1 /* SPDX-License-Identifier: GPL-2.0 */ 2 #ifndef __NET_SCHED_RED_H 3 #define __NET_SCHED_RED_H 4 5 #include <linux/types.h> 6 #include <linux/bug.h> 7 #include <net/pkt_sched.h> 8 #include <net/inet_ecn.h> 9 #include <net/dsfield.h> 10 #include <linux/reciprocal_div.h> 11 12 /* Random Early Detection (RED) algorithm. 13 ======================================= 14 15 Source: Sally Floyd and Van Jacobson, "Random Early Detection Gateways 16 for Congestion Avoidance", 1993, IEEE/ACM Transactions on Networking. 17 18 This file codes a "divisionless" version of RED algorithm 19 as written down in Fig.17 of the paper. 20 21 Short description. 22 ------------------ 23 24 When a new packet arrives we calculate the average queue length: 25 26 avg = (1-W)*avg + W*current_queue_len, 27 28 W is the filter time constant (chosen as 2^(-Wlog)), it controls 29 the inertia of the algorithm. To allow larger bursts, W should be 30 decreased. 31 32 if (avg > th_max) -> packet marked (dropped). 33 if (avg < th_min) -> packet passes. 34 if (th_min < avg < th_max) we calculate probability: 35 36 Pb = max_P * (avg - th_min)/(th_max-th_min) 37 38 and mark (drop) packet with this probability. 39 Pb changes from 0 (at avg==th_min) to max_P (avg==th_max). 40 max_P should be small (not 1), usually 0.01..0.02 is good value. 41 42 max_P is chosen as a number, so that max_P/(th_max-th_min) 43 is a negative power of two in order arithmetic to contain 44 only shifts. 45 46 47 Parameters, settable by user: 48 ----------------------------- 49 50 qth_min - bytes (should be < qth_max/2) 51 qth_max - bytes (should be at least 2*qth_min and less limit) 52 Wlog - bits (<32) log(1/W). 53 Plog - bits (<32) 54 55 Plog is related to max_P by formula: 56 57 max_P = (qth_max-qth_min)/2^Plog; 58 59 F.e. if qth_max=128K and qth_min=32K, then Plog=22 60 corresponds to max_P=0.02 61 62 Scell_log 63 Stab 64 65 Lookup table for log((1-W)^(t/t_ave). 66 67 68 NOTES: 69 70 Upper bound on W. 71 ----------------- 72 73 If you want to allow bursts of L packets of size S, 74 you should choose W: 75 76 L + 1 - th_min/S < (1-(1-W)^L)/W 77 78 th_min/S = 32 th_min/S = 4 79 80 log(W) L 81 -1 33 82 -2 35 83 -3 39 84 -4 46 85 -5 57 86 -6 75 87 -7 101 88 -8 135 89 -9 190 90 etc. 91 */ 92 93 /* 94 * Adaptative RED : An Algorithm for Increasing the Robustness of RED's AQM 95 * (Sally FLoyd, Ramakrishna Gummadi, and Scott Shenker) August 2001 96 * 97 * Every 500 ms: 98 * if (avg > target and max_p <= 0.5) 99 * increase max_p : max_p += alpha; 100 * else if (avg < target and max_p >= 0.01) 101 * decrease max_p : max_p *= beta; 102 * 103 * target :[qth_min + 0.4*(qth_min - qth_max), 104 * qth_min + 0.6*(qth_min - qth_max)]. 