blk-throttle.c

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/*
 * Interface for controlling IO bandwidth on a request queue
 *
 * Copyright (C) 2010 Vivek Goyal <[email protected]>
 */

#include <linux/module.h>
#include <linux/slab.h>
#include <linux/blkdev.h>
#include <linux/bio.h>
#include <linux/blktrace_api.h>
#include "blk-cgroup.h"
#include "blk.h"

/* Max dispatch from a group in 1 round */
static int throtl_grp_quantum = 8;

/* Total max dispatch from all groups in one round */
static int throtl_quantum = 32;

/* Throttling is performed over 100ms slice and after that slice is renewed */
static unsigned long throtl_slice = HZ/10;  /* 100 ms */

static struct blkcg_policy blkcg_policy_throtl;

/* A workqueue to queue throttle related work */
static struct workqueue_struct *kthrotld_workqueue;
static void throtl_schedule_delayed_work(struct throtl_data *td,
                unsigned long delay);

struct throtl_rb_root {
    struct rb_root rb;
    struct rb_node *left;
    unsigned int count;
    unsigned long min_disptime;
};

#define THROTL_RB_ROOT  (struct throtl_rb_root) { .rb = RB_ROOT, .left = NULL, \
            .count = 0, .min_disptime = 0}

#define rb_entry_tg(node)   rb_entry((node), struct throtl_grp, rb_node)

/* Per-cpu group stats */
struct tg_stats_cpu {
    /* total bytes transferred */
    struct blkg_rwstat      service_bytes;
    /* total IOs serviced, post merge */
    struct blkg_rwstat      serviced;
};

struct throtl_grp {
    /* must be the first member */
    struct blkg_policy_data pd;

    /* active throtl group service_tree member */
    struct rb_node rb_node;

    /*
     * Dispatch time in jiffies. This is the estimated time when group
     * will unthrottle and is ready to dispatch more bio. It is used as
     * key to sort active groups in service tree.
     */
    unsigned long disptime;

    unsigned int flags;

    /* Two lists for READ and WRITE */
    struct bio_list bio_lists[2];

    /* Number of queued bios on READ and WRITE lists */
    unsigned int nr_queued[2];

    /* bytes per second rate limits */
    uint64_t bps[2];

    /* IOPS limits */
    unsigned int iops[2];

    /* Number of bytes disptached in current slice */
    uint64_t bytes_disp[2];
    /* Number of bio's dispatched in current slice */
    unsigned int io_disp[2];

    /* When did we start a new slice */
    unsigned long slice_start[2];
    unsigned long slice_end[2];

    /* Some throttle limits got updated for the group */
    int limits_changed;

    /* Per cpu stats pointer */
    struct tg_stats_cpu __percpu *stats_cpu;

    /* List of tgs waiting for per cpu stats memory to be allocated */
    struct list_head stats_alloc_node;
};

struct throtl_data
{
    /* service tree for active throtl groups */
    struct throtl_rb_root tg_service_tree;

    struct request_queue *queue;

    /* Total Number of queued bios on READ and WRITE lists */
    unsigned int nr_queued[2];

    /*
     * number of total undestroyed groups
     */
    unsigned int nr_undestroyed_grps;

    /* Work for dispatching throttled bios */
    struct delayed_work throtl_work;

    int limits_changed;
};

/* list and work item to allocate percpu group stats */
static DEFINE_SPINLOCK(tg_stats_alloc_lock);
static LIST_HEAD(tg_stats_alloc_list);

static void tg_stats_alloc_fn(struct work_struct *);
static DECLARE_DELAYED_WORK(tg_stats_alloc_work, tg_stats_alloc_fn);

static inline struct throtl_grp *pd_to_tg(struct blkg_policy_data *pd)
{
    return pd ? container_of(pd, struct throtl_grp, pd) : NULL;
}

static inline struct throtl_grp *blkg_to_tg(struct blkcg_gq *blkg)
{
    return pd_to_tg(blkg_to_pd(blkg, &blkcg_policy_throtl));
}

static inline struct blkcg_gq *tg_to_blkg(struct throtl_grp *tg)
{
    return pd_to_blkg(&tg->pd);
}

static inline struct throtl_grp *td_root_tg(struct throtl_data *td)
{
    return blkg_to_tg(td->queue->root_blkg);
}

enum tg_state_flags {
    THROTL_TG_FLAG_on_rr = 0,   /* on round-robin busy list */
};

#define THROTL_TG_FNS(name)                     \
static inline void throtl_mark_tg_##name(struct throtl_grp *tg)     \
{                                   \
    (tg)->flags |= (1 << THROTL_TG_FLAG_##name);            \
}                                   \
static inline void throtl_clear_tg_##name(struct throtl_grp *tg)    \
{                                   \
    (tg)->flags &= ~(1 << THROTL_TG_FLAG_##name);           \
}                                   \
static inline int throtl_tg_##name(const struct throtl_grp *tg)     \
{                                   \
    return ((tg)->flags & (1 << THROTL_TG_FLAG_##name)) != 0;   \
}

