1/*
2 * sch_plug.c Queue traffic until an explicit release command
3 *
4 *             This program is free software; you can redistribute it and/or
5 *             modify it under the terms of the GNU General Public License
6 *             as published by the Free Software Foundation; either version
7 *             2 of the License, or (at your option) any later version.
8 *
9 * There are two ways to use this qdisc:
10 * 1. A simple "instantaneous" plug/unplug operation, by issuing an alternating
11 *    sequence of TCQ_PLUG_BUFFER & TCQ_PLUG_RELEASE_INDEFINITE commands.
12 *
13 * 2. For network output buffering (a.k.a output commit) functionality.
14 *    Output commit property is commonly used by applications using checkpoint
15 *    based fault-tolerance to ensure that the checkpoint from which a system
16 *    is being restored is consistent w.r.t outside world.
17 *
18 *    Consider for e.g. Remus - a Virtual Machine checkpointing system,
19 *    wherein a VM is checkpointed, say every 50ms. The checkpoint is replicated
20 *    asynchronously to the backup host, while the VM continues executing the
21 *    next epoch speculatively.
22 *
23 *    The following is a typical sequence of output buffer operations:
24 *       1.At epoch i, start_buffer(i)
25 *       2. At end of epoch i (i.e. after 50ms):
26 *          2.1 Stop VM and take checkpoint(i).
27 *          2.2 start_buffer(i+1) and Resume VM
28 *       3. While speculatively executing epoch(i+1), asynchronously replicate
29 *          checkpoint(i) to backup host.
30 *       4. When checkpoint_ack(i) is received from backup, release_buffer(i)
31 *    Thus, this Qdisc would receive the following sequence of commands:
32 *       TCQ_PLUG_BUFFER (epoch i)
33 *       .. TCQ_PLUG_BUFFER (epoch i+1)
34 *       ....TCQ_PLUG_RELEASE_ONE (epoch i)
35 *       ......TCQ_PLUG_BUFFER (epoch i+2)
36 *       ........
37 */
38
39#include <linux/module.h>
40#include <linux/types.h>
41#include <linux/kernel.h>
42#include <linux/errno.h>
43#include <linux/netdevice.h>
44#include <linux/skbuff.h>
45#include <net/pkt_sched.h>
46
47/*
48 * State of the queue, when used for network output buffering:
49 *
50 *                 plug(i+1)            plug(i)          head
51 * ------------------+--------------------+---------------->
52 *                   |                    |
53 *                   |                    |
54 * pkts_current_epoch| pkts_last_epoch    |pkts_to_release
55 * ----------------->|<--------+--------->|+--------------->
56 *                   v                    v
57 *
58 */
59
60struct plug_sched_data {
61	/* If true, the dequeue function releases all packets
62	 * from head to end of the queue. The queue turns into
63	 * a pass-through queue for newly arriving packets.
64	 */
65	bool unplug_indefinite;
66
67	/* Queue Limit in bytes */
68	u32 limit;
69
70	/* Number of packets (output) from the current speculatively
71	 * executing epoch.
72	 */
73	u32 pkts_current_epoch;
74
75	/* Number of packets corresponding to the recently finished
76	 * epoch. These will be released when we receive a
77	 * TCQ_PLUG_RELEASE_ONE command. This command is typically
78	 * issued after committing a checkpoint at the target.
79	 */
80	u32 pkts_last_epoch;
81
82	/*
83	 * Number of packets from the head of the queue, that can
84	 * be released (committed checkpoint).
85	 */
86	u32 pkts_to_release;
87};
88
89static int plug_enqueue(struct sk_buff *skb, struct Qdisc *sch)
90{
91	struct plug_sched_data *q = qdisc_priv(sch);
92
93	if (likely(sch->qstats.backlog + skb->len <= q->limit)) {
94		if (!q->unplug_indefinite)
95			q->pkts_current_epoch++;
96		return qdisc_enqueue_tail(skb, sch);
97	}
98
99	return qdisc_reshape_fail(skb, sch);
100}
101
102static struct sk_buff *plug_dequeue(struct Qdisc *sch)
103{
104	struct plug_sched_data *q = qdisc_priv(sch);
105
106	if (qdisc_is_throttled(sch))
107		return NULL;
108
109	if (!q->unplug_indefinite) {
110		if (!q->pkts_to_release) {
111			/* No more packets to dequeue. Block the queue
112			 * and wait for the next release command.
