ftrace: Documentation
[linux-2.6.git] / include / net / red.h
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 {
95         u32             prob_drop;      /* Early probability drops */
96         u32             prob_mark;      /* Early probability marks */
97         u32             forced_drop;    /* Forced drops, qavg > max_thresh */
98         u32             forced_mark;    /* Forced marks, qavg > max_thresh */
99         u32             pdrop;          /* Drops due to queue limits */
100         u32             other;          /* Drops due to drop() calls */
101         u32             backlog;
102 };
103
104 struct red_parms
105 {
106         /* Parameters */
107         u32             qth_min;        /* Min avg length threshold: A scaled */
108         u32             qth_max;        /* Max avg length threshold: A scaled */
109         u32             Scell_max;
110         u32             Rmask;          /* Cached random mask, see red_rmask */
111         u8              Scell_log;
112         u8              Wlog;           /* log(W)               */
113         u8              Plog;           /* random number bits   */
114         u8              Stab[RED_STAB_SIZE];
115
116         /* Variables */
117         int             qcount;         /* Number of packets since last random
118                                            number generation */
119         u32             qR;             /* Cached random number */
120
121         unsigned long   qavg;           /* Average queue length: A scaled */
122         psched_time_t   qidlestart;     /* Start of current idle period */
123 };
124
125 static inline u32 red_rmask(u8 Plog)
126 {
127         return Plog < 32 ? ((1 << Plog) - 1) : ~0UL;
128 }
129
130 static inline void red_set_parms(struct red_parms *p,
131                                  u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
132                                  u8 Scell_log, u8 *stab)
133 {
134         /* Reset average queue length, the value is strictly bound
135          * to the parameters below, reseting hurts a bit but leaving
136          * it might result in an unreasonable qavg for a while. --TGR
137          */
138         p->qavg         = 0;
139
140         p->qcount       = -1;
141         p->qth_min      = qth_min << Wlog;
142         p->qth_max      = qth_max << Wlog;
143         p->Wlog         = Wlog;
144         p->Plog         = Plog;
145         p->Rmask        = red_rmask(Plog);
146         p->Scell_log    = Scell_log;
147         p->Scell_max    = (255 << Scell_log);
148
149         memcpy(p->Stab, stab, sizeof(p->Stab));
150 }
151
152 static inline int red_is_idling(struct red_parms *p)
153 {
154         return p->qidlestart != PSCHED_PASTPERFECT;
155 }
156
157 static inline void red_start_of_idle_period(struct red_parms *p)
158 {
159         p->qidlestart = psched_get_time();
160 }
161
162 static inline void red_end_of_idle_period(struct red_parms *p)
163 {
164         p->qidlestart = PSCHED_PASTPERFECT;
165 }
166
167 static inline void red_restart(struct red_parms *p)
168 {
169         red_end_of_idle_period(p);
170         p->qavg = 0;
171         p->qcount = -1;
172 }
173
174 static inline unsigned long red_calc_qavg_from_idle_time(struct red_parms *p)
175 {
176         psched_time_t now;
177         long us_idle;
178         int  shift;
179
180         now = psched_get_time();
181         us_idle = psched_tdiff_bounded(now, p->qidlestart, p->Scell_max);
182
183         /*
184          * The problem: ideally, average length queue recalcultion should
185          * be done over constant clock intervals. This is too expensive, so
186          * that the calculation is driven by outgoing packets.
187          * When the queue is idle we have to model this clock by hand.
188          *
189          * SF+VJ proposed to "generate":
190          *
191          *      m = idletime / (average_pkt_size / bandwidth)
192          *
193          * dummy packets as a burst after idle time, i.e.
194          *
195          *      p->qavg *= (1-W)^m
196          *
197          * This is an apparently overcomplicated solution (f.e. we have to
198          * precompute a table to make this calculation in reasonable time)
199          * I believe that a simpler model may be used here,
200          * but it is field for experiments.
