net/sched/sch_fq_pie.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /* Flow Queue PIE discipline
   3  *
   4  * Copyright (C) 2019 Mohit P. Tahiliani <tahiliani@nitk.edu.in>
   5  * Copyright (C) 2019 Sachin D. Patil <sdp.sachin@gmail.com>
   6  * Copyright (C) 2019 V. Saicharan <vsaicharan1998@gmail.com>
   7  * Copyright (C) 2019 Mohit Bhasi <mohitbhasi1998@gmail.com>
   8  * Copyright (C) 2019 Leslie Monis <lesliemonis@gmail.com>
   9  * Copyright (C) 2019 Gautam Ramakrishnan <gautamramk@gmail.com>
  10  */
  11
  12 #include <linux/jhash.h>
  13 #include <linux/sizes.h>
  14 #include <linux/vmalloc.h>
  15 #include <net/pkt_cls.h>
  16 #include <net/pie.h>
  17
  18 /* Flow Queue PIE
  19  *
  20  * Principles:
  21  *   - Packets are classified on flows.
  22  *   - This is a Stochastic model (as we use a hash, several flows might
  23  *                                 be hashed to the same slot)
  24  *   - Each flow has a PIE managed queue.
  25  *   - Flows are linked onto two (Round Robin) lists,
  26  *     so that new flows have priority on old ones.
  27  *   - For a given flow, packets are not reordered.
  28  *   - Drops during enqueue only.
  29  *   - ECN capability is off by default.
  30  *   - ECN threshold (if ECN is enabled) is at 10% by default.
  31  *   - Uses timestamps to calculate queue delay by default.
  32  */
  33
  34 /**
  35  * struct fq_pie_flow - contains data for each flow
  36  * @vars:       pie vars associated with the flow
  37  * @deficit:    number of remaining byte credits
  38  * @backlog:    size of data in the flow
  39  * @qlen:       number of packets in the flow
  40  * @flowchain:  flowchain for the flow
  41  * @head:       first packet in the flow
  42  * @tail:       last packet in the flow
  43  */
  44 struct fq_pie_flow {
  45         struct pie_vars vars;
  46         s32 deficit;
  47         u32 backlog;
  48         u32 qlen;
  49         struct list_head flowchain;
  50         struct sk_buff *head;
  51         struct sk_buff *tail;
  52 };
  53
  54 struct fq_pie_sched_data {
  55         struct tcf_proto __rcu *filter_list; /* optional external classifier */
  56         struct tcf_block *block;
  57         struct fq_pie_flow *flows;
  58         struct Qdisc *sch;
  59         struct list_head old_flows;
  60         struct list_head new_flows;
  61         struct pie_params p_params;
  62         u32 ecn_prob;
  63         u32 flows_cnt;
  64         u32 flows_cursor;
  65         u32 quantum;
  66         u32 memory_limit;
  67         u32 new_flow_count;
  68         u32 memory_usage;
  69         u32 overmemory;
  70         struct pie_stats stats;
  71         struct timer_list adapt_timer;
  72 };
  73
  74 static unsigned int fq_pie_hash(const struct fq_pie_sched_data *q,
  75                                 struct sk_buff *skb)
  76 {
  77         return reciprocal_scale(skb_get_hash(skb), q->flows_cnt);
  78 }
  79
  80 static unsigned int fq_pie_classify(struct sk_buff *skb, struct Qdisc *sch,
  81                                     int *qerr)
  82 {
  83         struct fq_pie_sched_data *q = qdisc_priv(sch);
  84         struct tcf_proto *filter;
  85         struct tcf_result res;
  86         int result;
  87
  88         if (TC_H_MAJ(skb->priority) == sch->handle &&
  89             TC_H_MIN(skb->priority) > 0 &&
  90             TC_H_MIN(skb->priority) <= q->flows_cnt)
  91                 return TC_H_MIN(skb->priority);
  92
  93         filter = rcu_dereference_bh(q->filter_list);
  94         if (!filter)
  95                 return fq_pie_hash(q, skb) + 1;
  96
  97         *qerr = NET_XMIT_SUCCESS | __NET_XMIT_BYPASS;
  98         result = tcf_classify(skb, NULL, filter, &res, false);
