xref: /openbmc/linux/include/net/codel.h (revision e23feb16) 
1 #ifndef __NET_SCHED_CODEL_H
2 #define __NET_SCHED_CODEL_H
3 
4 /*
5  * Codel - The Controlled-Delay Active Queue Management algorithm
6  *
7  *  Copyright (C) 2011-2012 Kathleen Nichols <nichols@pollere.com>
8  *  Copyright (C) 2011-2012 Van Jacobson <van@pollere.net>
9  *  Copyright (C) 2012 Michael D. Taht <dave.taht@bufferbloat.net>
10  *  Copyright (C) 2012 Eric Dumazet <edumazet@google.com>
11  *
12  * Redistribution and use in source and binary forms, with or without
13  * modification, are permitted provided that the following conditions
14  * are met:
15  * 1. Redistributions of source code must retain the above copyright
16  *    notice, this list of conditions, and the following disclaimer,
17  *    without modification.
18  * 2. Redistributions in binary form must reproduce the above copyright
19  *    notice, this list of conditions and the following disclaimer in the
20  *    documentation and/or other materials provided with the distribution.
21  * 3. The names of the authors may not be used to endorse or promote products
22  *    derived from this software without specific prior written permission.
23  *
24  * Alternatively, provided that this notice is retained in full, this
25  * software may be distributed under the terms of the GNU General
26  * Public License ("GPL") version 2, in which case the provisions of the
27  * GPL apply INSTEAD OF those given above.
28  *
29  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
30  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
31  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
32  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
33  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
34  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
35  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
36  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
37  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
38  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
39  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
40  * DAMAGE.
41  *
42  */
43 
44 #include <linux/types.h>
45 #include <linux/ktime.h>
46 #include <linux/skbuff.h>
47 #include <net/pkt_sched.h>
48 #include <net/inet_ecn.h>
49 #include <linux/reciprocal_div.h>
50 
51 /* Controlling Queue Delay (CoDel) algorithm
52  * =========================================
53  * Source : Kathleen Nichols and Van Jacobson
54  * http://queue.acm.org/detail.cfm?id=2209336
55  *
56  * Implemented on linux by Dave Taht and Eric Dumazet
57  */
58 
59 
60 /* CoDel uses a 1024 nsec clock, encoded in u32
61  * This gives a range of 2199 seconds, because of signed compares
62  */
63 typedef u32 codel_time_t;
64 typedef s32 codel_tdiff_t;
65 #define CODEL_SHIFT 10
66 #define MS2TIME(a) ((a * NSEC_PER_MSEC) >> CODEL_SHIFT)
67 
68 static inline codel_time_t codel_get_time(void)
69 {
70 	u64 ns = ktime_to_ns(ktime_get());
71 
72 	return ns >> CODEL_SHIFT;
73 }
74 
75 #define codel_time_after(a, b)		((s32)(a) - (s32)(b) > 0)
76 #define codel_time_after_eq(a, b)	((s32)(a) - (s32)(b) >= 0)
77 #define codel_time_before(a, b)		((s32)(a) - (s32)(b) < 0)
78 #define codel_time_before_eq(a, b)	((s32)(a) - (s32)(b) <= 0)
79 
80 /* Qdiscs using codel plugin must use codel_skb_cb in their own cb[] */
81 struct codel_skb_cb {
82 	codel_time_t enqueue_time;
83 };
84 
85 static struct codel_skb_cb *get_codel_cb(const struct sk_buff *skb)
86 {
