net/sched: add skbprio scheduler

Skbprio (SKB Priority Queue) is a queueing discipline that prioritizes packets
according to their skb->priority field. Under congestion, already-enqueued lower
priority packets will be dropped to make space available for higher priority
packets. Skbprio was conceived as a solution for denial-of-service defenses that
need to route packets with different priorities as a means to overcome DoS
attacks.

v5
*Do not reference qdisc_dev(sch)->tx_queue_len for setting limit. Instead set
default sch->limit to 64.

v4
*Drop Documentation/networking/sch_skbprio.txt doc file to move it to tc man
page for Skbprio, in iproute2.

v3
*Drop max_limit parameter in struct skbprio_sched_data and instead use
sch->limit.

*Reference qdisc_dev(sch)->tx_queue_len only once, during initialisation for
qdisc (previously being referenced every time qdisc changes).

*Move qdisc's detailed description from in-code to Documentation/networking.

*When qdisc is saturated, enqueue incoming packet first before dequeueing
lowest priority packet in queue - improves usage of call stack registers.

*Introduce and use overlimit stat to keep track of number of dropped packets.

v2
*Use skb->priority field rather than DS field. Rename queueing discipline as
SKB Priority Queue (previously Gatekeeper Priority Queue).

*Queueing discipline is made classful to expose Skbprio's internal priority
queues.

Signed-off-by: Nishanth Devarajan <ndev2021@gmail.com>
Reviewed-by: Sachin Paryani <sachin.paryani@gmail.com>
Reviewed-by: Cody Doucette <doucette@bu.edu>
Reviewed-by: Michel Machado <michel@digirati.com.br>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/pkt_sched.h

@@ -124,6 +124,21 @@ struct tc_fifo_qopt {
 	__u32	limit;	/* Queue length: bytes for bfifo, packets for pfifo */
 };
 
+/* SKBPRIO section */
+
+/*
+ * Priorities go from zero to (SKBPRIO_MAX_PRIORITY - 1).
+ * SKBPRIO_MAX_PRIORITY should be at least 64 in order for skbprio to be able
+ * to map one to one the DS field of IPV4 and IPV6 headers.
+ * Memory allocation grows linearly with SKBPRIO_MAX_PRIORITY.
+ */
+
+#define SKBPRIO_MAX_PRIORITY 64
+
+struct tc_skbprio_qopt {
+	__u32	limit;		/* Queue length in packets. */
+};
+
 /* PRIO section */
 
 #define TCQ_PRIO_BANDS	16

net/sched/Kconfig

@@ -251,6 +251,19 @@ config NET_SCH_MQPRIO
 
 	  If unsure, say N.
 
+config NET_SCH_SKBPRIO
+	tristate "SKB priority queue scheduler (SKBPRIO)"
+	help
+	  Say Y here if you want to use the SKB priority queue
+	  scheduler. This schedules packets according to skb->priority,
+	  which is useful for request packets in DoS mitigation systems such
+	  as Gatekeeper.
+
+	  To compile this driver as a module, choose M here: the module will
+	  be called sch_skbprio.
+
+	  If unsure, say N.
+
 config NET_SCH_CHOKE
 	tristate "CHOose and Keep responsive flow scheduler (CHOKE)"
 	help

net/sched/Makefile

@@ -46,6 +46,7 @@ obj-$(CONFIG_NET_SCH_NETEM)	+= sch_netem.o
 obj-$(CONFIG_NET_SCH_DRR)	+= sch_drr.o
 obj-$(CONFIG_NET_SCH_PLUG)	+= sch_plug.o
 obj-$(CONFIG_NET_SCH_MQPRIO)	+= sch_mqprio.o
+obj-$(CONFIG_NET_SCH_SKBPRIO)	+= sch_skbprio.o
 obj-$(CONFIG_NET_SCH_CHOKE)	+= sch_choke.o
 obj-$(CONFIG_NET_SCH_QFQ)	+= sch_qfq.o
 obj-$(CONFIG_NET_SCH_CODEL)	+= sch_codel.o