113			 */
114			qdisc_throttled(sch);
115			return NULL;
116		}
117		q->pkts_to_release--;
118	}
119
120	return qdisc_dequeue_head(sch);
121}
122
123static int plug_init(struct Qdisc *sch, struct nlattr *opt)
124{
125	struct plug_sched_data *q = qdisc_priv(sch);
126
127	q->pkts_current_epoch = 0;
128	q->pkts_last_epoch = 0;
129	q->pkts_to_release = 0;
130	q->unplug_indefinite = false;
131
132	if (opt == NULL) {
133		/* We will set a default limit of 100 pkts (~150kB)
134		 * in case tx_queue_len is not available. The
135		 * default value is completely arbitrary.
136		 */
137		u32 pkt_limit = qdisc_dev(sch)->tx_queue_len ? : 100;
138		q->limit = pkt_limit * psched_mtu(qdisc_dev(sch));
139	} else {
140		struct tc_plug_qopt *ctl = nla_data(opt);
141
142		if (nla_len(opt) < sizeof(*ctl))
143			return -EINVAL;
144
145		q->limit = ctl->limit;
146	}
147
148	qdisc_throttled(sch);
149	return 0;
150}
151
152/* Receives 4 types of messages:
153 * TCQ_PLUG_BUFFER: Inset a plug into the queue and
154 *  buffer any incoming packets
155 * TCQ_PLUG_RELEASE_ONE: Dequeue packets from queue head
156 *   to beginning of the next plug.
157 * TCQ_PLUG_RELEASE_INDEFINITE: Dequeue all packets from queue.
158 *   Stop buffering packets until the next TCQ_PLUG_BUFFER
159 *   command is received (just act as a pass-thru queue).
160 * TCQ_PLUG_LIMIT: Increase/decrease queue size
161 */
162static int plug_change(struct Qdisc *sch, struct nlattr *opt)
163{
164	struct plug_sched_data *q = qdisc_priv(sch);
165	struct tc_plug_qopt *msg;
166
167	if (opt == NULL)
168		return -EINVAL;
169
170	msg = nla_data(opt);
171	if (nla_len(opt) < sizeof(*msg))
172		return -EINVAL;
173
174	switch (msg->action) {
175	case TCQ_PLUG_BUFFER:
176		/* Save size of the current buffer */
177		q->pkts_last_epoch = q->pkts_current_epoch;
178		q->pkts_current_epoch = 0;
179		if (q->unplug_indefinite)
180			qdisc_throttled(sch);
181		q->unplug_indefinite = false;
182		break;
183	case TCQ_PLUG_RELEASE_ONE:
184		/* Add packets from the last complete buffer to the
185		 * packets to be released set.
186		 */
187		q->pkts_to_release += q->pkts_last_epoch;
188		q->pkts_last_epoch = 0;
189		qdisc_unthrottled(sch);
190		netif_schedule_queue(sch->dev_queue);
191		break;
192	case TCQ_PLUG_RELEASE_INDEFINITE:
193		q->unplug_indefinite = true;
194		q->pkts_to_release = 0;
195		q->pkts_last_epoch = 0;
196		q->pkts_current_epoch = 0;
197		qdisc_unthrottled(sch);
198		netif_schedule_queue(sch->dev_queue);
199		break;
200	case TCQ_PLUG_LIMIT:
201		/* Limit is supplied in bytes */
202		q->limit = msg->limit;
203		break;
204	default:
205		return -EINVAL;
206	}
207
208	return 0;
209}
210
211static struct Qdisc_ops plug_qdisc_ops __read_mostly = {
212	.id          =       "plug",
213	.priv_size   =       sizeof(struct plug_sched_data),
214	.enqueue     =       plug_enqueue,
215	.dequeue     =       plug_dequeue,
216	.peek        =       qdisc_peek_head,
217	.init        =       plug_init,
218	.change      =       plug_change,
219	.owner       =       THIS_MODULE,
220};
221
222static int __init plug_module_init(void)
223{
224	return register_qdisc(&plug_qdisc_ops);
225}
226
227static void __exit plug_module_exit(void)
228{
229	unregister_qdisc(&plug_qdisc_ops);
230}
231module_init(plug_module_init)
232module_exit(plug_module_exit)
233MODULE_LICENSE("GPL");
234