87 	qdisc_cb_private_validate(skb, sizeof(struct codel_skb_cb));
88 	return (struct codel_skb_cb *)qdisc_skb_cb(skb)->data;
89 }
90 
91 static codel_time_t codel_get_enqueue_time(const struct sk_buff *skb)
92 {
93 	return get_codel_cb(skb)->enqueue_time;
94 }
95 
96 static void codel_set_enqueue_time(struct sk_buff *skb)
97 {
98 	get_codel_cb(skb)->enqueue_time = codel_get_time();
99 }
100 
101 static inline u32 codel_time_to_us(codel_time_t val)
102 {
103 	u64 valns = ((u64)val << CODEL_SHIFT);
104 
105 	do_div(valns, NSEC_PER_USEC);
106 	return (u32)valns;
107 }
108 
109 /**
110  * struct codel_params - contains codel parameters
111  * @target:	target queue size (in time units)
112  * @interval:	width of moving time window
113  * @ecn:	is Explicit Congestion Notification enabled
114  */
115 struct codel_params {
116 	codel_time_t	target;
117 	codel_time_t	interval;
118 	bool		ecn;
119 };
120 
121 /**
122  * struct codel_vars - contains codel variables
123  * @count:		how many drops we've done since the last time we
124  *			entered dropping state
125  * @lastcount:		count at entry to dropping state
126  * @dropping:		set to true if in dropping state
127  * @rec_inv_sqrt:	reciprocal value of sqrt(count) >> 1
128  * @first_above_time:	when we went (or will go) continuously above target
129  *			for interval
130  * @drop_next:		time to drop next packet, or when we dropped last
131  * @ldelay:		sojourn time of last dequeued packet
132  */
133 struct codel_vars {
134 	u32		count;
135 	u32		lastcount;
136 	bool		dropping;
137 	u16		rec_inv_sqrt;
138 	codel_time_t	first_above_time;
139 	codel_time_t	drop_next;
140 	codel_time_t	ldelay;
141 };
142 
143 #define REC_INV_SQRT_BITS (8 * sizeof(u16)) /* or sizeof_in_bits(rec_inv_sqrt) */
144 /* needed shift to get a Q0.32 number from rec_inv_sqrt */
145 #define REC_INV_SQRT_SHIFT (32 - REC_INV_SQRT_BITS)
146 
147 /**
148  * struct codel_stats - contains codel shared variables and stats
149  * @maxpacket:	largest packet we've seen so far
150  * @drop_count:	temp count of dropped packets in dequeue()
151  * ecn_mark:	number of packets we ECN marked instead of dropping
152  */
153 struct codel_stats {
154 	u32		maxpacket;
155 	u32		drop_count;
156 	u32		ecn_mark;
157 };
158 
159 static void codel_params_init(struct codel_params *params)
160 {
161 	params->interval = MS2TIME(100);
162 	params->target = MS2TIME(5);
163 	params->ecn = false;
164 }
165 
166 static void codel_vars_init(struct codel_vars *vars)
167 {
168 	memset(vars, 0, sizeof(*vars));
169 }
170 
171 static void codel_stats_init(struct codel_stats *stats)
172 {
173 	stats->maxpacket = 256;
174 }
175 
176 /*
177  * http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
178  * new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
179  *
180  * Here, invsqrt is a fixed point number (< 1.0), 32bit mantissa, aka Q0.32
181  */
182 static void codel_Newton_step(struct codel_vars *vars)
183 {
184 	u32 invsqrt = ((u32)vars->rec_inv_sqrt) << REC_INV_SQRT_SHIFT;
185 	u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 32;
186 	u64 val = (3LL << 32) - ((u64)vars->count * invsqrt2);
187 
188 	val >>= 2; /* avoid overflow in following multiply */
189 	val = (val * invsqrt) >> (32 - 2 + 1);
190 
191 	vars->rec_inv_sqrt = val >> REC_INV_SQRT_SHIFT;
192 }
193 
194 /*
195  * CoDel control_law is t + interval/sqrt(count)
196  * We maintain in rec_inv_sqrt the reciprocal value of sqrt(count) to avoid
197  * both sqrt() and divide operation.
