sch_plug.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * sch_plug.c Queue traffic until an explicit release command
 *
 * There are two ways to use this qdisc:
 * 1. A simple "instantaneous" plug/unplug operation, by issuing an alternating
 *    sequence of TCQ_PLUG_BUFFER & TCQ_PLUG_RELEASE_INDEFINITE commands.
 *
 * 2. For network output buffering (a.k.a output commit) functionality.
 *    Output commit property is commonly used by applications using checkpoint
 *    based fault-tolerance to ensure that the checkpoint from which a system
 *    is being restored is consistent w.r.t outside world.
 *
 *    Consider for e.g. Remus - a Virtual Machine checkpointing system,
 *    wherein a VM is checkpointed, say every 50ms. The checkpoint is replicated
 *    asynchronously to the backup host, while the VM continues executing the
 *    next epoch speculatively.
 *
 *    The following is a typical sequence of output buffer operations:
 *       1.At epoch i, start_buffer(i)
 *       2. At end of epoch i (i.e. after 50ms):
 *          2.1 Stop VM and take checkpoint(i).
 *          2.2 start_buffer(i+1) and Resume VM
 *       3. While speculatively executing epoch(i+1), asynchronously replicate
 *          checkpoint(i) to backup host.
 *       4. When checkpoint_ack(i) is received from backup, release_buffer(i)
 *    Thus, this Qdisc would receive the following sequence of commands:
 *       TCQ_PLUG_BUFFER (epoch i)
 *       .. TCQ_PLUG_BUFFER (epoch i+1)
 *       ....TCQ_PLUG_RELEASE_ONE (epoch i)
 *       ......TCQ_PLUG_BUFFER (epoch i+2)
 *       ........
 */

#include <linux/module.h>
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/netdevice.h>
#include <linux/skbuff.h>
#include <net/pkt_sched.h>

/*
 * State of the queue, when used for network output buffering:
 *
 *                 plug(i+1)            plug(i)          head
 * ------------------+--------------------+---------------->
 *                   |                    |
 *                   |                    |
 * pkts_current_epoch| pkts_last_epoch    |pkts_to_release
 * ----------------->|<--------+--------->|+--------------->
 *                   v                    v
 *
 */

struct plug_sched_data {
	/* If true, the dequeue function releases all packets
	 * from head to end of the queue. The queue turns into
	 * a pass-through queue for newly arriving packets.
	 */
	bool unplug_indefinite;

	bool throttled;

	/* Queue Limit in bytes */
	u32 limit;

	/* Number of packets (output) from the current speculatively
	 * executing epoch.
	 */
	u32 pkts_current_epoch;

	/* Number of packets corresponding to the recently finished
	 * epoch. These will be released when we receive a
	 * TCQ_PLUG_RELEASE_ONE command. This command is typically
	 * issued after committing a checkpoint at the target.
	 */
	u32 pkts_last_epoch;

	/*
	 * Number of packets from the head of the queue, that can
	 * be released (committed checkpoint).
	 */
	u32 pkts_to_release;
};

static int plug_enqueue(struct sk_buff *skb, struct Qdisc *sch,
			struct sk_buff **to_free)
{
	struct plug_sched_data *q = qdisc_priv(sch);

	if (likely(sch->qstats.backlog + skb->len <= q->limit)) {
		if (!q->unplug_indefinite)
			q->pkts_current_epoch++;
		return qdisc_enqueue_tail(skb, sch);
	}

	return qdisc_drop(skb, sch, to_free);
}

static struct sk_buff *plug_dequeue(struct Qdisc *sch)
{
	struct plug_sched_data *q = qdisc_priv(sch);

	if (q->throttled)
		return NULL;

	if (!q->unplug_indefinite) {
		if (!q->pkts_to_release) {
			/* No more packets to dequeue. Block the queue
			 * and wait for the next release command.
			 */
			q->throttled = true;
			return NULL;
		}
		q->pkts_to_release--;
	}

	return qdisc_dequeue_head(sch);
}

static int plug_init(struct Qdisc *sch, struct nlattr *opt,
		     struct netlink_ext_ack *extack)
{
	struct plug_sched_data *q = qdisc_priv(sch);

	q->pkts_current_epoch = 0;
	q->pkts_last_epoch = 0;
	q->pkts_to_release = 0;
	q->unplug_indefinite = false;

	if (opt == NULL) {
		q->limit = qdisc_dev(sch)->tx_queue_len
		           * psched_mtu(qdisc_dev(sch));
	} else {
		struct tc_plug_qopt *ctl = nla_data(opt);

		if (nla_len(opt) < sizeof(*ctl))
			return -EINVAL;

		q->limit = ctl->limit;
	}

	q->throttled = true;
	return 0;
}

/* Receives 4 types of messages:
 * TCQ_PLUG_BUFFER: Inset a plug into the queue and
 *  buffer any incoming packets
 * TCQ_PLUG_RELEASE_ONE: Dequeue packets from queue head
 *   to beginning of the next plug.
 * TCQ_PLUG_RELEASE_INDEFINITE: Dequeue all packets from queue.
 *   Stop buffering packets until the next TCQ_PLUG_BUFFER
 *   command is received (just act as a pass-thru queue).
 * TCQ_PLUG_LIMIT: Increase/decrease queue size
 */
static int plug_change(struct Qdisc *sch, struct nlattr *opt,
		       struct netlink_ext_ack *extack)
{
	struct plug_sched_data *q = qdisc_priv(sch);
	struct tc_plug_qopt *msg;

	if (opt == NULL)
		return -EINVAL;

	msg = nla_data(opt);
	if (nla_len(opt) < sizeof(*msg))
		return -EINVAL;

	switch (msg->action) {
	case TCQ_PLUG_BUFFER:
		/* Save size of the current buffer */
		q->pkts_last_epoch = q->pkts_current_epoch;
		q->pkts_current_epoch = 0;
		if (q->unplug_indefinite)
			q->throttled = true;
		q->unplug_indefinite = false;
		break;
	case TCQ_PLUG_RELEASE_ONE:
		/* Add packets from the last complete buffer to the
		 * packets to be released set.
		 */
		q->pkts_to_release += q->pkts_last_epoch;
		q->pkts_last_epoch = 0;
		q->throttled = false;
		netif_schedule_queue(sch->dev_queue);
		break;
	case TCQ_PLUG_RELEASE_INDEFINITE:
		q->unplug_indefinite = true;
		q->pkts_to_release = 0;
		q->pkts_last_epoch = 0;
		q->pkts_current_epoch = 0;
		q->throttled = false;
		netif_schedule_queue(sch->dev_queue);
		break;
	case TCQ_PLUG_LIMIT:
		/* Limit is supplied in bytes */
		q->limit = msg->limit;
		break;
	default:
		return -EINVAL;
	}

	return 0;
}

static struct Qdisc_ops plug_qdisc_ops __read_mostly = {
	.id          =       "plug",
	.priv_size   =       sizeof(struct plug_sched_data),
	.enqueue     =       plug_enqueue,
	.dequeue     =       plug_dequeue,
	.peek        =       qdisc_peek_head,
	.init        =       plug_init,
	.change      =       plug_change,
	.reset       =	     qdisc_reset_queue,
	.owner       =       THIS_MODULE,
};

static int __init plug_module_init(void)
{
	return register_qdisc(&plug_qdisc_ops);
}

static void __exit plug_module_exit(void)
{
	unregister_qdisc(&plug_qdisc_ops);
}
module_init(plug_module_init)
module_exit(plug_module_exit)
MODULE_LICENSE("GPL");