net/sched/sch_skbprio.c

@@ -0,0 +1,320 @@
+/*
+ * net/sched/sch_skbprio.c  SKB Priority Queue.
+ *
+ *		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, or (at your option) any later version.
+ *
+ * Authors:	Nishanth Devarajan, <ndev2021@gmail.com>
+ *		Cody Doucette, <doucette@bu.edu>
+ *	        original idea by Michel Machado, Cody Doucette, and Qiaobin Fu
+ */
+
+#include <linux/string.h>
+#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/sch_generic.h>
+#include <net/inet_ecn.h>
+
+/*		SKB Priority Queue
+ *	=================================
+ *
+ * Skbprio (SKB Priority Queue) is a queueing discipline that prioritizes
+ * packets according to their skb->priority field. Under congestion,
+ * Skbprio drops already-enqueued lower priority packets to make space
+ * available for higher priority packets; it was conceived as a solution
+ * for denial-of-service defenses that need to route packets with different
+ * priorities as a mean to overcome DoS attacks.
+ */
+
+struct skbprio_sched_data {
+	/* Queue state. */
+	struct sk_buff_head qdiscs[SKBPRIO_MAX_PRIORITY];
+	struct gnet_stats_queue qstats[SKBPRIO_MAX_PRIORITY];
+	u16 highest_prio;
+	u16 lowest_prio;
+};
+
+static u16 calc_new_high_prio(const struct skbprio_sched_data *q)
+{
+	int prio;
+
+	for (prio = q->highest_prio - 1; prio >= q->lowest_prio; prio--) {
+		if (!skb_queue_empty(&q->qdiscs[prio]))
+			return prio;
+	}
+
+	/* SKB queue is empty, return 0 (default highest priority setting). */
+	return 0;
+}
+
+static u16 calc_new_low_prio(const struct skbprio_sched_data *q)
+{
+	int prio;
+
+	for (prio = q->lowest_prio + 1; prio <= q->highest_prio; prio++) {
+		if (!skb_queue_empty(&q->qdiscs[prio]))
+			return prio;
+	}
+
+	/* SKB queue is empty, return SKBPRIO_MAX_PRIORITY - 1
+	 * (default lowest priority setting).
+	 */
+	return SKBPRIO_MAX_PRIORITY - 1;
+}
+
+static int skbprio_enqueue(struct sk_buff *skb, struct Qdisc *sch,
+			  struct sk_buff **to_free)
+{
+	const unsigned int max_priority = SKBPRIO_MAX_PRIORITY - 1;
+	struct skbprio_sched_data *q = qdisc_priv(sch);
+	struct sk_buff_head *qdisc;
+	struct sk_buff_head *lp_qdisc;
+	struct sk_buff *to_drop;
+	u16 prio, lp;
+
+	/* Obtain the priority of @skb. */
+	prio = min(skb->priority, max_priority);
+
+	qdisc = &q->qdiscs[prio];
+	if (sch->q.qlen < sch->limit) {
+		__skb_queue_tail(qdisc, skb);
+		qdisc_qstats_backlog_inc(sch, skb);
+		q->qstats[prio].backlog += qdisc_pkt_len(skb);
+
+		/* Check to update highest and lowest priorities. */
+		if (prio > q->highest_prio)
+			q->highest_prio = prio;
+
+		if (prio < q->lowest_prio)
+			q->lowest_prio = prio;
+
+		sch->q.qlen++;
+		return NET_XMIT_SUCCESS;
+	}
+
+	/* If this packet has the lowest priority, drop it. */
+	lp = q->lowest_prio;
+	if (prio <= lp) {
+		q->qstats[prio].drops++;
+		q->qstats[prio].overlimits++;
+		return qdisc_drop(skb, sch, to_free);
+	}
+
+	__skb_queue_tail(qdisc, skb);
+	qdisc_qstats_backlog_inc(sch, skb);
+	q->qstats[prio].backlog += qdisc_pkt_len(skb);
+
+	/* Drop the packet at the tail of the lowest priority qdisc. */
+	lp_qdisc = &q->qdiscs[lp];
+	to_drop = __skb_dequeue_tail(lp_qdisc);
+	BUG_ON(!to_drop);
+	qdisc_qstats_backlog_dec(sch, to_drop);
+	qdisc_drop(to_drop, sch, to_free);
+
+	q->qstats[lp].backlog -= qdisc_pkt_len(to_drop);
+	q->qstats[lp].drops++;
+	q->qstats[lp].overlimits++;
+
+	/* Check to update highest and lowest priorities. */
+	if (skb_queue_empty(lp_qdisc)) {
+		if (q->lowest_prio == q->highest_prio) {
+			/* The incoming packet is the only packet in queue. */
+			BUG_ON(sch->q.qlen != 1);
+			q->lowest_prio = prio;
+			q->highest_prio = prio;
+		} else {
+			q->lowest_prio = calc_new_low_prio(q);
+		}
+	}
+
+	if (prio > q->highest_prio)
+		q->highest_prio = prio;
+
+	return NET_XMIT_CN;
+}
+
+static struct sk_buff *skbprio_dequeue(struct Qdisc *sch)
+{
+	struct skbprio_sched_data *q = qdisc_priv(sch);
+	struct sk_buff_head *hpq = &q->qdiscs[q->highest_prio];
