/* Copyright (C) 2013 Cisco Systems, Inc, 2013.
   *
   * This program is free software; you can redistribute it and/or
   * modify it under the terms of the GNU General Public License
   * as published by the Free Software Foundation; either version 2
   * of the License.
   *
   * This program is distributed in the hope that it will be useful,
   * but WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   * GNU General Public License for more details.
   *
   * Author: Vijay Subramanian <vijaynsu@cisco.com>
   * Author: Mythili Prabhu <mysuryan@cisco.com>
   *
   * ECN support is added by Naeem Khademi <naeemk@ifi.uio.no>
   * University of Oslo, Norway.
   *
   * References:
   * IETF draft submission: http://tools.ietf.org/html/draft-pan-aqm-pie-00
   * IEEE  Conference on High Performance Switching and Routing 2013 :
   * "PIE: A * Lightweight Control Scheme to Address the Bufferbloat Problem"
   */
  
  #include <linux/module.h>
  #include <linux/slab.h>
  #include <linux/types.h>
  #include <linux/kernel.h>
  #include <linux/errno.h>
  #include <linux/skbuff.h>
  #include <net/pkt_sched.h>
  #include <net/inet_ecn.h>
  
  #define QUEUE_THRESHOLD 10000
  #define DQCOUNT_INVALID -1
  #define MAX_PROB  0xffffffff
  #define PIE_SCALE 8
  
  /* parameters used */
  struct pie_params {
  	psched_time_t target;	/* user specified target delay in pschedtime */
  	u32 tupdate;		/* timer frequency (in jiffies) */
  	u32 limit;		/* number of packets that can be enqueued */
  	u32 alpha;		/* alpha and beta are between 0 and 32 */
  	u32 beta;		/* and are used for shift relative to 1 */
  	bool ecn;		/* true if ecn is enabled */
  	bool bytemode;		/* to scale drop early prob based on pkt size */
  };
  
  /* variables used */
  struct pie_vars {
  	u32 prob;		/* probability but scaled by u32 limit. */
  	psched_time_t burst_time;
  	psched_time_t qdelay;
  	psched_time_t qdelay_old;
  	u64 dq_count;		/* measured in bytes */
  	psched_time_t dq_tstamp;	/* drain rate */
  	u32 avg_dq_rate;	/* bytes per pschedtime tick,scaled */
  	u32 qlen_old;		/* in bytes */
  };
  
  /* statistics gathering */
  struct pie_stats {
  	u32 packets_in;		/* total number of packets enqueued */
  	u32 dropped;		/* packets dropped due to pie_action */
  	u32 overlimit;		/* dropped due to lack of space in queue */
  	u32 maxq;		/* maximum queue size */
  	u32 ecn_mark;		/* packets marked with ECN */
  };
  
  /* private data for the Qdisc */
  struct pie_sched_data {
  	struct pie_params params;
  	struct pie_vars vars;
  	struct pie_stats stats;
  	struct timer_list adapt_timer;
  };
  
  static void pie_params_init(struct pie_params *params)
  {
  	params->alpha = 2;
  	params->beta = 20;
  	params->tupdate = usecs_to_jiffies(30 * USEC_PER_MSEC);	/* 30 ms */
  	params->limit = 1000;	/* default of 1000 packets */
  	params->target = PSCHED_NS2TICKS(20 * NSEC_PER_MSEC);	/* 20 ms */
  	params->ecn = false;
  	params->bytemode = false;
  }
  
  static void pie_vars_init(struct pie_vars *vars)
  {
  	vars->dq_count = DQCOUNT_INVALID;
  	vars->avg_dq_rate = 0;
  	/* default of 100 ms in pschedtime */
  	vars->burst_time = PSCHED_NS2TICKS(100 * NSEC_PER_MSEC);
  }
  
  static bool drop_early(struct Qdisc *sch, u32 packet_size)
  {
  	struct pie_sched_data *q = qdisc_priv(sch);
  	u32 rnd;
  	u32 local_prob = q->vars.prob;
  	u32 mtu = psched_mtu(qdisc_dev(sch));
  
  	/* If there is still burst allowance left skip random early drop */
  	if (q->vars.burst_time > 0)
  		return false;
  
  	/* If current delay is less than half of target, and
  	 * if drop prob is low already, disable early_drop
  	 */
  	if ((q->vars.qdelay < q->params.target / 2)
  	    && (q->vars.prob < MAX_PROB / 5))
  		return false;
  
