include/net/red.h

   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
 118 struct 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
 127 struct 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
 143 struct 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
 153 static inline u32 red_maxp(u8 Plog)
 154 {
 155         return Plog < 32 ? (~0U >> Plog) : ~0U;
 156 }
 157
 158 static 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
 169 static 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
 207 static inline int red_is_idling(const struct red_vars *v)
 208 {
 209         return v->qidlestart.tv64 != 0;
 210 }
 211
 212 static inline void red_start_of_idle_period(struct red_vars *v)
 213 {
 214         v->qidlestart = ktime_get();
 215 }
 216
 217 static inline void red_end_of_idle_period(struct red_vars *v)
 218 {
 219         v->qidlestart.tv64 = 0;
 220 }
 221
 222 static 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
 229 static 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
 277 static 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
 293 static 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
 304 static inline u32 red_random(const struct red_parms *p)
 305 {
 306         return reciprocal_divide(net_random(), p->max_P_reciprocal);
 307 }
 308
 309 static 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
 332 enum {
 333         RED_BELOW_MIN_THRESH,
 334         RED_BETWEEN_TRESH,
 335         RED_ABOVE_MAX_TRESH,
 336 };
 337
 338 static 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
 348 enum {
 349         RED_DONT_MARK,
 350         RED_PROB_MARK,
 351         RED_HARD_MARK,
 352 };
 353
 354 static 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
 384 static 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