// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Fair Queue CoDel discipline
 *
 *  Copyright (C) 2012,2015 Eric Dumazet <edumazet@google.com>
 */

#include <linux/module.h>
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/jiffies.h>
#include <linux/string.h>
#include <linux/in.h>
#include <linux/errno.h>
#include <linux/init.h>
#include <linux/skbuff.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <net/netlink.h>
#include <net/pkt_sched.h>
#include <net/pkt_cls.h>
#include <net/codel.h>
#include <net/codel_impl.h>
#include <net/codel_qdisc.h>

/*      Fair Queue CoDel.
 *
 * Principles :
 * Packets are classified (internal classifier or external) on flows.
 * This is a Stochastic model (as we use a hash, several flows
 *                             might be hashed on same slot)
 * Each flow has a CoDel managed queue.
 * Flows are linked onto two (Round Robin) lists,
 * so that new flows have priority on old ones.
 *
 * For a given flow, packets are not reordered (CoDel uses a FIFO)
 * head drops only.
 * ECN capability is on by default.
 * Low memory footprint (64 bytes per flow)
 */

struct fq_codel_flow {
        struct sk_buff    *head;
        struct sk_buff    *tail;
        struct list_head  flowchain;
        int               deficit;
        struct codel_vars cvars;
}; /* please try to keep this structure <= 64 bytes */

struct fq_codel_sched_data {
        struct tcf_proto __rcu *filter_list; /* optional external classifier */
        struct tcf_block *block;
        struct fq_codel_flow *flows;    /* Flows table [flows_cnt] */
        u32             *backlogs;      /* backlog table [flows_cnt] */
        u32             flows_cnt;      /* number of flows */
        u32             quantum;        /* psched_mtu(qdisc_dev(sch)); */
        u32             drop_batch_size;
        u32             memory_limit;
        struct codel_params cparams;
        struct codel_stats cstats;
        u32             memory_usage;
        u32             drop_overmemory;
        u32             drop_overlimit;
        u32             new_flow_count;

        struct list_head new_flows;     /* list of new flows */
        struct list_head old_flows;     /* list of old flows */
};

static unsigned int fq_codel_hash(const struct fq_codel_sched_data *q,
                                  struct sk_buff *skb)
{
        return reciprocal_scale(skb_get_hash(skb), q->flows_cnt);
}

static unsigned int fq_codel_classify(struct sk_buff *skb, struct Qdisc *sch,
                                      int *qerr)
{
        struct fq_codel_sched_data *q = qdisc_priv(sch);
        struct tcf_proto *filter;
        struct tcf_result res;
        int result;

        if (TC_H_MAJ(skb->priority) == sch->handle &&
            TC_H_MIN(skb->priority) > 0 &&
            TC_H_MIN(skb->priority) <= q->flows_cnt)
                return TC_H_MIN(skb->priority);

        filter = rcu_dereference_bh(q->filter_list);
        if (!filter)
                return fq_codel_hash(q, skb) + 1;

        *qerr = NET_XMIT_SUCCESS | __NET_XMIT_BYPASS;
        result = tcf_classify(skb, NULL, filter, &res, false);
        if (result >= 0) {
#ifdef CONFIG_NET_CLS_ACT
                switch (result) {
                case TC_ACT_STOLEN:
                case TC_ACT_QUEUED:
                case TC_ACT_TRAP:
                        *qerr = NET_XMIT_SUCCESS | __NET_XMIT_STOLEN;
                        fallthrough;
                case TC_ACT_SHOT:
                        return 0;
                }
#endif
                if (TC_H_MIN(res.classid) <= q->flows_cnt)
                        return TC_H_MIN(res.classid);
        }
        return 0;
}