  99         if (result >= 0) {
 100 #ifdef CONFIG_NET_CLS_ACT
 101                 switch (result) {
 102                 case TC_ACT_STOLEN:
 103                 case TC_ACT_QUEUED:
 104                 case TC_ACT_TRAP:
 105                         *qerr = NET_XMIT_SUCCESS | __NET_XMIT_STOLEN;
 106                         fallthrough;
 107                 case TC_ACT_SHOT:
 108                         return 0;
 109                 }
 110 #endif
 111                 if (TC_H_MIN(res.classid) <= q->flows_cnt)
 112                         return TC_H_MIN(res.classid);
 113         }
 114         return 0;
 115 }
 116
 117 /* add skb to flow queue (tail add) */
 118 static inline void flow_queue_add(struct fq_pie_flow *flow,
 119                                   struct sk_buff *skb)
 120 {
 121         if (!flow->head)
 122                 flow->head = skb;
 123         else
 124                 flow->tail->next = skb;
 125         flow->tail = skb;
 126         skb->next = NULL;
 127 }
 128
 129 static int fq_pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
 130                                 struct sk_buff **to_free)
 131 {
 132         struct fq_pie_sched_data *q = qdisc_priv(sch);
 133         struct fq_pie_flow *sel_flow;
 134         int ret;
 135         u8 memory_limited = false;
 136         u8 enqueue = false;
 137         u32 pkt_len;
 138         u32 idx;
 139
 140         /* Classifies packet into corresponding flow */
 141         idx = fq_pie_classify(skb, sch, &ret);
 142         if (idx == 0) {
 143                 if (ret & __NET_XMIT_BYPASS)
 144                         qdisc_qstats_drop(sch);
 145                 __qdisc_drop(skb, to_free);
 146                 return ret;
 147         }
 148         idx--;
 149
 150         sel_flow = &q->flows[idx];
 151         /* Checks whether adding a new packet would exceed memory limit */
 152         get_pie_cb(skb)->mem_usage = skb->truesize;
 153         memory_limited = q->memory_usage > q->memory_limit + skb->truesize;
 154
 155         /* Checks if the qdisc is full */
 156         if (unlikely(qdisc_qlen(sch) >= sch->limit)) {
 157                 q->stats.overlimit++;
 158                 goto out;
 159         } else if (unlikely(memory_limited)) {
 160                 q->overmemory++;
 161         }
 162
 163         if (!pie_drop_early(sch, &q->p_params, &sel_flow->vars,
 164                             sel_flow->backlog, skb->len)) {
 165                 enqueue = true;
 166         } else if (q->p_params.ecn &&
 167                    sel_flow->vars.prob <= (MAX_PROB / 100) * q->ecn_prob &&
 168                    INET_ECN_set_ce(skb)) {
 169                 /* If packet is ecn capable, mark it if drop probability
 170                  * is lower than the parameter ecn_prob, else drop it.
 171                  */
 172                 q->stats.ecn_mark++;
 173                 enqueue = true;
 174         }
 175         if (enqueue) {
 176                 /* Set enqueue time only when dq_rate_estimator is disabled. */
 177                 if (!q->p_params.dq_rate_estimator)
 178                         pie_set_enqueue_time(skb);
 179
 180                 pkt_len = qdisc_pkt_len(skb);
 181                 q->stats.packets_in++;
 182                 q->memory_usage += skb->truesize;
 183                 sch->qstats.backlog += pkt_len;
 184                 sch->q.qlen++;
 185                 flow_queue_add(sel_flow, skb);
 186                 if (list_empty(&sel_flow->flowchain)) {
 187                         list_add_tail(&sel_flow->flowchain, &q->new_flows);
 188                         q->new_flow_count++;
 189                         sel_flow->deficit = q->quantum;
