--- /dev/null
+/*
+ * sch_plug.c Queue traffic until an explicit release command
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version
+ * 2 of the License, or (at your option) any later version.
+ *
+ * There are two ways to use this qdisc:
+ * 1. A simple "instantaneous" plug/unplug operation, by issuing an alternating
+ * sequence of TCQ_PLUG_BUFFER & TCQ_PLUG_RELEASE_INDEFINITE commands.
+ *
+ * 2. For network output buffering (a.k.a output commit) functionality.
+ * Output commit property is commonly used by applications using checkpoint
+ * based fault-tolerance to ensure that the checkpoint from which a system
+ * is being restored is consistent w.r.t outside world.
+ *
+ * Consider for e.g. Remus - a Virtual Machine checkpointing system,
+ * wherein a VM is checkpointed, say every 50ms. The checkpoint is replicated
+ * asynchronously to the backup host, while the VM continues executing the
+ * next epoch speculatively.
+ *
+ * The following is a typical sequence of output buffer operations:
+ * 1.At epoch i, start_buffer(i)
+ * 2. At end of epoch i (i.e. after 50ms):
+ * 2.1 Stop VM and take checkpoint(i).
+ * 2.2 start_buffer(i+1) and Resume VM
+ * 3. While speculatively executing epoch(i+1), asynchronously replicate
+ * checkpoint(i) to backup host.
+ * 4. When checkpoint_ack(i) is received from backup, release_buffer(i)
+ * Thus, this Qdisc would receive the following sequence of commands:
+ * TCQ_PLUG_BUFFER (epoch i)
+ * .. TCQ_PLUG_BUFFER (epoch i+1)
+ * ....TCQ_PLUG_RELEASE_ONE (epoch i)
+ * ......TCQ_PLUG_BUFFER (epoch i+2)
+ * ........
+ */
+
+#include <linux/module.h>
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/errno.h>
+#include <linux/netdevice.h>
+#include <linux/skbuff.h>
+#include <net/pkt_sched.h>
+
+/*
+ * State of the queue, when used for network output buffering:
+ *
+ * plug(i+1) plug(i) head
+ * ------------------+--------------------+---------------->
+ * | |
+ * | |
+ * pkts_current_epoch| pkts_last_epoch |pkts_to_release
+ * ----------------->|<--------+--------->|+--------------->
+ * v v
+ *
+ */
+
+struct plug_sched_data {
+ /* If true, the dequeue function releases all packets
+ * from head to end of the queue. The queue turns into
+ * a pass-through queue for newly arriving packets.
+ */
+ bool unplug_indefinite;
+
+ /* Queue Limit in bytes */
+ u32 limit;
+
+ /* Number of packets (output) from the current speculatively
+ * executing epoch.
+ */
+ u32 pkts_current_epoch;
+
+ /* Number of packets corresponding to the recently finished
+ * epoch. These will be released when we receive a
+ * TCQ_PLUG_RELEASE_ONE command. This command is typically
+ * issued after committing a checkpoint at the target.
+ */
+ u32 pkts_last_epoch;
+
+ /*
+ * Number of packets from the head of the queue, that can
+ * be released (committed checkpoint).
+ */
+ u32 pkts_to_release;
+};
+
+static int plug_enqueue(struct sk_buff *skb, struct Qdisc *sch)
+{
+ struct plug_sched_data *q = qdisc_priv(sch);
+
+ if (likely(sch->qstats.backlog + skb->len <= q->limit)) {
+ if (!q->unplug_indefinite)
+ q->pkts_current_epoch++;
+ return qdisc_enqueue_tail(skb, sch);
+ }
+
+ return qdisc_reshape_fail(skb, sch);
+}
+
+static struct sk_buff *plug_dequeue(struct Qdisc *sch)
+{
+ struct plug_sched_data *q = qdisc_priv(sch);
+
+ if (qdisc_is_throttled(sch))
+ return NULL;
+
+ if (!q->unplug_indefinite) {
+ if (!q->pkts_to_release) {
+ /* No more packets to dequeue. Block the queue
+ * and wait for the next release command.