/* helper functions : might be changed when/if skb use a standard list_head */

/* remove one skb from head of slot queue */
static inline struct sk_buff *dequeue_head(struct fq_codel_flow *flow)
{
        struct sk_buff *skb = flow->head;

        flow->head = skb->next;
        skb_mark_not_on_list(skb);
        return skb;
}

/* add skb to flow queue (tail add) */
static inline void flow_queue_add(struct fq_codel_flow *flow,
                                  struct sk_buff *skb)
{
        if (flow->head == NULL)
                flow->head = skb;
        else
                flow->tail->next = skb;
        flow->tail = skb;
        skb->next = NULL;
}

static unsigned int fq_codel_drop(struct Qdisc *sch, unsigned int max_packets,
                                  struct sk_buff **to_free)
{
        struct fq_codel_sched_data *q = qdisc_priv(sch);
        struct sk_buff *skb;
        unsigned int maxbacklog = 0, idx = 0, i, len;
        struct fq_codel_flow *flow;
        unsigned int threshold;
        unsigned int mem = 0;

        /* Queue is full! Find the fat flow and drop packet(s) from it.
         * This might sound expensive, but with 1024 flows, we scan
         * 4KB of memory, and we dont need to handle a complex tree
         * in fast path (packet queue/enqueue) with many cache misses.
         * In stress mode, we'll try to drop 64 packets from the flow,
         * amortizing this linear lookup to one cache line per drop.
         */
        for (i = 0; i < q->flows_cnt; i++) {
                if (q->backlogs[i] > maxbacklog) {
                        maxbacklog = q->backlogs[i];
                        idx = i;
                }
        }

        /* Our goal is to drop half of this fat flow backlog */
        threshold = maxbacklog >> 1;

        flow = &q->flows[idx];
        len = 0;
        i = 0;
        do {
                skb = dequeue_head(flow);
                len += qdisc_pkt_len(skb);
                mem += get_codel_cb(skb)->mem_usage;
                tcf_set_drop_reason(skb, SKB_DROP_REASON_QDISC_OVERLIMIT);
                __qdisc_drop(skb, to_free);
        } while (++i < max_packets && len < threshold);

        /* Tell codel to increase its signal strength also */
        flow->cvars.count += i;
        q->backlogs[idx] -= len;
        q->memory_usage -= mem;
        sch->qstats.drops += i;
        sch->qstats.backlog -= len;
        sch->q.qlen -= i;
        return idx;
}

static int fq_codel_enqueue(struct sk_buff *skb, struct Qdisc *sch,
                            struct sk_buff **to_free)
{
        struct fq_codel_sched_data *q = qdisc_priv(sch);
        unsigned int idx, prev_backlog, prev_qlen;
        struct fq_codel_flow *flow;
        int ret;
        unsigned int pkt_len;
        bool memory_limited;

        idx = fq_codel_classify(skb, sch, &ret);
        if (idx == 0) {
                if (ret & __NET_XMIT_BYPASS)
                        qdisc_qstats_drop(sch);
                __qdisc_drop(skb, to_free);
                return ret;
        }
        idx--;

        codel_set_enqueue_time(skb);
        flow = &q->flows[idx];
        flow_queue_add(flow, skb);
        q->backlogs[idx] += qdisc_pkt_len(skb);
        qdisc_qstats_backlog_inc(sch, skb);

        if (list_empty(&flow->flowchain)) {
                list_add_tail(&flow->flowchain, &q->new_flows);
                q->new_flow_count++;
                flow->deficit = q->quantum;
        }
        get_codel_cb(skb)->mem_usage = skb->truesize;
        q->memory_usage += get_codel_cb(skb)->mem_usage;
        memory_limited = q->memory_usage > q->memory_limit;
        if (++sch->q.qlen <= sch->limit && !memory_limited)
                return NET_XMIT_SUCCESS;

        prev_backlog = sch->qstats.backlog;
        prev_qlen = sch->q.qlen;

        /* save this packet length as it might be dropped by fq_codel_drop() */
        pkt_len = qdisc_pkt_len(skb);
        /* fq_codel_drop() is quite expensive, as it performs a linear search
         * in q->backlogs[] to find a fat flow.
         * So instead of dropping a single packet, drop half of its backlog
         * with a 64 packets limit to not add a too big cpu spike here.
         */
        ret = fq_codel_drop(sch, q->drop_batch_size, to_free);

        prev_qlen -= sch->q.qlen;
        prev_backlog -= sch->qstats.backlog;
        q->drop_overlimit += prev_qlen;
        if (memory_limited)
                q->drop_overmemory += prev_qlen;

