net/sched/sch_plug.c

   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");