include/net/red.h

   1 /* SPDX-License-Identifier: GPL-2.0 */
   2 #ifndef __NET_SCHED_RED_H
   3 #define __NET_SCHED_RED_H
   4
   5 #include <linux/types.h>
   6 #include <linux/bug.h>
   7 #include <net/pkt_sched.h>
   8 #include <net/inet_ecn.h>
   9 #include <net/dsfield.h>
  10 #include <linux/reciprocal_div.h>
  11
  12 /*      Random Early Detection (RED) algorithm.
  13         =======================================
  14
  15         Source: Sally Floyd and Van Jacobson, "Random Early Detection Gateways
  16         for Congestion Avoidance", 1993, IEEE/ACM Transactions on Networking.
  17
  18         This file codes a "divisionless" version of RED algorithm
  19         as written down in Fig.17 of the paper.
  20
  21         Short description.
  22         ------------------
  23
  24         When a new packet arrives we calculate the average queue length:
  25
  26         avg = (1-W)*avg + W*current_queue_len,
  27
  28         W is the filter time constant (chosen as 2^(-Wlog)), it controls
  29         the inertia of the algorithm. To allow larger bursts, W should be
  30         decreased.
  31
  32         if (avg > th_max) -> packet marked (dropped).
  33         if (avg < th_min) -> packet passes.
  34         if (th_min < avg < th_max) we calculate probability:
  35
  36         Pb = max_P * (avg - th_min)/(th_max-th_min)
  37
  38         and mark (drop) packet with this probability.
  39         Pb changes from 0 (at avg==th_min) to max_P (avg==th_max).
  40         max_P should be small (not 1), usually 0.01..0.02 is good value.
  41
  42         max_P is chosen as a number, so that max_P/(th_max-th_min)
  43         is a negative power of two in order arithmetics to contain
  44         only shifts.
  45
  46
  47         Parameters, settable by user:
  48         -----------------------------
  49
  50         qth_min         - bytes (should be < qth_max/2)
  51         qth_max         - bytes (should be at least 2*qth_min and less limit)
  52         Wlog            - bits (<32) log(1/W).
  53         Plog            - bits (<32)
  54
  55         Plog is related to max_P by formula:
  56
  57         max_P = (qth_max-qth_min)/2^Plog;
  58
  59         F.e. if qth_max=128K and qth_min=32K, then Plog=22
  60         corresponds to max_P=0.02
  61
  62         Scell_log
  63         Stab
  64
  65         Lookup table for log((1-W)^(t/t_ave).
  66
  67
  68         NOTES:
  69
  70         Upper bound on W.
  71         -----------------
  72
  73         If you want to allow bursts of L packets of size S,
  74         you should choose W:
  75
  76         L + 1 - th_min/S < (1-(1-W)^L)/W
  77
  78         th_min/S = 32         th_min/S = 4
  79
  80         log(W)  L
  81         -1      33
  82         -2      35
  83         -3      39
  84         -4      46
  85         -5      57
  86         -6      75
  87         -7      101
  88         -8      135
  89         -9      190
  90         etc.
  91  */
  92
  93 /*
  94  * Adaptative RED : An Algorithm for Increasing the Robustness of RED's AQM
  95  * (Sally FLoyd, Ramakrishna Gummadi, and Scott Shenker) August 2001
  96  *
  97  * Every 500 ms:
  98  *  if (avg > target and max_p <= 0.5)
  99  *   increase max_p : max_p += alpha;
 100  *  else if (avg < target and max_p >= 0.01)
 101  *   decrease max_p : max_p *= beta;
 102  *
 103  * target :[qth_min + 0.4*(qth_min - qth_max),
 104  *          qth_min + 0.6*(qth_min - qth_max)].