        /* As we dropped packet(s), better let upper stack know this.
         * If we dropped a packet for this flow, return NET_XMIT_CN,
         * but in this case, our parents wont increase their backlogs.
         */
        if (ret == idx) {
                qdisc_tree_reduce_backlog(sch, prev_qlen - 1,
                                          prev_backlog - pkt_len);
                return NET_XMIT_CN;
        }
        qdisc_tree_reduce_backlog(sch, prev_qlen, prev_backlog);
        return NET_XMIT_SUCCESS;
}

/* This is the specific function called from codel_dequeue()
 * to dequeue a packet from queue. Note: backlog is handled in
 * codel, we dont need to reduce it here.
 */
static struct sk_buff *dequeue_func(struct codel_vars *vars, void *ctx)
{
        struct Qdisc *sch = ctx;
        struct fq_codel_sched_data *q = qdisc_priv(sch);
        struct fq_codel_flow *flow;
        struct sk_buff *skb = NULL;

        flow = container_of(vars, struct fq_codel_flow, cvars);
        if (flow->head) {
                skb = dequeue_head(flow);
                q->backlogs[flow - q->flows] -= qdisc_pkt_len(skb);
                q->memory_usage -= get_codel_cb(skb)->mem_usage;
                sch->q.qlen--;
                sch->qstats.backlog -= qdisc_pkt_len(skb);
        }
        return skb;
}

static void drop_func(struct sk_buff *skb, void *ctx)
{
        struct Qdisc *sch = ctx;

        qdisc_dequeue_drop(sch, skb, SKB_DROP_REASON_QDISC_CONGESTED);
        qdisc_qstats_drop(sch);
}

static struct sk_buff *fq_codel_dequeue(struct Qdisc *sch)
{
        struct fq_codel_sched_data *q = qdisc_priv(sch);
        struct sk_buff *skb;
        struct fq_codel_flow *flow;
        struct list_head *head;

begin:
        head = &q->new_flows;
        if (list_empty(head)) {
                head = &q->old_flows;
                if (list_empty(head))
                        return NULL;
        }
        flow = list_first_entry(head, struct fq_codel_flow, flowchain);

        if (flow->deficit <= 0) {
                flow->deficit += q->quantum;
                list_move_tail(&flow->flowchain, &q->old_flows);
                goto begin;
        }

        skb = codel_dequeue(sch, &sch->qstats.backlog, &q->cparams,
                            &flow->cvars, &q->cstats, qdisc_pkt_len,
                            codel_get_enqueue_time, drop_func, dequeue_func);

        if (!skb) {
                /* force a pass through old_flows to prevent starvation */
                if ((head == &q->new_flows) && !list_empty(&q->old_flows))
                        list_move_tail(&flow->flowchain, &q->old_flows);
                else
                        list_del_init(&flow->flowchain);
                goto begin;
        }
        qdisc_bstats_update(sch, skb);
        flow->deficit -= qdisc_pkt_len(skb);

        if (q->cstats.drop_count) {
                qdisc_tree_reduce_backlog(sch, q->cstats.drop_count,
                                          q->cstats.drop_len);
                q->cstats.drop_count = 0;
                q->cstats.drop_len = 0;
        }
        return skb;
}

static void fq_codel_flow_purge(struct fq_codel_flow *flow)
{
        rtnl_kfree_skbs(flow->head, flow->tail);
        flow->head = NULL;
}

static void fq_codel_reset(struct Qdisc *sch)
{
        struct fq_codel_sched_data *q = qdisc_priv(sch);
        int i;

