Contributors: 3
Author Tokens Token Proportion Commits Commit Proportion
Chia-Yu Chang 3424 63.24% 2 50.00%
Koen De Schepper 1988 36.72% 1 25.00%
Victor Nogueira 2 0.04% 1 25.00%
Total 5414 4 


// SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
/* Copyright (C) 2024 Nokia
 *
 * Author: Koen De Schepper <koen.de_schepper@nokia-bell-labs.com>
 * Author: Olga Albisser <olga@albisser.org>
 * Author: Henrik Steen <henrist@henrist.net>
 * Author: Olivier Tilmans <olivier.tilmans@nokia.com>
 * Author: Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>
 *
 * DualPI Improved with a Square (dualpi2):
 * - Supports congestion controls that comply with the Prague requirements
 *   in RFC9331 (e.g. TCP-Prague)
 * - Supports coupled dual-queue with PI2 as defined in RFC9332
 * - Supports ECN L4S-identifier (IP.ECN==0b*1)
 *
 * note: Although DCTCP and BBRv3 can use shallow-threshold ECN marks,
 *   they do not meet the 'Prague L4S Requirements' listed in RFC 9331
 *   Section 4, so they can only be used with DualPI2 in a datacenter
 *   context.
 *
 * References:
 * - RFC9332: https://datatracker.ietf.org/doc/html/rfc9332
 * - De Schepper, Koen, et al. "PI 2: A linearized AQM for both classic and
 *   scalable TCP."  in proc. ACM CoNEXT'16, 2016.
 */

#include <linux/errno.h>
#include <linux/hrtimer.h>
#include <linux/if_vlan.h>
#include <linux/kernel.h>
#include <linux/limits.h>
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/types.h>

#include <net/gso.h>
#include <net/inet_ecn.h>
#include <net/pkt_cls.h>
#include <net/pkt_sched.h>

/* 32b enable to support flows with windows up to ~8.6 * 1e9 packets
 * i.e., twice the maximal snd_cwnd.
 * MAX_PROB must be consistent with the RNG in dualpi2_roll().
 */
#define MAX_PROB U32_MAX

/* alpha/beta values exchanged over netlink are in units of 256ns */
#define ALPHA_BETA_SHIFT 8

/* Scaled values of alpha/beta must fit in 32b to avoid overflow in later
 * computations. Consequently (see and dualpi2_scale_alpha_beta()), their
 * netlink-provided values can use at most 31b, i.e. be at most (2^23)-1
 * (~4MHz) as those are given in 1/256th. This enable to tune alpha/beta to
 * control flows whose maximal RTTs can be in usec up to few secs.
 */
#define ALPHA_BETA_MAX ((1U << 31) - 1)

/* Internal alpha/beta are in units of 64ns.
 * This enables to use all alpha/beta values in the allowed range without loss
 * of precision due to rounding when scaling them internally, e.g.,
 * scale_alpha_beta(1) will not round down to 0.
 */
#define ALPHA_BETA_GRANULARITY 6

#define ALPHA_BETA_SCALING (ALPHA_BETA_SHIFT - ALPHA_BETA_GRANULARITY)

/* We express the weights (wc, wl) in %, i.e., wc + wl = 100 */
#define MAX_WC 100

struct dualpi2_sched_data {
	struct Qdisc *l_queue;	/* The L4S Low latency queue (L-queue) */
	struct Qdisc *sch;	/* The Classic queue (C-queue) */

	/* Registered tc filters */
	struct tcf_proto __rcu *tcf_filters;
	struct tcf_block *tcf_block;

	/* PI2 parameters */
	u64	pi2_target;	/* Target delay in nanoseconds */
	u32	pi2_tupdate;	/* Timer frequency in nanoseconds */
	u32	pi2_prob;	/* Base PI probability */
	u32	pi2_alpha;	/* Gain factor for the integral rate response */
	u32	pi2_beta;	/* Gain factor for the proportional response */
	struct hrtimer pi2_timer; /* prob update timer */

	/* Step AQM (L-queue only) parameters */
	u32	step_thresh;	/* Step threshold */
	bool	step_in_packets; /* Step thresh in packets (1) or time (0) */

	/* C-queue starvation protection */
	s32	c_protection_credit; /* Credit (sign indicates which queue) */
	s32	c_protection_init; /* Reset value of the credit */
	u8	c_protection_wc; /* C-queue weight (between 0 and MAX_WC) */
	u8	c_protection_wl; /* L-queue weight (MAX_WC - wc) */

	/* General dualQ parameters */
	u32	memory_limit;	/* Memory limit of both queues */
	u8	coupling_factor;/* Coupling factor (k) between both queues */
	u8	ecn_mask;	/* Mask to match packets into L-queue */
	u32	min_qlen_step;	/* Minimum queue length to apply step thresh */
	bool	drop_early;	/* Drop at enqueue (1) instead of dequeue  (0) */
	bool	drop_overload;	/* Drop (1) on overload, or overflow (0) */
	bool	split_gso;	/* Split aggregated skb (1) or leave as is (0) */

	/* Statistics */
	u64	c_head_ts;	/* Enqueue timestamp of the C-queue head */
	u64	l_head_ts;	/* Enqueue timestamp of the L-queue head */
	u64	last_qdelay;	/* Q delay val at the last probability update */
	u32	packets_in_c;	/* Enqueue packet counter of the C-queue */
	u32	packets_in_l;	/* Enqueue packet counter of the L-queue */
	u32	maxq;		/* Maximum queue size of the C-queue */
	u32	ecn_mark;	/* ECN mark pkt counter due to PI probability */
	u32	step_marks;	/* ECN mark pkt counter due to step AQM */
	u32	memory_used;	/* Memory used of both queues */
	u32	max_memory_used;/* Maximum used memory */

