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1 #ifndef __NET_SCHED_RED_H
2 #define __NET_SCHED_RED_H
3
4 #include <linux/config.h>
5 #include <linux/types.h>
6 #include <net/pkt_sched.h>
7 #include <net/inet_ecn.h>
8 #include <net/dsfield.h>
9
10 /*      Random Early Detection (RED) algorithm.
11         =======================================
12
13         Source: Sally Floyd and Van Jacobson, "Random Early Detection Gateways
14         for Congestion Avoidance", 1993, IEEE/ACM Transactions on Networking.
15
16         This file codes a "divisionless" version of RED algorithm
17         as written down in Fig.17 of the paper.
18
19         Short description.
20         ------------------
21
22         When a new packet arrives we calculate the average queue length:
23
24         avg = (1-W)*avg + W*current_queue_len,
25
26         W is the filter time constant (chosen as 2^(-Wlog)), it controls
27         the inertia of the algorithm. To allow larger bursts, W should be
28         decreased.
29
30         if (avg > th_max) -> packet marked (dropped).
31         if (avg < th_min) -> packet passes.
32         if (th_min < avg < th_max) we calculate probability:
33
34         Pb = max_P * (avg - th_min)/(th_max-th_min)
35
36         and mark (drop) packet with this probability.
37         Pb changes from 0 (at avg==th_min) to max_P (avg==th_max).
38         max_P should be small (not 1), usually 0.01..0.02 is good value.
39
40         max_P is chosen as a number, so that max_P/(th_max-th_min)
41         is a negative power of two in order arithmetics to contain
42         only shifts.
43
44
45         Parameters, settable by user:
46         -----------------------------
47
48         qth_min         - bytes (should be < qth_max/2)
49         qth_max         - bytes (should be at least 2*qth_min and less limit)
50         Wlog            - bits (<32) log(1/W).
51         Plog            - bits (<32)
52
53         Plog is related to max_P by formula:
54
55         max_P = (qth_max-qth_min)/2^Plog;
56
57         F.e. if qth_max=128K and qth_min=32K, then Plog=22
58         corresponds to max_P=0.02
59
60         Scell_log
61         Stab
62
63         Lookup table for log((1-W)^(t/t_ave).
64
65
66         NOTES:
67
68         Upper bound on W.
69         -----------------
70
71         If you want to allow bursts of L packets of size S,
72         you should choose W:
73
74         L + 1 - th_min/S < (1-(1-W)^L)/W
75
76         th_min/S = 32         th_min/S = 4
77
78         log(W)  L
79         -1      33
80         -2      35
81         -3      39
82         -4      46
83         -5      57
84         -6      75
85         -7      101
86         -8      135
87         -9      190
88         etc.
89  */
90
91 #define RED_STAB_SIZE   256
92 #define RED_STAB_MASK   (RED_STAB_SIZE - 1)
93
94 struct red_stats
95 {
96         u32             prob_drop;      /* Early probability drops */
97         u32             prob_mark;      /* Early probability marks */
98         u32             forced_drop;    /* Forced drops, qavg > max_thresh */
99         u32             forced_mark;    /* Forced marks, qavg > max_thresh */
100         u32             pdrop;          /* Drops due to queue limits */
101         u32             other;          /* Drops due to drop() calls */
102         u32             backlog;
103 };
104
105 struct red_parms
106 {
107         /* Parameters */
108         u32             qth_min;        /* Min avg length threshold: A scaled */
109         u32             qth_max;        /* Max avg length threshold: A scaled */
110         u32             Scell_max;
111         u32             Rmask;          /* Cached random mask, see red_rmask */
112         u8              Scell_log;
113         u8              Wlog;           /* log(W)               */
114         u8              Plog;           /* random number bits   */
115         u8              Stab[RED_STAB_SIZE];
116
117         /* Variables */
118         int             qcount;         /* Number of packets since last random
119                                            number generation */
120         u32             qR;             /* Cached random number */
121
122         unsigned long   qavg;           /* Average queue length: A scaled */
123         psched_time_t   qidlestart;     /* Start of current idle period */
124 };
125
126 static inline u32 red_rmask(u8 Plog)
127 {
128         return Plog < 32 ? ((1 << Plog) - 1) : ~0UL;
129 }
130
131 static inline void red_set_parms(struct red_parms *p,
132                                  u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
133                                  u8 Scell_log, u8 *stab)
134 {
135         /* Reset average queue length, the value is strictly bound
136          * to the parameters below, reseting hurts a bit but leaving
137          * it might result in an unreasonable qavg for a while. --TGR
138          */
139         p->qavg         = 0;
140
141         p->qcount       = -1;
142         p->qth_min      = qth_min << Wlog;
143         p->qth_max      = qth_max << Wlog;
144         p->Wlog         = Wlog;
145         p->Plog         = Plog;
146         p->Rmask        = red_rmask(Plog);
147         p->Scell_log    = Scell_log;
148         p->Scell_max    = (255 << Scell_log);
149
150         memcpy(p->Stab, stab, sizeof(p->Stab));
151 }
152
153 static inline int red_is_idling(struct red_parms *p)
154 {
155         return !PSCHED_IS_PASTPERFECT(p->qidlestart);
156 }
157
158 static inline void red_start_of_idle_period(struct red_parms *p)
159 {
160         PSCHED_GET_TIME(p->qidlestart);
161 }
162
163 static inline void red_end_of_idle_period(struct red_parms *p)
164 {
165         PSCHED_SET_PASTPERFECT(p->qidlestart);
166 }
167
168 static inline void red_restart(struct red_parms *p)
169 {
170         red_end_of_idle_period(p);
171         p->qavg = 0;
172         p->qcount = -1;
173 }
174
175 static inline unsigned long red_calc_qavg_from_idle_time(struct red_parms *p)
176 {
177         psched_time_t now;
178         long us_idle;
179         int  shift;
180
181         PSCHED_GET_TIME(now);
182         us_idle = PSCHED_TDIFF_SAFE(now, p->qidlestart, p->Scell_max);
183
184         /*
185          * The problem: ideally, average length queue recalcultion should
186          * be done over constant clock intervals. This is too expensive, so
187          * that the calculation is driven by outgoing packets.
