linux/include/net/red.h
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   1#ifndef __NET_SCHED_RED_H
   2#define __NET_SCHED_RED_H
   3
   4#include <linux/types.h>
   5#include <linux/bug.h>
   6#include <net/pkt_sched.h>
   7#include <net/inet_ecn.h>
   8#include <net/dsfield.h>
   9#include <linux/reciprocal_div.h>
  10
  11/*      Random Early Detection (RED) algorithm.
  12        =======================================
  13
  14        Source: Sally Floyd and Van Jacobson, "Random Early Detection Gateways
  15        for Congestion Avoidance", 1993, IEEE/ACM Transactions on Networking.
  16
  17        This file codes a "divisionless" version of RED algorithm
  18        as written down in Fig.17 of the paper.
  19
  20        Short description.
  21        ------------------
  22
  23        When a new packet arrives we calculate the average queue length:
  24
  25        avg = (1-W)*avg + W*current_queue_len,
  26
  27        W is the filter time constant (chosen as 2^(-Wlog)), it controls
  28        the inertia of the algorithm. To allow larger bursts, W should be
  29        decreased.
  30
  31        if (avg > th_max) -> packet marked (dropped).
  32        if (avg < th_min) -> packet passes.
  33        if (th_min < avg < th_max) we calculate probability:
  34
  35        Pb = max_P * (avg - th_min)/(th_max-th_min)
  36
  37        and mark (drop) packet with this probability.
  38        Pb changes from 0 (at avg==th_min) to max_P (avg==th_max).
  39        max_P should be small (not 1), usually 0.01..0.02 is good value.
  40
  41        max_P is chosen as a number, so that max_P/(th_max-th_min)
  42        is a negative power of two in order arithmetics to contain
  43        only shifts.
  44
  45
  46        Parameters, settable by user:
  47        -----------------------------
  48
  49        qth_min         - bytes (should be < qth_max/2)
  50        qth_max         - bytes (should be at least 2*qth_min and less limit)
  51        Wlog            - bits (<32) log(1/W).
  52        Plog            - bits (<32)
  53
  54        Plog is related to max_P by formula:
  55
  56        max_P = (qth_max-qth_min)/2^Plog;
  57
  58        F.e. if qth_max=128K and qth_min=32K, then Plog=22
  59        corresponds to max_P=0.02
  60
  61        Scell_log
  62        Stab
  63
  64        Lookup table for log((1-W)^(t/t_ave).
  65
  66
  67        NOTES:
  68
  69        Upper bound on W.
  70        -----------------
  71
  72        If you want to allow bursts of L packets of size S,
  73        you should choose W:
  74
  75        L + 1 - th_min/S < (1-(1-W)^L)/W
  76
  77        th_min/S = 32         th_min/S = 4
  78
  79        log(W)  L
  80        -1      33
  81        -2      35
  82        -3      39
  83        -4      46
  84        -5      57
  85        -6      75
  86        -7      101
  87        -8      135
  88        -9      190
  89        etc.
  90 */
  91
  92/*
  93 * Adaptative RED : An Algorithm for Increasing the Robustness of RED's AQM
  94 * (Sally FLoyd, Ramakrishna Gummadi, and Scott Shenker) August 2001
  95 *
  96 * Every 500 ms:
  97 *  if (avg > target and max_p <= 0.5)
  98 *   increase max_p : max_p += alpha;
  99 *  else if (avg < target and max_p >= 0.01)
 100 *   decrease max_p : max_p *= beta;
 101 *
 102 * target :[qth_min + 0.4*(qth_min - qth_max),
 103 *          qth_min + 0.6*(qth_min - qth_max)].