        INIT_LIST_HEAD(&q->new_flows);
        INIT_LIST_HEAD(&q->old_flows);
        for (i = 0; i < q->flows_cnt; i++) {
                struct fq_codel_flow *flow = q->flows + i;

                fq_codel_flow_purge(flow);
                INIT_LIST_HEAD(&flow->flowchain);
                codel_vars_init(&flow->cvars);
        }
        memset(q->backlogs, 0, q->flows_cnt * sizeof(u32));
        q->memory_usage = 0;
}

static const struct nla_policy fq_codel_policy[TCA_FQ_CODEL_MAX + 1] = {
        [TCA_FQ_CODEL_TARGET]   = { .type = NLA_U32 },
        [TCA_FQ_CODEL_LIMIT]    = { .type = NLA_U32 },
        [TCA_FQ_CODEL_INTERVAL] = { .type = NLA_U32 },
        [TCA_FQ_CODEL_ECN]      = { .type = NLA_U32 },
        [TCA_FQ_CODEL_FLOWS]    = { .type = NLA_U32 },
        [TCA_FQ_CODEL_QUANTUM]  = { .type = NLA_U32 },
        [TCA_FQ_CODEL_CE_THRESHOLD] = { .type = NLA_U32 },
        [TCA_FQ_CODEL_DROP_BATCH_SIZE] = { .type = NLA_U32 },
        [TCA_FQ_CODEL_MEMORY_LIMIT] = { .type = NLA_U32 },
        [TCA_FQ_CODEL_CE_THRESHOLD_SELECTOR] = { .type = NLA_U8 },
        [TCA_FQ_CODEL_CE_THRESHOLD_MASK] = { .type = NLA_U8 },
};

static int fq_codel_change(struct Qdisc *sch, struct nlattr *opt,
                           struct netlink_ext_ack *extack)
{
        unsigned int dropped_pkts = 0, dropped_bytes = 0;
        struct fq_codel_sched_data *q = qdisc_priv(sch);
        struct nlattr *tb[TCA_FQ_CODEL_MAX + 1];
        u32 quantum = 0;
        int err;

        err = nla_parse_nested_deprecated(tb, TCA_FQ_CODEL_MAX, opt,
                                          fq_codel_policy, NULL);
        if (err < 0)
                return err;
        if (tb[TCA_FQ_CODEL_FLOWS]) {
                if (q->flows)
                        return -EINVAL;
                q->flows_cnt = nla_get_u32(tb[TCA_FQ_CODEL_FLOWS]);
                if (!q->flows_cnt ||
                    q->flows_cnt > 65536)
                        return -EINVAL;
        }
        if (tb[TCA_FQ_CODEL_QUANTUM]) {
                quantum = max(256U, nla_get_u32(tb[TCA_FQ_CODEL_QUANTUM]));
                if (quantum > FQ_CODEL_QUANTUM_MAX) {
                        NL_SET_ERR_MSG(extack, "Invalid quantum");
                        return -EINVAL;
                }
        }
        sch_tree_lock(sch);

        if (tb[TCA_FQ_CODEL_TARGET]) {
                u64 target = nla_get_u32(tb[TCA_FQ_CODEL_TARGET]);

                WRITE_ONCE(q->cparams.target,
                           (target * NSEC_PER_USEC) >> CODEL_SHIFT);
        }

        if (tb[TCA_FQ_CODEL_CE_THRESHOLD]) {
                u64 val = nla_get_u32(tb[TCA_FQ_CODEL_CE_THRESHOLD]);

                WRITE_ONCE(q->cparams.ce_threshold,
                           (val * NSEC_PER_USEC) >> CODEL_SHIFT);
        }

        if (tb[TCA_FQ_CODEL_CE_THRESHOLD_SELECTOR])
                WRITE_ONCE(q->cparams.ce_threshold_selector,
                           nla_get_u8(tb[TCA_FQ_CODEL_CE_THRESHOLD_SELECTOR]));
        if (tb[TCA_FQ_CODEL_CE_THRESHOLD_MASK])
                WRITE_ONCE(q->cparams.ce_threshold_mask,
                           nla_get_u8(tb[TCA_FQ_CODEL_CE_THRESHOLD_MASK]));

        if (tb[TCA_FQ_CODEL_INTERVAL]) {
                u64 interval = nla_get_u32(tb[TCA_FQ_CODEL_INTERVAL]);

