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");