THROTL_TG_FNS(on_rr);

#define throtl_log_tg(td, tg, fmt, args...) do {            \
    char __pbuf[128];                       \
                                    \
    blkg_path(tg_to_blkg(tg), __pbuf, sizeof(__pbuf));      \
    blk_add_trace_msg((td)->queue, "throtl %s " fmt, __pbuf, ##args); \
} while (0)

#define throtl_log(td, fmt, args...)    \
    blk_add_trace_msg((td)->queue, "throtl " fmt, ##args)

static inline unsigned int total_nr_queued(struct throtl_data *td)
{
    return td->nr_queued[0] + td->nr_queued[1];
}

/*
 * Worker for allocating per cpu stat for tgs. This is scheduled on the
 * system_wq once there are some groups on the alloc_list waiting for
 * allocation.
 */
static void tg_stats_alloc_fn(struct work_struct *work)
{
    static struct tg_stats_cpu *stats_cpu;  /* this fn is non-reentrant */
    struct delayed_work *dwork = to_delayed_work(work);
    bool empty = false;

alloc_stats:
    if (!stats_cpu) {
        stats_cpu = alloc_percpu(struct tg_stats_cpu);
        if (!stats_cpu) {
            /* allocation failed, try again after some time */
            schedule_delayed_work(dwork, msecs_to_jiffies(10));
            return;
        }
    }

    spin_lock_irq(&tg_stats_alloc_lock);

    if (!list_empty(&tg_stats_alloc_list)) {
        struct throtl_grp *tg = list_first_entry(&tg_stats_alloc_list,
                             struct throtl_grp,
                             stats_alloc_node);
        swap(tg->stats_cpu, stats_cpu);
        list_del_init(&tg->stats_alloc_node);
    }

    empty = list_empty(&tg_stats_alloc_list);
    spin_unlock_irq(&tg_stats_alloc_lock);
    if (!empty)
        goto alloc_stats;
}

static void throtl_pd_init(struct blkcg_gq *blkg)
{
    struct throtl_grp *tg = blkg_to_tg(blkg);
    unsigned long flags;

    RB_CLEAR_NODE(&tg->rb_node);
    bio_list_init(&tg->bio_lists[0]);
    bio_list_init(&tg->bio_lists[1]);
    tg->limits_changed = false;

    tg->bps[READ] = -1;
    tg->bps[WRITE] = -1;
    tg->iops[READ] = -1;
    tg->iops[WRITE] = -1;

    /*
     * Ugh... We need to perform per-cpu allocation for tg->stats_cpu
     * but percpu allocator can't be called from IO path.  Queue tg on
     * tg_stats_alloc_list and allocate from work item.
     */
    spin_lock_irqsave(&tg_stats_alloc_lock, flags);
    list_add(&tg->stats_alloc_node, &tg_stats_alloc_list);
    schedule_delayed_work(&tg_stats_alloc_work, 0);
    spin_unlock_irqrestore(&tg_stats_alloc_lock, flags);
}

static void throtl_pd_exit(struct blkcg_gq *blkg)
{
    struct throtl_grp *tg = blkg_to_tg(blkg);
    unsigned long flags;

    spin_lock_irqsave(&tg_stats_alloc_lock, flags);
    list_del_init(&tg->stats_alloc_node);
    spin_unlock_irqrestore(&tg_stats_alloc_lock, flags);

    free_percpu(tg->stats_cpu);
}

static void throtl_pd_reset_stats(struct blkcg_gq *blkg)
{
    struct throtl_grp *tg = blkg_to_tg(blkg);
    int cpu;

    if (tg->stats_cpu == NULL)
        return;

    for_each_possible_cpu(cpu) {
        struct tg_stats_cpu *sc = per_cpu_ptr(tg->stats_cpu, cpu);

        blkg_rwstat_reset(&sc->service_bytes);
        blkg_rwstat_reset(&sc->serviced);
    }
}

static struct throtl_grp *throtl_lookup_tg(struct throtl_data *td,
                       struct blkcg *blkcg)
{
    /*
     * This is the common case when there are no blkcgs.  Avoid lookup
     * in this case
     */
    if (blkcg == &blkcg_root)
        return td_root_tg(td);

    return blkg_to_tg(blkg_lookup(blkcg, td->queue));
}

static struct throtl_grp *throtl_lookup_create_tg(struct throtl_data *td,
                          struct blkcg *blkcg)
{
    struct request_queue *q = td->queue;
    struct throtl_grp *tg = NULL;