105 * alpha : min(0.01, max_p / 4) 106 * beta : 0.9 107 * max_P is a Q0.32 fixed point number (with 32 bits mantissa) 108 * max_P between 0.01 and 0.5 (1% - 50%) [ Its no longer a negative power of two ] 109 */ 110 #define RED_ONE_PERCENT ((u32)DIV_ROUND_CLOSEST(1ULL<<32, 100)) 111 112 #define MAX_P_MIN (1 * RED_ONE_PERCENT) 113 #define MAX_P_MAX (50 * RED_ONE_PERCENT) 114 #define MAX_P_ALPHA(val) min(MAX_P_MIN, val / 4) 115 116 #define RED_STAB_SIZE 256 117 #define RED_STAB_MASK (RED_STAB_SIZE - 1) 118 119 struct red_stats { 120 u32 prob_drop; /* Early probability drops */ 121 u32 prob_mark; /* Early probability marks */ 122 u32 forced_drop; /* Forced drops, qavg > max_thresh */ 123 u32 forced_mark; /* Forced marks, qavg > max_thresh */ 124 u32 pdrop; /* Drops due to queue limits */ 125 }; 126 127 struct red_parms { 128 /* Parameters */ 129 u32 qth_min; /* Min avg length threshold: Wlog scaled */ 130 u32 qth_max; /* Max avg length threshold: Wlog scaled */ 131 u32 Scell_max; 132 u32 max_P; /* probability, [0 .. 1.0] 32 scaled */ 133 /* reciprocal_value(max_P / qth_delta) */ 134 struct reciprocal_value max_P_reciprocal; 135 u32 qth_delta; /* max_th - min_th */ 136 u32 target_min; /* min_th + 0.4*(max_th - min_th) */ 137 u32 target_max; /* min_th + 0.6*(max_th - min_th) */ 138 u8 Scell_log; 139 u8 Wlog; /* log(W) */ 140 u8 Plog; /* random number bits */ 141 u8 Stab[RED_STAB_SIZE]; 142 }; 143 144 struct red_vars { 145 /* Variables */ 146 int qcount; /* Number of packets since last random 147 number generation */ 148 u32 qR; /* Cached random number */ 149 150 unsigned long qavg; /* Average queue length: Wlog scaled */ 151 ktime_t qidlestart; /* Start of current idle period */ 152 }; 153 154 static inline u32 red_maxp(u8 Plog) 155 { 156 return Plog < 32 ? (~0U >> Plog) : ~0U; 157 } 158 159 static inline void red_set_vars(struct red_vars *v) 160 { 161 /* Reset average queue length, the value is strictly bound 162 * to the parameters below, resetting hurts a bit but leaving 163 * it might result in an unreasonable qavg for a while. --TGR 164 */ 165 v->qavg = 0; 166 167 v->qcount = -1; 168 } 169 170 static inline bool red_check_params(u32 qth_min, u32 qth_max, u8 Wlog, 171 u8 Scell_log, u8 *stab) 172 { 173 if (fls(qth_min) + Wlog >= 32) 174 return false; 175 if (fls(qth_max) + Wlog >= 32) 176 return false; 177 if (Scell_log >= 32) 178 return false; 179 if (qth_max < qth_min) 180 return false; 181 if (stab) { 182 int i; 183 184 for (i = 0; i < RED_STAB_SIZE; i++) 185 if (stab[i] >= 32) 186 return false; 187 } 188 return true; 189 } 190 191 static inline int red_get_flags(unsigned char qopt_flags, 192 unsigned char historic_mask, 193 struct nlattr *flags_attr, 194 unsigned char supported_mask, 195 struct nla_bitfield32 *p_flags, 196 unsigned char *p_userbits, 197 struct netlink_ext_ack *extack) 198 { 199 struct nla_bitfield32 flags; 200 201 if (qopt_flags && flags_attr) { 202 NL_SET_ERR_MSG_MOD(extack, "flags should be passed either through qopt, or through a dedicated attribute"); 203 return -EINVAL; 204 } 205 206 if (flags_attr) { 207 flags = nla_get_bitfield32(flags_attr); 208 } else { 209 flags.selector = historic_mask; 210 flags.value = qopt_flags & historic_mask; 211 } 212 213 *p_flags = flags; 214 *p_userbits = qopt_flags & ~historic_mask; 215 return 0; 216 } 217 218 static inline int red_validate_flags(unsigned char flags, 219 struct netlink_ext_ack *extack) 220 { 221 if ((flags & TC_RED_NODROP) && !