                WRITE_ONCE(q->cparams.interval,
                           (interval * NSEC_PER_USEC) >> CODEL_SHIFT);
        }

        if (tb[TCA_FQ_CODEL_LIMIT])
                WRITE_ONCE(sch->limit,
                           nla_get_u32(tb[TCA_FQ_CODEL_LIMIT]));

        if (tb[TCA_FQ_CODEL_ECN])
                WRITE_ONCE(q->cparams.ecn,
                           !!nla_get_u32(tb[TCA_FQ_CODEL_ECN]));

        if (quantum)
                WRITE_ONCE(q->quantum, quantum);

        if (tb[TCA_FQ_CODEL_DROP_BATCH_SIZE])
                WRITE_ONCE(q->drop_batch_size,
                           max(1U, nla_get_u32(tb[TCA_FQ_CODEL_DROP_BATCH_SIZE])));

        if (tb[TCA_FQ_CODEL_MEMORY_LIMIT])
                WRITE_ONCE(q->memory_limit,
                           min(1U << 31, nla_get_u32(tb[TCA_FQ_CODEL_MEMORY_LIMIT])));

        while (sch->q.qlen > sch->limit ||
               q->memory_usage > q->memory_limit) {
                struct sk_buff *skb = qdisc_dequeue_internal(sch, false);

                if (!skb)
                        break;

                dropped_pkts++;
                dropped_bytes += qdisc_pkt_len(skb);
                rtnl_kfree_skbs(skb, skb);
        }
        qdisc_tree_reduce_backlog(sch, dropped_pkts, dropped_bytes);

        sch_tree_unlock(sch);
        return 0;
}

static void fq_codel_destroy(struct Qdisc *sch)
{
        struct fq_codel_sched_data *q = qdisc_priv(sch);

        tcf_block_put(q->block);
        kvfree(q->backlogs);
        kvfree(q->flows);
}

static int fq_codel_init(struct Qdisc *sch, struct nlattr *opt,
                         struct netlink_ext_ack *extack)
{
        struct fq_codel_sched_data *q = qdisc_priv(sch);
        int i;
        int err;

        sch->limit = 10*1024;
        q->flows_cnt = 1024;
        q->memory_limit = 32 << 20; /* 32 MBytes */
        q->drop_batch_size = 64;
        q->quantum = psched_mtu(qdisc_dev(sch));
        INIT_LIST_HEAD(&q->new_flows);
        INIT_LIST_HEAD(&q->old_flows);
        codel_params_init(&q->cparams);
        codel_stats_init(&q->cstats);
        q->cparams.ecn = true;
        q->cparams.mtu = psched_mtu(qdisc_dev(sch));

        if (opt) {
                err = fq_codel_change(sch, opt, extack);
                if (err)
                        goto init_failure;
        }

        err = tcf_block_get(&q->block, &q->filter_list, sch, extack);
        if (err)
                goto init_failure;

        if (!q->flows) {
                q->flows = kvzalloc_objs(struct fq_codel_flow, q->flows_cnt);
                if (!q->flows) {
                        err = -ENOMEM;
                        goto init_failure;
                }
                q->backlogs = kvcalloc(q->flows_cnt, sizeof(u32), GFP_KERNEL);
                if (!q->backlogs) {
                        err = -ENOMEM;
                        goto alloc_failure;
                }
                for (i = 0; i < q->flows_cnt; i++) {
                        struct fq_codel_flow *flow = q->flows + i;

                        INIT_LIST_HEAD(&flow->flowchain);
                        codel_vars_init(&flow->cvars);
                }
        }
        if (sch->limit >= 1)
                sch->flags |= TCQ_F_CAN_BYPASS;
        else
                sch->flags &= ~TCQ_F_CAN_BYPASS;

        sch->flags |= TCQ_F_DEQUEUE_DROPS;

        return 0;

alloc_failure:
        kvfree(q->flows);
        q->flows = NULL;
init_failure:
        q->flows_cnt = 0;
        return err;
}

static int fq_codel_dump(struct Qdisc *sch, struct sk_buff *skb)
{
        struct fq_codel_sched_data *q = qdisc_priv(sch);
        codel_time_t ce_threshold;
        struct nlattr *opts;

        opts = nla_nest_start_noflag(skb, TCA_OPTIONS);
        if (opts == NULL)
                goto nla_put_failure;