 190                         sel_flow->qlen = 0;
 191                         sel_flow->backlog = 0;
 192                 }
 193                 sel_flow->qlen++;
 194                 sel_flow->backlog += pkt_len;
 195                 return NET_XMIT_SUCCESS;
 196         }
 197 out:
 198         q->stats.dropped++;
 199         sel_flow->vars.accu_prob = 0;
 200         __qdisc_drop(skb, to_free);
 201         qdisc_qstats_drop(sch);
 202         return NET_XMIT_CN;
 203 }
 204
 205 static const struct netlink_range_validation fq_pie_q_range = {
 206         .min = 1,
 207         .max = 1 << 20,
 208 };
 209
 210 static const struct nla_policy fq_pie_policy[TCA_FQ_PIE_MAX + 1] = {
 211         [TCA_FQ_PIE_LIMIT]              = {.type = NLA_U32},
 212         [TCA_FQ_PIE_FLOWS]              = {.type = NLA_U32},
 213         [TCA_FQ_PIE_TARGET]             = {.type = NLA_U32},
 214         [TCA_FQ_PIE_TUPDATE]            = {.type = NLA_U32},
 215         [TCA_FQ_PIE_ALPHA]              = {.type = NLA_U32},
 216         [TCA_FQ_PIE_BETA]               = {.type = NLA_U32},
 217         [TCA_FQ_PIE_QUANTUM]            =
 218                         NLA_POLICY_FULL_RANGE(NLA_U32, &fq_pie_q_range),
 219         [TCA_FQ_PIE_MEMORY_LIMIT]       = {.type = NLA_U32},
 220         [TCA_FQ_PIE_ECN_PROB]           = {.type = NLA_U32},
 221         [TCA_FQ_PIE_ECN]                = {.type = NLA_U32},
 222         [TCA_FQ_PIE_BYTEMODE]           = {.type = NLA_U32},
 223         [TCA_FQ_PIE_DQ_RATE_ESTIMATOR]  = {.type = NLA_U32},
 224 };
 225
 226 static inline struct sk_buff *dequeue_head(struct fq_pie_flow *flow)
 227 {
 228         struct sk_buff *skb = flow->head;
 229
 230         flow->head = skb->next;
 231         skb->next = NULL;
 232         return skb;
 233 }
 234
 235 static struct sk_buff *fq_pie_qdisc_dequeue(struct Qdisc *sch)
 236 {
 237         struct fq_pie_sched_data *q = qdisc_priv(sch);
 238         struct sk_buff *skb = NULL;
 239         struct fq_pie_flow *flow;
 240         struct list_head *head;
 241         u32 pkt_len;
 242
 243 begin:
 244         head = &q->new_flows;
 245         if (list_empty(head)) {
 246                 head = &q->old_flows;
 247                 if (list_empty(head))
 248                         return NULL;
 249         }
 250
 251         flow = list_first_entry(head, struct fq_pie_flow, flowchain);
 252         /* Flow has exhausted all its credits */
 253         if (flow->deficit <= 0) {
 254                 flow->deficit += q->quantum;
 255                 list_move_tail(&flow->flowchain, &q->old_flows);
 256                 goto begin;
 257         }
 258
 259         if (flow->head) {
 260                 skb = dequeue_head(flow);
 261                 pkt_len = qdisc_pkt_len(skb);
 262                 sch->qstats.backlog -= pkt_len;
 263                 sch->q.qlen--;
 264                 qdisc_bstats_update(sch, skb);
 265         }
 266
 267         if (!skb) {
 268                 /* force a pass through old_flows to prevent starvation */
 269                 if (head == &q->new_flows && !list_empty(&q->old_flows))
 270                         list_move_tail(&flow->flowchain, &q->old_flows);
 271                 else
 272                         list_del_init(&flow->flowchain);
 273                 goto begin;
 274         }
 275
 276         flow->qlen--;
 277         flow->deficit -= pkt_len;
 278         flow->backlog -= pkt_len;
 279         q->memory_usage -= get_pie_cb(skb)->mem_usage;
 280         pie_process_dequeue(skb, &q->p_params, &flow->vars, flow->backlog);
 281         return skb;
 282 }
 283
 284 static int fq_pie_change(struct Qdisc *sch, struct nlattr *opt,
 285                          struct netlink_ext_ack *extack)
 286 {
