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Linux/net/sched/sch_plug.c

  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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