amplifying qdisc

[Russell Strong]

Hi,

I'm attempting to fill a link with background traffic that is sent
whenever the link is idle.  To do this I've creates a qdisc that will
repeat the last packet in the queue for a defined number of times
(possibly infinite in the future). I am able to control the contents of
the fill traffic by sending the occasional packet through this qdisc.

This is works as the root qdisc and below a TBF.  When I try it as a
leaf of HTB unexpected behaviour ensues.  I suspect my approach is
violating some rules for qdiscs?  Any help/ideas/pointers would be
appreciated.

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * net/sched/sch_amp.c  amplifying qdisc
 *
 * Authors:     Russell Strong <russ...@strong.id.au>
 */

#include <linux/module.h>
#include <linux/slab.h>
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/errno.h>
#include <linux/skbuff.h>
#include <net/pkt_sched.h>

struct amp_sched_data {
        u32 duplicates;
        u32 remaining;
};

static int amp_enqueue(struct sk_buff *skb, struct Qdisc *sch,
                         struct sk_buff **to_free)
{
        struct amp_sched_data *asd = qdisc_priv(sch);

        printk(KERN_DEBUG "amp_enqueue: qlen %d\n", sch->q.qlen);

        asd->remaining = asd->duplicates;

        if (likely(sch->q.qlen < sch->limit))
                return qdisc_enqueue_tail(skb, sch);

        return qdisc_drop(skb, sch, to_free);
}

static struct sk_buff *amp_dequeue(struct Qdisc *sch)
{
        struct sk_buff *skb;
        struct amp_sched_data *asd = qdisc_priv(sch);

        printk(KERN_DEBUG "amp_dequeue: qlen %d, remaining %d\n",
                sch->q.qlen, asd->remaining);

        if (sch->q.qlen == 1 && asd->remaining > 0) {
                skb = qdisc_peek_head(sch);
                skb = skb_clone(skb, GFP_ATOMIC);
                asd->remaining -= 1;
        } else
                skb = qdisc_dequeue_head(sch);

        return skb;
}

static int amp_init(struct Qdisc *sch, struct nlattr *opt,
                     struct netlink_ext_ack *extack)
{
        struct amp_sched_data *asd = qdisc_priv(sch);
        u32 limit = qdisc_dev(sch)->tx_queue_len;

        sch->limit = limit;
        asd->duplicates = 10;
        asd->remaining = 0;

        printk(KERN_DEBUG "amp_init\n");

        return 0;
}

static void amp_reset_queue(struct Qdisc *sch)
{
        struct amp_sched_data *asd = qdisc_priv(sch);
        asd->remaining = 0;

        qdisc_reset_queue(sch);
}

static int amp_dump(struct Qdisc *sch, struct sk_buff *skb)
{
        return skb->len;
}

struct Qdisc_ops amp_qdisc_ops __read_mostly = {
        .id             =       "amp",
        .priv_size      =       sizeof(struct amp_sched_data),
        .enqueue        =       amp_enqueue,
        .dequeue        =       amp_dequeue,
        .peek           =       qdisc_peek_head,
        .init           =       amp_init,
        .reset          =       amp_reset_queue,
        .change         =       amp_init,
        .dump           =       amp_dump,
        .owner          =       THIS_MODULE,
};

static int __init amp_module_init(void)
{
        return register_qdisc(&amp_qdisc_ops);
}

static void __exit amp_module_exit(void)
{
        unregister_qdisc(&amp_qdisc_ops);
}

module_init(amp_module_init)
module_exit(amp_module_exit)
MODULE_LICENSE("GPL");