 287         struct fq_pie_sched_data *q = qdisc_priv(sch);
 288         struct nlattr *tb[TCA_FQ_PIE_MAX + 1];
 289         unsigned int len_dropped = 0;
 290         unsigned int num_dropped = 0;
 291         int err;
 292
 293         err = nla_parse_nested(tb, TCA_FQ_PIE_MAX, opt, fq_pie_policy, extack);
 294         if (err < 0)
 295                 return err;
 296
 297         sch_tree_lock(sch);
 298         if (tb[TCA_FQ_PIE_LIMIT]) {
 299                 u32 limit = nla_get_u32(tb[TCA_FQ_PIE_LIMIT]);
 300
 301                 q->p_params.limit = limit;
 302                 sch->limit = limit;
 303         }
 304         if (tb[TCA_FQ_PIE_FLOWS]) {
 305                 if (q->flows) {
 306                         NL_SET_ERR_MSG_MOD(extack,
 307                                            "Number of flows cannot be changed");
 308                         goto flow_error;
 309                 }
 310                 q->flows_cnt = nla_get_u32(tb[TCA_FQ_PIE_FLOWS]);
 311                 if (!q->flows_cnt || q->flows_cnt > 65536) {
 312                         NL_SET_ERR_MSG_MOD(extack,
 313                                            "Number of flows must range in [1..65536]");
 314                         goto flow_error;
 315                 }
 316         }
 317
 318         /* convert from microseconds to pschedtime */
 319         if (tb[TCA_FQ_PIE_TARGET]) {
 320                 /* target is in us */
 321                 u32 target = nla_get_u32(tb[TCA_FQ_PIE_TARGET]);
 322
 323                 /* convert to pschedtime */
 324                 q->p_params.target =
 325                         PSCHED_NS2TICKS((u64)target * NSEC_PER_USEC);
 326         }
 327
 328         /* tupdate is in jiffies */
 329         if (tb[TCA_FQ_PIE_TUPDATE])
 330                 q->p_params.tupdate =
 331                         usecs_to_jiffies(nla_get_u32(tb[TCA_FQ_PIE_TUPDATE]));
 332
 333         if (tb[TCA_FQ_PIE_ALPHA])
 334                 q->p_params.alpha = nla_get_u32(tb[TCA_FQ_PIE_ALPHA]);
 335
 336         if (tb[TCA_FQ_PIE_BETA])
 337                 q->p_params.beta = nla_get_u32(tb[TCA_FQ_PIE_BETA]);
 338
 339         if (tb[TCA_FQ_PIE_QUANTUM])
 340                 q->quantum = nla_get_u32(tb[TCA_FQ_PIE_QUANTUM]);
 341
 342         if (tb[TCA_FQ_PIE_MEMORY_LIMIT])
 343                 q->memory_limit = nla_get_u32(tb[TCA_FQ_PIE_MEMORY_LIMIT]);
 344
 345         if (tb[TCA_FQ_PIE_ECN_PROB])
 346                 q->ecn_prob = nla_get_u32(tb[TCA_FQ_PIE_ECN_PROB]);
 347
 348         if (tb[TCA_FQ_PIE_ECN])
 349                 q->p_params.ecn = nla_get_u32(tb[TCA_FQ_PIE_ECN]);
 350
 351         if (tb[TCA_FQ_PIE_BYTEMODE])
 352                 q->p_params.bytemode = nla_get_u32(tb[TCA_FQ_PIE_BYTEMODE]);
 353
 354         if (tb[TCA_FQ_PIE_DQ_RATE_ESTIMATOR])
 355                 q->p_params.dq_rate_estimator =
 356                         nla_get_u32(tb[TCA_FQ_PIE_DQ_RATE_ESTIMATOR]);
 357
 358         /* Drop excess packets if new limit is lower */
 359         while (sch->q.qlen > sch->limit) {
 360                 struct sk_buff *skb = fq_pie_qdisc_dequeue(sch);
 361
 362                 len_dropped += qdisc_pkt_len(skb);
 363                 num_dropped += 1;
 364                 rtnl_kfree_skbs(skb, skb);
 365         }
 366         qdisc_tree_reduce_backlog(sch, num_dropped, len_dropped);
 367
 368         sch_tree_unlock(sch);
 369         return 0;
 370
 371 flow_error:
 372         sch_tree_unlock(sch);
 373         return -EINVAL;
 374 }
 375
 376 static void fq_pie_timer(struct timer_list *t)
 377 {
 378         struct fq_pie_sched_data *q = from_timer(q, t, adapt_timer);
 379         unsigned long next, tupdate;
 380         struct Qdisc *sch = q->sch;
