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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 | 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 | |
103 | struct 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 | |
123 | static inline u32 red_rmask(u8 Plog) |
124 | { |
125 | return Plog < 32 ? ((1 << Plog) - 1) : ~0UL; |
126 | } |
127 | |
128 | static 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 | |
150 | static inline int red_is_idling(struct red_parms *p) |
151 | { |
152 | return p->qidlestart != PSCHED_PASTPERFECT; |
153 | } |
154 | |
155 | static inline void red_start_of_idle_period(struct red_parms *p) |
156 | { |
157 | p->qidlestart = psched_get_time(); |
158 | } |
159 | |
160 | static inline void red_end_of_idle_period(struct red_parms *p) |
161 | { |
162 | p->qidlestart = PSCHED_PASTPERFECT; |
163 | } |
164 | |
165 | static 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 | |
172 | static 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 | |
222 | static 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 | |
237 | static 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 | |
246 | static inline u32 red_random(struct red_parms *p) |
247 | { |
248 | return net_random() & p->Rmask; |
249 | } |
250 | |
251 | static 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 | |
272 | enum { |
273 | RED_BELOW_MIN_THRESH, |
274 | RED_BETWEEN_TRESH, |
275 | RED_ABOVE_MAX_TRESH, |
276 | }; |
277 | |
278 | static 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 | |
288 | enum { |
289 | RED_DONT_MARK, |
290 | RED_PROB_MARK, |
291 | RED_HARD_MARK, |
292 | }; |
293 | |
294 | static 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 |
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