include/net/codel.h

   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
  50 /* Controlling Queue Delay (CoDel) algorithm
  51  * =========================================
  52  * Source : Kathleen Nichols and Van Jacobson
  53  * http://queue.acm.org/detail.cfm?id=2209336
  54  *
  55  * Implemented on linux by Dave Taht and Eric Dumazet
  56  */
  57
  58
  59 /* CoDel uses a 1024 nsec clock, encoded in u32
  60  * This gives a range of 2199 seconds, because of signed compares
  61  */
  62 typedef u32 codel_time_t;
  63 typedef s32 codel_tdiff_t;
  64 #define CODEL_SHIFT 10
  65 #define MS2TIME(a) ((a * NSEC_PER_MSEC) >> CODEL_SHIFT)
  66
  67 static inline codel_time_t codel_get_time(void)
  68 {
  69         u64 ns = ktime_get_ns();
  70
  71         return ns >> CODEL_SHIFT;
  72 }
  73
  74 /* Dealing with timer wrapping, according to RFC 1982, as desc in wikipedia:
  75  *  https://en.wikipedia.org/wiki/Serial_number_arithmetic#General_Solution
  76  * codel_time_after(a,b) returns true if the time a is after time b.
  77  */
  78 #define codel_time_after(a, b)                                          \
  79         (typecheck(codel_time_t, a) &&                                  \
  80          typecheck(codel_time_t, b) &&                                  \
  81          ((s32)((a) - (b)) > 0))
  82 #define codel_time_before(a, b)         codel_time_after(b, a)
  83
  84 #define codel_time_after_eq(a, b)                                       \
  85         (typecheck(codel_time_t, a) &&                                  \
  86          typecheck(codel_time_t, b) &&                                  \
  87          ((s32)((a) - (b)) >= 0))
  88 #define codel_time_before_eq(a, b)      codel_time_after_eq(b, a)
  89
  90 /* Qdiscs using codel plugin must use codel_skb_cb in their own cb[] */
  91 struct codel_skb_cb {
  92         codel_time_t enqueue_time;
  93 };
  94
  95 static struct codel_skb_cb *get_codel_cb(const struct sk_buff *skb)
  96 {
  97         qdisc_cb_private_validate(skb, sizeof(struct codel_skb_cb));
  98         return (struct codel_skb_cb *)qdisc_skb_cb(skb)->data;
  99 }
 100
 101 static codel_time_t codel_get_enqueue_time(const struct sk_buff *skb)
 102 {
 103         return get_codel_cb(skb)->enqueue_time;
 104 }
 105
 106 static void codel_set_enqueue_time(struct sk_buff *skb)
 107 {
 108         get_codel_cb(skb)->enqueue_time = codel_get_time();
 109 }
 110
 111 static inline u32 codel_time_to_us(codel_time_t val)
 112 {
 113         u64 valns = ((u64)val << CODEL_SHIFT);
 114
 115         do_div(valns, NSEC_PER_USEC);
 116         return (u32)valns;
 117 }
 118
 119 /**
 120  * struct codel_params - contains codel parameters
 121  * @target:     target queue size (in time units)
 122  * @interval:   width of moving time window
 123  * @ecn:        is Explicit Congestion Notification enabled
 124  */
 125 struct codel_params {
 126         codel_time_t    target;
 127         codel_time_t    interval;
 128         bool            ecn;
 129 };
 130
 131 /**
 132  * struct codel_vars - contains codel variables
 133  * @count:              how many drops we've done since the last time we
 134  *                      entered dropping state
 135  * @lastcount:          count at entry to dropping state
 136  * @dropping:           set to true if in dropping state
 137  * @rec_inv_sqrt:       reciprocal value of sqrt(count) >> 1
 138  * @first_above_time:   when we went (or will go) continuously above target
 139  *                      for interval
 140  * @drop_next:          time to drop next packet, or when we dropped last
 141  * @ldelay:             sojourn time of last dequeued packet
 142  */
 143 struct codel_vars {
 144         u32             count;
 145         u32             lastcount;
 146         bool            dropping;
 147         u16             rec_inv_sqrt;
 148         codel_time_t    first_above_time;
 149         codel_time_t    drop_next;
 150         codel_time_t    ldelay;
 151 };
 152
 153 #define REC_INV_SQRT_BITS (8 * sizeof(u16)) /* or sizeof_in_bits(rec_inv_sqrt) */
 154 /* needed shift to get a Q0.32 number from rec_inv_sqrt */
 155 #define REC_INV_SQRT_SHIFT (32 - REC_INV_SQRT_BITS)
 156
 157 /**
 158  * struct codel_stats - contains codel shared variables and stats
 159  * @maxpacket:  largest packet we've seen so far
 160  * @drop_count: temp count of dropped packets in dequeue()
 161  * ecn_mark:    number of packets we ECN marked instead of dropping
 162  */
 163 struct codel_stats {
 164         u32             maxpacket;
 165         u32             drop_count;