	/* Deferred drop statistics */
	u32	deferred_drops_cnt;	/* Packets dropped */
	u32	deferred_drops_len;	/* Bytes dropped */
};

struct dualpi2_skb_cb {
	u64 ts;			/* Timestamp at enqueue */
	u8 apply_step:1,	/* Can we apply the step threshold */
	   classified:2,	/* Packet classification results */
	   ect:2;		/* Packet ECT codepoint */
};

enum dualpi2_classification_results {
	DUALPI2_C_CLASSIC	= 0,	/* C-queue */
	DUALPI2_C_L4S		= 1,	/* L-queue (scale mark/classic drop) */
	DUALPI2_C_LLLL		= 2,	/* L-queue (no drops/marks) */
	__DUALPI2_C_MAX			/* Keep last*/
};

static struct dualpi2_skb_cb *dualpi2_skb_cb(struct sk_buff *skb)
{
	qdisc_cb_private_validate(skb, sizeof(struct dualpi2_skb_cb));
	return (struct dualpi2_skb_cb *)qdisc_skb_cb(skb)->data;
}

static u64 dualpi2_sojourn_time(struct sk_buff *skb, u64 reference)
{
	return reference - dualpi2_skb_cb(skb)->ts;
}

static u64 head_enqueue_time(struct Qdisc *q)
{
	struct sk_buff *skb = qdisc_peek_head(q);

	return skb ? dualpi2_skb_cb(skb)->ts : 0;
}

static u32 dualpi2_scale_alpha_beta(u32 param)
{
	u64 tmp = ((u64)param * MAX_PROB >> ALPHA_BETA_SCALING);

	do_div(tmp, NSEC_PER_SEC);
	return tmp;
}

static u32 dualpi2_unscale_alpha_beta(u32 param)
{
	u64 tmp = ((u64)param * NSEC_PER_SEC << ALPHA_BETA_SCALING);

	do_div(tmp, MAX_PROB);
	return tmp;
}

static ktime_t next_pi2_timeout(struct dualpi2_sched_data *q)
{
	return ktime_add_ns(ktime_get_ns(), q->pi2_tupdate);
}

static bool skb_is_l4s(struct sk_buff *skb)
{
	return dualpi2_skb_cb(skb)->classified == DUALPI2_C_L4S;
}

static bool skb_in_l_queue(struct sk_buff *skb)
{
	return dualpi2_skb_cb(skb)->classified != DUALPI2_C_CLASSIC;
}

static bool skb_apply_step(struct sk_buff *skb, struct dualpi2_sched_data *q)
{
	return skb_is_l4s(skb) && qdisc_qlen(q->l_queue) >= q->min_qlen_step;
}

static bool dualpi2_mark(struct dualpi2_sched_data *q, struct sk_buff *skb)
{
	if (INET_ECN_set_ce(skb)) {
		q->ecn_mark++;
		return true;
	}
	return false;
}

static void dualpi2_reset_c_protection(struct dualpi2_sched_data *q)
{
	q->c_protection_credit = q->c_protection_init;
}

/* This computes the initial credit value and WRR weight for the L queue (wl)
 * from the weight of the C queue (wc).
 * If wl > wc, the scheduler will start with the L queue when reset.
 */
static void dualpi2_calculate_c_protection(struct Qdisc *sch,
					   struct dualpi2_sched_data *q, u32 wc)
{
	q->c_protection_wc = wc;
	q->c_protection_wl = MAX_WC - wc;
	q->c_protection_init = (s32)psched_mtu(qdisc_dev(sch)) *
		((int)q->c_protection_wc - (int)q->c_protection_wl);
	dualpi2_reset_c_protection(q);
}

static bool dualpi2_roll(u32 prob)
{
	return get_random_u32() <= prob;
}

/* Packets in the C-queue are subject to a marking probability pC, which is the
 * square of the internal PI probability (i.e., have an overall lower mark/drop
 * probability). If the qdisc is overloaded, ignore ECT values and only drop.
 *
 * Note that this marking scheme is also applied to L4S packets during overload.
 * Return true if packet dropping is required in C queue
 */
static bool dualpi2_classic_marking(struct dualpi2_sched_data *q,
				    struct sk_buff *skb, u32 prob,
				    bool overload)
{
	if (dualpi2_roll(prob) && dualpi2_roll(prob)) {
		if (overload || dualpi2_skb_cb(skb)->ect == INET_ECN_NOT_ECT)
			return true;
		dualpi2_mark(q, skb);
	}
	return false;
}

/* Packets in the L-queue are subject to a marking probability pL given by the
 * internal PI probability scaled by the coupling factor.
 *
 * On overload (i.e., @local_l_prob is >= 100%):
 * - if the qdisc is configured to trade losses to preserve latency (i.e.,
 *   @q->drop_overload), apply classic drops first before marking.
 * - otherwise, preserve the "no loss" property of ECN at the cost of queueing
 *   delay, eventually resulting in taildrop behavior once sch->limit is
 *   reached.
 * Return true if packet dropping is required in L queue
 */
static bool dualpi2_scalable_marking(struct dualpi2_sched_data *q,
				     struct sk_buff *skb,
				     u64 local_l_prob, u32 prob,
				     bool overload)
{
	if (overload) {
		/* Apply classic drop */
		if (!q->drop_overload ||
		    !(dualpi2_roll(prob) && dualpi2_roll(prob)))
			goto mark;
		return true;
	}

	/* We can safely cut the upper 32b as overload==false */
	if (dualpi2_roll(local_l_prob)) {
		/* Non-ECT packets could have classified as L4S by filters. */
		if (dualpi2_skb_cb(skb)->ect == INET_ECN_NOT_ECT)
			return true;
mark:
		dualpi2_mark(q, skb);
	}
	return false;
}