 381         spinlock_t *root_lock; /* to lock qdisc for probability calculations */
 382         int max_cnt, i;
 383
 384         rcu_read_lock();
 385         root_lock = qdisc_lock(qdisc_root_sleeping(sch));
 386         spin_lock(root_lock);
 387
 388         /* Limit this expensive loop to 2048 flows per round. */
 389         max_cnt = min_t(int, q->flows_cnt - q->flows_cursor, 2048);
 390         for (i = 0; i < max_cnt; i++) {
 391                 pie_calculate_probability(&q->p_params,
 392                                           &q->flows[q->flows_cursor].vars,
 393                                           q->flows[q->flows_cursor].backlog);
 394                 q->flows_cursor++;
 395         }
 396
 397         tupdate = q->p_params.tupdate;
 398         next = 0;
 399         if (q->flows_cursor >= q->flows_cnt) {
 400                 q->flows_cursor = 0;
 401                 next = tupdate;
 402         }
 403         if (tupdate)
 404                 mod_timer(&q->adapt_timer, jiffies + next);
 405         spin_unlock(root_lock);
 406         rcu_read_unlock();
 407 }
 408
 409 static int fq_pie_init(struct Qdisc *sch, struct nlattr *opt,
 410                        struct netlink_ext_ack *extack)
 411 {
 412         struct fq_pie_sched_data *q = qdisc_priv(sch);
 413         int err;
 414         u32 idx;
 415
 416         pie_params_init(&q->p_params);
 417         sch->limit = 10 * 1024;
 418         q->p_params.limit = sch->limit;
 419         q->quantum = psched_mtu(qdisc_dev(sch));
 420         q->sch = sch;
 421         q->ecn_prob = 10;
 422         q->flows_cnt = 1024;
 423         q->memory_limit = SZ_32M;
 424
 425         INIT_LIST_HEAD(&q->new_flows);
 426         INIT_LIST_HEAD(&q->old_flows);
 427         timer_setup(&q->adapt_timer, fq_pie_timer, 0);
 428
 429         if (opt) {
 430                 err = fq_pie_change(sch, opt, extack);
 431
 432                 if (err)
 433                         return err;
 434         }
 435
 436         err = tcf_block_get(&q->block, &q->filter_list, sch, extack);
 437         if (err)
 438                 goto init_failure;
 439
 440         q->flows = kvcalloc(q->flows_cnt, sizeof(struct fq_pie_flow),
 441                             GFP_KERNEL);
 442         if (!q->flows) {
 443                 err = -ENOMEM;
 444                 goto init_failure;
 445         }
 446         for (idx = 0; idx < q->flows_cnt; idx++) {
 447                 struct fq_pie_flow *flow = q->flows + idx;
 448
 449                 INIT_LIST_HEAD(&flow->flowchain);
 450                 pie_vars_init(&flow->vars);
 451         }
 452
 453         mod_timer(&q->adapt_timer, jiffies + HZ / 2);
 454
 455         return 0;
 456
 457 init_failure:
 458         q->flows_cnt = 0;
 459
 460         return err;
 461 }
 462
 463 static int fq_pie_dump(struct Qdisc *sch, struct sk_buff *skb)
 464 {
 465         struct fq_pie_sched_data *q = qdisc_priv(sch);
 466         struct nlattr *opts;
 467
 468         opts = nla_nest_start(skb, TCA_OPTIONS);
 469         if (!opts)
 470                 return -EMSGSIZE;
 471
 472         /* convert target from pschedtime to us */
 473         if (nla_put_u32(skb, TCA_FQ_PIE_LIMIT, sch->limit) ||
 474             nla_put_u32(skb, TCA_FQ_PIE_FLOWS, q->flows_cnt) ||
 475             nla_put_u32(skb, TCA_FQ_PIE_TARGET,
 476                         ((u32)PSCHED_TICKS2NS(q->p_params.target)) /
 477                         NSEC_PER_USEC) ||
 478             nla_put_u32(skb, TCA_FQ_PIE_TUPDATE,
 479                         jiffies_to_usecs(q->p_params.tupdate)) ||
 480             nla_put_u32(skb, TCA_FQ_PIE_ALPHA, q->p_params.alpha) ||
 481             nla_put_u32(skb, TCA_FQ_PIE_BETA, q->p_params.beta) ||