 166         u32             ecn_mark;
 167 };
 168
 169 static void codel_params_init(struct codel_params *params)
 170 {
 171         params->interval = MS2TIME(100);
 172         params->target = MS2TIME(5);
 173         params->ecn = false;
 174 }
 175
 176 static void codel_vars_init(struct codel_vars *vars)
 177 {
 178         memset(vars, 0, sizeof(*vars));
 179 }
 180
 181 static void codel_stats_init(struct codel_stats *stats)
 182 {
 183         stats->maxpacket = 256;
 184 }
 185
 186 /*
 187  * http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
 188  * new_invsqrt = (invsqrt / 2) * (3 - count * invsqrt^2)
 189  *
 190  * Here, invsqrt is a fixed point number (< 1.0), 32bit mantissa, aka Q0.32
 191  */
 192 static void codel_Newton_step(struct codel_vars *vars)
 193 {
 194         u32 invsqrt = ((u32)vars->rec_inv_sqrt) << REC_INV_SQRT_SHIFT;
 195         u32 invsqrt2 = ((u64)invsqrt * invsqrt) >> 32;
 196         u64 val = (3LL << 32) - ((u64)vars->count * invsqrt2);
 197
 198         val >>= 2; /* avoid overflow in following multiply */
 199         val = (val * invsqrt) >> (32 - 2 + 1);
 200
 201         vars->rec_inv_sqrt = val >> REC_INV_SQRT_SHIFT;
 202 }
 203
 204 /*
 205  * CoDel control_law is t + interval/sqrt(count)
 206  * We maintain in rec_inv_sqrt the reciprocal value of sqrt(count) to avoid
 207  * both sqrt() and divide operation.
 208  */
 209 static codel_time_t codel_control_law(codel_time_t t,
 210                                       codel_time_t interval,
 211                                       u32 rec_inv_sqrt)
 212 {
 213         return t + reciprocal_scale(interval, rec_inv_sqrt << REC_INV_SQRT_SHIFT);
 214 }
 215
 216 static bool codel_should_drop(const struct sk_buff *skb,
 217                               struct Qdisc *sch,
 218                               struct codel_vars *vars,
 219                               struct codel_params *params,
 220                               struct codel_stats *stats,
 221                               codel_time_t now)
 222 {
 223         bool ok_to_drop;
 224
 225         if (!skb) {
 226                 vars->first_above_time = 0;
 227                 return false;
 228         }
 229
 230         vars->ldelay = now - codel_get_enqueue_time(skb);
 231         sch->qstats.backlog -= qdisc_pkt_len(skb);
 232
 233         if (unlikely(qdisc_pkt_len(skb) > stats->maxpacket))
 234                 stats->maxpacket = qdisc_pkt_len(skb);
 235
 236         if (codel_time_before(vars->ldelay, params->target) ||
 237             sch->qstats.backlog <= stats->maxpacket) {
 238                 /* went below - stay below for at least interval */
 239                 vars->first_above_time = 0;
 240                 return false;
 241         }
 242         ok_to_drop = false;
 243         if (vars->first_above_time == 0) {
 244                 /* just went above from below. If we stay above
 245                  * for at least interval we'll say it's ok to drop
 246                  */
 247                 vars->first_above_time = now + params->interval;
 248         } else if (codel_time_after(now, vars->first_above_time)) {
 249                 ok_to_drop = true;
 250         }
 251         return ok_to_drop;
 252 }
 253
 254 typedef struct sk_buff * (*codel_skb_dequeue_t)(struct codel_vars *vars,
 255                                                 struct Qdisc *sch);
 256
 257 static struct sk_buff *codel_dequeue(struct Qdisc *sch,
 258                                      struct codel_params *params,
 259                                      struct codel_vars *vars,
 260                                      struct codel_stats *stats,
 261                                      codel_skb_dequeue_t dequeue_func)
 262 {
 263         struct sk_buff *skb = dequeue_func(vars, sch);
 264         codel_time_t now;
 265         bool drop;
 266
 267         if (!skb) {
 268                 vars->dropping = false;
 269                 return skb;
 270         }
 271         now = codel_get_time();
 272         drop = codel_should_drop(skb, sch, vars, params, stats, now);
 273         if (vars->dropping) {
 274                 if (!drop) {
 275                         /* sojourn time below target - leave dropping state */
 276                         vars->dropping = false;
 277                 } else if (codel_time_after_eq(now, vars->drop_next)) {
 278                         /* It's time for the next drop. Drop the current
 279                          * packet and dequeue the next. The dequeue might
 280                          * take us out of dropping state.
