include/net/red.h

   1 #ifndef __NET_SCHED_RED_H
   2 #define __NET_SCHED_RED_H
   3
   4 #include <linux/config.h>
   5 #include <linux/types.h>
   6 #include <net/pkt_sched.h>
   7 #include <net/inet_ecn.h>
   8 #include <net/dsfield.h>
   9
  10 /*      Random Early Detection (RED) algorithm.
  11         =======================================
  12
  13         Source: Sally Floyd and Van Jacobson, "Random Early Detection Gateways
  14         for Congestion Avoidance", 1993, IEEE/ACM Transactions on Networking.
  15
  16         This file codes a "divisionless" version of RED algorithm
  17         as written down in Fig.17 of the paper.
  18
  19         Short description.
  20         ------------------
  21
  22         When a new packet arrives we calculate the average queue length:
  23
  24         avg = (1-W)*avg + W*current_queue_len,
  25
  26         W is the filter time constant (chosen as 2^(-Wlog)), it controls
  27         the inertia of the algorithm. To allow larger bursts, W should be
  28         decreased.
  29
  30         if (avg > th_max) -> packet marked (dropped).
  31         if (avg < th_min) -> packet passes.
  32         if (th_min < avg < th_max) we calculate probability:
  33
  34         Pb = max_P * (avg - th_min)/(th_max-th_min)
  35
  36         and mark (drop) packet with this probability.
  37         Pb changes from 0 (at avg==th_min) to max_P (avg==th_max).
  38         max_P should be small (not 1), usually 0.01..0.02 is good value.
  39
  40         max_P is chosen as a number, so that max_P/(th_max-th_min)
  41         is a negative power of two in order arithmetics to contain
  42         only shifts.
  43
  44
  45         Parameters, settable by user:
  46         -----------------------------
  47
  48         qth_min         - bytes (should be < qth_max/2)
  49         qth_max         - bytes (should be at least 2*qth_min and less limit)
  50         Wlog            - bits (<32) log(1/W).
  51         Plog            - bits (<32)
  52
  53         Plog is related to max_P by formula:
  54
  55         max_P = (qth_max-qth_min)/2^Plog;
  56
  57         F.e. if qth_max=128K and qth_min=32K, then Plog=22
  58         corresponds to max_P=0.02
  59
  60         Scell_log
  61         Stab
  62
  63         Lookup table for log((1-W)^(t/t_ave).
  64
  65
  66         NOTES:
  67
  68         Upper bound on W.
  69         -----------------
  70
  71         If you want to allow bursts of L packets of size S,
  72         you should choose W:
  73
  74         L + 1 - th_min/S < (1-(1-W)^L)/W
  75
  76         th_min/S = 32         th_min/S = 4
  77
  78         log(W)  L
  79         -1      33
  80         -2      35
  81         -3      39
  82         -4      46
  83         -5      57
  84         -6      75
  85         -7      101
  86         -8      135
  87         -9      190
  88         etc.
  89  */
  90
  91 #define RED_STAB_SIZE   256
  92 #define RED_STAB_MASK   (RED_STAB_SIZE - 1)
  93
  94 struct red_stats
  95 {
  96         u32             prob_drop;      /* Early probability drops */
  97         u32             prob_mark;      /* Early probability marks */
  98         u32             forced_drop;    /* Forced drops, qavg > max_thresh */
  99         u32             forced_mark;    /* Forced marks, qavg > max_thresh */
 100         u32             pdrop;          /* Drops due to queue limits */
 101         u32             other;          /* Drops due to drop() calls */
 102         u32             backlog;
 103 };
 104
 105 struct red_parms
 106 {
 107         /* Parameters */
 108         u32             qth_min;        /* Min avg length threshold: A scaled */
 109         u32             qth_max;        /* Max avg length threshold: A scaled */
 110         u32             Scell_max;
 111         u32             Rmask;          /* Cached random mask, see red_rmask */
 112         u8              Scell_log;
 113         u8              Wlog;           /* log(W)               */
 114         u8              Plog;           /* random number bits   */
 115         u8              Stab[RED_STAB_SIZE];
 116
 117         /* Variables */
 118         int             qcount;         /* Number of packets since last random
 119                                            number generation */
 120         u32             qR;             /* Cached random number */
 121
 122         unsigned long   qavg;           /* Average queue length: A scaled */
 123         psched_time_t   qidlestart;     /* Start of current idle period */
 124 };
 125
 126 static inline u32 red_rmask(u8 Plog)
 127 {
 128         return Plog < 32 ? ((1 << Plog) - 1) : ~0UL;
 129 }
 130
 131 static inline void red_set_parms(struct red_parms *p,
 132                                  u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
 133                                  u8 Scell_log, u8 *stab)
 134 {
 135         /* Reset average queue length, the value is strictly bound
 136          * to the parameters below, reseting hurts a bit but leaving
 137          * it might result in an unreasonable qavg for a while. --TGR
 138          */
 139         p->qavg         = 0;
 140
 141         p->qcount       = -1;
 142         p->qth_min      = qth_min << Wlog;
 143         p->qth_max      = qth_max << Wlog;
 144         p->Wlog         = Wlog;
 145         p->Plog         = Plog;
 146         p->Rmask        = red_rmask(Plog);
 147         p->Scell_log    = Scell_log;
 148         p->Scell_max    = (255 << Scell_log);
 149
 150         memcpy(p->Stab, stab, sizeof(p->Stab));
 151 }
 152
 153 static inline int red_is_idling(struct red_parms *p)
 154 {
 155         return !PSCHED_IS_PASTPERFECT(p->qidlestart);
 156 }
 157
 158 static inline void red_start_of_idle_period(struct red_parms *p)
 159 {
 160         PSCHED_GET_TIME(p->qidlestart);
 161 }
 162
 163 static inline void red_end_of_idle_period(struct red_parms *p)
 164 {
 165         PSCHED_SET_PASTPERFECT(p->qidlestart);
 166 }
 167
 168 static inline void red_restart(struct red_parms *p)
 169 {
 170         red_end_of_idle_period(p);
 171         p->qavg = 0;
 172         p->qcount = -1;
 173 }
 174
 175 static inline unsigned long red_calc_qavg_from_idle_time(struct red_parms *p)
 176 {
 177         psched_time_t now;
 178         long us_idle;
 179         int  shift;
 180
 181         PSCHED_GET_TIME(now);
 182         us_idle = PSCHED_TDIFF_SAFE(now, p->qidlestart, p->Scell_max);
 183
 184         /*
 185          * The problem: ideally, average length queue recalcultion should
 186          * be done over constant clock intervals. This is too expensive, so
 187          * that the calculation is driven by outgoing packets.