188          * When the queue is idle we have to model this clock by hand.
189          *
190          * SF+VJ proposed to "generate":
191          *
192          *      m = idletime / (average_pkt_size / bandwidth)
193          *
194          * dummy packets as a burst after idle time, i.e.
195          *
196          *      p->qavg *= (1-W)^m
197          *
198          * This is an apparently overcomplicated solution (f.e. we have to
199          * precompute a table to make this calculation in reasonable time)
200          * I believe that a simpler model may be used here,
201          * but it is field for experiments.
202          */
203
204         shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK];
205
206         if (shift)
207                 return p->qavg >> shift;
208         else {
209                 /* Approximate initial part of exponent with linear function:
210                  *
211                  *      (1-W)^m ~= 1-mW + ...
212                  *
213                  * Seems, it is the best solution to
214                  * problem of too coarse exponent tabulation.
215                  */
216                 us_idle = (p->qavg * us_idle) >> p->Scell_log;
217
218                 if (us_idle < (p->qavg >> 1))
219                         return p->qavg - us_idle;
220                 else
221                         return p->qavg >> 1;
222         }
223 }
224
225 static inline unsigned long red_calc_qavg_no_idle_time(struct red_parms *p,
226                                                        unsigned int backlog)
227 {
228         /*
229          * NOTE: p->qavg is fixed point number with point at Wlog.
230          * The formula below is equvalent to floating point
231          * version:
232          *
233          *      qavg = qavg*(1-W) + backlog*W;
234          *
235          * --ANK (980924)
236          */
237         return p->qavg + (backlog - (p->qavg >> p->Wlog));
238 }
239
240 static inline unsigned long red_calc_qavg(struct red_parms *p,
241                                           unsigned int backlog)
242 {
243         if (!red_is_idling(p))
244                 return red_calc_qavg_no_idle_time(p, backlog);
245         else
246                 return red_calc_qavg_from_idle_time(p);
247 }
248
249 static inline u32 red_random(struct red_parms *p)
250 {
251         return net_random() & p->Rmask;
252 }
253
254 static inline int red_mark_probability(struct red_parms *p, unsigned long qavg)
255 {
256         /* The formula used below causes questions.
257
258            OK. qR is random number in the interval 0..Rmask
259            i.e. 0..(2^Plog). If we used floating point
260            arithmetics, it would be: (2^Plog)*rnd_num,
261            where rnd_num is less 1.
262
263            Taking into account, that qavg have fixed
264            point at Wlog, and Plog is related to max_P by
265            max_P = (qth_max-qth_min)/2^Plog; two lines
266            below have the following floating point equivalent:
267
268            max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount
269
270            Any questions? --ANK (980924)
271          */
272         return !(((qavg - p->qth_min) >> p->Wlog) * p->qcount < p->qR);
273 }
274
275 enum {
276         RED_BELOW_MIN_THRESH,
277         RED_BETWEEN_TRESH,
278         RED_ABOVE_MAX_TRESH,
279 };
280
281 static inline int red_cmp_thresh(struct red_parms *p, unsigned long qavg)
282 {
283         if (qavg < p->qth_min)
284                 return RED_BELOW_MIN_THRESH;
285         else if (qavg >= p->qth_max)
286                 return RED_ABOVE_MAX_TRESH;
287         else
288                 return RED_BETWEEN_TRESH;
289 }
290
291 enum {
292         RED_DONT_MARK,
293         RED_PROB_MARK,
294         RED_HARD_MARK,
295 };
296
297 static inline int red_action(struct red_parms *p, unsigned long qavg)
298 {
299         switch (red_cmp_thresh(p, qavg)) {
300                 case RED_BELOW_MIN_THRESH:
301                         p->qcount = -1;
302                         return RED_DONT_MARK;
303
304                 case RED_BETWEEN_TRESH:
305                         if (++p->qcount) {
306                                 if (red_mark_probability(p, qavg)) {
307                                         p->qcount = 0;
308                                         p->qR = red_random(p);
309                                         return RED_PROB_MARK;
310                                 }
311                         } else
312                                 p->qR = red_random(p);
313
314                         return RED_DONT_MARK;
315
316                 case RED_ABOVE_MAX_TRESH:
317                         p->qcount = -1;
318                         return RED_HARD_MARK;
319         }
320
321         BUG();
322         return RED_DONT_MARK;
323 }
324
325 #endif