/* Decide whether a given packet must be dropped (or marked if ECT), according
 * to the PI2 probability.
 *
 * Never mark/drop if we have a standing queue of less than 2 MTUs.
 */
static bool must_drop(struct Qdisc *sch, struct dualpi2_sched_data *q,
		      struct sk_buff *skb)
{
	u64 local_l_prob;
	bool overload;
	u32 prob;

	if (sch->qstats.backlog < 2 * psched_mtu(qdisc_dev(sch)))
		return false;

	prob = READ_ONCE(q->pi2_prob);
	local_l_prob = (u64)prob * q->coupling_factor;
	overload = local_l_prob > MAX_PROB;

	switch (dualpi2_skb_cb(skb)->classified) {
	case DUALPI2_C_CLASSIC:
		return dualpi2_classic_marking(q, skb, prob, overload);
	case DUALPI2_C_L4S:
		return dualpi2_scalable_marking(q, skb, local_l_prob, prob,
						overload);
	default: /* DUALPI2_C_LLLL */
		return false;
	}
}

static void dualpi2_read_ect(struct sk_buff *skb)
{
	struct dualpi2_skb_cb *cb = dualpi2_skb_cb(skb);
	int wlen = skb_network_offset(skb);

	switch (skb_protocol(skb, true)) {
	case htons(ETH_P_IP):
		wlen += sizeof(struct iphdr);
		if (!pskb_may_pull(skb, wlen) ||
		    skb_try_make_writable(skb, wlen))
			goto not_ecn;

		cb->ect = ipv4_get_dsfield(ip_hdr(skb)) & INET_ECN_MASK;
		break;
	case htons(ETH_P_IPV6):
		wlen += sizeof(struct ipv6hdr);
		if (!pskb_may_pull(skb, wlen) ||
		    skb_try_make_writable(skb, wlen))
			goto not_ecn;

		cb->ect = ipv6_get_dsfield(ipv6_hdr(skb)) & INET_ECN_MASK;
		break;
	default:
		goto not_ecn;
	}
	return;

not_ecn:
	/* Non pullable/writable packets can only be dropped hence are
	 * classified as not ECT.
	 */
	cb->ect = INET_ECN_NOT_ECT;
}

static int dualpi2_skb_classify(struct dualpi2_sched_data *q,
				struct sk_buff *skb)
{
	struct dualpi2_skb_cb *cb = dualpi2_skb_cb(skb);
	struct tcf_result res;
	struct tcf_proto *fl;
	int result;

	dualpi2_read_ect(skb);
	if (cb->ect & q->ecn_mask) {
		cb->classified = DUALPI2_C_L4S;
		return NET_XMIT_SUCCESS;
	}

	if (TC_H_MAJ(skb->priority) == q->sch->handle &&
	    TC_H_MIN(skb->priority) < __DUALPI2_C_MAX) {
		cb->classified = TC_H_MIN(skb->priority);
		return NET_XMIT_SUCCESS;
	}

	fl = rcu_dereference_bh(q->tcf_filters);
	if (!fl) {
		cb->classified = DUALPI2_C_CLASSIC;
		return NET_XMIT_SUCCESS;
	}

	result = tcf_classify(skb, NULL, fl, &res, false);
	if (result >= 0) {
#ifdef CONFIG_NET_CLS_ACT
		switch (result) {
		case TC_ACT_STOLEN:
		case TC_ACT_QUEUED:
		case TC_ACT_TRAP:
			return NET_XMIT_SUCCESS | __NET_XMIT_STOLEN;
		case TC_ACT_SHOT:
			return NET_XMIT_SUCCESS | __NET_XMIT_BYPASS;
		}
#endif
		cb->classified = TC_H_MIN(res.classid) < __DUALPI2_C_MAX ?
			TC_H_MIN(res.classid) : DUALPI2_C_CLASSIC;
	}
	return NET_XMIT_SUCCESS;
}

static int dualpi2_enqueue_skb(struct sk_buff *skb, struct Qdisc *sch,
			       struct sk_buff **to_free)
{
	struct dualpi2_sched_data *q = qdisc_priv(sch);
	struct dualpi2_skb_cb *cb;

	if (unlikely(qdisc_qlen(sch) >= sch->limit) ||
	    unlikely((u64)q->memory_used + skb->truesize > q->memory_limit)) {
		qdisc_qstats_overlimit(sch);
		if (skb_in_l_queue(skb))
			qdisc_qstats_overlimit(q->l_queue);
		return qdisc_drop_reason(skb, sch, to_free,
					 SKB_DROP_REASON_QDISC_OVERLIMIT);
	}

	if (q->drop_early && must_drop(sch, q, skb)) {
		qdisc_drop_reason(skb, sch, to_free,
				  SKB_DROP_REASON_QDISC_CONGESTED);
		return NET_XMIT_SUCCESS | __NET_XMIT_BYPASS;
	}

	cb = dualpi2_skb_cb(skb);
	cb->ts = ktime_get_ns();
	q->memory_used += skb->truesize;
	if (q->memory_used > q->max_memory_used)
		q->max_memory_used = q->memory_used;

	if (qdisc_qlen(sch) > q->maxq)
		q->maxq = qdisc_qlen(sch);

	if (skb_in_l_queue(skb)) {
		/* Apply step thresh if skb is L4S && L-queue len >= min_qlen */
		dualpi2_skb_cb(skb)->apply_step = skb_apply_step(skb, q);

		/* Keep the overall qdisc stats consistent */
		++sch->q.qlen;
		qdisc_qstats_backlog_inc(sch, skb);
		++q->packets_in_l;
		if (!q->l_head_ts)
			q->l_head_ts = cb->ts;
		return qdisc_enqueue_tail(skb, q->l_queue);
	}
	++q->packets_in_c;
	if (!q->c_head_ts)
		q->c_head_ts = cb->ts;
	return qdisc_enqueue_tail(skb, sch);
}

/* By default, dualpi2 will split GSO skbs into independent skbs and enqueue
 * each of those individually. This yields the following benefits, at the
 * expense of CPU usage:
 * - Finer-grained AQM actions as the sub-packets of a burst no longer share the
 *   same fate (e.g., the random mark/drop probability is applied individually)
 * - Improved precision of the starvation protection/WRR scheduler at dequeue,
 *   as the size of the dequeued packets will be smaller.
 */
static int dualpi2_qdisc_enqueue(struct sk_buff *skb, struct Qdisc *sch,
				 struct sk_buff **to_free)
{
	struct dualpi2_sched_data *q = qdisc_priv(sch);
	int err;

	err = dualpi2_skb_classify(q, skb);
	if (err != NET_XMIT_SUCCESS) {
		if (err & __NET_XMIT_BYPASS)
			qdisc_qstats_drop(sch);
		__qdisc_drop(skb, to_free);
		return err;
	}

	if (q->split_gso && skb_is_gso(skb)) {
		netdev_features_t features;
		struct sk_buff *nskb, *next;
		int cnt, byte_len, orig_len;
		int err;

		features = netif_skb_features(skb);
		nskb = skb_gso_segment(skb, features & ~NETIF_F_GSO_MASK);
		if (IS_ERR_OR_NULL(nskb))
			return qdisc_drop(skb, sch, to_free);

		cnt = 1;
		byte_len = 0;
		orig_len = qdisc_pkt_len(skb);
		skb_list_walk_safe(nskb, nskb, next) {
			skb_mark_not_on_list(nskb);