  	/* If we have fewer than 2 mtu-sized packets, disable drop_early,
  	 * similar to min_th in RED
  	 */
  	if (sch->qstats.backlog < 2 * mtu)
  		return false;
  
  	/* If bytemode is turned on, use packet size to compute new
  	 * probablity. Smaller packets will have lower drop prob in this case
  	 */
  	if (q->params.bytemode && packet_size <= mtu)
  		local_prob = (local_prob / mtu) * packet_size;
  	else
  		local_prob = q->vars.prob;
  
  	rnd = prandom_u32();
  	if (rnd < local_prob)
  		return true;
  
  	return false;
  }
  
  static int pie_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch)
  {
  	struct pie_sched_data *q = qdisc_priv(sch);
  	bool enqueue = false;
  
  	if (unlikely(qdisc_qlen(sch) >= sch->limit)) {
  		q->stats.overlimit++;
  		goto out;
  	}
  
  	if (!drop_early(sch, skb->len)) {
  		enqueue = true;
  	} else if (q->params.ecn && (q->vars.prob <= MAX_PROB / 10) &&
  		   INET_ECN_set_ce(skb)) {
  		/* If packet is ecn capable, mark it if drop probability
  		 * is lower than 10%, else drop it.
  		 */
  		q->stats.ecn_mark++;
  		enqueue = true;
  	}
  
  	/* we can enqueue the packet */
  	if (enqueue) {
  		q->stats.packets_in++;
  		if (qdisc_qlen(sch) > q->stats.maxq)
  			q->stats.maxq = qdisc_qlen(sch);
  
  		return qdisc_enqueue_tail(skb, sch);
  	}
  
  out:
  	q->stats.dropped++;
  	return qdisc_drop(skb, sch);
  }
  
  static const struct nla_policy pie_policy[TCA_PIE_MAX + 1] = {
  	[TCA_PIE_TARGET] = {.type = NLA_U32},
  	[TCA_PIE_LIMIT] = {.type = NLA_U32},
  	[TCA_PIE_TUPDATE] = {.type = NLA_U32},
  	[TCA_PIE_ALPHA] = {.type = NLA_U32},
  	[TCA_PIE_BETA] = {.type = NLA_U32},
  	[TCA_PIE_ECN] = {.type = NLA_U32},
  	[TCA_PIE_BYTEMODE] = {.type = NLA_U32},
  };
  
  static int pie_change(struct Qdisc *sch, struct nlattr *opt)
  {
  	struct pie_sched_data *q = qdisc_priv(sch);
  	struct nlattr *tb[TCA_PIE_MAX + 1];
  	unsigned int qlen;
  	int err;
  
  	if (!opt)
  		return -EINVAL;
  
  	err = nla_parse_nested(tb, TCA_PIE_MAX, opt, pie_policy);
  	if (err < 0)
  		return err;
  
  	sch_tree_lock(sch);
  
  	/* convert from microseconds to pschedtime */
  	if (tb[TCA_PIE_TARGET]) {
  		/* target is in us */
  		u32 target = nla_get_u32(tb[TCA_PIE_TARGET]);
  
  		/* convert to pschedtime */
  		q->params.target = PSCHED_NS2TICKS((u64)target * NSEC_PER_USEC);
  	}
  
  	/* tupdate is in jiffies */
  	if (tb[TCA_PIE_TUPDATE])
  		q->params.tupdate = usecs_to_jiffies(nla_get_u32(tb[TCA_PIE_TUPDATE]));
  
  	if (tb[TCA_PIE_LIMIT]) {
  		u32 limit = nla_get_u32(tb[TCA_PIE_LIMIT]);
  
  		q->params.limit = limit;
  		sch->limit = limit;
  	}
  
  	if (tb[TCA_PIE_ALPHA])
  		q->params.alpha = nla_get_u32(tb[TCA_PIE_ALPHA]);
  
  	if (tb[TCA_PIE_BETA])
  		q->params.beta = nla_get_u32(tb[TCA_PIE_BETA]);
  
  	if (tb[TCA_PIE_ECN])
  		q->params.ecn = nla_get_u32(tb[TCA_PIE_ECN]);
  
  	if (tb[TCA_PIE_BYTEMODE])
  		q->params.bytemode = nla_get_u32(tb[TCA_PIE_BYTEMODE]);
  
