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 326