 105  * alpha : min(0.01, max_p / 4)
 106  * beta : 0.9
 107  * max_P is a Q0.32 fixed point number (with 32 bits mantissa)
 108  * max_P between 0.01 and 0.5 (1% - 50%) [ Its no longer a negative power of two ]
 109  */
 110 #define RED_ONE_PERCENT ((u32)DIV_ROUND_CLOSEST(1ULL<<32, 100))
 111
 112 #define MAX_P_MIN (1 * RED_ONE_PERCENT)
 113 #define MAX_P_MAX (50 * RED_ONE_PERCENT)
 114 #define MAX_P_ALPHA(val) min(MAX_P_MIN, val / 4)
 115
 116 #define RED_STAB_SIZE   256
 117 #define RED_STAB_MASK   (RED_STAB_SIZE - 1)
 118
 119 struct red_stats {
 120         u32             prob_drop;      /* Early probability drops */
 121         u32             prob_mark;      /* Early probability marks */
 122         u32             forced_drop;    /* Forced drops, qavg > max_thresh */
 123         u32             forced_mark;    /* Forced marks, qavg > max_thresh */
 124         u32             pdrop;          /* Drops due to queue limits */
 125         u32             other;          /* Drops due to drop() calls */
 126 };
 127
 128 struct red_parms {
 129         /* Parameters */
 130         u32             qth_min;        /* Min avg length threshold: Wlog scaled */
 131         u32             qth_max;        /* Max avg length threshold: Wlog scaled */
 132         u32             Scell_max;
 133         u32             max_P;          /* probability, [0 .. 1.0] 32 scaled */
 134         /* reciprocal_value(max_P / qth_delta) */
 135         struct reciprocal_value max_P_reciprocal;
 136         u32             qth_delta;      /* max_th - min_th */
 137         u32             target_min;     /* min_th + 0.4*(max_th - min_th) */
 138         u32             target_max;     /* min_th + 0.6*(max_th - min_th) */
 139         u8              Scell_log;
 140         u8              Wlog;           /* log(W)               */
 141         u8              Plog;           /* random number bits   */
 142         u8              Stab[RED_STAB_SIZE];
 143 };
 144
 145 struct red_vars {
 146         /* Variables */
 147         int             qcount;         /* Number of packets since last random
 148                                            number generation */
 149         u32             qR;             /* Cached random number */
 150
 151         unsigned long   qavg;           /* Average queue length: Wlog scaled */
 152         ktime_t         qidlestart;     /* Start of current idle period */
 153 };
 154
 155 static inline u32 red_maxp(u8 Plog)
 156 {
 157         return Plog < 32 ? (~0U >> Plog) : ~0U;
 158 }
 159
 160 static inline void red_set_vars(struct red_vars *v)
 161 {
 162         /* Reset average queue length, the value is strictly bound
 163          * to the parameters below, reseting hurts a bit but leaving
 164          * it might result in an unreasonable qavg for a while. --TGR
 165          */
 166         v->qavg         = 0;
 167
 168         v->qcount       = -1;
 169 }
 170
 171 static inline bool red_check_params(u32 qth_min, u32 qth_max, u8 Wlog)
 172 {
 173         if (fls(qth_min) + Wlog > 32)
 174                 return false;
 175         if (fls(qth_max) + Wlog > 32)
 176                 return false;
 177         if (qth_max < qth_min)
 178                 return false;
 179         return true;
 180 }
 181
 182 static inline void red_set_parms(struct red_parms *p,
 183                                  u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
 184                                  u8 Scell_log, u8 *stab, u32 max_P)
 185 {
 186         int delta = qth_max - qth_min;
 187         u32 max_p_delta;
 188
 189         p->qth_min      = qth_min << Wlog;
 190         p->qth_max      = qth_max << Wlog;
 191         p->Wlog         = Wlog;
 192         p->Plog         = Plog;
 193         if (delta <= 0)
 194                 delta = 1;
 195         p->qth_delta    = delta;
 196         if (!max_P) {
 197                 max_P = red_maxp(Plog);
 198                 max_P *= delta; /* max_P = (qth_max - qth_min)/2^Plog */
 199         }
 200         p->max_P = max_P;
 201         max_p_delta = max_P / delta;
 202         max_p_delta = max(max_p_delta, 1U);
 203         p->max_P_reciprocal  = reciprocal_value(max_p_delta);
 204
 205         /* RED Adaptative target :
 206          * [min_th + 0.4*(min_th - max_th),
 207          *  min_th + 0.6*(min_th - max_th)].
