xref: /openbmc/linux/net/sched/sch_plug.c (revision 8fc134fe)
1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /*
3  * sch_plug.c Queue traffic until an explicit release command
4  *
5  * There are two ways to use this qdisc:
6  * 1. A simple "instantaneous" plug/unplug operation, by issuing an alternating
7  *    sequence of TCQ_PLUG_BUFFER & TCQ_PLUG_RELEASE_INDEFINITE commands.
8  *
9  * 2. For network output buffering (a.k.a output commit) functionality.
10  *    Output commit property is commonly used by applications using checkpoint
11  *    based fault-tolerance to ensure that the checkpoint from which a system
12  *    is being restored is consistent w.r.t outside world.
13  *
14  *    Consider for e.g. Remus - a Virtual Machine checkpointing system,
15  *    wherein a VM is checkpointed, say every 50ms. The checkpoint is replicated
16  *    asynchronously to the backup host, while the VM continues executing the
17  *    next epoch speculatively.
18  *
19  *    The following is a typical sequence of output buffer operations:
20  *       1.At epoch i, start_buffer(i)
21  *       2. At end of epoch i (i.e. after 50ms):
22  *          2.1 Stop VM and take checkpoint(i).
23  *          2.2 start_buffer(i+1) and Resume VM
24  *       3. While speculatively executing epoch(i+1), asynchronously replicate
25  *          checkpoint(i) to backup host.
26  *       4. When checkpoint_ack(i) is received from backup, release_buffer(i)
27  *    Thus, this Qdisc would receive the following sequence of commands:
28  *       TCQ_PLUG_BUFFER (epoch i)
29  *       .. TCQ_PLUG_BUFFER (epoch i+1)
30  *       ....TCQ_PLUG_RELEASE_ONE (epoch i)
31  *       ......TCQ_PLUG_BUFFER (epoch i+2)
32  *       ........
33  */
34 
35 #include <linux/module.h>
36 #include <linux/types.h>
37 #include <linux/kernel.h>
38 #include <linux/errno.h>
39 #include <linux/netdevice.h>
40 #include <linux/skbuff.h>
41 #include <net/pkt_sched.h>
42 
43 /*
44  * State of the queue, when used for network output buffering:
45  *
46  *                 plug(i+1)            plug(i)          head
47  * ------------------+--------------------+---------------->
48  *                   |                    |
49  *                   |                    |
50  * pkts_current_epoch| pkts_last_epoch    |pkts_to_release
51  * ----------------->|<--------+--------->|+--------------->
52  *                   v                    v
53  *
54  */
55 
56 struct plug_sched_data {
57 	/* If true, the dequeue function releases all packets
58 	 * from head to end of the queue. The queue turns into
59 	 * a pass-through queue for newly arriving packets.
60 	 */
61 	bool unplug_indefinite;
62 
63 	bool throttled;
64 
65 	/* Queue Limit in bytes */
66 	u32 limit;
67 
68 	/* Number of packets (output) from the current speculatively
69 	 * executing epoch.
70 	 */
71 	u32 pkts_current_epoch;
72 
73 	/* Number of packets corresponding to the recently finished
74 	 * epoch. These will be released when we receive a
75 	 * TCQ_PLUG_RELEASE_ONE command. This command is typically
76 	 * issued after committing a checkpoint at the target.
77 	 */
78 	u32 pkts_last_epoch;
79 
80 	/*
81 	 * Number of packets from the head of the queue, that can
82 	 * be released (committed checkpoint).
83 	 */
84 	u32 pkts_to_release;
85 };
86 
plug_enqueue(struct sk_buff * skb,struct Qdisc * sch,struct sk_buff ** to_free)87 static int plug_enqueue(struct sk_buff *skb, struct Qdisc *sch,
88 			struct sk_buff **to_free)
89 {
90 	struct plug_sched_data *q = qdisc_priv(sch);
91 
92 	if (likely(sch->qstats.backlog + skb->len <= q->limit)) {
93 		if (!q->unplug_indefinite)
94 			q->pkts_current_epoch++;
95 		return qdisc_enqueue_tail(skb, sch);
96 	}
97 
98 	return qdisc_drop(skb, sch, to_free);
99 }
100 
plug_dequeue(struct Qdisc * sch)101 static struct sk_buff *plug_dequeue(struct Qdisc *sch)
102 {
103 	struct plug_sched_data *q = qdisc_priv(sch);
104 
105 	if (q->throttled)
106 		return NULL;
107 
108 	if (!q->unplug_indefinite) {
109 		if (!q->pkts_to_release) {
110 			/* No more packets to dequeue. Block the queue
111 			 * and wait for the next release command.