    /*
     * This is the common case when there are no blkcgs.  Avoid lookup
     * in this case
     */
    if (blkcg == &blkcg_root) {
        tg = td_root_tg(td);
    } else {
        struct blkcg_gq *blkg;

        blkg = blkg_lookup_create(blkcg, q);

        /* if %NULL and @q is alive, fall back to root_tg */
        if (!IS_ERR(blkg))
            tg = blkg_to_tg(blkg);
        else if (!blk_queue_dead(q))
            tg = td_root_tg(td);
    }

    return tg;
}

static struct throtl_grp *throtl_rb_first(struct throtl_rb_root *root)
{
    /* Service tree is empty */
    if (!root->count)
        return NULL;

    if (!root->left)
        root->left = rb_first(&root->rb);

    if (root->left)
        return rb_entry_tg(root->left);

    return NULL;
}

static void rb_erase_init(struct rb_node *n, struct rb_root *root)
{
    rb_erase(n, root);
    RB_CLEAR_NODE(n);
}

static void throtl_rb_erase(struct rb_node *n, struct throtl_rb_root *root)
{
    if (root->left == n)
        root->left = NULL;
    rb_erase_init(n, &root->rb);
    --root->count;
}

static void update_min_dispatch_time(struct throtl_rb_root *st)
{
    struct throtl_grp *tg;

    tg = throtl_rb_first(st);
    if (!tg)
        return;

    st->min_disptime = tg->disptime;
}

static void
tg_service_tree_add(struct throtl_rb_root *st, struct throtl_grp *tg)
{
    struct rb_node **node = &st->rb.rb_node;
    struct rb_node *parent = NULL;
    struct throtl_grp *__tg;
    unsigned long key = tg->disptime;
    int left = 1;

    while (*node != NULL) {
        parent = *node;
        __tg = rb_entry_tg(parent);

        if (time_before(key, __tg->disptime))
            node = &parent->rb_left;
        else {
            node = &parent->rb_right;
            left = 0;
        }
    }

    if (left)
        st->left = &tg->rb_node;

    rb_link_node(&tg->rb_node, parent, node);
    rb_insert_color(&tg->rb_node, &st->rb);
}

static void __throtl_enqueue_tg(struct throtl_data *td, struct throtl_grp *tg)
{
    struct throtl_rb_root *st = &td->tg_service_tree;

    tg_service_tree_add(st, tg);
    throtl_mark_tg_on_rr(tg);
    st->count++;
}

static void throtl_enqueue_tg(struct throtl_data *td, struct throtl_grp *tg)
{
    if (!throtl_tg_on_rr(tg))
        __throtl_enqueue_tg(td, tg);
}

static void __throtl_dequeue_tg(struct throtl_data *td, struct throtl_grp *tg)
{
    throtl_rb_erase(&tg->rb_node, &td->tg_service_tree);
    throtl_clear_tg_on_rr(tg);
}

static void throtl_dequeue_tg(struct throtl_data *td, struct throtl_grp *tg)
{
    if (throtl_tg_on_rr(tg))
        __throtl_dequeue_tg(td, tg);
}

static void throtl_schedule_next_dispatch(struct throtl_data *td)
{
    struct throtl_rb_root *st = &td->tg_service_tree;

    /*
     * If there are more bios pending, schedule more work.
     */
    if (!total_nr_queued(td))
        return;

    BUG_ON(!st->count);

    update_min_dispatch_time(st);

    if (time_before_eq(st->min_disptime, jiffies))
        throtl_schedule_delayed_work(td, 0);
    else
        throtl_schedule_delayed_work(td, (st->min_disptime - jiffies));
}

static inline void
throtl_start_new_slice(struct throtl_data *td, struct throtl_grp *tg, bool rw)
{
    tg->bytes_disp[rw] = 0;
    tg->io_disp[rw] = 0;
    tg->slice_start[rw] = jiffies;
    tg->slice_end[rw] = jiffies + throtl_slice;
    throtl_log_tg(td, tg, "[%c] new slice start=%lu end=%lu jiffies=%lu",
            rw == READ ? 'R' : 'W', tg->slice_start[rw],
            tg->slice_end[rw], jiffies);
}

static inline void throtl_set_slice_end(struct throtl_data *td,
        struct throtl_grp *tg, bool rw, unsigned long jiffy_end)
{
    tg->slice_end[rw] = roundup(jiffy_end, throtl_slice);
}

static inline void throtl_extend_slice(struct throtl_data *td,
        struct throtl_grp *tg, bool rw, unsigned long jiffy_end)
{
    tg->slice_end[rw] = roundup(jiffy_end, throtl_slice);
    throtl_log_tg(td, tg, "[%c] extend slice start=%lu end=%lu jiffies=%lu",
            rw == READ ? 'R' : 'W', tg->slice_start[rw],
            tg->slice_end[rw], jiffies);
}