(flags & TC_RED_ECN)) { 222 NL_SET_ERR_MSG_MOD(extack, "nodrop mode is only meaningful with ECN"); 223 return -EINVAL; 224 } 225 226 return 0; 227 } 228 229 static inline void red_set_parms(struct red_parms *p, 230 u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog, 231 u8 Scell_log, u8 *stab, u32 max_P) 232 { 233 int delta = qth_max - qth_min; 234 u32 max_p_delta; 235 236 WRITE_ONCE(p->qth_min, qth_min << Wlog); 237 WRITE_ONCE(p->qth_max, qth_max << Wlog); 238 WRITE_ONCE(p->Wlog, Wlog); 239 WRITE_ONCE(p->Plog, Plog); 240 if (delta <= 0) 241 delta = 1; 242 p->qth_delta = delta; 243 if (!max_P) { 244 max_P = red_maxp(Plog); 245 max_P *= delta; /* max_P = (qth_max - qth_min)/2^Plog */ 246 } 247 WRITE_ONCE(p->max_P, max_P); 248 max_p_delta = max_P / delta; 249 max_p_delta = max(max_p_delta, 1U); 250 p->max_P_reciprocal = reciprocal_value(max_p_delta); 251 252 /* RED Adaptative target : 253 * [min_th + 0.4*(min_th - max_th), 254 * min_th + 0.6*(min_th - max_th)]. 255 */ 256 delta /= 5; 257 p->target_min = qth_min + 2*delta; 258 p->target_max = qth_min + 3*delta; 259 260 WRITE_ONCE(p->Scell_log, Scell_log); 261 p->Scell_max = (255 << Scell_log); 262 263 if (stab) 264 memcpy(p->Stab, stab, sizeof(p->Stab)); 265 } 266 267 static inline int red_is_idling(const struct red_vars *v) 268 { 269 return v->qidlestart != 0; 270 } 271 272 static inline void red_start_of_idle_period(struct red_vars *v) 273 { 274 v->qidlestart = ktime_get(); 275 } 276 277 static inline void red_end_of_idle_period(struct red_vars *v) 278 { 279 v->qidlestart = 0; 280 } 281 282 static inline void red_restart(struct red_vars *v) 283 { 284 red_end_of_idle_period(v); 285 v->qavg = 0; 286 v->qcount = -1; 287 } 288 289 static inline unsigned long red_calc_qavg_from_idle_time(const struct red_parms *p, 290 const struct red_vars *v) 291 { 292 s64 delta = ktime_us_delta(ktime_get(), v->qidlestart); 293 long us_idle = min_t(s64, delta, p->Scell_max); 294 int shift; 295 296 /* 297 * The problem: ideally, average length queue recalculation should 298 * be done over constant clock intervals. This is too expensive, so 299 * that the calculation is driven by outgoing packets. 300 * When the queue is idle we have to model this clock by hand. 301 * 302 * SF+VJ proposed to "generate": 303 * 304 * m = idletime / (average_pkt_size / bandwidth) 305 * 306 * dummy packets as a burst after idle time, i.e. 307 * 308 * v->qavg *= (1-W)^m 309 * 310 * This is an apparently overcomplicated solution (f.e. we have to 311 * precompute a table to make this calculation in reasonable time) 312 * I believe that a simpler model may be used here, 313 * but it is field for experiments. 314 */ 315 316 shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK]; 317 318 if (shift) 319 return v->qavg >> shift; 320 else { 321 /* Approximate initial part of exponent with linear function: 322 * 323 * (1-W)^m ~= 1-mW + ... 324 * 325 * Seems, it is the best solution to 326 * problem of too coarse exponent tabulation. 