a number, so that max_P/(th_max-th_min)
    is a negative power of two in order arithmetics to contain
    only shifts.


    Parameters, settable by user:
    -----------------------------

    qth_min     - bytes (should be < qth_max/2)
    qth_max     - bytes (should be at least 2*qth_min and less limit)
    Wlog            - bits (<32) log(1/W).
    Plog            - bits (<32)

    Plog is related to max_P by formula:

    max_P = (qth_max-qth_min)/2^Plog;

    F.e. if qth_max=128K and qth_min=32K, then Plog=22
    corresponds to max_P=0.02

    Scell_log
    Stab

    Lookup table for log((1-W)^(t/t_ave).


    NOTES:

    Upper bound on W.
    -----------------

    If you want to allow bursts of L packets of size S,
    you should choose W:

    L + 1 - th_min/S < (1-(1-W)^L)/W

    th_min/S = 32         th_min/S = 4

    log(W)  L
    -1  33
    -2  35
    -3  39
    -4  46
    -5  57
    -6  75
    -7  101
    -8  135
    -9  190
    etc.
 */

/*
 * Adaptative RED : An Algorithm for Increasing the Robustness of RED's AQM
 * (Sally FLoyd, Ramakrishna Gummadi, and Scott Shenker) August 2001
 *
 * Every 500 ms:
 *  if (avg > target and max_p <= 0.5)
 *   increase max_p : max_p += alpha;
 *  else if (avg < target and max_p >= 0.01)
 *   decrease max_p : max_p *= beta;
 *
 * target :[qth_min + 0.4*(qth_min - qth_max),
 *          qth_min + 0.6*(qth_min - qth_max)].
 * alpha : min(0.01, max_p / 4)
 * beta : 0.9
 * max_P is a Q0.32 fixed point number (with 32 bits mantissa)
 * max_P between 0.01 and 0.5 (1% - 50%) [ Its no longer a negative power of two ]
 */
#define RED_ONE_PERCENT ((u32)DIV_ROUND_CLOSEST(1ULL<<32, 100))

#define MAX_P_MIN (1 * RED_ONE_PERCENT)
#define MAX_P_MAX (50 * RED_ONE_PERCENT)
#define MAX_P_ALPHA(val) min(MAX_P_MIN, val / 4)

#define RED_STAB_SIZE   256
#define RED_STAB_MASK   (RED_STAB_SIZE - 1)

struct red_stats {
    u32     prob_drop;  /* Early probability drops */
    u32     prob_mark;  /* Early probability marks */
    u32     forced_drop;    /* Forced drops, qavg > max_thresh */
    u32     forced_mark;    /* Forced marks, qavg > max_thresh */
    u32     pdrop;          /* Drops due to queue limits */
    u32     other;          /* Drops due to drop() calls */
};

struct red_parms {
    /* Parameters */
    u32     qth_min;    /* Min avg length threshold: Wlog scaled */
    u32     qth_max;    /* Max avg length threshold: Wlog scaled */
    u32     Scell_max;
    u32     max_P;      /* probability, [0 .. 1.0] 32 scaled */
    u32     max_P_reciprocal; /* reciprocal_value(max_P / qth_delta) */
    u32     qth_delta;  /* max_th - min_th */
    u32     target_min; /* min_th + 0.4*(max_th - min_th) */
    u32     target_max; /* min_th + 0.6*(max_th - min_th) */
    u8      Scell_log;
    u8      Wlog;       /* log(W)       */
    u8      Plog;       /* random number bits   */
    u8      Stab[RED_STAB_SIZE];
};

struct red_vars {
    /* Variables */
    int     qcount;     /* Number of packets since last random
                       number generation */
    u32     qR;     /* Cached random number */

    unsigned long   qavg;       /* Average queue length: Wlog scaled */
    ktime_t     qidlestart; /* Start of current idle period */
};

static inline u32 red_maxp(u8 Plog)
{
    return Plog < 32 ? (~0U >> Plog) : ~0U;
}

static inline void red_set_vars(struct red_vars *v)
{
    /* Reset average queue length, the value is strictly bound
     * to the parameters below, reseting hurts a bit but leaving
     * it might result in an unreasonable qavg for a while. --TGR
     */
    v->qavg     = 0;

    v->qcount   = -1;
}

static inline void red_set_parms(struct red_parms *p,
                 u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
                 u8 Scell_log, u8 *stab, u32 max_P)
{
    int delta = qth_max - qth_min;
    u32 max_p_delta;

    p->qth_min  = qth_min << Wlog;
    p->qth_max  = qth_max << Wlog;
    p->Wlog     = Wlog;
    p->Plog     = Plog;
    if (delta < 0)
        delta = 1;
    p->qth_delta    = delta;
    if (!max_P) {
        max_P = red_maxp(Plog);
        max_P *= delta; /* max_P = (qth_max - qth_min)/2^Plog */
    }
    p->max_P = max_P;
    max_p_delta = max_P / delta;
    max_p_delta = max(max_p_delta, 1U);
    p->max_P_reciprocal  = reciprocal_value(max_p_delta);

    /* RED Adaptative target :
     * [min_th + 0.4*(min_th - max_th),
     *  min_th + 0.6*(min_th - max_th)].
     */
    delta /= 5;
    p->target_min = qth_min + 2*delta;
    p->target_max = qth_min + 3*delta;

    p->Scell_log    = Scell_log;
    p->Scell_max    = (255 << Scell_log);

    if (stab)
        memcpy(p->Stab, stab, sizeof(p->Stab));
}

static inline int red_is_idling(const struct red_vars *v)
{
    return v->qidlestart.tv64 != 0;
}

static inline void red_start_of_idle_period(struct red_vars *v)
{
    v->qidlestart = ktime_get();
}

static inline void red_end_of_idle_period(struct red_vars *v)
{
    v->qidlestart.tv64 = 0;
}

static inline void red_restart(struct red_vars *v)
{
    red_end_of_idle_period(v);
    v->qavg = 0;
    v->qcount = -1;
}

static inline unsigned long red_calc_qavg_from_idle_time(const struct red_parms *p,
                             const struct red_vars *v)
{
    s64 delta = ktime_us_delta(ktime_get(), v->qidlestart);
    long us_idle = min_t(s64, delta, p->Scell_max);
    int  shift;