112 			 */
113 			q->throttled = true;
114 			return NULL;
115 		}
116 		q->pkts_to_release--;
117 	}
118 
119 	return qdisc_dequeue_head(sch);
120 }
121 
plug_init(struct Qdisc * sch,struct nlattr * opt,struct netlink_ext_ack * extack)122 static int plug_init(struct Qdisc *sch, struct nlattr *opt,
123 		     struct netlink_ext_ack *extack)
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 		q->limit = qdisc_dev(sch)->tx_queue_len
134 		           * psched_mtu(qdisc_dev(sch));
135 	} else {
136 		struct tc_plug_qopt *ctl = nla_data(opt);
137 
138 		if (nla_len(opt) < sizeof(*ctl))
139 			return -EINVAL;
140 
141 		q->limit = ctl->limit;
142 	}
143 
144 	q->throttled = true;
145 	return 0;
146 }
147 
148 /* Receives 4 types of messages:
149  * TCQ_PLUG_BUFFER: Inset a plug into the queue and
150  *  buffer any incoming packets
151  * TCQ_PLUG_RELEASE_ONE: Dequeue packets from queue head
152  *   to beginning of the next plug.
153  * TCQ_PLUG_RELEASE_INDEFINITE: Dequeue all packets from queue.
154  *   Stop buffering packets until the next TCQ_PLUG_BUFFER
155  *   command is received (just act as a pass-thru queue).
156  * TCQ_PLUG_LIMIT: Increase/decrease queue size
157  */
plug_change(struct Qdisc * sch,struct nlattr * opt,struct netlink_ext_ack * extack)158 static int plug_change(struct Qdisc *sch, struct nlattr *opt,
159 		       struct netlink_ext_ack *extack)
160 {
161 	struct plug_sched_data *q = qdisc_priv(sch);
162 	struct tc_plug_qopt *msg;
163 
164 	msg = nla_data(opt);
165 	if (nla_len(opt) < sizeof(*msg))
166 		return -EINVAL;
167 
168 	switch (msg->action) {
169 	case TCQ_PLUG_BUFFER:
170 		/* Save size of the current buffer */
171 		q->pkts_last_epoch = q->pkts_current_epoch;
172 		q->pkts_current_epoch = 0;
173 		if (q->unplug_indefinite)
174 			q->throttled = true;
175 		q->unplug_indefinite = false;
176 		break;
177 	case TCQ_PLUG_RELEASE_ONE:
178 		/* Add packets from the last complete buffer to the
179 		 * packets to be released set.
180 		 */
181 		q->pkts_to_release += q->pkts_last_epoch;
182 		q->pkts_last_epoch = 0;
183 		q->throttled = false;
184 		netif_schedule_queue(sch->dev_queue);
185 		break;
186 	case TCQ_PLUG_RELEASE_INDEFINITE:
187 		q->unplug_indefinite = true;
188 		q->pkts_to_release = 0;
189 		q->pkts_last_epoch = 0;
190 		q->pkts_current_epoch = 0;
191 		q->throttled = false;
192 		netif_schedule_queue(sch->dev_queue);
193 		break;
194 	case TCQ_PLUG_LIMIT:
195 		/* Limit is supplied in bytes */
196 		q->limit = msg->limit;
197 		break;
198 	default:
199 		return -EINVAL;
200 	}
201 
202 	return 0;
203 }
204 
205 static struct Qdisc_ops plug_qdisc_ops __read_mostly = {
206 	.id          =       "plug",
207 	.priv_size   =       sizeof(struct plug_sched_data),
208 	.enqueue     =       plug_enqueue,
209 	.dequeue     =       plug_dequeue,
210 	.peek        =       qdisc_peek_dequeued,
211 	.init        =       plug_init,
212 	.change      =       plug_change,
213 	.reset       =	     qdisc_reset_queue,
214 	.owner       =       THIS_MODULE,
215 };
216 
plug_module_init(void)217 static int __init plug_module_init(void)
218 {
219 	return register_qdisc(&plug_qdisc_ops);
220 }
221 
plug_module_exit(void)222 static void __exit plug_module_exit(void)
223 {
224 	unregister_qdisc(&plug_qdisc_ops);
225 }
226 module_init(plug_module_init)
227 module_exit(plug_module_exit)
228 MODULE_LICENSE("GPL");
229