			/* Iterate through GSO fragments of an skb:
			 * (1) Set pkt_len from the single GSO fragments
			 * (2) Copy classified and ect values of an skb
			 * (3) Enqueue fragment & set ts in dualpi2_enqueue_skb
			 */
			qdisc_skb_cb(nskb)->pkt_len = nskb->len;
			dualpi2_skb_cb(nskb)->classified =
				dualpi2_skb_cb(skb)->classified;
			dualpi2_skb_cb(nskb)->ect = dualpi2_skb_cb(skb)->ect;
			err = dualpi2_enqueue_skb(nskb, sch, to_free);

			if (err == NET_XMIT_SUCCESS) {
				/* Compute the backlog adjustment that needs
				 * to be propagated in the qdisc tree to reflect
				 * all new skbs successfully enqueued.
				 */
				++cnt;
				byte_len += nskb->len;
			}
		}
		if (cnt > 1) {
			/* The caller will add the original skb stats to its
			 * backlog, compensate this if any nskb is enqueued.
			 */
			--cnt;
			byte_len -= orig_len;
		}
		qdisc_tree_reduce_backlog(sch, -cnt, -byte_len);
		consume_skb(skb);
		return err;
	}
	return dualpi2_enqueue_skb(skb, sch, to_free);
}

/* Select the queue from which the next packet can be dequeued, ensuring that
 * neither queue can starve the other with a WRR scheduler.
 *
 * The sign of the WRR credit determines the next queue, while the size of
 * the dequeued packet determines the magnitude of the WRR credit change. If
 * either queue is empty, the WRR credit is kept unchanged.
 *
 * As the dequeued packet can be dropped later, the caller has to perform the
 * qdisc_bstats_update() calls.
 */
static struct sk_buff *dequeue_packet(struct Qdisc *sch,
				      struct dualpi2_sched_data *q,
				      int *credit_change,
				      u64 now)
{
	struct sk_buff *skb = NULL;
	int c_len;

	*credit_change = 0;
	c_len = qdisc_qlen(sch) - qdisc_qlen(q->l_queue);
	if (qdisc_qlen(q->l_queue) && (!c_len || q->c_protection_credit <= 0)) {
		skb = __qdisc_dequeue_head(&q->l_queue->q);
		WRITE_ONCE(q->l_head_ts, head_enqueue_time(q->l_queue));
		if (c_len)
			*credit_change = q->c_protection_wc;
		qdisc_qstats_backlog_dec(q->l_queue, skb);

		/* Keep the global queue size consistent */
		--sch->q.qlen;
		q->memory_used -= skb->truesize;
	} else if (c_len) {
		skb = __qdisc_dequeue_head(&sch->q);
		WRITE_ONCE(q->c_head_ts, head_enqueue_time(sch));
		if (qdisc_qlen(q->l_queue))
			*credit_change = ~((s32)q->c_protection_wl) + 1;
		q->memory_used -= skb->truesize;
	} else {
		dualpi2_reset_c_protection(q);
		return NULL;
	}
	*credit_change *= qdisc_pkt_len(skb);
	qdisc_qstats_backlog_dec(sch, skb);
	return skb;
}

static int do_step_aqm(struct dualpi2_sched_data *q, struct sk_buff *skb,
		       u64 now)
{
	u64 qdelay = 0;

	if (q->step_in_packets)
		qdelay = qdisc_qlen(q->l_queue);
	else
		qdelay = dualpi2_sojourn_time(skb, now);

	if (dualpi2_skb_cb(skb)->apply_step && qdelay > q->step_thresh) {
		if (!dualpi2_skb_cb(skb)->ect) {
			/* Drop this non-ECT packet */
			return 1;
		}

		if (dualpi2_mark(q, skb))
			++q->step_marks;
	}
	qdisc_bstats_update(q->l_queue, skb);
	return 0;
}

static void drop_and_retry(struct dualpi2_sched_data *q, struct sk_buff *skb,
			   struct Qdisc *sch, enum skb_drop_reason reason)
{
	++q->deferred_drops_cnt;
	q->deferred_drops_len += qdisc_pkt_len(skb);
	kfree_skb_reason(skb, reason);
	qdisc_qstats_drop(sch);
}

static struct sk_buff *dualpi2_qdisc_dequeue(struct Qdisc *sch)
{
	struct dualpi2_sched_data *q = qdisc_priv(sch);
	struct sk_buff *skb;
	int credit_change;
	u64 now;

	now = ktime_get_ns();

	while ((skb = dequeue_packet(sch, q, &credit_change, now))) {
		if (!q->drop_early && must_drop(sch, q, skb)) {
			drop_and_retry(q, skb, sch,
				       SKB_DROP_REASON_QDISC_CONGESTED);
			continue;
		}

		if (skb_in_l_queue(skb) && do_step_aqm(q, skb, now)) {
			qdisc_qstats_drop(q->l_queue);
			drop_and_retry(q, skb, sch,
				       SKB_DROP_REASON_DUALPI2_STEP_DROP);
			continue;
		}

		q->c_protection_credit += credit_change;
		qdisc_bstats_update(sch, skb);
		break;
	}

	if (q->deferred_drops_cnt) {
		qdisc_tree_reduce_backlog(sch, q->deferred_drops_cnt,
					  q->deferred_drops_len);
		q->deferred_drops_cnt = 0;
		q->deferred_drops_len = 0;
	}
	return skb;
}

static s64 __scale_delta(u64 diff)
{
	do_div(diff, 1 << ALPHA_BETA_GRANULARITY);
	return diff;
}

static void get_queue_delays(struct dualpi2_sched_data *q, u64 *qdelay_c,
			     u64 *qdelay_l)
{
	u64 now, qc, ql;

	now = ktime_get_ns();
	qc = READ_ONCE(q->c_head_ts);
	ql = READ_ONCE(q->l_head_ts);