 188          * When the queue is idle we have to model this clock by hand.
 189          *
 190          * SF+VJ proposed to "generate":
 191          *
 192          *      m = idletime / (average_pkt_size / bandwidth)
 193          *
 194          * dummy packets as a burst after idle time, i.e.
 195          *
 196          *      p->qavg *= (1-W)^m
 197          *
 198          * This is an apparently overcomplicated solution (f.e. we have to
 199          * precompute a table to make this calculation in reasonable time)
 200          * I believe that a simpler model may be used here,
 201          * but it is field for experiments.
 202          */
 203
 204         shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK];
 205
 206         if (shift)
 207                 return p->qavg >> shift;
 208         else {
 209                 /* Approximate initial part of exponent with linear function:
 210                  *
 211                  *      (1-W)^m ~= 1-mW + ...
 212                  *
 213                  * Seems, it is the best solution to
 214                  * problem of too coarse exponent tabulation.
 215                  */
 216                 us_idle = (p->qavg * us_idle) >> p->Scell_log;
 217
 218                 if (us_idle < (p->qavg >> 1))
 219                         return p->qavg - us_idle;
 220                 else
 221                         return p->qavg >> 1;
 222         }
 223 }
 224
 225 static inline unsigned long red_calc_qavg_no_idle_time(struct red_parms *p,
 226                                                        unsigned int backlog)
 227 {
 228         /*
 229          * NOTE: p->qavg is fixed point number with point at Wlog.
 230          * The formula below is equvalent to floating point
 231          * version:
 232          *
 233          *      qavg = qavg*(1-W) + backlog*W;
 234          *
 235          * --ANK (980924)
 236          */
 237         return p->qavg + (backlog - (p->qavg >> p->Wlog));
 238 }
 239
 240 static inline unsigned long red_calc_qavg(struct red_parms *p,
 241                                           unsigned int backlog)
 242 {
 243         if (!red_is_idling(p))
 244                 return red_calc_qavg_no_idle_time(p, backlog);
 245         else
 246                 return red_calc_qavg_from_idle_time(p);
 247 }
 248
 249 static inline u32 red_random(struct red_parms *p)
 250 {
 251         return net_random() & p->Rmask;
 252 }
 253
 254 static inline int red_mark_probability(struct red_parms *p, unsigned long qavg)
 255 {
 256         /* The formula used below causes questions.
 257
 258            OK. qR is random number in the interval 0..Rmask
 259            i.e. 0..(2^Plog). If we used floating point
 260            arithmetics, it would be: (2^Plog)*rnd_num,
 261            where rnd_num is less 1.
 262
 263            Taking into account, that qavg have fixed
 264            point at Wlog, and Plog is related to max_P by
 265            max_P = (qth_max-qth_min)/2^Plog; two lines
 266            below have the following floating point equivalent:
 267
 268            max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount
 269
 270            Any questions? --ANK (980924)
 271          */
 272         return !(((qavg - p->qth_min) >> p->Wlog) * p->qcount < p->qR);
 273 }
 274
 275 enum {
 276         RED_BELOW_MIN_THRESH,
 277         RED_BETWEEN_TRESH,
 278         RED_ABOVE_MAX_TRESH,
 279 };
 280
 281 static inline int red_cmp_thresh(struct red_parms *p, unsigned long qavg)
 282 {
 283         if (qavg < p->qth_min)
 284                 return RED_BELOW_MIN_THRESH;
 285         else if (qavg >= p->qth_max)
 286                 return RED_ABOVE_MAX_TRESH;
 287         else
 288                 return RED_BETWEEN_TRESH;
 289 }
 290
 291 enum {
 292         RED_DONT_MARK,
 293         RED_PROB_MARK,
 294         RED_HARD_MARK,
 295 };
 296
 297 static inline int red_action(struct red_parms *p, unsigned long qavg)
 298 {
 299         switch (red_cmp_thresh(p, qavg)) {
 300                 case RED_BELOW_MIN_THRESH:
 301                         p->qcount = -1;
 302                         return RED_DONT_MARK;
 303
 304                 case RED_BETWEEN_TRESH:
 305                         if (++p->qcount) {
 306                                 if (red_mark_probability(p, qavg)) {
 307                                         p->qcount = 0;
 308                                         p->qR = red_random(p);
 309                                         return RED_PROB_MARK;
 310                                 }
 311                         } else
 312                                 p->qR = red_random(p);
 313
 314                         return RED_DONT_MARK;
 315
 316                 case RED_ABOVE_MAX_TRESH:
 317                         p->qcount = -1;
 318                         return RED_HARD_MARK;
 319         }
 320
 321         BUG();
 322         return RED_DONT_MARK;
 323 }
 324
 325 #endif