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 arithmetics 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 u32 other; /* Drops due to drop() calls */
126 };
127
128 struct red_parms {
129 /* Parameters */
130 u32 qth_min; /* Min avg length threshold: Wlog scaled */
131 u32 qth_max; /* Max avg length threshold: Wlog scaled */
132 u32 Scell_max;
133 u32 max_P; /* probability, [0 .. 1.0] 32 scaled */
134 /* reciprocal_value(max_P / qth_delta) */
135 struct reciprocal_value max_P_reciprocal;
136 u32 qth_delta; /* max_th - min_th */
137 u32 target_min; /* min_th + 0.4*(max_th - min_th) */
138 u32 target_max; /* min_th + 0.6*(max_th - min_th) */
139 u8 Scell_log;
140 u8 Wlog; /* log(W) */
141 u8 Plog; /* random number bits */
142 u8 Stab[RED_STAB_SIZE];
143 };
144
145 struct red_vars {
146 /* Variables */
147 int qcount; /* Number of packets since last random
148 number generation */
149 u32 qR; /* Cached random number */
150
151 unsigned long qavg; /* Average queue length: Wlog scaled */
152 ktime_t qidlestart; /* Start of current idle period */
153 };
154
155 static inline u32 red_maxp(u8 Plog)
156 {
157 return Plog < 32 ? (~0U >> Plog) : ~0U;
158 }
159
160 static inline void red_set_vars(struct red_vars *v)
161 {
162 /* Reset average queue length, the value is strictly bound
163 * to the parameters below, reseting hurts a bit but leaving
164 * it might result in an unreasonable qavg for a while. --TGR
165 */
166 v->qavg = 0;
167
168 v->qcount = -1;
169 }
170
171 static inline bool red_check_params(u32 qth_min, u32 qth_max, u8 Wlog)
172 {
173 if (fls(qth_min) + Wlog > 32)
174 return false;
175 if (fls(qth_max) + Wlog > 32)
176 return false;
177 if (qth_max < qth_min)
178 return false;
179 return true;
180 }
181
182 static inline void red_set_parms(struct red_parms *p,
183 u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
184 u8 Scell_log, u8 *stab, u32 max_P)
185 {
186 int delta = qth_max - qth_min;
187 u32 max_p_delta;
188
189 p->qth_min = qth_min << Wlog;
190 p->qth_max = qth_max << Wlog;
191 p->Wlog = Wlog;
192 p->Plog = Plog;
193 if (delta <= 0)
194 delta = 1;
195 p->qth_delta = delta;
196 if (!max_P) {
197 max_P = red_maxp(Plog);
198 max_P *= delta; /* max_P = (qth_max - qth_min)/2^Plog */
199 }
200 p->max_P = max_P;
201 max_p_delta = max_P / delta;
202 max_p_delta = max(max_p_delta, 1U);
203 p->max_P_reciprocal = reciprocal_value(max_p_delta);
204
205 /* RED Adaptative target :
206 * [min_th + 0.4*(min_th - max_th),
207 * min_th + 0.6*(min_th - max_th)].
208 */
209 delta /= 5;
210 p->target_min = qth_min + 2*delta;
211 p->target_max = qth_min + 3*delta;
212
213 p->Scell_log = Scell_log;
214 p->Scell_max = (255 << Scell_log);
215
216 if (stab)
217 memcpy(p->Stab, stab, sizeof(p->Stab));
218 }
219
220 static inline int red_is_idling(const struct red_vars *v)
221 {
222 return v->qidlestart != 0;
223 }
224
225 static inline void red_start_of_idle_period(struct red_vars *v)
226 {
227 v->qidlestart = ktime_get();
228 }
229
230 static inline void red_end_of_idle_period(struct red_vars *v)
231 {
232 v->qidlestart = 0;
233 }
234
235 static inline void red_restart(struct red_vars *v)
236 {
237 red_end_of_idle_period(v);
238 v->qavg = 0;
239 v->qcount = -1;
240 }
241
242 static inline unsigned long red_calc_qavg_from_idle_time(const struct red_parms *p,
243 const struct red_vars *v)
244 {
245 s64 delta = ktime_us_delta(ktime_get(), v->qidlestart);
246 long us_idle = min_t(s64, delta, p->Scell_max);
247 int shift;
248
249 /*
250 * The problem: ideally, average length queue recalcultion should
251 * be done over constant clock intervals. This is too expensive, so
252 * that the calculation is driven by outgoing packets.
253 * When the queue is idle we have to model this clock by hand.
254 *
255 * SF+VJ proposed to "generate":
256 *
257 * m = idletime / (average_pkt_size / bandwidth)
258 *
259 * dummy packets as a burst after idle time, i.e.
260 *
261 * v->qavg *= (1-W)^m
262 *
263 * This is an apparently overcomplicated solution (f.e. we have to
264 * precompute a table to make this calculation in reasonable time)
265 * I believe that a simpler model may be used here,
266 * but it is field for experiments.
