sch_plug.c

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/*
 * 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");