/* Determine if previously allocated or extended slice is complete or not */
static bool
throtl_slice_used(struct throtl_data *td, struct throtl_grp *tg, bool rw)
{
    if (time_in_range(jiffies, tg->slice_start[rw], tg->slice_end[rw]))
        return 0;

    return 1;
}

/* Trim the used slices and adjust slice start accordingly */
static inline void
throtl_trim_slice(struct throtl_data *td, struct throtl_grp *tg, bool rw)
{
    unsigned long nr_slices, time_elapsed, io_trim;
    u64 bytes_trim, tmp;

    BUG_ON(time_before(tg->slice_end[rw], tg->slice_start[rw]));

    /*
     * If bps are unlimited (-1), then time slice don't get
     * renewed. Don't try to trim the slice if slice is used. A new
     * slice will start when appropriate.
     */
    if (throtl_slice_used(td, tg, rw))
        return;

    /*
     * A bio has been dispatched. Also adjust slice_end. It might happen
     * that initially cgroup limit was very low resulting in high
     * slice_end, but later limit was bumped up and bio was dispached
     * sooner, then we need to reduce slice_end. A high bogus slice_end
     * is bad because it does not allow new slice to start.
     */

    throtl_set_slice_end(td, tg, rw, jiffies + throtl_slice);

    time_elapsed = jiffies - tg->slice_start[rw];

    nr_slices = time_elapsed / throtl_slice;

    if (!nr_slices)
        return;
    tmp = tg->bps[rw] * throtl_slice * nr_slices;
    do_div(tmp, HZ);
    bytes_trim = tmp;

    io_trim = (tg->iops[rw] * throtl_slice * nr_slices)/HZ;

    if (!bytes_trim && !io_trim)
        return;

    if (tg->bytes_disp[rw] >= bytes_trim)
        tg->bytes_disp[rw] -= bytes_trim;
    else
        tg->bytes_disp[rw] = 0;

    if (tg->io_disp[rw] >= io_trim)
        tg->io_disp[rw] -= io_trim;
    else
        tg->io_disp[rw] = 0;

    tg->slice_start[rw] += nr_slices * throtl_slice;

    throtl_log_tg(td, tg, "[%c] trim slice nr=%lu bytes=%llu io=%lu"
            " start=%lu end=%lu jiffies=%lu",
            rw == READ ? 'R' : 'W', nr_slices, bytes_trim, io_trim,
            tg->slice_start[rw], tg->slice_end[rw], jiffies);
}

static bool tg_with_in_iops_limit(struct throtl_data *td, struct throtl_grp *tg,
        struct bio *bio, unsigned long *wait)
{
    bool rw = bio_data_dir(bio);
    unsigned int io_allowed;
    unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
    u64 tmp;

    jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];

    /* Slice has just started. Consider one slice interval */
    if (!jiffy_elapsed)
        jiffy_elapsed_rnd = throtl_slice;

    jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, throtl_slice);

    /*
     * jiffy_elapsed_rnd should not be a big value as minimum iops can be
     * 1 then at max jiffy elapsed should be equivalent of 1 second as we
     * will allow dispatch after 1 second and after that slice should
     * have been trimmed.
     */

    tmp = (u64)tg->iops[rw] * jiffy_elapsed_rnd;
    do_div(tmp, HZ);

    if (tmp > UINT_MAX)
        io_allowed = UINT_MAX;
    else
        io_allowed = tmp;

    if (tg->io_disp[rw] + 1 <= io_allowed) {
        if (wait)
            *wait = 0;
        return 1;
    }

    /* Calc approx time to dispatch */
    jiffy_wait = ((tg->io_disp[rw] + 1) * HZ)/tg->iops[rw] + 1;

    if (jiffy_wait > jiffy_elapsed)
        jiffy_wait = jiffy_wait - jiffy_elapsed;
    else
        jiffy_wait = 1;

    if (wait)
        *wait = jiffy_wait;
    return 0;
}

static bool tg_with_in_bps_limit(struct throtl_data *td, struct throtl_grp *tg,
        struct bio *bio, unsigned long *wait)
{
    bool rw = bio_data_dir(bio);
    u64 bytes_allowed, extra_bytes, tmp;
    unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;

    jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];

    /* Slice has just started. Consider one slice interval */
    if (!jiffy_elapsed)
        jiffy_elapsed_rnd = throtl_slice;

    jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, throtl_slice);

    tmp = tg->bps[rw] * jiffy_elapsed_rnd;
    do_div(tmp, HZ);
    bytes_allowed = tmp;

    if (tg->bytes_disp[rw] + bio->bi_size <= bytes_allowed) {
        if (wait)
            *wait = 0;
        return 1;
    }

    /* Calc approx time to dispatch */
    extra_bytes = tg->bytes_disp[rw] + bio->bi_size - bytes_allowed;
    jiffy_wait = div64_u64(extra_bytes * HZ, tg->bps[rw]);

    if (!jiffy_wait)
        jiffy_wait = 1;