201          */
202
203         shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK];
204
205         if (shift)
206                 return p->qavg >> shift;
207         else {
208                 /* Approximate initial part of exponent with linear function:
209                  *
210                  *      (1-W)^m ~= 1-mW + ...
211                  *
212                  * Seems, it is the best solution to
213                  * problem of too coarse exponent tabulation.
214                  */
215                 us_idle = (p->qavg * (u64)us_idle) >> p->Scell_log;
216
217                 if (us_idle < (p->qavg >> 1))
218                         return p->qavg - us_idle;
219                 else
220                         return p->qavg >> 1;
221         }
222 }
223
224 static inline unsigned long red_calc_qavg_no_idle_time(struct red_parms *p,
225                                                        unsigned int backlog)
226 {
227         /*
228          * NOTE: p->qavg is fixed point number with point at Wlog.
229          * The formula below is equvalent to floating point
230          * version:
231          *
232          *      qavg = qavg*(1-W) + backlog*W;
233          *
234          * --ANK (980924)
235          */
236         return p->qavg + (backlog - (p->qavg >> p->Wlog));
237 }
238
239 static inline unsigned long red_calc_qavg(struct red_parms *p,
240                                           unsigned int backlog)
241 {
242         if (!red_is_idling(p))
243                 return red_calc_qavg_no_idle_time(p, backlog);
244         else
245                 return red_calc_qavg_from_idle_time(p);
246 }
247
248 static inline u32 red_random(struct red_parms *p)
249 {
250         return net_random() & p->Rmask;
251 }
252
253 static inline int red_mark_probability(struct red_parms *p, unsigned long qavg)
254 {
255         /* The formula used below causes questions.
256
257            OK. qR is random number in the interval 0..Rmask
258            i.e. 0..(2^Plog). If we used floating point
259            arithmetics, it would be: (2^Plog)*rnd_num,
260            where rnd_num is less 1.
261
262            Taking into account, that qavg have fixed
263            point at Wlog, and Plog is related to max_P by
264            max_P = (qth_max-qth_min)/2^Plog; two lines
265            below have the following floating point equivalent:
266
267            max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount
268
269            Any questions? --ANK (980924)
270          */
271         return !(((qavg - p->qth_min) >> p->Wlog) * p->qcount < p->qR);
272 }
273
274 enum {
275         RED_BELOW_MIN_THRESH,
276         RED_BETWEEN_TRESH,
277         RED_ABOVE_MAX_TRESH,
278 };
279
280 static inline int red_cmp_thresh(struct red_parms *p, unsigned long qavg)
281 {
282         if (qavg < p->qth_min)
283                 return RED_BELOW_MIN_THRESH;
284         else if (qavg >= p->qth_max)
285                 return RED_ABOVE_MAX_TRESH;
286         else
287                 return RED_BETWEEN_TRESH;
288 }
289
290 enum {
291         RED_DONT_MARK,
292         RED_PROB_MARK,
293         RED_HARD_MARK,
294 };
295
296 static inline int red_action(struct red_parms *p, unsigned long qavg)
297 {
298         switch (red_cmp_thresh(p, qavg)) {
299                 case RED_BELOW_MIN_THRESH:
300                         p->qcount = -1;
301                         return RED_DONT_MARK;
302
303                 case RED_BETWEEN_TRESH:
304                         if (++p->qcount) {
305                                 if (red_mark_probability(p, qavg)) {
306                                         p->qcount = 0;
307                                         p->qR = red_random(p);
308                                         return RED_PROB_MARK;
309                                 }
310                         } else
311                                 p->qR = red_random(p);
312
313                         return RED_DONT_MARK;
314
315                 case RED_ABOVE_MAX_TRESH:
316                         p->qcount = -1;
317                         return RED_HARD_MARK;
318         }
319
320         BUG();
321         return RED_DONT_MARK;
322 }
323
324 #endif