 208          */
 209         delta /= 5;
 210         p->target_min = qth_min + 2*delta;
 211         p->target_max = qth_min + 3*delta;
 212
 213         p->Scell_log    = Scell_log;
 214         p->Scell_max    = (255 << Scell_log);
 215
 216         if (stab)
 217                 memcpy(p->Stab, stab, sizeof(p->Stab));
 218 }
 219
 220 static inline int red_is_idling(const struct red_vars *v)
 221 {
 222         return v->qidlestart != 0;
 223 }
 224
 225 static inline void red_start_of_idle_period(struct red_vars *v)
 226 {
 227         v->qidlestart = ktime_get();
 228 }
 229
 230 static inline void red_end_of_idle_period(struct red_vars *v)
 231 {
 232         v->qidlestart = 0;
 233 }
 234
 235 static inline void red_restart(struct red_vars *v)
 236 {
 237         red_end_of_idle_period(v);
 238         v->qavg = 0;
 239         v->qcount = -1;
 240 }
 241
 242 static inline unsigned long red_calc_qavg_from_idle_time(const struct red_parms *p,
 243                                                          const struct red_vars *v)
 244 {
 245         s64 delta = ktime_us_delta(ktime_get(), v->qidlestart);
 246         long us_idle = min_t(s64, delta, p->Scell_max);
 247         int  shift;
 248
 249         /*
 250          * The problem: ideally, average length queue recalcultion should
 251          * be done over constant clock intervals. This is too expensive, so
 252          * that the calculation is driven by outgoing packets.
 253          * When the queue is idle we have to model this clock by hand.
 254          *
 255          * SF+VJ proposed to "generate":
 256          *
 257          *      m = idletime / (average_pkt_size / bandwidth)
 258          *
 259          * dummy packets as a burst after idle time, i.e.
 260          *
 261          *      v->qavg *= (1-W)^m
 262          *
 263          * This is an apparently overcomplicated solution (f.e. we have to
 264          * precompute a table to make this calculation in reasonable time)
 265          * I believe that a simpler model may be used here,
 266          * but it is field for experiments.
 267          */
 268
 269         shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK];
 270
 271         if (shift)
 272                 return v->qavg >> shift;
 273         else {
 274                 /* Approximate initial part of exponent with linear function:
 275                  *
 276                  *      (1-W)^m ~= 1-mW + ...
 277                  *
 278                  * Seems, it is the best solution to
 279                  * problem of too coarse exponent tabulation.
 280                  */
 281                 us_idle = (v->qavg * (u64)us_idle) >> p->Scell_log;
 282
 283                 if (us_idle < (v->qavg >> 1))
 284                         return v->qavg - us_idle;
 285                 else
 286                         return v->qavg >> 1;
 287         }
 288 }
 289
 290 static inline unsigned long red_calc_qavg_no_idle_time(const struct red_parms *p,
 291                                                        const struct red_vars *v,
 292                                                        unsigned int backlog)
 293 {
 294         /*
 295          * NOTE: v->qavg is fixed point number with point at Wlog.
 296          * The formula below is equvalent to floating point
 297          * version:
 298          *
 299          *      qavg = qavg*(1-W) + backlog*W;
 300          *
 301          * --ANK (980924)
 302          */
 303         return v->qavg + (backlog - (v->qavg >> p->Wlog));
 304 }
 305
 306 static inline unsigned long red_calc_qavg(const struct red_parms *p,
 307                                           const struct red_vars *v,
 308                                           unsigned int backlog)
 309 {
 310         if (!red_is_idling(v))
 311                 return red_calc_qavg_no_idle_time(p, v, backlog);
 312         else
 313                 return red_calc_qavg_from_idle_time(p, v);
 314 }
 315
 316
 317 static inline u32 red_random(const struct red_parms *p)
 318 {
 319         return reciprocal_divide(prandom_u32(), p->max_P_reciprocal);
 320 }
 321
 322 static inline int red_mark_probability(const struct red_parms *p,
 323                                        const struct red_vars *v,
 324                                        unsigned long qavg)
 325 {
 326         /* The formula used below causes questions.
