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 bool throttled;
68
69 /* Queue Limit in bytes */
70 u32 limit;
71
72 /* Number of packets (output) from the current speculatively
73 * executing epoch.
74 */
75 u32 pkts_current_epoch;
76
77 /* Number of packets corresponding to the recently finished
78 * epoch. These will be released when we receive a
79 * TCQ_PLUG_RELEASE_ONE command. This command is typically
80 * issued after committing a checkpoint at the target.
81 */
82 u32 pkts_last_epoch;
83
84 /*
85 * Number of packets from the head of the queue, that can
86 * be released (committed checkpoint).
87 */
88 u32 pkts_to_release;
89};
90
91static int plug_enqueue(struct sk_buff *skb, struct Qdisc *sch,
92 struct sk_buff **to_free)
93{
94 struct plug_sched_data *q = qdisc_priv(sch);
95
96 if (likely(sch->qstats.backlog + skb->len <= q->limit)) {
97 if (!q->unplug_indefinite)
98 q->pkts_current_epoch++;
99 return qdisc_enqueue_tail(skb, sch);
100 }
101
102 return qdisc_drop(skb, sch, to_free);
103}
104
105static struct sk_buff *plug_dequeue(struct Qdisc *sch)
106{
107 struct plug_sched_data *q = qdisc_priv(sch);
108
109 if (q->throttled)
110 return NULL;
111
112 if (!q->unplug_indefinite) {
113 if (!q->pkts_to_release) {
114 /* No more packets to dequeue. Block the queue
115 * and wait for the next release command.
116 */
117 q->throttled = true;
118 return NULL;
119 }
120 q->pkts_to_release--;
121 }
122
123 return qdisc_dequeue_head(sch);
124}
125
126static int plug_init(struct Qdisc *sch, struct nlattr *opt,
127 struct netlink_ext_ack *extack)
128{
129 struct plug_sched_data *q = qdisc_priv(sch);
130
131 q->pkts_current_epoch = 0;
132 q->pkts_last_epoch = 0;
133 q->pkts_to_release = 0;
134 q->unplug_indefinite = false;
135
136 if (opt == NULL) {
137 q->limit = qdisc_dev(sch)->tx_queue_len
138 * 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 q->throttled = true;
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 struct netlink_ext_ack *extack)
164{
165 struct plug_sched_data *q = qdisc_priv(sch);
166 struct tc_plug_qopt *msg;
167
168 if (opt == NULL)
169 return -EINVAL;
170
171 msg = nla_data(opt);
172 if (nla_len(opt) < sizeof(*msg))
173 return -EINVAL;
174
175 switch (msg->action) {
176 case TCQ_PLUG_BUFFER:
177 /* Save size of the current buffer */
178 q->pkts_last_epoch = q->pkts_current_epoch;
179 q->pkts_current_epoch = 0;
180 if (q->unplug_indefinite)
181 q->throttled = true;
182 q->unplug_indefinite = false;
183 break;
184 case TCQ_PLUG_RELEASE_ONE:
185 /* Add packets from the last complete buffer to the
186 * packets to be released set.
187 */
188 q->pkts_to_release += q->pkts_last_epoch;
189 q->pkts_last_epoch = 0;
190 q->throttled = false;
191 netif_schedule_queue(sch->dev_queue);
192 break;
193 case TCQ_PLUG_RELEASE_INDEFINITE:
194 q->unplug_indefinite = true;
195 q->pkts_to_release = 0;
196 q->pkts_last_epoch = 0;
197 q->pkts_current_epoch = 0;
198 q->throttled = false;
199 netif_schedule_queue(sch->dev_queue);
200 break;
201 case TCQ_PLUG_LIMIT:
202 /* Limit is supplied in bytes */
203 q->limit = msg->limit;
204 break;
205 default:
206 return -EINVAL;
207 }
208
209 return 0;
210}
211
212static struct Qdisc_ops plug_qdisc_ops __read_mostly = {
213 .id = "plug",
214 .priv_size = sizeof(struct plug_sched_data),
215 .enqueue = plug_enqueue,
216 .dequeue = plug_dequeue,
217 .peek = qdisc_peek_head,
218 .init = plug_init,
219 .change = plug_change,
220 .reset = qdisc_reset_queue,
221 .owner = THIS_MODULE,
222};
223
224static int __init plug_module_init(void)
225{
226 return register_qdisc(&plug_qdisc_ops);
227}
228
229static void __exit plug_module_exit(void)
230{
231 unregister_qdisc(&plug_qdisc_ops);
232}
233module_init(plug_module_init)
234module_exit(plug_module_exit)
235MODULE_LICENSE("GPL");
236