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 93struct red_stats { 94 u32 prob_drop; /* Early probability drops */ 95 u32 prob_mark; /* Early probability marks */ 96 u32 forced_drop; /* Forced drops, qavg > max_thresh */ 97 u32 forced_mark; /* Forced marks, qavg > max_thresh */ 98 u32 pdrop; /* Drops due to queue limits */ 99 u32 other; /* Drops due to drop() calls */ 100 u32 backlog; 101}; 102 103struct red_parms { 104 /* Parameters */ 105 u32 qth_min; /* Min avg length threshold: A scaled */ 106 u32 qth_max; /* Max avg length threshold: A scaled */ 107 u32 Scell_max; 108 u32 Rmask; /* Cached random mask, see red_rmask */ 109 u8 Scell_log; 110 u8 Wlog; /* log(W) */ 111 u8 Plog; /* random number bits */ 112 u8 Stab[RED_STAB_SIZE]; 113 114 /* Variables */ 115 int qcount; /* Number of packets since last random 116 number generation */ 117 u32 qR; /* Cached random number */ 118 119 unsigned long qavg; /* Average queue length: A scaled */ 120 psched_time_t qidlestart; /* Start of current idle period */ 121}; 122 123static inline u32 red_rmask(u8 Plog) 124{ 125 return Plog < 32 ? ((1 << Plog) - 1) : ~0UL; 126} 127 128static inline void red_set_parms(struct red_parms *p, 129 u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog, 130 u8 Scell_log, u8 *stab) 131{ 132 /* Reset average queue length, the value is strictly bound 133 * to the parameters below, reseting hurts a bit but leaving 134 * it might result in an unreasonable qavg for a while. --TGR 135 */ 136 p->qavg = 0; 137 138 p->qcount = -1; 139 p->qth_min = qth_min << Wlog; 140 p->qth_max = qth_max << Wlog; 141 p->Wlog = Wlog; 142 p->Plog = Plog; 143 p->Rmask = red_rmask(Plog); 144 p->Scell_log = Scell_log; 145 p->Scell_max = (255 << Scell_log); 146 147 memcpy(p->Stab, stab, sizeof(p->Stab)); 148} 149 150static inline int red_is_idling(struct red_parms *p) 151{ 152 return p->qidlestart != PSCHED_PASTPERFECT; 153} 154 155static inline void red_start_of_idle_period(struct red_parms *p) 156{ 157 p->qidlestart = psched_get_time(); 158} 159 160static inline void red_end_of_idle_period(struct red_parms *p) 161{ 162 p->qidlestart = PSCHED_PASTPERFECT; 163} 164 165static inline void red_restart(struct red_parms *p) 166{ 167 red_end_of_idle_period(p); 168 p->qavg = 0; 169 p->qcount = -1; 170} 171 172static inline unsigned long red_calc_qavg_from_idle_time(struct red_parms *p) 173{ 174 psched_time_t now; 175 long us_idle; 176 int shift; 177 178 now = psched_get_time(); 179 us_idle = psched_tdiff_bounded(now, p->qidlestart, p->Scell_max); 180 181 /* 182 * The problem: ideally, average length queue recalcultion should 183 * be done over constant clock intervals. This is too expensive, so 184 * that the calculation is driven by outgoing packets. 185 * When the queue is idle we have to model this clock by hand. 186 * 187 * SF+VJ proposed to "generate": 188 * 189 * m = idletime / (average_pkt_size / bandwidth) 190 * 191 * dummy packets as a burst after idle time, i.e. 192 * 193 * p->qavg *= (1-W)^m 194 * 195 * This is an apparently overcomplicated solution (f.e. we have to 196 * precompute a table to make this calculation in reasonable time) 197 * I believe that a simpler model may be used here, 198 * but it is field for experiments. 199 */ 200 201 shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK]; 202 203 if (shift) 204 return p->qavg >> shift; 205 else { 206 /* Approximate initial part of exponent with linear function: 207 * 208 * (1-W)^m ~= 1-mW + ... 209 * 210 * Seems, it is the best solution to 211 * problem of too coarse exponent tabulation. 212 */ 213 us_idle = (p->qavg * (u64)us_idle) >> p->Scell_log; 214 215 if (us_idle < (p->qavg >> 1)) 216 return p->qavg - us_idle; 217 else 218 return p->qavg >> 1; 219 } 220} 221 222static inline unsigned long red_calc_qavg_no_idle_time(struct red_parms *p, 223 unsigned int backlog) 224{ 225 /* 226 * NOTE: p->qavg is fixed point number with point at Wlog. 227 * The formula below is equvalent to floating point 228 * version: 229 * 230 * qavg = qavg*(1-W) + backlog*W; 231 * 232 * --ANK (980924) 233 */ 234 return p->qavg + (backlog - (p->qavg >> p->Wlog)); 235} 236 237static inline unsigned long red_calc_qavg(struct red_parms *p, 238 unsigned int backlog) 239{ 240 if (!red_is_idling(p)) 241 return red_calc_qavg_no_idle_time(p, backlog); 242 else 243 return red_calc_qavg_from_idle_time(p); 244} 245 246static inline u32 red_random(struct red_parms *p) 247{ 248 return net_random() & p->Rmask; 249} 250 251static inline int red_mark_probability(struct red_parms *p, unsigned long qavg) 252{ 253 /* The formula used below causes questions. 254 255 OK. qR is random number in the interval 0..Rmask 256 i.e. 0..(2^Plog). If we used floating point 257 arithmetics, it would be: (2^Plog)*rnd_num, 258 where rnd_num is less 1. 259 260 Taking into account, that qavg have fixed 261 point at Wlog, and Plog is related to max_P by 262 max_P = (qth_max-qth_min)/2^Plog; two lines 263 below have the following floating point equivalent: 264 265 max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount 266 267 Any questions? --ANK (980924) 268 */ 269 return !(((qavg - p->qth_min) >> p->Wlog) * p->qcount < p->qR); 270} 271 272enum { 273 RED_BELOW_MIN_THRESH, 274 RED_BETWEEN_TRESH, 275 RED_ABOVE_MAX_TRESH, 276}; 277 278static inline int red_cmp_thresh(struct red_parms *p, unsigned long qavg) 279{ 280 if (qavg < p->qth_min) 281 return RED_BELOW_MIN_THRESH; 282 else if (qavg >= p->qth_max) 283 return RED_ABOVE_MAX_TRESH; 284 else 285 return RED_BETWEEN_TRESH; 286} 287 288enum { 289 RED_DONT_MARK, 290 RED_PROB_MARK, 291 RED_HARD_MARK, 292}; 293 294static inline int red_action(struct red_parms *p, unsigned long qavg) 295{ 296 switch (red_cmp_thresh(p, qavg)) { 297 case RED_BELOW_MIN_THRESH: 298 p->qcount = -1; 299 return RED_DONT_MARK; 300 301 case RED_BETWEEN_TRESH: 302 if (++p->qcount) { 303 if (red_mark_probability(p, qavg)) { 304 p->qcount = 0; 305 p->qR = red_random(p); 306 return RED_PROB_MARK; 307 } 308 } else 309 p->qR = red_random(p); 310 311 return RED_DONT_MARK; 312 313 case RED_ABOVE_MAX_TRESH: 314 p->qcount = -1; 315 return RED_HARD_MARK; 316 } 317 318 BUG(); 319 return RED_DONT_MARK; 320} 321 322#endif 323