    /*
     * This wait time is without taking into consideration the rounding
     * up we did. Add that time also.
     */
    jiffy_wait = jiffy_wait + (jiffy_elapsed_rnd - jiffy_elapsed);
    if (wait)
        *wait = jiffy_wait;
    return 0;
}

static bool tg_no_rule_group(struct throtl_grp *tg, bool rw) {
    if (tg->bps[rw] == -1 && tg->iops[rw] == -1)
        return 1;
    return 0;
}

/*
 * Returns whether one can dispatch a bio or not. Also returns approx number
 * of jiffies to wait before this bio is with-in IO rate and can be dispatched
 */
static bool tg_may_dispatch(struct throtl_data *td, struct throtl_grp *tg,
                struct bio *bio, unsigned long *wait)
{
    bool rw = bio_data_dir(bio);
    unsigned long bps_wait = 0, iops_wait = 0, max_wait = 0;

    /*
     * Currently whole state machine of group depends on first bio
     * queued in the group bio list. So one should not be calling
     * this function with a different bio if there are other bios
     * queued.
     */
    BUG_ON(tg->nr_queued[rw] && bio != bio_list_peek(&tg->bio_lists[rw]));

    /* If tg->bps = -1, then BW is unlimited */
    if (tg->bps[rw] == -1 && tg->iops[rw] == -1) {
        if (wait)
            *wait = 0;
        return 1;
    }

    /*
     * If previous slice expired, start a new one otherwise renew/extend
     * existing slice to make sure it is at least throtl_slice interval
     * long since now.
     */
    if (throtl_slice_used(td, tg, rw))
        throtl_start_new_slice(td, tg, rw);
    else {
        if (time_before(tg->slice_end[rw], jiffies + throtl_slice))
            throtl_extend_slice(td, tg, rw, jiffies + throtl_slice);
    }

    if (tg_with_in_bps_limit(td, tg, bio, &bps_wait)
        && tg_with_in_iops_limit(td, tg, bio, &iops_wait)) {
        if (wait)
            *wait = 0;
        return 1;
    }

    max_wait = max(bps_wait, iops_wait);

    if (wait)
        *wait = max_wait;

    if (time_before(tg->slice_end[rw], jiffies + max_wait))
        throtl_extend_slice(td, tg, rw, jiffies + max_wait);

    return 0;
}

static void throtl_update_dispatch_stats(struct blkcg_gq *blkg, u64 bytes,
                     int rw)
{
    struct throtl_grp *tg = blkg_to_tg(blkg);
    struct tg_stats_cpu *stats_cpu;
    unsigned long flags;

    /* If per cpu stats are not allocated yet, don't do any accounting. */
    if (tg->stats_cpu == NULL)
        return;

    /*
     * Disabling interrupts to provide mutual exclusion between two
     * writes on same cpu. It probably is not needed for 64bit. Not
     * optimizing that case yet.
     */
    local_irq_save(flags);

    stats_cpu = this_cpu_ptr(tg->stats_cpu);

    blkg_rwstat_add(&stats_cpu->serviced, rw, 1);
    blkg_rwstat_add(&stats_cpu->service_bytes, rw, bytes);

    local_irq_restore(flags);
}

static void throtl_charge_bio(struct throtl_grp *tg, struct bio *bio)
{
    bool rw = bio_data_dir(bio);

    /* Charge the bio to the group */
    tg->bytes_disp[rw] += bio->bi_size;
    tg->io_disp[rw]++;

    throtl_update_dispatch_stats(tg_to_blkg(tg), bio->bi_size, bio->bi_rw);
}

static void throtl_add_bio_tg(struct throtl_data *td, struct throtl_grp *tg,
            struct bio *bio)
{
    bool rw = bio_data_dir(bio);

    bio_list_add(&tg->bio_lists[rw], bio);
    /* Take a bio reference on tg */
    blkg_get(tg_to_blkg(tg));
    tg->nr_queued[rw]++;
    td->nr_queued[rw]++;
    throtl_enqueue_tg(td, tg);
}

static void tg_update_disptime(struct throtl_data *td, struct throtl_grp *tg)
{
    unsigned long read_wait = -1, write_wait = -1, min_wait = -1, disptime;
    struct bio *bio;

    if ((bio = bio_list_peek(&tg->bio_lists[READ])))
        tg_may_dispatch(td, tg, bio, &read_wait);

    if ((bio = bio_list_peek(&tg->bio_lists[WRITE])))
        tg_may_dispatch(td, tg, bio, &write_wait);

    min_wait = min(read_wait, write_wait);
    disptime = jiffies + min_wait;

