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TOMOYO Linux Cross Reference
Linux/net/sched/sch_plug.c

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  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(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  */
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         if (opt == NULL)
165                 return -EINVAL;
166 
167         msg = nla_data(opt);
168         if (nla_len(opt) < sizeof(*msg))
169                 return -EINVAL;
170 
171         switch (msg->action) {
172         case TCQ_PLUG_BUFFER:
173                 /* Save size of the current buffer */
174                 q->pkts_last_epoch = q->pkts_current_epoch;
175                 q->pkts_current_epoch = 0;
176                 if (q->unplug_indefinite)
177                         q->throttled = true;
178                 q->unplug_indefinite = false;
179                 break;
180         case TCQ_PLUG_RELEASE_ONE:
181                 /* Add packets from the last complete buffer to the
182                  * packets to be released set.
183                  */
184                 q->pkts_to_release += q->pkts_last_epoch;
185                 q->pkts_last_epoch = 0;
186                 q->throttled = false;
187                 netif_schedule_queue(sch->dev_queue);
188                 break;
189         case TCQ_PLUG_RELEASE_INDEFINITE:
190                 q->unplug_indefinite = true;
191                 q->pkts_to_release = 0;
192                 q->pkts_last_epoch = 0;
193                 q->pkts_current_epoch = 0;
194                 q->throttled = false;
195                 netif_schedule_queue(sch->dev_queue);
196                 break;
197         case TCQ_PLUG_LIMIT:
198                 /* Limit is supplied in bytes */
199                 q->limit = msg->limit;
200                 break;
201         default:
202                 return -EINVAL;
203         }
204 
205         return 0;
206 }
207 
208 static struct Qdisc_ops plug_qdisc_ops __read_mostly = {
209         .id          =       "plug",
210         .priv_size   =       sizeof(struct plug_sched_data),
211         .enqueue     =       plug_enqueue,
212         .dequeue     =       plug_dequeue,
213         .peek        =       qdisc_peek_head,
214         .init        =       plug_init,
215         .change      =       plug_change,
216         .reset       =       qdisc_reset_queue,
217         .owner       =       THIS_MODULE,
218 };
219 
220 static int __init plug_module_init(void)
221 {
222         return register_qdisc(&plug_qdisc_ops);
223 }
224 
225 static void __exit plug_module_exit(void)
226 {
227         unregister_qdisc(&plug_qdisc_ops);
228 }
229 module_init(plug_module_init)
230 module_exit(plug_module_exit)
231 MODULE_LICENSE("GPL");
232 

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