	*qdelay_c = qc ? now - qc : 0;
	*qdelay_l = ql ? now - ql : 0;
}

static u32 calculate_probability(struct Qdisc *sch)
{
	struct dualpi2_sched_data *q = qdisc_priv(sch);
	u32 new_prob;
	u64 qdelay_c;
	u64 qdelay_l;
	u64 qdelay;
	s64 delta;

	get_queue_delays(q, &qdelay_c, &qdelay_l);
	qdelay = max(qdelay_l, qdelay_c);

	/* Alpha and beta take at most 32b, i.e, the delay difference would
	 * overflow for queuing delay differences > ~4.2sec.
	 */
	delta = ((s64)qdelay - (s64)q->pi2_target) * q->pi2_alpha;
	delta += ((s64)qdelay - (s64)q->last_qdelay) * q->pi2_beta;
	q->last_qdelay = qdelay;

	/* Bound new_prob between 0 and MAX_PROB */
	if (delta > 0) {
		new_prob = __scale_delta(delta) + q->pi2_prob;
		if (new_prob < q->pi2_prob)
			new_prob = MAX_PROB;
	} else {
		new_prob = q->pi2_prob - __scale_delta(~delta + 1);
		if (new_prob > q->pi2_prob)
			new_prob = 0;
	}

	/* If we do not drop on overload, ensure we cap the L4S probability to
	 * 100% to keep window fairness when overflowing.
	 */
	if (!q->drop_overload)
		return min_t(u32, new_prob, MAX_PROB / q->coupling_factor);
	return new_prob;
}

static u32 get_memory_limit(struct Qdisc *sch, u32 limit)
{
	/* Apply rule of thumb, i.e., doubling the packet length,
	 * to further include per packet overhead in memory_limit.
	 */
	u64 memlim = mul_u32_u32(limit, 2 * psched_mtu(qdisc_dev(sch)));

	if (upper_32_bits(memlim))
		return U32_MAX;
	else
		return lower_32_bits(memlim);
}

static u32 convert_us_to_nsec(u32 us)
{
	u64 ns = mul_u32_u32(us, NSEC_PER_USEC);

	if (upper_32_bits(ns))
		return U32_MAX;

	return lower_32_bits(ns);
}

static u32 convert_ns_to_usec(u64 ns)
{
	do_div(ns, NSEC_PER_USEC);
	if (upper_32_bits(ns))
		return U32_MAX;

	return lower_32_bits(ns);
}

static enum hrtimer_restart dualpi2_timer(struct hrtimer *timer)
{
	struct dualpi2_sched_data *q = timer_container_of(q, timer, pi2_timer);
	struct Qdisc *sch = q->sch;
	spinlock_t *root_lock; /* to lock qdisc for probability calculations */

	rcu_read_lock();
	root_lock = qdisc_lock(qdisc_root_sleeping(sch));
	spin_lock(root_lock);

	WRITE_ONCE(q->pi2_prob, calculate_probability(sch));
	hrtimer_set_expires(&q->pi2_timer, next_pi2_timeout(q));

	spin_unlock(root_lock);
	rcu_read_unlock();
	return HRTIMER_RESTART;
}

static struct netlink_range_validation dualpi2_alpha_beta_range = {
	.min = 1,
	.max = ALPHA_BETA_MAX,
};

static const struct nla_policy dualpi2_policy[TCA_DUALPI2_MAX + 1] = {
	[TCA_DUALPI2_LIMIT]		= NLA_POLICY_MIN(NLA_U32, 1),
	[TCA_DUALPI2_MEMORY_LIMIT]	= NLA_POLICY_MIN(NLA_U32, 1),
	[TCA_DUALPI2_TARGET]		= { .type = NLA_U32 },
	[TCA_DUALPI2_TUPDATE]		= NLA_POLICY_MIN(NLA_U32, 1),
	[TCA_DUALPI2_ALPHA]		=
		NLA_POLICY_FULL_RANGE(NLA_U32, &dualpi2_alpha_beta_range),
	[TCA_DUALPI2_BETA]		=
		NLA_POLICY_FULL_RANGE(NLA_U32, &dualpi2_alpha_beta_range),
	[TCA_DUALPI2_STEP_THRESH_PKTS]	= { .type = NLA_U32 },
	[TCA_DUALPI2_STEP_THRESH_US]	= { .type = NLA_U32 },
	[TCA_DUALPI2_MIN_QLEN_STEP]	= { .type = NLA_U32 },
	[TCA_DUALPI2_COUPLING]		= NLA_POLICY_MIN(NLA_U8, 1),
	[TCA_DUALPI2_DROP_OVERLOAD]	=
		NLA_POLICY_MAX(NLA_U8, TCA_DUALPI2_DROP_OVERLOAD_MAX),
	[TCA_DUALPI2_DROP_EARLY]	=
		NLA_POLICY_MAX(NLA_U8, TCA_DUALPI2_DROP_EARLY_MAX),
	[TCA_DUALPI2_C_PROTECTION]	=
		NLA_POLICY_RANGE(NLA_U8, 0, MAX_WC),
	[TCA_DUALPI2_ECN_MASK]		=
		NLA_POLICY_RANGE(NLA_U8, TC_DUALPI2_ECN_MASK_L4S_ECT,
				 TCA_DUALPI2_ECN_MASK_MAX),
	[TCA_DUALPI2_SPLIT_GSO]		=
		NLA_POLICY_MAX(NLA_U8, TCA_DUALPI2_SPLIT_GSO_MAX),
};

static int dualpi2_change(struct Qdisc *sch, struct nlattr *opt,
			  struct netlink_ext_ack *extack)
{
	struct nlattr *tb[TCA_DUALPI2_MAX + 1];
	struct dualpi2_sched_data *q;
	int old_backlog;
	int old_qlen;
	int err;

