include/net/red.h

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