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 325