  	/* Drop excess packets if new limit is lower */
  	qlen = sch->q.qlen;
  	while (sch->q.qlen > sch->limit) {
  		struct sk_buff *skb = __skb_dequeue(&sch->q);
  
  		sch->qstats.backlog -= qdisc_pkt_len(skb);
  		qdisc_drop(skb, sch);
  	}
  	qdisc_tree_decrease_qlen(sch, qlen - sch->q.qlen);
  
  	sch_tree_unlock(sch);
  	return 0;
  }
  
  static void pie_process_dequeue(struct Qdisc *sch, struct sk_buff *skb)
  {
  
  	struct pie_sched_data *q = qdisc_priv(sch);
  	int qlen = sch->qstats.backlog;	/* current queue size in bytes */
  
  	/* If current queue is about 10 packets or more and dq_count is unset
  	 * we have enough packets to calculate the drain rate. Save
  	 * current time as dq_tstamp and start measurement cycle.
  	 */
  	if (qlen >= QUEUE_THRESHOLD && q->vars.dq_count == DQCOUNT_INVALID) {
  		q->vars.dq_tstamp = psched_get_time();
  		q->vars.dq_count = 0;
  	}
  
  	/* Calculate the average drain rate from this value.  If queue length
  	 * has receded to a small value viz., <= QUEUE_THRESHOLD bytes,reset
  	 * the dq_count to -1 as we don't have enough packets to calculate the
  	 * drain rate anymore The following if block is entered only when we
  	 * have a substantial queue built up (QUEUE_THRESHOLD bytes or more)
  	 * and we calculate the drain rate for the threshold here.  dq_count is
  	 * in bytes, time difference in psched_time, hence rate is in
  	 * bytes/psched_time.
  	 */
  	if (q->vars.dq_count != DQCOUNT_INVALID) {
  		q->vars.dq_count += skb->len;
  
  		if (q->vars.dq_count >= QUEUE_THRESHOLD) {
  			psched_time_t now = psched_get_time();
  			u32 dtime = now - q->vars.dq_tstamp;
  			u32 count = q->vars.dq_count << PIE_SCALE;
  
  			if (dtime == 0)
  				return;
  
  			count = count / dtime;
  
  			if (q->vars.avg_dq_rate == 0)
  				q->vars.avg_dq_rate = count;
  			else
  				q->vars.avg_dq_rate =
  				    (q->vars.avg_dq_rate -
  				     (q->vars.avg_dq_rate >> 3)) + (count >> 3);
  
  			/* If the queue has receded below the threshold, we hold
  			 * on to the last drain rate calculated, else we reset
  			 * dq_count to 0 to re-enter the if block when the next
  			 * packet is dequeued
  			 */
  			if (qlen < QUEUE_THRESHOLD)
  				q->vars.dq_count = DQCOUNT_INVALID;
  			else {
  				q->vars.dq_count = 0;
  				q->vars.dq_tstamp = psched_get_time();
  			}
  
  			if (q->vars.burst_time > 0) {
  				if (q->vars.burst_time > dtime)
  					q->vars.burst_time -= dtime;
  				else
  					q->vars.burst_time = 0;
  			}
  		}
  	}
  }
  
  static void calculate_probability(struct Qdisc *sch)
  {
  	struct pie_sched_data *q = qdisc_priv(sch);
  	u32 qlen = sch->qstats.backlog;	/* queue size in bytes */
  	psched_time_t qdelay = 0;	/* in pschedtime */
  	psched_time_t qdelay_old = q->vars.qdelay;	/* in pschedtime */
  	s32 delta = 0;		/* determines the change in probability */
  	u32 oldprob;
  	u32 alpha, beta;
  	bool update_prob = true;
  
  	q->vars.qdelay_old = q->vars.qdelay;
  
  	if (q->vars.avg_dq_rate > 0)
  		qdelay = (qlen << PIE_SCALE) / q->vars.avg_dq_rate;
  	else
  		qdelay = 0;
  
  	/* If qdelay is zero and qlen is not, it means qlen is very small, less
  	 * than dequeue_rate, so we do not update probabilty in this round
  	 */
  	if (qdelay == 0 && qlen != 0)
  		update_prob = false;
  