    /* Update dispatch time */
    throtl_dequeue_tg(td, tg);
    tg->disptime = disptime;
    throtl_enqueue_tg(td, tg);
}

static void tg_dispatch_one_bio(struct throtl_data *td, struct throtl_grp *tg,
                bool rw, struct bio_list *bl)
{
    struct bio *bio;

    bio = bio_list_pop(&tg->bio_lists[rw]);
    tg->nr_queued[rw]--;
    /* Drop bio reference on blkg */
    blkg_put(tg_to_blkg(tg));

    BUG_ON(td->nr_queued[rw] <= 0);
    td->nr_queued[rw]--;

    throtl_charge_bio(tg, bio);
    bio_list_add(bl, bio);
    bio->bi_rw |= REQ_THROTTLED;

    throtl_trim_slice(td, tg, rw);
}

static int throtl_dispatch_tg(struct throtl_data *td, struct throtl_grp *tg,
                struct bio_list *bl)
{
    unsigned int nr_reads = 0, nr_writes = 0;
    unsigned int max_nr_reads = throtl_grp_quantum*3/4;
    unsigned int max_nr_writes = throtl_grp_quantum - max_nr_reads;
    struct bio *bio;

    /* Try to dispatch 75% READS and 25% WRITES */

    while ((bio = bio_list_peek(&tg->bio_lists[READ]))
        && tg_may_dispatch(td, tg, bio, NULL)) {

        tg_dispatch_one_bio(td, tg, bio_data_dir(bio), bl);
        nr_reads++;

        if (nr_reads >= max_nr_reads)
            break;
    }

    while ((bio = bio_list_peek(&tg->bio_lists[WRITE]))
        && tg_may_dispatch(td, tg, bio, NULL)) {

        tg_dispatch_one_bio(td, tg, bio_data_dir(bio), bl);
        nr_writes++;

        if (nr_writes >= max_nr_writes)
            break;
    }

    return nr_reads + nr_writes;
}

static int throtl_select_dispatch(struct throtl_data *td, struct bio_list *bl)
{
    unsigned int nr_disp = 0;
    struct throtl_grp *tg;
    struct throtl_rb_root *st = &td->tg_service_tree;

    while (1) {
        tg = throtl_rb_first(st);

        if (!tg)
            break;

        if (time_before(jiffies, tg->disptime))
            break;

        throtl_dequeue_tg(td, tg);

        nr_disp += throtl_dispatch_tg(td, tg, bl);

        if (tg->nr_queued[0] || tg->nr_queued[1]) {
            tg_update_disptime(td, tg);
            throtl_enqueue_tg(td, tg);
        }

        if (nr_disp >= throtl_quantum)
            break;
    }

    return nr_disp;
}

static void throtl_process_limit_change(struct throtl_data *td)
{
    struct request_queue *q = td->queue;
    struct blkcg_gq *blkg, *n;

    if (!td->limits_changed)
        return;

    xchg(&td->limits_changed, false);

    throtl_log(td, "limits changed");

    list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
        struct throtl_grp *tg = blkg_to_tg(blkg);

        if (!tg->limits_changed)
            continue;

        if (!xchg(&tg->limits_changed, false))
            continue;

        throtl_log_tg(td, tg, "limit change rbps=%llu wbps=%llu"
            " riops=%u wiops=%u", tg->bps[READ], tg->bps[WRITE],
            tg->iops[READ], tg->iops[WRITE]);

        /*
         * Restart the slices for both READ and WRITES. It
         * might happen that a group's limit are dropped
         * suddenly and we don't want to account recently
         * dispatched IO with new low rate
         */
        throtl_start_new_slice(td, tg, 0);
        throtl_start_new_slice(td, tg, 1);

        if (throtl_tg_on_rr(tg))
            tg_update_disptime(td, tg);
    }
}

/* Dispatch throttled bios. Should be called without queue lock held. */
static int throtl_dispatch(struct request_queue *q)
{
    struct throtl_data *td = q->td;
    unsigned int nr_disp = 0;
    struct bio_list bio_list_on_stack;
    struct bio *bio;
    struct blk_plug plug;

    spin_lock_irq(q->queue_lock);

    throtl_process_limit_change(td);

    if (!total_nr_queued(td))
        goto out;

    bio_list_init(&bio_list_on_stack);

    throtl_log(td, "dispatch nr_queued=%u read=%u write=%u",
            total_nr_queued(td), td->nr_queued[READ],
            td->nr_queued[WRITE]);

    nr_disp = throtl_select_dispatch(td, &bio_list_on_stack);

    if (nr_disp)
        throtl_log(td, "bios disp=%u", nr_disp);

    throtl_schedule_next_dispatch(td);
out:
    spin_unlock_irq(q->queue_lock);

