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// SPDX-License-Identifier: GPL-2.0-or-later
/*
* sch_plug.c Queue traffic until an explicit release command
*
* 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;
bool throttled;
/* 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 sk_buff **to_free)
{
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_drop(skb, sch, to_free);
}
static struct sk_buff *plug_dequeue(struct Qdisc *sch)
{
struct plug_sched_data *q = qdisc_priv(sch);
if (q->throttled)
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.
*/
q->throttled = true;
return NULL;
}
q->pkts_to_release--;
}
return qdisc_dequeue_head(sch);
}
static int plug_init(struct Qdisc *sch, struct nlattr *opt,
struct netlink_ext_ack *extack)
{
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) {
q->limit = qdisc_dev(sch)->tx_queue_len
* 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;
}
q->throttled = true;
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 netlink_ext_ack *extack)
{
struct plug_sched_data *q = qdisc_priv(sch);
struct tc_plug_qopt *msg;
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)
q->throttled = true;
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;
q->throttled = false;
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;
q->throttled = false;
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,
.reset = qdisc_reset_queue,
.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");
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