  	/* In the algorithm, alpha and beta are between 0 and 2 with typical
  	 * value for alpha as 0.125. In this implementation, we use values 0-32
  	 * passed from user space to represent this. Also, alpha and beta have
  	 * unit of HZ and need to be scaled before they can used to update
  	 * probability. alpha/beta are updated locally below by 1) scaling them
  	 * appropriately 2) scaling down by 16 to come to 0-2 range.
  	 * Please see paper for details.
  	 *
  	 * We scale alpha and beta differently depending on whether we are in
  	 * light, medium or high dropping mode.
  	 */
  	if (q->vars.prob < MAX_PROB / 100) {
  		alpha =
  		    (q->params.alpha * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 7;
  		beta =
  		    (q->params.beta * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 7;
  	} else if (q->vars.prob < MAX_PROB / 10) {
  		alpha =
  		    (q->params.alpha * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 5;
  		beta =
  		    (q->params.beta * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 5;
  	} else {
  		alpha =
  		    (q->params.alpha * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 4;
  		beta =
  		    (q->params.beta * (MAX_PROB / PSCHED_TICKS_PER_SEC)) >> 4;
  	}
  
  	/* alpha and beta should be between 0 and 32, in multiples of 1/16 */
  	delta += alpha * ((qdelay - q->params.target));
  	delta += beta * ((qdelay - qdelay_old));
  
  	oldprob = q->vars.prob;
  
  	/* to ensure we increase probability in steps of no more than 2% */
  	if (delta > (s32) (MAX_PROB / (100 / 2)) &&
  	    q->vars.prob >= MAX_PROB / 10)
  		delta = (MAX_PROB / 100) * 2;
  
  	/* Non-linear drop:
  	 * Tune drop probability to increase quickly for high delays(>= 250ms)
  	 * 250ms is derived through experiments and provides error protection
  	 */
  
  	if (qdelay > (PSCHED_NS2TICKS(250 * NSEC_PER_MSEC)))
  		delta += MAX_PROB / (100 / 2);
  
  	q->vars.prob += delta;
  
  	if (delta > 0) {
  		/* prevent overflow */
  		if (q->vars.prob < oldprob) {
  			q->vars.prob = MAX_PROB;
  			/* Prevent normalization error. If probability is at
  			 * maximum value already, we normalize it here, and
  			 * skip the check to do a non-linear drop in the next
  			 * section.
  			 */
  			update_prob = false;
  		}
  	} else {
  		/* prevent underflow */
  		if (q->vars.prob > oldprob)
  			q->vars.prob = 0;
  	}
  
  	/* Non-linear drop in probability: Reduce drop probability quickly if
  	 * delay is 0 for 2 consecutive Tupdate periods.
  	 */
  
  	if ((qdelay == 0) && (qdelay_old == 0) && update_prob)
  		q->vars.prob = (q->vars.prob * 98) / 100;
  
  	q->vars.qdelay = qdelay;
  	q->vars.qlen_old = qlen;
  
  	/* We restart the measurement cycle if the following conditions are met
  	 * 1. If the delay has been low for 2 consecutive Tupdate periods
  	 * 2. Calculated drop probability is zero
  	 * 3. We have atleast one estimate for the avg_dq_rate ie.,
  	 *    is a non-zero value
  	 */
  	if ((q->vars.qdelay < q->params.target / 2) &&
  	    (q->vars.qdelay_old < q->params.target / 2) &&
  	    (q->vars.prob == 0) &&
  	    (q->vars.avg_dq_rate > 0))
  		pie_vars_init(&q->vars);
  }
  
  static void pie_timer(unsigned long arg)
  {
  	struct Qdisc *sch = (struct Qdisc *)arg;
  	struct pie_sched_data *q = qdisc_priv(sch);
  	spinlock_t *root_lock = qdisc_lock(qdisc_root_sleeping(sch));
  
  	spin_lock(root_lock);
  	calculate_probability(sch);
  
  	/* reset the timer to fire after 'tupdate'. tupdate is in jiffies. */
  	if (q->params.tupdate)
  		mod_timer(&q->adapt_timer, jiffies + q->params.tupdate);
  	spin_unlock(root_lock);
  