 104 * alpha : min(0.01, max_p / 4)
 105 * beta : 0.9
 106 * max_P is a Q0.32 fixed point number (with 32 bits mantissa)
 107 * max_P between 0.01 and 0.5 (1% - 50%) [ Its no longer a negative power of two ]
 108 */
 109#define RED_ONE_PERCENT ((u32)DIV_ROUND_CLOSEST(1ULL<<32, 100))
 110
 111#define MAX_P_MIN (1 * RED_ONE_PERCENT)
 112#define MAX_P_MAX (50 * RED_ONE_PERCENT)
 113#define MAX_P_ALPHA(val) min(MAX_P_MIN, val / 4)
 114
 115#define RED_STAB_SIZE   256
 116#define RED_STAB_MASK   (RED_STAB_SIZE - 1)
 117
 118struct red_stats {
 119        u32             prob_drop;      /* Early probability drops */
 120        u32             prob_mark;      /* Early probability marks */
 121        u32             forced_drop;    /* Forced drops, qavg > max_thresh */
 122        u32             forced_mark;    /* Forced marks, qavg > max_thresh */
 123        u32             pdrop;          /* Drops due to queue limits */
 124        u32             other;          /* Drops due to drop() calls */
 125};
 126
 127struct red_parms {
 128        /* Parameters */
 129        u32             qth_min;        /* Min avg length threshold: Wlog scaled */
 130        u32             qth_max;        /* Max avg length threshold: Wlog scaled */
 131        u32             Scell_max;
 132        u32             max_P;          /* probability, [0 .. 1.0] 32 scaled */
 133        u32             max_P_reciprocal; /* reciprocal_value(max_P / qth_delta) */
 134        u32             qth_delta;      /* max_th - min_th */
 135        u32             target_min;     /* min_th + 0.4*(max_th - min_th) */
 136        u32             target_max;     /* min_th + 0.6*(max_th - min_th) */
 137        u8              Scell_log;
 138        u8              Wlog;           /* log(W)               */
 139        u8              Plog;           /* random number bits   */
 140        u8              Stab[RED_STAB_SIZE];
 141};
 142
 143struct red_vars {
 144        /* Variables */
 145        int             qcount;         /* Number of packets since last random
 146                                           number generation */
 147        u32             qR;             /* Cached random number */
 148
 149        unsigned long   qavg;           /* Average queue length: Wlog scaled */
 150        ktime_t         qidlestart;     /* Start of current idle period */
 151};
 152
 153static inline u32 red_maxp(u8 Plog)
 154{
 155        return Plog < 32 ? (~0U >> Plog) : ~0U;
 156}
 157
 158static inline void red_set_vars(struct red_vars *v)
 159{
 160        /* Reset average queue length, the value is strictly bound
 161         * to the parameters below, reseting hurts a bit but leaving
 162         * it might result in an unreasonable qavg for a while. --TGR
 163         */
 164        v->qavg         = 0;
 165
 166        v->qcount       = -1;
 167}
 168
 169static inline void red_set_parms(struct red_parms *p,
 170                                 u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
 171                                 u8 Scell_log, u8 *stab, u32 max_P)
 172{
 173        int delta = qth_max - qth_min;
 174        u32 max_p_delta;
 175
 176        p->qth_min      = qth_min << Wlog;
 177        p->qth_max      = qth_max << Wlog;
 178        p->Wlog         = Wlog;
 179        p->Plog         = Plog;
 180        if (delta < 0)
 181                delta = 1;
 182        p->qth_delta    = delta;
 183        if (!max_P) {
 184                max_P = red_maxp(Plog);
 185                max_P *= delta; /* max_P = (qth_max - qth_min)/2^Plog */
 186        }
 187        p->max_P = max_P;
 188        max_p_delta = max_P / delta;
 189        max_p_delta = max(max_p_delta, 1U);
 190        p->max_P_reciprocal  = reciprocal_value(max_p_delta);
 191
 192        /* RED Adaptative target :
 193         * [min_th + 0.4*(min_th - max_th),
 194         *  min_th + 0.6*(min_th - max_th)].
 195         */
 196        delta /= 5;
 197        p->target_min = qth_min + 2*delta;
 198        p->target_max = qth_min + 3*delta;
 199
 200        p->Scell_log    = Scell_log;
 201        p->Scell_max    = (255 << Scell_log);
 202
 203        if (stab)
 204                memcpy(p->Stab, stab, sizeof(p->Stab));
 205}
 206
 207static inline int red_is_idling(const struct red_vars *v)
 208{
 209        return v->qidlestart.tv64 != 0;
 210}
 211
 212static inline void red_start_of_idle_period(struct red_vars *v)
 213{
 214        v->qidlestart = ktime_get();
 215}
 216
 217static inline void red_end_of_idle_period(struct red_vars *v)
 218{
 219        v->qidlestart.tv64 = 0;
 220}
 221
 222static inline void red_restart(struct red_vars *v)
 223{
 224        red_end_of_idle_period(v);
 225        v->qavg = 0;
 226        v->qcount = -1;
 227}
 228
 229static inline unsigned long red_calc_qavg_from_idle_time(const struct red_parms *p,
 230                                                         const struct red_vars *v)
 231{
 232        s64 delta = ktime_us_delta(ktime_get(), v->qidlestart);
 233        long us_idle = min_t(s64, delta, p->Scell_max);
 234        int  shift;
 235
 236        /*
 237         * The problem: ideally, average length queue recalcultion should
 238         * be done over constant clock intervals. This is too expensive, so
 239         * that the calculation is driven by outgoing packets.
 240         * When the queue is idle we have to model this clock by hand.
 241         *
 242         * SF+VJ proposed to "generate":
 243         *
 244         *      m = idletime / (average_pkt_size / bandwidth)
 245         *
 246         * dummy packets as a burst after idle time, i.e.
 247         *
 248         *      v->qavg *= (1-W)^m
 249         *
 250         * This is an apparently overcomplicated solution (f.e. we have to
 251         * precompute a table to make this calculation in reasonable time)
 252         * I believe that a simpler model may be used here,
 253         * but it is field for experiments.
 254         */
 255
 256        shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK];
 257
 258        if (shift)
 259                return v->qavg >> shift;
 260        else {
 261                /* Approximate initial part of exponent with linear function:
 262                 *
 263                 *      (1-W)^m ~= 1-mW + ...