198  */
199 static codel_time_t codel_control_law(codel_time_t t,
200 				      codel_time_t interval,
201 				      u32 rec_inv_sqrt)
202 {
203 	return t + reciprocal_divide(interval, rec_inv_sqrt << REC_INV_SQRT_SHIFT);
204 }
205 
206 
207 static bool codel_should_drop(const struct sk_buff *skb,
208 			      struct Qdisc *sch,
209 			      struct codel_vars *vars,
210 			      struct codel_params *params,
211 			      struct codel_stats *stats,
212 			      codel_time_t now)
213 {
214 	bool ok_to_drop;
215 
216 	if (!skb) {
217 		vars->first_above_time = 0;
218 		return false;
219 	}
220 
221 	vars->ldelay = now - codel_get_enqueue_time(skb);
222 	sch->qstats.backlog -= qdisc_pkt_len(skb);
223 
224 	if (unlikely(qdisc_pkt_len(skb) > stats->maxpacket))
225 		stats->maxpacket = qdisc_pkt_len(skb);
226 
227 	if (codel_time_before(vars->ldelay, params->target) ||
228 	    sch->qstats.backlog <= stats->maxpacket) {
229 		/* went below - stay below for at least interval */
230 		vars->first_above_time = 0;
231 		return false;
232 	}
233 	ok_to_drop = false;
234 	if (vars->first_above_time == 0) {
235 		/* just went above from below. If we stay above
236 		 * for at least interval we'll say it's ok to drop
237 		 */
238 		vars->first_above_time = now + params->interval;
239 	} else if (codel_time_after(now, vars->first_above_time)) {
240 		ok_to_drop = true;
241 	}
242 	return ok_to_drop;
243 }
244 
245 typedef struct sk_buff * (*codel_skb_dequeue_t)(struct codel_vars *vars,
246 						struct Qdisc *sch);
247 
248 static struct sk_buff *codel_dequeue(struct Qdisc *sch,
249 				     struct codel_params *params,
250 				     struct codel_vars *vars,
251 				     struct codel_stats *stats,
252 				     codel_skb_dequeue_t dequeue_func)
253 {
254 	struct sk_buff *skb = dequeue_func(vars, sch);
255 	codel_time_t now;
256 	bool drop;
257 
258 	if (!skb) {
259 		vars->dropping = false;
260 		return skb;
261 	}
262 	now = codel_get_time();
263 	drop = codel_should_drop(skb, sch, vars, params, stats, now);
264 	if (vars->dropping) {
265 		if (!drop) {
266 			/* sojourn time below target - leave dropping state */
267 			vars->dropping = false;
268 		} else if (codel_time_after_eq(now, vars->drop_next)) {
269 			/* It's time for the next drop. Drop the current
270 			 * packet and dequeue the next. The dequeue might
271 			 * take us out of dropping state.
272 			 * If not, schedule the next drop.
273 			 * A large backlog might result in drop rates so high
274 			 * that the next drop should happen now,
275 			 * hence the while loop.
276 			 */
277 			while (vars->dropping &&
278 			       codel_time_after_eq(now, vars->drop_next)) {
279 				vars->count++; /* dont care of possible wrap
280 						* since there is no more divide
281 						*/
282 				codel_Newton_step(vars);
283 				if (params->ecn && INET_ECN_set_ce(skb)) {
284 					stats->ecn_mark++;
285 					vars->drop_next =
286 						codel_control_law(vars->drop_next,
287 								  params->interval,
288 								  vars->rec_inv_sqrt);
289 					goto end;
290 				}
291 				qdisc_drop(skb, sch);
292 				stats->drop_count++;
293 				skb = dequeue_func(vars, sch);
294 				if (!codel_should_drop(skb, sch,
295 						       vars, params, stats, now)) {
296 					/* leave dropping state */
297 					vars->dropping = false;
298 				} else {
299 					/* and schedule the next drop */
300 					vars->drop_next =
301 						codel_control_law(vars->drop_next,
302 								  params->interval,
303 								  vars->rec_inv_sqrt);
304 				}
305 			}
306 		}
307 	} else if (drop) {
308 		u32 delta;
309 
310 		if (params->ecn && INET_ECN_set_ce(skb)) {
311 			stats->ecn_mark++;
312 		} else {
313 			qdisc_drop(skb, sch);
314 			stats->drop_count++;
315 
316 			skb = dequeue_func(vars, sch);
317 			drop = codel_should_drop(skb, sch, vars, params,
318 						 stats, now);
319 		}
320 		vars->dropping = true;
321 		/* if min went above target close to when we last went below it
322 		 * assume that the drop rate that controlled the queue on the
323 		 * last cycle is a good starting point to control it now.
324 		 */
325 		delta = vars->count - vars->lastcount;
326 		if (delta > 1 &&
327 		    codel_time_before(now - vars->drop_next,
328 				      16 * params->interval)) {
329 			vars->count = delta;
330 			/* we dont care if rec_inv_sqrt approximation
331 			 * is not very precise :
332 			 * Next Newton steps will correct it quadratically.
333 			 */
334 			codel_Newton_step(vars);
335 		} else {
336 			vars->count = 1;
337 			vars->rec_inv_sqrt = ~0U >> REC_INV_SQRT_SHIFT;
338 		}
339 		vars->lastcount = vars->count;
340 		vars->drop_next = codel_control_law(now, params->interval,
341 						    vars->rec_inv_sqrt);
342 	}
343 end:
344 	return skb;
345 }
346 #endif
347