  }
  
  static int pie_init(struct Qdisc *sch, struct nlattr *opt)
  {
  	struct pie_sched_data *q = qdisc_priv(sch);
  
  	pie_params_init(&q->params);
  	pie_vars_init(&q->vars);
  	sch->limit = q->params.limit;
  
  	setup_timer(&q->adapt_timer, pie_timer, (unsigned long)sch);
  	mod_timer(&q->adapt_timer, jiffies + HZ / 2);
  
  	if (opt) {
  		int err = pie_change(sch, opt);
  
  		if (err)
  			return err;
  	}
  
  	return 0;
  }
  
  static int pie_dump(struct Qdisc *sch, struct sk_buff *skb)
  {
  	struct pie_sched_data *q = qdisc_priv(sch);
  	struct nlattr *opts;
  
  	opts = nla_nest_start(skb, TCA_OPTIONS);
  	if (opts == NULL)
  		goto nla_put_failure;
  
  	/* convert target from pschedtime to us */
  	if (nla_put_u32(skb, TCA_PIE_TARGET,
  			((u32) PSCHED_TICKS2NS(q->params.target)) /
  			NSEC_PER_USEC) ||
  	    nla_put_u32(skb, TCA_PIE_LIMIT, sch->limit) ||
  	    nla_put_u32(skb, TCA_PIE_TUPDATE, jiffies_to_usecs(q->params.tupdate)) ||
  	    nla_put_u32(skb, TCA_PIE_ALPHA, q->params.alpha) ||
  	    nla_put_u32(skb, TCA_PIE_BETA, q->params.beta) ||
  	    nla_put_u32(skb, TCA_PIE_ECN, q->params.ecn) ||
  	    nla_put_u32(skb, TCA_PIE_BYTEMODE, q->params.bytemode))
  		goto nla_put_failure;
  
  	return nla_nest_end(skb, opts);
  
  nla_put_failure:
  	nla_nest_cancel(skb, opts);
  	return -1;
  
  }
  
  static int pie_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
  {
  	struct pie_sched_data *q = qdisc_priv(sch);
  	struct tc_pie_xstats st = {
  		.prob		= q->vars.prob,
  		.delay		= ((u32) PSCHED_TICKS2NS(q->vars.qdelay)) /
  				   NSEC_PER_USEC,
  		/* unscale and return dq_rate in bytes per sec */
  		.avg_dq_rate	= q->vars.avg_dq_rate *
  				  (PSCHED_TICKS_PER_SEC) >> PIE_SCALE,
  		.packets_in	= q->stats.packets_in,
  		.overlimit	= q->stats.overlimit,
  		.maxq		= q->stats.maxq,
  		.dropped	= q->stats.dropped,
  		.ecn_mark	= q->stats.ecn_mark,
  	};
  
  	return gnet_stats_copy_app(d, &st, sizeof(st));
  }
  
  static struct sk_buff *pie_qdisc_dequeue(struct Qdisc *sch)
  {
  	struct sk_buff *skb;
  	skb = __qdisc_dequeue_head(sch, &sch->q);
  
  	if (!skb)
  		return NULL;
  
  	pie_process_dequeue(sch, skb);
  	return skb;
  }
  
  static void pie_reset(struct Qdisc *sch)
  {
  	struct pie_sched_data *q = qdisc_priv(sch);
  	qdisc_reset_queue(sch);
  	pie_vars_init(&q->vars);
  }
  
  static void pie_destroy(struct Qdisc *sch)
  {
  	struct pie_sched_data *q = qdisc_priv(sch);
  	q->params.tupdate = 0;
  	del_timer_sync(&q->adapt_timer);
  }
  
  static struct Qdisc_ops pie_qdisc_ops __read_mostly = {
  	.id = "pie",
  	.priv_size	= sizeof(struct pie_sched_data),
  	.enqueue	= pie_qdisc_enqueue,
  	.dequeue	= pie_qdisc_dequeue,
  	.peek		= qdisc_peek_dequeued,
  	.init		= pie_init,
  	.destroy	= pie_destroy,
  	.reset		= pie_reset,
  	.change		= pie_change,
  	.dump		= pie_dump,
  	.dump_stats	= pie_dump_stats,
  	.owner		= THIS_MODULE,
  };
  
  static int __init pie_module_init(void)
  {
  	return register_qdisc(&pie_qdisc_ops);
  }
  
  static void __exit pie_module_exit(void)
  {
  	unregister_qdisc(&pie_qdisc_ops);
  }
  
  module_init(pie_module_init);
  module_exit(pie_module_exit);
  
  MODULE_DESCRIPTION("Proportional Integral controller Enhanced (PIE) scheduler");
  MODULE_AUTHOR("Vijay Subramanian");
  MODULE_AUTHOR("Mythili Prabhu");
  MODULE_LICENSE("GPL");