        if (nla_put_u32(skb, TCA_FQ_CODEL_TARGET,
                        codel_time_to_us(READ_ONCE(q->cparams.target))) ||
            nla_put_u32(skb, TCA_FQ_CODEL_LIMIT,
                        READ_ONCE(sch->limit)) ||
            nla_put_u32(skb, TCA_FQ_CODEL_INTERVAL,
                        codel_time_to_us(READ_ONCE(q->cparams.interval))) ||
            nla_put_u32(skb, TCA_FQ_CODEL_ECN,
                        READ_ONCE(q->cparams.ecn)) ||
            nla_put_u32(skb, TCA_FQ_CODEL_QUANTUM,
                        READ_ONCE(q->quantum)) ||
            nla_put_u32(skb, TCA_FQ_CODEL_DROP_BATCH_SIZE,
                        READ_ONCE(q->drop_batch_size)) ||
            nla_put_u32(skb, TCA_FQ_CODEL_MEMORY_LIMIT,
                        READ_ONCE(q->memory_limit)) ||
            nla_put_u32(skb, TCA_FQ_CODEL_FLOWS,
                        READ_ONCE(q->flows_cnt)))
                goto nla_put_failure;

        ce_threshold = READ_ONCE(q->cparams.ce_threshold);
        if (ce_threshold != CODEL_DISABLED_THRESHOLD) {
                if (nla_put_u32(skb, TCA_FQ_CODEL_CE_THRESHOLD,
                                codel_time_to_us(ce_threshold)))
                        goto nla_put_failure;
                if (nla_put_u8(skb, TCA_FQ_CODEL_CE_THRESHOLD_SELECTOR,
                               READ_ONCE(q->cparams.ce_threshold_selector)))
                        goto nla_put_failure;
                if (nla_put_u8(skb, TCA_FQ_CODEL_CE_THRESHOLD_MASK,
                               READ_ONCE(q->cparams.ce_threshold_mask)))
                        goto nla_put_failure;
        }

        return nla_nest_end(skb, opts);

nla_put_failure:
        return -1;
}

static int fq_codel_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
{
        struct fq_codel_sched_data *q = qdisc_priv(sch);
        struct tc_fq_codel_xstats st = {
                .type                           = TCA_FQ_CODEL_XSTATS_QDISC,
        };
        struct list_head *pos;

        st.qdisc_stats.maxpacket = q->cstats.maxpacket;
        st.qdisc_stats.drop_overlimit = q->drop_overlimit;
        st.qdisc_stats.ecn_mark = q->cstats.ecn_mark;
        st.qdisc_stats.new_flow_count = q->new_flow_count;
        st.qdisc_stats.ce_mark = q->cstats.ce_mark;
        st.qdisc_stats.memory_usage  = q->memory_usage;
        st.qdisc_stats.drop_overmemory = q->drop_overmemory;

        sch_tree_lock(sch);
        list_for_each(pos, &q->new_flows)
                st.qdisc_stats.new_flows_len++;

        list_for_each(pos, &q->old_flows)
                st.qdisc_stats.old_flows_len++;
        sch_tree_unlock(sch);

        return gnet_stats_copy_app(d, &st, sizeof(st));
}

static struct Qdisc *fq_codel_leaf(struct Qdisc *sch, unsigned long arg)
{
        return NULL;
}

static unsigned long fq_codel_find(struct Qdisc *sch, u32 classid)
{
        return 0;
}

static unsigned long fq_codel_bind(struct Qdisc *sch, unsigned long parent,
                              u32 classid)
{
        return 0;
}

static void fq_codel_unbind(struct Qdisc *q, unsigned long cl)
{
}

static struct tcf_block *fq_codel_tcf_block(struct Qdisc *sch, unsigned long cl,
                                            struct netlink_ext_ack *extack)
{
        struct fq_codel_sched_data *q = qdisc_priv(sch);

        if (cl)
                return NULL;
        return q->block;
}

static int fq_codel_dump_class(struct Qdisc *sch, unsigned long cl,
                          struct sk_buff *skb, struct tcmsg *tcm)
{
        tcm->tcm_handle |= TC_H_MIN(cl);
        return 0;
}