 264                 *
 265                 * Seems, it is the best solution to
 266                 * problem of too coarse exponent tabulation.
 267                 */
 268                us_idle = (v->qavg * (u64)us_idle) >> p->Scell_log;
 269
 270                if (us_idle < (v->qavg >> 1))
 271                        return v->qavg - us_idle;
 272                else
 273                        return v->qavg >> 1;
 274        }
 275}
 276
 277static inline unsigned long red_calc_qavg_no_idle_time(const struct red_parms *p,
 278                                                       const struct red_vars *v,
 279                                                       unsigned int backlog)
 280{
 281        /*
 282         * NOTE: v->qavg is fixed point number with point at Wlog.
 283         * The formula below is equvalent to floating point
 284         * version:
 285         *
 286         *      qavg = qavg*(1-W) + backlog*W;
 287         *
 288         * --ANK (980924)
 289         */
 290        return v->qavg + (backlog - (v->qavg >> p->Wlog));
 291}
 292
 293static inline unsigned long red_calc_qavg(const struct red_parms *p,
 294                                          const struct red_vars *v,
 295                                          unsigned int backlog)
 296{
 297        if (!red_is_idling(v))
 298                return red_calc_qavg_no_idle_time(p, v, backlog);
 299        else
 300                return red_calc_qavg_from_idle_time(p, v);
 301}
 302
 303
 304static inline u32 red_random(const struct red_parms *p)
 305{
 306        return reciprocal_divide(net_random(), p->max_P_reciprocal);
 307}
 308
 309static inline int red_mark_probability(const struct red_parms *p,
 310                                       const struct red_vars *v,
 311                                       unsigned long qavg)
 312{
 313        /* The formula used below causes questions.
 314
 315           OK. qR is random number in the interval
 316                (0..1/max_P)*(qth_max-qth_min)
 317           i.e. 0..(2^Plog). If we used floating point
 318           arithmetics, it would be: (2^Plog)*rnd_num,
 319           where rnd_num is less 1.
 320
 321           Taking into account, that qavg have fixed
 322           point at Wlog, two lines
 323           below have the following floating point equivalent:
 324
 325           max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount
 326
 327           Any questions? --ANK (980924)
 328         */
 329        return !(((qavg - p->qth_min) >> p->Wlog) * v->qcount < v->qR);
 330}
 331
 332enum {
 333        RED_BELOW_MIN_THRESH,
 334        RED_BETWEEN_TRESH,
 335        RED_ABOVE_MAX_TRESH,
 336};
 337
 338static inline int red_cmp_thresh(const struct red_parms *p, unsigned long qavg)
 339{
 340        if (qavg < p->qth_min)
 341                return RED_BELOW_MIN_THRESH;
 342        else if (qavg >= p->qth_max)
 343                return RED_ABOVE_MAX_TRESH;
 344        else
 345                return RED_BETWEEN_TRESH;
 346}
 347
 348enum {
 349        RED_DONT_MARK,
 350        RED_PROB_MARK,
 351        RED_HARD_MARK,
 352};
 353
 354static inline int red_action(const struct red_parms *p,
 355                             struct red_vars *v,
 356                             unsigned long qavg)
 357{
 358        switch (red_cmp_thresh(p, qavg)) {
 359                case RED_BELOW_MIN_THRESH:
 360                        v->qcount = -1;
 361                        return RED_DONT_MARK;
 362
 363                case RED_BETWEEN_TRESH:
 364                        if (++v->qcount) {
 365                                if (red_mark_probability(p, v, qavg)) {
 366                                        v->qcount = 0;
 367                                        v->qR = red_random(p);
 368                                        return RED_PROB_MARK;
 369                                }
 370                        } else
 371                                v->qR = red_random(p);
 372
 373                        return RED_DONT_MARK;
 374
 375                case RED_ABOVE_MAX_TRESH:
 376                        v->qcount = -1;
 377                        return RED_HARD_MARK;
 378        }
 379
 380        BUG();
 381        return RED_DONT_MARK;
 382}
 383
 384static inline void red_adaptative_algo(struct red_parms *p, struct red_vars *v)
 385{
 386        unsigned long qavg;
 387        u32 max_p_delta;
 388
 389        qavg = v->qavg;
 390        if (red_is_idling(v))
 391                qavg = red_calc_qavg_from_idle_time(p, v);
 392
 393        /* v->qavg is fixed point number with point at Wlog */
 394        qavg >>= p->Wlog;
 395
 396        if (qavg > p->target_max && p->max_P <= MAX_P_MAX)
 397                p->max_P += MAX_P_ALPHA(p->max_P); /* maxp = maxp + alpha */
 398        else if (qavg < p->target_min && p->max_P >= MAX_P_MIN)
 399                p->max_P = (p->max_P/10)*9; /* maxp = maxp * Beta */
 400
 401        max_p_delta = DIV_ROUND_CLOSEST(p->max_P, p->qth_delta);
 402        max_p_delta = max(max_p_delta, 1U);
 403        p->max_P_reciprocal = reciprocal_value(max_p_delta);
 404}
 405#endif
 406
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