    /*
     * If we dispatched some requests, unplug the queue to make sure
     * immediate dispatch
     */
    if (nr_disp) {
        blk_start_plug(&plug);
        while((bio = bio_list_pop(&bio_list_on_stack)))
            generic_make_request(bio);
        blk_finish_plug(&plug);
    }
    return nr_disp;
}

void blk_throtl_work(struct work_struct *work)
{
    struct throtl_data *td = container_of(work, struct throtl_data,
                    throtl_work.work);
    struct request_queue *q = td->queue;

    throtl_dispatch(q);
}

/* Call with queue lock held */
static void
throtl_schedule_delayed_work(struct throtl_data *td, unsigned long delay)
{

    struct delayed_work *dwork = &td->throtl_work;

    /* schedule work if limits changed even if no bio is queued */
    if (total_nr_queued(td) || td->limits_changed) {
        mod_delayed_work(kthrotld_workqueue, dwork, delay);
        throtl_log(td, "schedule work. delay=%lu jiffies=%lu",
                delay, jiffies);
    }
}

static u64 tg_prfill_cpu_rwstat(struct seq_file *sf,
                struct blkg_policy_data *pd, int off)
{
    struct throtl_grp *tg = pd_to_tg(pd);
    struct blkg_rwstat rwstat = { }, tmp;
    int i, cpu;

    for_each_possible_cpu(cpu) {
        struct tg_stats_cpu *sc = per_cpu_ptr(tg->stats_cpu, cpu);

        tmp = blkg_rwstat_read((void *)sc + off);
        for (i = 0; i < BLKG_RWSTAT_NR; i++)
            rwstat.cnt[i] += tmp.cnt[i];
    }

    return __blkg_prfill_rwstat(sf, pd, &rwstat);
}

static int tg_print_cpu_rwstat(struct cgroup *cgrp, struct cftype *cft,
                   struct seq_file *sf)
{
    struct blkcg *blkcg = cgroup_to_blkcg(cgrp);

    blkcg_print_blkgs(sf, blkcg, tg_prfill_cpu_rwstat, &blkcg_policy_throtl,
              cft->private, true);
    return 0;
}

static u64 tg_prfill_conf_u64(struct seq_file *sf, struct blkg_policy_data *pd,
                  int off)
{
    struct throtl_grp *tg = pd_to_tg(pd);
    u64 v = *(u64 *)((void *)tg + off);

    if (v == -1)
        return 0;
    return __blkg_prfill_u64(sf, pd, v);
}

static u64 tg_prfill_conf_uint(struct seq_file *sf, struct blkg_policy_data *pd,
                   int off)
{
    struct throtl_grp *tg = pd_to_tg(pd);
    unsigned int v = *(unsigned int *)((void *)tg + off);

    if (v == -1)
        return 0;
    return __blkg_prfill_u64(sf, pd, v);
}

static int tg_print_conf_u64(struct cgroup *cgrp, struct cftype *cft,
                 struct seq_file *sf)
{
    blkcg_print_blkgs(sf, cgroup_to_blkcg(cgrp), tg_prfill_conf_u64,
              &blkcg_policy_throtl, cft->private, false);
    return 0;
}

static int tg_print_conf_uint(struct cgroup *cgrp, struct cftype *cft,
                  struct seq_file *sf)
{
    blkcg_print_blkgs(sf, cgroup_to_blkcg(cgrp), tg_prfill_conf_uint,
              &blkcg_policy_throtl, cft->private, false);
    return 0;
}

static int tg_set_conf(struct cgroup *cgrp, struct cftype *cft, const char *buf,
               bool is_u64)
{
    struct blkcg *blkcg = cgroup_to_blkcg(cgrp);
    struct blkg_conf_ctx ctx;
    struct throtl_grp *tg;
    struct throtl_data *td;
    int ret;

    ret = blkg_conf_prep(blkcg, &blkcg_policy_throtl, buf, &ctx);
    if (ret)
        return ret;

    tg = blkg_to_tg(ctx.blkg);
    td = ctx.blkg->q->td;

    if (!ctx.v)
        ctx.v = -1;

    if (is_u64)
        *(u64 *)((void *)tg + cft->private) = ctx.v;
    else
        *(unsigned int *)((void *)tg + cft->private) = ctx.v;