    /*
     * The problem: ideally, average length queue recalcultion should
     * be done over constant clock intervals. This is too expensive, so
     * that the calculation is driven by outgoing packets.
     * When the queue is idle we have to model this clock by hand.
     *
     * SF+VJ proposed to "generate":
     *
     *  m = idletime / (average_pkt_size / bandwidth)
     *
     * dummy packets as a burst after idle time, i.e.
     *
     *  v->qavg *= (1-W)^m
     *
     * This is an apparently overcomplicated solution (f.e. we have to
     * precompute a table to make this calculation in reasonable time)
     * I believe that a simpler model may be used here,
     * but it is field for experiments.
     */

    shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK];

    if (shift)
        return v->qavg >> shift;
    else {
        /* Approximate initial part of exponent with linear function:
         *
         *  (1-W)^m ~= 1-mW + ...
         *
         * Seems, it is the best solution to
         * problem of too coarse exponent tabulation.
         */
        us_idle = (v->qavg * (u64)us_idle) >> p->Scell_log;

        if (us_idle < (v->qavg >> 1))
            return v->qavg - us_idle;
        else
            return v->qavg >> 1;
    }
}

static inline unsigned long red_calc_qavg_no_idle_time(const struct red_parms *p,
                               const struct red_vars *v,
                               unsigned int backlog)
{
    /*
     * NOTE: v->qavg is fixed point number with point at Wlog.
     * The formula below is equvalent to floating point
     * version:
     *
     *  qavg = qavg*(1-W) + backlog*W;
     *
     * --ANK (980924)
     */
    return v->qavg + (backlog - (v->qavg >> p->Wlog));
}

static inline unsigned long red_calc_qavg(const struct red_parms *p,
                      const struct red_vars *v,
                      unsigned int backlog)
{
    if (!red_is_idling(v))
        return red_calc_qavg_no_idle_time(p, v, backlog);
    else
        return red_calc_qavg_from_idle_time(p, v);
}


static inline u32 red_random(const struct red_parms *p)
{
    return reciprocal_divide(net_random(), p->max_P_reciprocal);
}

static inline int red_mark_probability(const struct red_parms *p,
                       const struct red_vars *v,
                       unsigned long qavg)
{
    /* The formula used below causes questions.

       OK. qR is random number in the interval
        (0..1/max_P)*(qth_max-qth_min)
       i.e. 0..(2^Plog). If we used floating point
       arithmetics, it would be: (2^Plog)*rnd_num,
       where rnd_num is less 1.

       Taking into account, that qavg have fixed
       point at Wlog, two lines
       below have the following floating point equivalent:

       max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount

       Any questions? --ANK (980924)
     */
    return !(((qavg - p->qth_min) >> p->Wlog) * v->qcount < v->qR);
}

enum {
    RED_BELOW_MIN_THRESH,
    RED_BETWEEN_TRESH,
    RED_ABOVE_MAX_TRESH,
};

static inline int red_cmp_thresh(const struct red_parms *p, unsigned long qavg)
{
    if (qavg < p->qth_min)
        return RED_BELOW_MIN_THRESH;
    else if (qavg >= p->qth_max)
        return RED_ABOVE_MAX_TRESH;
    else
        return RED_BETWEEN_TRESH;
}

enum {
    RED_DONT_MARK,
    RED_PROB_MARK,
    RED_HARD_MARK,
};

static inline int red_action(const struct red_parms *p,
                 struct red_vars *v,
                 unsigned long qavg)
{
    switch (red_cmp_thresh(p, qavg)) {
        case RED_BELOW_MIN_THRESH:
            v->qcount = -1;
            return RED_DONT_MARK;

        case RED_BETWEEN_TRESH:
            if (++v->qcount) {
                if (red_mark_probability(p, v, qavg)) {
                    v->qcount = 0;
                    v->qR = red_random(p);
                    return RED_PROB_MARK;
                }
            } else
                v->qR = red_random(p);

            return RED_DONT_MARK;

        case RED_ABOVE_MAX_TRESH:
            v->qcount = -1;
            return RED_HARD_MARK;
    }

    BUG();
    return RED_DONT_MARK;
}

static inline void red_adaptative_algo(struct red_parms *p, struct red_vars *v)
{
    unsigned long qavg;
    u32 max_p_delta;

    qavg = v->qavg;
    if (red_is_idling(v))
        qavg = red_calc_qavg_from_idle_time(p, v);

    /* v->qavg is fixed point number with point at Wlog */
    qavg >>= p->Wlog;

    if (qavg > p->target_max && p->max_P <= MAX_P_MAX)
        p->max_P += MAX_P_ALPHA(p->max_P); /* maxp = maxp + alpha */
    else if (qavg < p->target_min && p->max_P >= MAX_P_MIN)
        p->max_P = (p->max_P/10)*9; /* maxp = maxp * Beta */

    max_p_delta = DIV_ROUND_CLOSEST(p->max_P, p->qth_delta);
    max_p_delta = max(max_p_delta, 1U);
    p->max_P_reciprocal = reciprocal_value(max_p_delta);
}
#endif