327 */ 328 us_idle = (v->qavg * (u64)us_idle) >> p->Scell_log; 329 330 if (us_idle < (v->qavg >> 1)) 331 return v->qavg - us_idle; 332 else 333 return v->qavg >> 1; 334 } 335 } 336 337 static inline unsigned long red_calc_qavg_no_idle_time(const struct red_parms *p, 338 const struct red_vars *v, 339 unsigned int backlog) 340 { 341 /* 342 * NOTE: v->qavg is fixed point number with point at Wlog. 343 * The formula below is equivalent to floating point 344 * version: 345 * 346 * qavg = qavg*(1-W) + backlog*W; 347 * 348 * --ANK (980924) 349 */ 350 return v->qavg + (backlog - (v->qavg >> p->Wlog)); 351 } 352 353 static inline unsigned long red_calc_qavg(const struct red_parms *p, 354 const struct red_vars *v, 355 unsigned int backlog) 356 { 357 if (!red_is_idling(v)) 358 return red_calc_qavg_no_idle_time(p, v, backlog); 359 else 360 return red_calc_qavg_from_idle_time(p, v); 361 } 362 363 364 static inline u32 red_random(const struct red_parms *p) 365 { 366 return reciprocal_divide(get_random_u32(), p->max_P_reciprocal); 367 } 368 369 static inline int red_mark_probability(const struct red_parms *p, 370 const struct red_vars *v, 371 unsigned long qavg) 372 { 373 /* The formula used below causes questions. 374 375 OK. qR is random number in the interval 376 (0..1/max_P)*(qth_max-qth_min) 377 i.e. 0..(2^Plog). If we used floating point 378 arithmetic, it would be: (2^Plog)*rnd_num, 379 where rnd_num is less 1. 380 381 Taking into account, that qavg have fixed 382 point at Wlog, two lines 383 below have the following floating point equivalent: 384 385 max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount 386 387 Any questions? --ANK (980924) 388 */ 389 return !(((qavg - p->qth_min) >> p->Wlog) * v->qcount < v->qR); 390 } 391 392 enum { 393 RED_BELOW_MIN_THRESH, 394 RED_BETWEEN_TRESH, 395 RED_ABOVE_MAX_TRESH, 396 }; 397 398 static inline int red_cmp_thresh(const struct red_parms *p, unsigned long qavg) 399 { 400 if (qavg < p->qth_min) 401 return RED_BELOW_MIN_THRESH; 402 else if (qavg >= p->qth_max) 403 return RED_ABOVE_MAX_TRESH; 404 else 405 return RED_BETWEEN_TRESH; 406 } 407 408 enum { 409 RED_DONT_MARK, 410 RED_PROB_MARK, 411 RED_HARD_MARK, 412 }; 413 414 static inline int red_action(const struct red_parms *p, 415 struct red_vars *v, 416 unsigned long qavg) 417 { 418 switch (red_cmp_thresh(p, qavg)) { 419 case RED_BELOW_MIN_THRESH: 420 v->qcount = -1; 421 return RED_DONT_MARK; 422 423 case RED_BETWEEN_TRESH: 424 if (++v->qcount) { 425 if (red_mark_probability(p, v, qavg)) { 426 v->qcount = 0; 427 v->qR = red_random(p); 428 return RED_PROB_MARK; 429 } 430 } else 431 v->qR = red_random(p); 432 433 return RED_DONT_MARK; 434 435 case RED_ABOVE_MAX_TRESH: 436 v->qcount = -1; 437 return RED_HARD_MARK; 438 } 439 440 BUG(); 441 return RED_DONT_MARK; 442 } 443 444 static inline void red_adaptative_algo(struct red_parms *p, struct red_vars *v) 445 { 446 unsigned long qavg; 447 u32 max_p_delta; 448 449 qavg = v->qavg; 450 if (red_is_idling(v)) 451 qavg = red_calc_qavg_from_idle_time(p, v); 452 453 /* v->qavg is fixed point number with point at Wlog */ 454 qavg >>= p->Wlog; 455 456 if (qavg > p->target_max && p->max_P <= MAX_P_MAX) 457 p->max_P += MAX_P_ALPHA(p->max_P); /* maxp = maxp + alpha */ 458 else if (qavg < p->target_min && p->max_P >= MAX_P_MIN) 459 p->max_P = (p->max_P/10)*9; /* maxp = maxp * Beta */ 460 461 max_p_delta = DIV_ROUND_CLOSEST(p->max_P, p->qth_delta); 462 max_p_delta = max(max_p_delta, 1U); 463 p->max_P_reciprocal = reciprocal_value(max_p_delta); 464 } 465 #endif 466