 327
 328            OK. qR is random number in the interval
 329                 (0..1/max_P)*(qth_max-qth_min)
 330            i.e. 0..(2^Plog). If we used floating point
 331            arithmetics, it would be: (2^Plog)*rnd_num,
 332            where rnd_num is less 1.
 333
 334            Taking into account, that qavg have fixed
 335            point at Wlog, two lines
 336            below have the following floating point equivalent:
 337
 338            max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount
 339
 340            Any questions? --ANK (980924)
 341          */
 342         return !(((qavg - p->qth_min) >> p->Wlog) * v->qcount < v->qR);
 343 }
 344
 345 enum {
 346         RED_BELOW_MIN_THRESH,
 347         RED_BETWEEN_TRESH,
 348         RED_ABOVE_MAX_TRESH,
 349 };
 350
 351 static inline int red_cmp_thresh(const struct red_parms *p, unsigned long qavg)
 352 {
 353         if (qavg < p->qth_min)
 354                 return RED_BELOW_MIN_THRESH;
 355         else if (qavg >= p->qth_max)
 356                 return RED_ABOVE_MAX_TRESH;
 357         else
 358                 return RED_BETWEEN_TRESH;
 359 }
 360
 361 enum {
 362         RED_DONT_MARK,
 363         RED_PROB_MARK,
 364         RED_HARD_MARK,
 365 };
 366
 367 static inline int red_action(const struct red_parms *p,
 368                              struct red_vars *v,
 369                              unsigned long qavg)
 370 {
 371         switch (red_cmp_thresh(p, qavg)) {
 372                 case RED_BELOW_MIN_THRESH:
 373                         v->qcount = -1;
 374                         return RED_DONT_MARK;
 375
 376                 case RED_BETWEEN_TRESH:
 377                         if (++v->qcount) {
 378                                 if (red_mark_probability(p, v, qavg)) {
 379                                         v->qcount = 0;
 380                                         v->qR = red_random(p);
 381                                         return RED_PROB_MARK;
 382                                 }
 383                         } else
 384                                 v->qR = red_random(p);
 385
 386                         return RED_DONT_MARK;
 387
 388                 case RED_ABOVE_MAX_TRESH:
 389                         v->qcount = -1;
 390                         return RED_HARD_MARK;
 391         }
 392
 393         BUG();
 394         return RED_DONT_MARK;
 395 }
 396
 397 static inline void red_adaptative_algo(struct red_parms *p, struct red_vars *v)
 398 {
 399         unsigned long qavg;
 400         u32 max_p_delta;
 401
 402         qavg = v->qavg;
 403         if (red_is_idling(v))
 404                 qavg = red_calc_qavg_from_idle_time(p, v);
 405
 406         /* v->qavg is fixed point number with point at Wlog */
 407         qavg >>= p->Wlog;
 408
 409         if (qavg > p->target_max && p->max_P <= MAX_P_MAX)
 410                 p->max_P += MAX_P_ALPHA(p->max_P); /* maxp = maxp + alpha */
 411         else if (qavg < p->target_min && p->max_P >= MAX_P_MIN)
 412                 p->max_P = (p->max_P/10)*9; /* maxp = maxp * Beta */
 413
 414         max_p_delta = DIV_ROUND_CLOSEST(p->max_P, p->qth_delta);
 415         max_p_delta = max(max_p_delta, 1U);
 416         p->max_P_reciprocal = reciprocal_value(max_p_delta);
 417 }
 418 #endif