static int fq_codel_dump_class_stats(struct Qdisc *sch, unsigned long cl,
                                     struct gnet_dump *d)
{
        struct fq_codel_sched_data *q = qdisc_priv(sch);
        u32 idx = cl - 1;
        struct gnet_stats_queue qs = { 0 };
        struct tc_fq_codel_xstats xstats;

        if (idx < q->flows_cnt) {
                const struct fq_codel_flow *flow = &q->flows[idx];
                const struct sk_buff *skb;

                memset(&xstats, 0, sizeof(xstats));
                xstats.type = TCA_FQ_CODEL_XSTATS_CLASS;
                xstats.class_stats.deficit = flow->deficit;
                xstats.class_stats.ldelay =
                        codel_time_to_us(flow->cvars.ldelay);
                xstats.class_stats.count = flow->cvars.count;
                xstats.class_stats.lastcount = flow->cvars.lastcount;
                xstats.class_stats.dropping = flow->cvars.dropping;
                if (flow->cvars.dropping) {
                        codel_tdiff_t delta = flow->cvars.drop_next -
                                              codel_get_time();

                        xstats.class_stats.drop_next = (delta >= 0) ?
                                codel_time_to_us(delta) :
                                -codel_time_to_us(-delta);
                }
                if (flow->head) {
                        sch_tree_lock(sch);
                        skb = flow->head;
                        while (skb) {
                                qs.qlen++;
                                skb = skb->next;
                        }
                        sch_tree_unlock(sch);
                }
                qs.backlog = q->backlogs[idx];
                qs.drops = 0;
        }
        if (gnet_stats_copy_queue(d, NULL, &qs, qs.qlen) < 0)
                return -1;
        if (idx < q->flows_cnt)
                return gnet_stats_copy_app(d, &xstats, sizeof(xstats));
        return 0;
}

static void fq_codel_walk(struct Qdisc *sch, struct qdisc_walker *arg)
{
        struct fq_codel_sched_data *q = qdisc_priv(sch);
        unsigned int i;

        if (arg->stop)
                return;

        for (i = 0; i < q->flows_cnt; i++) {
                if (list_empty(&q->flows[i].flowchain)) {
                        arg->count++;
                        continue;
                }
                if (!tc_qdisc_stats_dump(sch, i + 1, arg))
                        break;
        }
}

static const struct Qdisc_class_ops fq_codel_class_ops = {
        .leaf           =       fq_codel_leaf,
        .find           =       fq_codel_find,
        .tcf_block      =       fq_codel_tcf_block,
        .bind_tcf       =       fq_codel_bind,
        .unbind_tcf     =       fq_codel_unbind,
        .dump           =       fq_codel_dump_class,
        .dump_stats     =       fq_codel_dump_class_stats,
        .walk           =       fq_codel_walk,
};

static struct Qdisc_ops fq_codel_qdisc_ops __read_mostly = {
        .cl_ops         =       &fq_codel_class_ops,
        .id             =       "fq_codel",
        .priv_size      =       sizeof(struct fq_codel_sched_data),
        .enqueue        =       fq_codel_enqueue,
        .dequeue        =       fq_codel_dequeue,
        .peek           =       qdisc_peek_dequeued,
        .init           =       fq_codel_init,
        .reset          =       fq_codel_reset,
        .destroy        =       fq_codel_destroy,
        .change         =       fq_codel_change,
        .dump           =       fq_codel_dump,
        .dump_stats =   fq_codel_dump_stats,
        .owner          =       THIS_MODULE,
};
MODULE_ALIAS_NET_SCH("fq_codel");

static int __init fq_codel_module_init(void)
{
        return register_qdisc(&fq_codel_qdisc_ops);
}

static void __exit fq_codel_module_exit(void)
{
        unregister_qdisc(&fq_codel_qdisc_ops);
}

module_init(fq_codel_module_init)
module_exit(fq_codel_module_exit)
MODULE_AUTHOR("Eric Dumazet");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Fair Queue CoDel discipline");