267 */
268
269 shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK];
270
271 if (shift)
272 return v->qavg >> shift;
273 else {
274 /* Approximate initial part of exponent with linear function:
275 *
276 * (1-W)^m ~= 1-mW + ...
277 *
278 * Seems, it is the best solution to
279 * problem of too coarse exponent tabulation.
280 */
281 us_idle = (v->qavg * (u64)us_idle) >> p->Scell_log;
282
283 if (us_idle < (v->qavg >> 1))
284 return v->qavg - us_idle;
285 else
286 return v->qavg >> 1;
287 }
288 }
289
290 static inline unsigned long red_calc_qavg_no_idle_time(const struct red_parms *p,
291 const struct red_vars *v,
292 unsigned int backlog)
293 {
294 /*
295 * NOTE: v->qavg is fixed point number with point at Wlog.
296 * The formula below is equvalent to floating point
297 * version:
298 *
299 * qavg = qavg*(1-W) + backlog*W;
300 *
301 * --ANK (980924)
302 */
303 return v->qavg + (backlog - (v->qavg >> p->Wlog));
304 }
305
306 static inline unsigned long red_calc_qavg(const struct red_parms *p,
307 const struct red_vars *v,
308 unsigned int backlog)
309 {
310 if (!red_is_idling(v))
311 return red_calc_qavg_no_idle_time(p, v, backlog);
312 else
313 return red_calc_qavg_from_idle_time(p, v);
314 }
315
316
317 static inline u32 red_random(const struct red_parms *p)
318 {
319 return reciprocal_divide(prandom_u32(), p->max_P_reciprocal);
320 }
321
322 static inline int red_mark_probability(const struct red_parms *p,
323 const struct red_vars *v,
324 unsigned long qavg)
325 {
326 /* The formula used below causes questions.
327
328 OK. qR is random number in the interval
329 (0..1/max_P)*(qth_max-qth_min)
330 i.e. 0..(2^Plog). If we used floating point
331 arithmetics, it would be: (2^Plog)*rnd_num,
332 where rnd_num is less 1.
333
334 Taking into account, that qavg have fixed
335 point at Wlog, two lines
336 below have the following floating point equivalent:
337
338 max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount
339
340 Any questions? --ANK (980924)
341 */
342 return !(((qavg - p->qth_min) >> p->Wlog) * v->qcount < v->qR);
343 }
344
345 enum {
346 RED_BELOW_MIN_THRESH,
347 RED_BETWEEN_TRESH,
348 RED_ABOVE_MAX_TRESH,
349 };
350
351 static inline int red_cmp_thresh(const struct red_parms *p, unsigned long qavg)
352 {
353 if (qavg < p->qth_min)
354 return RED_BELOW_MIN_THRESH;
355 else if (qavg >= p->qth_max)
356 return RED_ABOVE_MAX_TRESH;
357 else
358 return RED_BETWEEN_TRESH;
359 }
360
361 enum {
362 RED_DONT_MARK,
363 RED_PROB_MARK,
364 RED_HARD_MARK,
365 };
366
367 static inline int red_action(const struct red_parms *p,
368 struct red_vars *v,
369 unsigned long qavg)
370 {
371 switch (red_cmp_thresh(p, qavg)) {
372 case RED_BELOW_MIN_THRESH:
373 v->qcount = -1;
374 return RED_DONT_MARK;
375
376 case RED_BETWEEN_TRESH:
377 if (++v->qcount) {
378 if (red_mark_probability(p, v, qavg)) {
379 v->qcount = 0;
380 v->qR = red_random(p);
381 return RED_PROB_MARK;
382 }
383 } else
384 v->qR = red_random(p);
385
386 return RED_DONT_MARK;
387
388 case RED_ABOVE_MAX_TRESH:
389 v->qcount = -1;
390 return RED_HARD_MARK;
391 }
392
393 BUG();
394 return RED_DONT_MARK;
395 }
396
397 static inline void red_adaptative_algo(struct red_parms *p, struct red_vars *v)
398 {
399 unsigned long qavg;
400 u32 max_p_delta;
401
402 qavg = v->qavg;
403 if (red_is_idling(v))
404 qavg = red_calc_qavg_from_idle_time(p, v);
405
406 /* v->qavg is fixed point number with point at Wlog */
407 qavg >>= p->Wlog;
408
409 if (qavg > p->target_max && p->max_P <= MAX_P_MAX)
410 p->max_P += MAX_P_ALPHA(p->max_P); /* maxp = maxp + alpha */
411 else if (qavg < p->target_min && p->max_P >= MAX_P_MIN)
412 p->max_P = (p->max_P/10)*9; /* maxp = maxp * Beta */
413
414 max_p_delta = DIV_ROUND_CLOSEST(p->max_P, p->qth_delta);
415 max_p_delta = max(max_p_delta, 1U);
416 p->max_P_reciprocal = reciprocal_value(max_p_delta);
417 }
418 #endif