	if (!opt || !nla_len(opt)) {
		NL_SET_ERR_MSG_MOD(extack, "Dualpi2 options are required");
		return -EINVAL;
	}
	err = nla_parse_nested(tb, TCA_DUALPI2_MAX, opt, dualpi2_policy,
			       extack);
	if (err < 0)
		return err;
	if (tb[TCA_DUALPI2_STEP_THRESH_PKTS] && tb[TCA_DUALPI2_STEP_THRESH_US]) {
		NL_SET_ERR_MSG_MOD(extack, "multiple step thresh attributes");
		return -EINVAL;
	}

	q = qdisc_priv(sch);
	sch_tree_lock(sch);

	if (tb[TCA_DUALPI2_LIMIT]) {
		u32 limit = nla_get_u32(tb[TCA_DUALPI2_LIMIT]);

		WRITE_ONCE(sch->limit, limit);
		WRITE_ONCE(q->memory_limit, get_memory_limit(sch, limit));
	}

	if (tb[TCA_DUALPI2_MEMORY_LIMIT])
		WRITE_ONCE(q->memory_limit,
			   nla_get_u32(tb[TCA_DUALPI2_MEMORY_LIMIT]));

	if (tb[TCA_DUALPI2_TARGET]) {
		u64 target = nla_get_u32(tb[TCA_DUALPI2_TARGET]);

		WRITE_ONCE(q->pi2_target, target * NSEC_PER_USEC);
	}

	if (tb[TCA_DUALPI2_TUPDATE]) {
		u64 tupdate = nla_get_u32(tb[TCA_DUALPI2_TUPDATE]);

		WRITE_ONCE(q->pi2_tupdate, convert_us_to_nsec(tupdate));
	}

	if (tb[TCA_DUALPI2_ALPHA]) {
		u32 alpha = nla_get_u32(tb[TCA_DUALPI2_ALPHA]);

		WRITE_ONCE(q->pi2_alpha, dualpi2_scale_alpha_beta(alpha));
	}

	if (tb[TCA_DUALPI2_BETA]) {
		u32 beta = nla_get_u32(tb[TCA_DUALPI2_BETA]);

		WRITE_ONCE(q->pi2_beta, dualpi2_scale_alpha_beta(beta));
	}

	if (tb[TCA_DUALPI2_STEP_THRESH_PKTS]) {
		u32 step_th = nla_get_u32(tb[TCA_DUALPI2_STEP_THRESH_PKTS]);

		WRITE_ONCE(q->step_in_packets, true);
		WRITE_ONCE(q->step_thresh, step_th);
	} else if (tb[TCA_DUALPI2_STEP_THRESH_US]) {
		u32 step_th = nla_get_u32(tb[TCA_DUALPI2_STEP_THRESH_US]);

		WRITE_ONCE(q->step_in_packets, false);
		WRITE_ONCE(q->step_thresh, convert_us_to_nsec(step_th));
	}

	if (tb[TCA_DUALPI2_MIN_QLEN_STEP])
		WRITE_ONCE(q->min_qlen_step,
			   nla_get_u32(tb[TCA_DUALPI2_MIN_QLEN_STEP]));

	if (tb[TCA_DUALPI2_COUPLING]) {
		u8 coupling = nla_get_u8(tb[TCA_DUALPI2_COUPLING]);

		WRITE_ONCE(q->coupling_factor, coupling);
	}

	if (tb[TCA_DUALPI2_DROP_OVERLOAD]) {
		u8 drop_overload = nla_get_u8(tb[TCA_DUALPI2_DROP_OVERLOAD]);

		WRITE_ONCE(q->drop_overload, (bool)drop_overload);
	}

	if (tb[TCA_DUALPI2_DROP_EARLY]) {
		u8 drop_early = nla_get_u8(tb[TCA_DUALPI2_DROP_EARLY]);

		WRITE_ONCE(q->drop_early, (bool)drop_early);
	}

	if (tb[TCA_DUALPI2_C_PROTECTION]) {
		u8 wc = nla_get_u8(tb[TCA_DUALPI2_C_PROTECTION]);

		dualpi2_calculate_c_protection(sch, q, wc);
	}

	if (tb[TCA_DUALPI2_ECN_MASK]) {
		u8 ecn_mask = nla_get_u8(tb[TCA_DUALPI2_ECN_MASK]);

		WRITE_ONCE(q->ecn_mask, ecn_mask);
	}

	if (tb[TCA_DUALPI2_SPLIT_GSO]) {
		u8 split_gso = nla_get_u8(tb[TCA_DUALPI2_SPLIT_GSO]);

		WRITE_ONCE(q->split_gso, (bool)split_gso);
	}

	old_qlen = qdisc_qlen(sch);
	old_backlog = sch->qstats.backlog;
	while (qdisc_qlen(sch) > sch->limit ||
	       q->memory_used > q->memory_limit) {
		struct sk_buff *skb = qdisc_dequeue_internal(sch, true);

		q->memory_used -= skb->truesize;
		qdisc_qstats_backlog_dec(sch, skb);
		rtnl_qdisc_drop(skb, sch);
	}
	qdisc_tree_reduce_backlog(sch, old_qlen - qdisc_qlen(sch),
				  old_backlog - sch->qstats.backlog);

	sch_tree_unlock(sch);
	return 0;
}

/* Default alpha/beta values give a 10dB stability margin with max_rtt=100ms. */
static void dualpi2_reset_default(struct Qdisc *sch)
{
	struct dualpi2_sched_data *q = qdisc_priv(sch);

	q->sch->limit = 10000;				/* Max 125ms at 1Gbps */
	q->memory_limit = get_memory_limit(sch, q->sch->limit);

	q->pi2_target = 15 * NSEC_PER_MSEC;
	q->pi2_tupdate = 16 * NSEC_PER_MSEC;
	q->pi2_alpha = dualpi2_scale_alpha_beta(41);	/* ~0.16 Hz * 256 */
	q->pi2_beta = dualpi2_scale_alpha_beta(819);	/* ~3.20 Hz * 256 */

