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 | |
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 | |
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 | |
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(qdisc: sch)->tx_queue_len |
134 | * psched_mtu(dev: qdisc_dev(qdisc: sch)); |
135 | } else { |
136 | struct tc_plug_qopt *ctl = nla_data(nla: opt); |
137 | |
138 | if (nla_len(nla: 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 | */ |
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(nla: opt); |
165 | if (nla_len(nla: 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(txq: 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(txq: 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 | |
217 | static int __init plug_module_init(void) |
218 | { |
219 | return register_qdisc(qops: &plug_qdisc_ops); |
220 | } |
221 | |
222 | static void __exit plug_module_exit(void) |
223 | { |
224 | unregister_qdisc(qops: &plug_qdisc_ops); |
225 | } |
226 | module_init(plug_module_init) |
227 | module_exit(plug_module_exit) |
228 | MODULE_LICENSE("GPL" ); |
229 | |