+ */
+ qdisc_throttled(sch);
+ return NULL;
+ }
+ q->pkts_to_release--;
+ }
+
+ return qdisc_dequeue_head(sch);
+}
+
+static int plug_init(struct Qdisc *sch, struct nlattr *opt)
+{
+ struct plug_sched_data *q = qdisc_priv(sch);
+
+ q->pkts_current_epoch = 0;
+ q->pkts_last_epoch = 0;
+ q->pkts_to_release = 0;
+ q->unplug_indefinite = false;
+
+ if (opt == NULL) {
+ /* We will set a default limit of 100 pkts (~150kB)
+ * in case tx_queue_len is not available. The
+ * default value is completely arbitrary.
+ */
+ u32 pkt_limit = qdisc_dev(sch)->tx_queue_len ? : 100;
+ q->limit = pkt_limit * psched_mtu(qdisc_dev(sch));
+ } else {
+ struct tc_plug_qopt *ctl = nla_data(opt);
+
+ if (nla_len(opt) < sizeof(*ctl))
+ return -EINVAL;
+
+ q->limit = ctl->limit;
+ }
+
+ qdisc_throttled(sch);
+ return 0;
+}
+
+/* Receives 4 types of messages:
+ * TCQ_PLUG_BUFFER: Inset a plug into the queue and
+ * buffer any incoming packets
+ * TCQ_PLUG_RELEASE_ONE: Dequeue packets from queue head
+ * to beginning of the next plug.
+ * TCQ_PLUG_RELEASE_INDEFINITE: Dequeue all packets from queue.
+ * Stop buffering packets until the next TCQ_PLUG_BUFFER
+ * command is received (just act as a pass-thru queue).
+ * TCQ_PLUG_LIMIT: Increase/decrease queue size
+ */
+static int plug_change(struct Qdisc *sch, struct nlattr *opt)
+{
+ struct plug_sched_data *q = qdisc_priv(sch);
+ struct tc_plug_qopt *msg;
+
+ if (opt == NULL)
+ return -EINVAL;
+
+ msg = nla_data(opt);
+ if (nla_len(opt) < sizeof(*msg))
+ return -EINVAL;
+
+ switch (msg->action) {
+ case TCQ_PLUG_BUFFER:
+ /* Save size of the current buffer */
+ q->pkts_last_epoch = q->pkts_current_epoch;
+ q->pkts_current_epoch = 0;
+ if (q->unplug_indefinite)
+ qdisc_throttled(sch);
+ q->unplug_indefinite = false;
+ break;
+ case TCQ_PLUG_RELEASE_ONE:
+ /* Add packets from the last complete buffer to the
+ * packets to be released set.
+ */
+ q->pkts_to_release += q->pkts_last_epoch;
+ q->pkts_last_epoch = 0;
+ qdisc_unthrottled(sch);
+ netif_schedule_queue(sch->dev_queue);
+ break;
+ case TCQ_PLUG_RELEASE_INDEFINITE:
+ q->unplug_indefinite = true;
+ q->pkts_to_release = 0;
+ q->pkts_last_epoch = 0;
+ q->pkts_current_epoch = 0;
+ qdisc_unthrottled(sch);
+ netif_schedule_queue(sch->dev_queue);
+ break;
+ case TCQ_PLUG_LIMIT:
+ /* Limit is supplied in bytes */
+ q->limit = msg->limit;
+ break;
+ default:
+ return -EINVAL;
+ }
+
+ return 0;
+}
+
+static struct Qdisc_ops plug_qdisc_ops __read_mostly = {
+ .id = "plug",
+ .priv_size = sizeof(struct plug_sched_data),
+ .enqueue = plug_enqueue,
+ .dequeue = plug_dequeue,
+ .peek = qdisc_peek_head,
+ .init = plug_init,
+ .change = plug_change,
+ .owner = THIS_MODULE,
+};
+
+static int __init plug_module_init(void)
+{
+ return register_qdisc(&plug_qdisc_ops);
+}
+
+static void __exit plug_module_exit(void)
+{
+ unregister_qdisc(&plug_qdisc_ops);
+}
+module_init(plug_module_init)
+module_exit(plug_module_exit)
+MODULE_LICENSE("GPL");
Code Review / kvmfornfv.git / blobdiff