	q->step_thresh = 1 * NSEC_PER_MSEC;
	q->step_in_packets = false;

	dualpi2_calculate_c_protection(q->sch, q, 10);	/* wc=10%, wl=90% */

	q->ecn_mask = TC_DUALPI2_ECN_MASK_L4S_ECT;	/* INET_ECN_ECT_1 */
	q->min_qlen_step = 0;		/* Always apply step mark in L-queue */
	q->coupling_factor = 2;		/* window fairness for equal RTTs */
	q->drop_overload = TC_DUALPI2_DROP_OVERLOAD_DROP; /* Drop overload */
	q->drop_early = TC_DUALPI2_DROP_EARLY_DROP_DEQUEUE; /* Drop dequeue */
	q->split_gso = TC_DUALPI2_SPLIT_GSO_SPLIT_GSO;	/* Split GSO */
}

static int dualpi2_init(struct Qdisc *sch, struct nlattr *opt,
			struct netlink_ext_ack *extack)
{
	struct dualpi2_sched_data *q = qdisc_priv(sch);
	int err;

	q->l_queue = qdisc_create_dflt(sch->dev_queue, &pfifo_qdisc_ops,
				       TC_H_MAKE(sch->handle, 1), extack);
	if (!q->l_queue)
		return -ENOMEM;

	err = tcf_block_get(&q->tcf_block, &q->tcf_filters, sch, extack);
	if (err)
		return err;

	q->sch = sch;
	dualpi2_reset_default(sch);
	hrtimer_setup(&q->pi2_timer, dualpi2_timer, CLOCK_MONOTONIC,
		      HRTIMER_MODE_ABS_PINNED_SOFT);

	if (opt && nla_len(opt)) {
		err = dualpi2_change(sch, opt, extack);

		if (err)
			return err;
	}

	hrtimer_start(&q->pi2_timer, next_pi2_timeout(q),
		      HRTIMER_MODE_ABS_PINNED_SOFT);
	return 0;
}

static int dualpi2_dump(struct Qdisc *sch, struct sk_buff *skb)
{
	struct dualpi2_sched_data *q = qdisc_priv(sch);
	struct nlattr *opts;
	bool step_in_pkts;
	u32 step_th;

	step_in_pkts = READ_ONCE(q->step_in_packets);
	step_th = READ_ONCE(q->step_thresh);

	opts = nla_nest_start_noflag(skb, TCA_OPTIONS);
	if (!opts)
		goto nla_put_failure;

	if (step_in_pkts &&
	    (nla_put_u32(skb, TCA_DUALPI2_LIMIT, READ_ONCE(sch->limit)) ||
	    nla_put_u32(skb, TCA_DUALPI2_MEMORY_LIMIT,
			READ_ONCE(q->memory_limit)) ||
	    nla_put_u32(skb, TCA_DUALPI2_TARGET,
			convert_ns_to_usec(READ_ONCE(q->pi2_target))) ||
	    nla_put_u32(skb, TCA_DUALPI2_TUPDATE,
			convert_ns_to_usec(READ_ONCE(q->pi2_tupdate))) ||
	    nla_put_u32(skb, TCA_DUALPI2_ALPHA,
			dualpi2_unscale_alpha_beta(READ_ONCE(q->pi2_alpha))) ||
	    nla_put_u32(skb, TCA_DUALPI2_BETA,
			dualpi2_unscale_alpha_beta(READ_ONCE(q->pi2_beta))) ||
	    nla_put_u32(skb, TCA_DUALPI2_STEP_THRESH_PKTS, step_th) ||
	    nla_put_u32(skb, TCA_DUALPI2_MIN_QLEN_STEP,
			READ_ONCE(q->min_qlen_step)) ||
	    nla_put_u8(skb, TCA_DUALPI2_COUPLING,
		       READ_ONCE(q->coupling_factor)) ||
	    nla_put_u8(skb, TCA_DUALPI2_DROP_OVERLOAD,
		       READ_ONCE(q->drop_overload)) ||
	    nla_put_u8(skb, TCA_DUALPI2_DROP_EARLY,
		       READ_ONCE(q->drop_early)) ||
	    nla_put_u8(skb, TCA_DUALPI2_C_PROTECTION,
		       READ_ONCE(q->c_protection_wc)) ||
	    nla_put_u8(skb, TCA_DUALPI2_ECN_MASK, READ_ONCE(q->ecn_mask)) ||
	    nla_put_u8(skb, TCA_DUALPI2_SPLIT_GSO, READ_ONCE(q->split_gso))))
		goto nla_put_failure;

	if (!step_in_pkts &&
	    (nla_put_u32(skb, TCA_DUALPI2_LIMIT, READ_ONCE(sch->limit)) ||
	    nla_put_u32(skb, TCA_DUALPI2_MEMORY_LIMIT,
			READ_ONCE(q->memory_limit)) ||
	    nla_put_u32(skb, TCA_DUALPI2_TARGET,
			convert_ns_to_usec(READ_ONCE(q->pi2_target))) ||
	    nla_put_u32(skb, TCA_DUALPI2_TUPDATE,
			convert_ns_to_usec(READ_ONCE(q->pi2_tupdate))) ||
	    nla_put_u32(skb, TCA_DUALPI2_ALPHA,
			dualpi2_unscale_alpha_beta(READ_ONCE(q->pi2_alpha))) ||
	    nla_put_u32(skb, TCA_DUALPI2_BETA,
			dualpi2_unscale_alpha_beta(READ_ONCE(q->pi2_beta))) ||
	    nla_put_u32(skb, TCA_DUALPI2_STEP_THRESH_US,
			convert_ns_to_usec(step_th)) ||
	    nla_put_u32(skb, TCA_DUALPI2_MIN_QLEN_STEP,
			READ_ONCE(q->min_qlen_step)) ||
	    nla_put_u8(skb, TCA_DUALPI2_COUPLING,
		       READ_ONCE(q->coupling_factor)) ||
	    nla_put_u8(skb, TCA_DUALPI2_DROP_OVERLOAD,
		       READ_ONCE(q->drop_overload)) ||
	    nla_put_u8(skb, TCA_DUALPI2_DROP_EARLY,
		       READ_ONCE(q->drop_early)) ||
	    nla_put_u8(skb, TCA_DUALPI2_C_PROTECTION,
		       READ_ONCE(q->c_protection_wc)) ||
	    nla_put_u8(skb, TCA_DUALPI2_ECN_MASK, READ_ONCE(q->ecn_mask)) ||
	    nla_put_u8(skb, TCA_DUALPI2_SPLIT_GSO, READ_ONCE(q->split_gso))))
		goto nla_put_failure;

