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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 | |
60 | struct 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 | |
89 | static 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 | |
102 | static 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 | |
123 | static 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 | */ |
162 | static 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 | |
211 | static 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 | |
222 | static int __init plug_module_init(void) |
223 | { |
224 | return register_qdisc(&plug_qdisc_ops); |
225 | } |
226 | |
227 | static void __exit plug_module_exit(void) |
228 | { |
229 | unregister_qdisc(&plug_qdisc_ops); |
230 | } |
231 | module_init(plug_module_init) |
232 | module_exit(plug_module_exit) |
233 | MODULE_LICENSE("GPL"); |
234 |
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