+	struct sk_buff *skb = __skb_dequeue(hpq);
+
+	if (unlikely(!skb))
+		return NULL;
+
+	sch->q.qlen--;
+	qdisc_qstats_backlog_dec(sch, skb);
+	qdisc_bstats_update(sch, skb);
+
+	q->qstats[q->highest_prio].backlog -= qdisc_pkt_len(skb);
+
+	/* Update highest priority field. */
+	if (skb_queue_empty(hpq)) {
+		if (q->lowest_prio == q->highest_prio) {
+			BUG_ON(sch->q.qlen);
+			q->highest_prio = 0;
+			q->lowest_prio = SKBPRIO_MAX_PRIORITY - 1;
+		} else {
+			q->highest_prio = calc_new_high_prio(q);
+		}
+	}
+	return skb;
+}
+
+static int skbprio_change(struct Qdisc *sch, struct nlattr *opt,
+			struct netlink_ext_ack *extack)
+{
+	struct tc_skbprio_qopt *ctl = nla_data(opt);
+
+	sch->limit = ctl->limit;
+	return 0;
+}
+
+static int skbprio_init(struct Qdisc *sch, struct nlattr *opt,
+			struct netlink_ext_ack *extack)
+{
+	struct skbprio_sched_data *q = qdisc_priv(sch);
+	int prio;
+
+	/* Initialise all queues, one for each possible priority. */
+	for (prio = 0; prio < SKBPRIO_MAX_PRIORITY; prio++)
+		__skb_queue_head_init(&q->qdiscs[prio]);
+
+	memset(&q->qstats, 0, sizeof(q->qstats));
+	q->highest_prio = 0;
+	q->lowest_prio = SKBPRIO_MAX_PRIORITY - 1;
+	sch->limit = 64;
+	if (!opt)
+		return 0;
+
+	return skbprio_change(sch, opt, extack);
+}
+
+static int skbprio_dump(struct Qdisc *sch, struct sk_buff *skb)
+{
+	struct tc_skbprio_qopt opt;
+
+	opt.limit = sch->limit;
+
+	if (nla_put(skb, TCA_OPTIONS, sizeof(opt), &opt))
+		return -1;
+
+	return skb->len;
+}
+
+static void skbprio_reset(struct Qdisc *sch)
+{
+	struct skbprio_sched_data *q = qdisc_priv(sch);
+	int prio;
+
+	sch->qstats.backlog = 0;
+	sch->q.qlen = 0;
+
+	for (prio = 0; prio < SKBPRIO_MAX_PRIORITY; prio++)
+		__skb_queue_purge(&q->qdiscs[prio]);
+
+	memset(&q->qstats, 0, sizeof(q->qstats));
+	q->highest_prio = 0;
+	q->lowest_prio = SKBPRIO_MAX_PRIORITY - 1;
+}
+
+static void skbprio_destroy(struct Qdisc *sch)
+{
+	struct skbprio_sched_data *q = qdisc_priv(sch);
+	int prio;
+
+	for (prio = 0; prio < SKBPRIO_MAX_PRIORITY; prio++)
+		__skb_queue_purge(&q->qdiscs[prio]);
+}
+
+static struct Qdisc *skbprio_leaf(struct Qdisc *sch, unsigned long arg)
+{
+	return NULL;
+}
+
+static unsigned long skbprio_find(struct Qdisc *sch, u32 classid)
+{
+	return 0;
+}
+
+static int skbprio_dump_class(struct Qdisc *sch, unsigned long cl,
+			     struct sk_buff *skb, struct tcmsg *tcm)
+{
+	tcm->tcm_handle |= TC_H_MIN(cl);
+	return 0;
+}
+
+static int skbprio_dump_class_stats(struct Qdisc *sch, unsigned long cl,
+				   struct gnet_dump *d)
+{
+	struct skbprio_sched_data *q = qdisc_priv(sch);
+	if (gnet_stats_copy_queue(d, NULL, &q->qstats[cl - 1],
+		q->qstats[cl - 1].qlen) < 0)
+		return -1;
+	return 0;
+}
+
+static void skbprio_walk(struct Qdisc *sch, struct qdisc_walker *arg)
+{
+	unsigned int i;
+
+	if (arg->stop)
+		return;
+
+	for (i = 0; i < SKBPRIO_MAX_PRIORITY; i++) {
+		if (arg->count < arg->skip) {
+			arg->count++;
+			continue;
+		}
+		if (arg->fn(sch, i + 1, arg) < 0) {
+			arg->stop = 1;
+			break;
+		}
+		arg->count++;
+	}
+}
+
+static const struct Qdisc_class_ops skbprio_class_ops = {
+	.leaf		=	skbprio_leaf,
+	.find		=	skbprio_find,
+	.dump		=	skbprio_dump_class,
+	.dump_stats	=	skbprio_dump_class_stats,
+	.walk		=	skbprio_walk,
+};
+
+static struct Qdisc_ops skbprio_qdisc_ops __read_mostly = {
+	.cl_ops		=	&skbprio_class_ops,
+	.id		=	"skbprio",
+	.priv_size	=	sizeof(struct skbprio_sched_data),
+	.enqueue	=	skbprio_enqueue,
+	.dequeue	=	skbprio_dequeue,
+	.peek		=	qdisc_peek_dequeued,
+	.init		=	skbprio_init,
+	.reset		=	skbprio_reset,
+	.change		=	skbprio_change,
+	.dump		=	skbprio_dump,
+	.destroy	=	skbprio_destroy,
+	.owner		=	THIS_MODULE,
+};
+
+static int __init skbprio_module_init(void)
+{
+	return register_qdisc(&skbprio_qdisc_ops);
+}
+
+static void __exit skbprio_module_exit(void)
+{
+	unregister_qdisc(&skbprio_qdisc_ops);
+}
+
+module_init(skbprio_module_init)
+module_exit(skbprio_module_exit)
+
+MODULE_LICENSE("GPL");