	return nla_nest_end(skb, opts);

nla_put_failure:
	nla_nest_cancel(skb, opts);
	return -1;
}

static int dualpi2_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
{
	struct dualpi2_sched_data *q = qdisc_priv(sch);
	struct tc_dualpi2_xstats st = {
		.prob			= READ_ONCE(q->pi2_prob),
		.packets_in_c		= q->packets_in_c,
		.packets_in_l		= q->packets_in_l,
		.maxq			= q->maxq,
		.ecn_mark		= q->ecn_mark,
		.credit			= q->c_protection_credit,
		.step_marks		= q->step_marks,
		.memory_used		= q->memory_used,
		.max_memory_used	= q->max_memory_used,
		.memory_limit		= q->memory_limit,
	};
	u64 qc, ql;

	get_queue_delays(q, &qc, &ql);
	st.delay_l = convert_ns_to_usec(ql);
	st.delay_c = convert_ns_to_usec(qc);
	return gnet_stats_copy_app(d, &st, sizeof(st));
}

/* Reset both L-queue and C-queue, internal packet counters, PI probability,
 * C-queue protection credit, and timestamps, while preserving current
 * configuration of DUALPI2.
 */
static void dualpi2_reset(struct Qdisc *sch)
{
	struct dualpi2_sched_data *q = qdisc_priv(sch);

	qdisc_reset_queue(sch);
	qdisc_reset_queue(q->l_queue);
	q->c_head_ts = 0;
	q->l_head_ts = 0;
	q->pi2_prob = 0;
	q->packets_in_c = 0;
	q->packets_in_l = 0;
	q->maxq = 0;
	q->ecn_mark = 0;
	q->step_marks = 0;
	q->memory_used = 0;
	q->max_memory_used = 0;
	dualpi2_reset_c_protection(q);
}

static void dualpi2_destroy(struct Qdisc *sch)
{
	struct dualpi2_sched_data *q = qdisc_priv(sch);

	q->pi2_tupdate = 0;
	hrtimer_cancel(&q->pi2_timer);
	if (q->l_queue)
		qdisc_put(q->l_queue);
	tcf_block_put(q->tcf_block);
}

static struct Qdisc *dualpi2_leaf(struct Qdisc *sch, unsigned long arg)
{
	return NULL;
}

static unsigned long dualpi2_find(struct Qdisc *sch, u32 classid)
{
	return 0;
}

static unsigned long dualpi2_bind(struct Qdisc *sch, unsigned long parent,
				  u32 classid)
{
	return 0;
}

static void dualpi2_unbind(struct Qdisc *q, unsigned long cl)
{
}

static struct tcf_block *dualpi2_tcf_block(struct Qdisc *sch, unsigned long cl,
					   struct netlink_ext_ack *extack)
{
	struct dualpi2_sched_data *q = qdisc_priv(sch);

	if (cl)
		return NULL;
	return q->tcf_block;
}

static void dualpi2_walk(struct Qdisc *sch, struct qdisc_walker *arg)
{
	unsigned int i;

	if (arg->stop)
		return;

	/* We statically define only 2 queues */
	for (i = 0; i < 2; i++) {
		if (arg->count < arg->skip) {
			arg->count++;
			continue;
		}
		if (arg->fn(sch, i + 1, arg) < 0) {
			arg->stop = 1;
			break;
		}
		arg->count++;
	}
}

/* Minimal class support to handle tc filters */
static const struct Qdisc_class_ops dualpi2_class_ops = {
	.leaf		= dualpi2_leaf,
	.find		= dualpi2_find,
	.tcf_block	= dualpi2_tcf_block,
	.bind_tcf	= dualpi2_bind,
	.unbind_tcf	= dualpi2_unbind,
	.walk		= dualpi2_walk,
};

static struct Qdisc_ops dualpi2_qdisc_ops __read_mostly = {
	.id		= "dualpi2",
	.cl_ops		= &dualpi2_class_ops,
	.priv_size	= sizeof(struct dualpi2_sched_data),
	.enqueue	= dualpi2_qdisc_enqueue,
	.dequeue	= dualpi2_qdisc_dequeue,
	.peek		= qdisc_peek_dequeued,
	.init		= dualpi2_init,
	.destroy	= dualpi2_destroy,
	.reset		= dualpi2_reset,
	.change		= dualpi2_change,
	.dump		= dualpi2_dump,
	.dump_stats	= dualpi2_dump_stats,
	.owner		= THIS_MODULE,
};

static int __init dualpi2_module_init(void)
{
	return register_qdisc(&dualpi2_qdisc_ops);
}

static void __exit dualpi2_module_exit(void)
{
	unregister_qdisc(&dualpi2_qdisc_ops);
}

module_init(dualpi2_module_init);
module_exit(dualpi2_module_exit);

MODULE_DESCRIPTION("Dual Queue with Proportional Integral controller Improved with a Square (dualpi2) scheduler");
MODULE_AUTHOR("Koen De Schepper <koen.de_schepper@nokia-bell-labs.com>");
MODULE_AUTHOR("Chia-Yu Chang <chia-yu.chang@nokia-bell-labs.com>");
MODULE_AUTHOR("Olga Albisser <olga@albisser.org>");
MODULE_AUTHOR("Henrik Steen <henrist@henrist.net>");
MODULE_AUTHOR("Olivier Tilmans <olivier.tilmans@nokia.com>");

MODULE_LICENSE("Dual BSD/GPL");
MODULE_VERSION("1.0");