 482             nla_put_u32(skb, TCA_FQ_PIE_QUANTUM, q->quantum) ||
 483             nla_put_u32(skb, TCA_FQ_PIE_MEMORY_LIMIT, q->memory_limit) ||
 484             nla_put_u32(skb, TCA_FQ_PIE_ECN_PROB, q->ecn_prob) ||
 485             nla_put_u32(skb, TCA_FQ_PIE_ECN, q->p_params.ecn) ||
 486             nla_put_u32(skb, TCA_FQ_PIE_BYTEMODE, q->p_params.bytemode) ||
 487             nla_put_u32(skb, TCA_FQ_PIE_DQ_RATE_ESTIMATOR,
 488                         q->p_params.dq_rate_estimator))
 489                 goto nla_put_failure;
 490
 491         return nla_nest_end(skb, opts);
 492
 493 nla_put_failure:
 494         nla_nest_cancel(skb, opts);
 495         return -EMSGSIZE;
 496 }
 497
 498 static int fq_pie_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
 499 {
 500         struct fq_pie_sched_data *q = qdisc_priv(sch);
 501         struct tc_fq_pie_xstats st = {
 502                 .packets_in     = q->stats.packets_in,
 503                 .overlimit      = q->stats.overlimit,
 504                 .overmemory     = q->overmemory,
 505                 .dropped        = q->stats.dropped,
 506                 .ecn_mark       = q->stats.ecn_mark,
 507                 .new_flow_count = q->new_flow_count,
 508                 .memory_usage   = q->memory_usage,
 509         };
 510         struct list_head *pos;
 511
 512         sch_tree_lock(sch);
 513         list_for_each(pos, &q->new_flows)
 514                 st.new_flows_len++;
 515
 516         list_for_each(pos, &q->old_flows)
 517                 st.old_flows_len++;
 518         sch_tree_unlock(sch);
 519
 520         return gnet_stats_copy_app(d, &st, sizeof(st));
 521 }
 522
 523 static void fq_pie_reset(struct Qdisc *sch)
 524 {
 525         struct fq_pie_sched_data *q = qdisc_priv(sch);
 526         u32 idx;
 527
 528         INIT_LIST_HEAD(&q->new_flows);
 529         INIT_LIST_HEAD(&q->old_flows);
 530         for (idx = 0; idx < q->flows_cnt; idx++) {
 531                 struct fq_pie_flow *flow = q->flows + idx;
 532
 533                 /* Removes all packets from flow */
 534                 rtnl_kfree_skbs(flow->head, flow->tail);
 535                 flow->head = NULL;
 536
 537                 INIT_LIST_HEAD(&flow->flowchain);
 538                 pie_vars_init(&flow->vars);
 539         }
 540 }
 541
 542 static void fq_pie_destroy(struct Qdisc *sch)
 543 {
 544         struct fq_pie_sched_data *q = qdisc_priv(sch);
 545
 546         tcf_block_put(q->block);
 547         q->p_params.tupdate = 0;
 548         del_timer_sync(&q->adapt_timer);
 549         kvfree(q->flows);
 550 }
 551
 552 static struct Qdisc_ops fq_pie_qdisc_ops __read_mostly = {
 553         .id             = "fq_pie",
 554         .priv_size      = sizeof(struct fq_pie_sched_data),
 555         .enqueue        = fq_pie_qdisc_enqueue,
 556         .dequeue        = fq_pie_qdisc_dequeue,
 557         .peek           = qdisc_peek_dequeued,
 558         .init           = fq_pie_init,
 559         .destroy        = fq_pie_destroy,
 560         .reset          = fq_pie_reset,
 561         .change         = fq_pie_change,
 562         .dump           = fq_pie_dump,
 563         .dump_stats     = fq_pie_dump_stats,
 564         .owner          = THIS_MODULE,
 565 };
 566
 567 static int __init fq_pie_module_init(void)
 568 {
 569         return register_qdisc(&fq_pie_qdisc_ops);
 570 }
 571
 572 static void __exit fq_pie_module_exit(void)
 573 {
 574         unregister_qdisc(&fq_pie_qdisc_ops);
 575 }
 576
 577 module_init(fq_pie_module_init);
 578 module_exit(fq_pie_module_exit);
 579
 580 MODULE_DESCRIPTION("Flow Queue Proportional Integral controller Enhanced (FQ-PIE)");
 581 MODULE_AUTHOR("Mohit P. Tahiliani");
 582 MODULE_LICENSE("GPL");