 281                          * If not, schedule the next drop.
 282                          * A large backlog might result in drop rates so high
 283                          * that the next drop should happen now,
 284                          * hence the while loop.
 285                          */
 286                         while (vars->dropping &&
 287                                codel_time_after_eq(now, vars->drop_next)) {
 288                                 vars->count++; /* dont care of possible wrap
 289                                                 * since there is no more divide
 290                                                 */
 291                                 codel_Newton_step(vars);
 292                                 if (params->ecn && INET_ECN_set_ce(skb)) {
 293                                         stats->ecn_mark++;
 294                                         vars->drop_next =
 295                                                 codel_control_law(vars->drop_next,
 296                                                                   params->interval,
 297                                                                   vars->rec_inv_sqrt);
 298                                         goto end;
 299                                 }
 300                                 qdisc_drop(skb, sch);
 301                                 stats->drop_count++;
 302                                 skb = dequeue_func(vars, sch);
 303                                 if (!codel_should_drop(skb, sch,
 304                                                        vars, params, stats, now)) {
 305                                         /* leave dropping state */
 306                                         vars->dropping = false;
 307                                 } else {
 308                                         /* and schedule the next drop */
 309                                         vars->drop_next =
 310                                                 codel_control_law(vars->drop_next,
 311                                                                   params->interval,
 312                                                                   vars->rec_inv_sqrt);
 313                                 }
 314                         }
 315                 }
 316         } else if (drop) {
 317                 u32 delta;
 318
 319                 if (params->ecn && INET_ECN_set_ce(skb)) {
 320                         stats->ecn_mark++;
 321                 } else {
 322                         qdisc_drop(skb, sch);
 323                         stats->drop_count++;
 324
 325                         skb = dequeue_func(vars, sch);
 326                         drop = codel_should_drop(skb, sch, vars, params,
 327                                                  stats, now);
 328                 }
 329                 vars->dropping = true;
 330                 /* if min went above target close to when we last went below it
 331                  * assume that the drop rate that controlled the queue on the
 332                  * last cycle is a good starting point to control it now.
 333                  */
 334                 delta = vars->count - vars->lastcount;
 335                 if (delta > 1 &&
 336                     codel_time_before(now - vars->drop_next,
 337                                       16 * params->interval)) {
 338                         vars->count = delta;
 339                         /* we dont care if rec_inv_sqrt approximation
 340                          * is not very precise :
 341                          * Next Newton steps will correct it quadratically.
 342                          */
 343                         codel_Newton_step(vars);
 344                 } else {
 345                         vars->count = 1;
 346                         vars->rec_inv_sqrt = ~0U >> REC_INV_SQRT_SHIFT;
 347                 }
 348                 vars->lastcount = vars->count;
 349                 vars->drop_next = codel_control_law(now, params->interval,
 350                                                     vars->rec_inv_sqrt);
 351         }
 352 end:
 353         return skb;
 354 }
 355 #endif