    /* XXX: we don't need the following deferred processing */
    xchg(&tg->limits_changed, true);
    xchg(&td->limits_changed, true);
    throtl_schedule_delayed_work(td, 0);

    blkg_conf_finish(&ctx);
    return 0;
}

static int tg_set_conf_u64(struct cgroup *cgrp, struct cftype *cft,
               const char *buf)
{
    return tg_set_conf(cgrp, cft, buf, true);
}

static int tg_set_conf_uint(struct cgroup *cgrp, struct cftype *cft,
                const char *buf)
{
    return tg_set_conf(cgrp, cft, buf, false);
}

static struct cftype throtl_files[] = {
    {
        .name = "throttle.read_bps_device",
        .private = offsetof(struct throtl_grp, bps[READ]),
        .read_seq_string = tg_print_conf_u64,
        .write_string = tg_set_conf_u64,
        .max_write_len = 256,
    },
    {
        .name = "throttle.write_bps_device",
        .private = offsetof(struct throtl_grp, bps[WRITE]),
        .read_seq_string = tg_print_conf_u64,
        .write_string = tg_set_conf_u64,
        .max_write_len = 256,
    },
    {
        .name = "throttle.read_iops_device",
        .private = offsetof(struct throtl_grp, iops[READ]),
        .read_seq_string = tg_print_conf_uint,
        .write_string = tg_set_conf_uint,
        .max_write_len = 256,
    },
    {
        .name = "throttle.write_iops_device",
        .private = offsetof(struct throtl_grp, iops[WRITE]),
        .read_seq_string = tg_print_conf_uint,
        .write_string = tg_set_conf_uint,
        .max_write_len = 256,
    },
    {
        .name = "throttle.io_service_bytes",
        .private = offsetof(struct tg_stats_cpu, service_bytes),
        .read_seq_string = tg_print_cpu_rwstat,
    },
    {
        .name = "throttle.io_serviced",
        .private = offsetof(struct tg_stats_cpu, serviced),
        .read_seq_string = tg_print_cpu_rwstat,
    },
    { } /* terminate */
};

static void throtl_shutdown_wq(struct request_queue *q)
{
    struct throtl_data *td = q->td;

    cancel_delayed_work_sync(&td->throtl_work);
}

static struct blkcg_policy blkcg_policy_throtl = {
    .pd_size        = sizeof(struct throtl_grp),
    .cftypes        = throtl_files,

    .pd_init_fn     = throtl_pd_init,
    .pd_exit_fn     = throtl_pd_exit,
    .pd_reset_stats_fn  = throtl_pd_reset_stats,
};

bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
{
    struct throtl_data *td = q->td;
    struct throtl_grp *tg;
    bool rw = bio_data_dir(bio), update_disptime = true;
    struct blkcg *blkcg;
    bool throttled = false;

    if (bio->bi_rw & REQ_THROTTLED) {
        bio->bi_rw &= ~REQ_THROTTLED;
        goto out;
    }

    /*
     * A throtl_grp pointer retrieved under rcu can be used to access
     * basic fields like stats and io rates. If a group has no rules,
     * just update the dispatch stats in lockless manner and return.
     */
    rcu_read_lock();
    blkcg = bio_blkcg(bio);
    tg = throtl_lookup_tg(td, blkcg);
    if (tg) {
        if (tg_no_rule_group(tg, rw)) {
            throtl_update_dispatch_stats(tg_to_blkg(tg),
                             bio->bi_size, bio->bi_rw);
            goto out_unlock_rcu;
        }
    }

    /*
     * Either group has not been allocated yet or it is not an unlimited
     * IO group
     */
    spin_lock_irq(q->queue_lock);
    tg = throtl_lookup_create_tg(td, blkcg);
    if (unlikely(!tg))
        goto out_unlock;

    if (tg->nr_queued[rw]) {
        /*
         * There is already another bio queued in same dir. No
         * need to update dispatch time.
         */
        update_disptime = false;
        goto queue_bio;

    }

    /* Bio is with-in rate limit of group */
    if (tg_may_dispatch(td, tg, bio, NULL)) {
        throtl_charge_bio(tg, bio);

        /*
         * We need to trim slice even when bios are not being queued
         * otherwise it might happen that a bio is not queued for
         * a long time and slice keeps on extending and trim is not
         * called for a long time. Now if limits are reduced suddenly
         * we take into account all the IO dispatched so far at new
         * low rate and * newly queued IO gets a really long dispatch
         * time.
         *
         * So keep on trimming slice even if bio is not queued.
         */
        throtl_trim_slice(td, tg, rw);
        goto out_unlock;
    }

queue_bio:
    throtl_log_tg(td, tg, "[%c] bio. bdisp=%llu sz=%u bps=%llu"
            " iodisp=%u iops=%u queued=%d/%d",
            rw == READ ? 'R' : 'W',
            tg->bytes_disp[rw], bio->bi_size, tg->bps[rw],
            tg->io_disp[rw], tg->iops[rw],
            tg->nr_queued[READ], tg->nr_queued[WRITE]);

    bio_associate_current(bio);
    throtl_add_bio_tg(q->td, tg, bio);
    throttled = true;

    if (update_disptime) {
        tg_update_disptime(td, tg);
        throtl_schedule_next_dispatch(td);
    }

out_unlock:
    spin_unlock_irq(q->queue_lock);
out_unlock_rcu:
    rcu_read_unlock();
out:
    return throttled;
}

void blk_throtl_drain(struct request_queue *q)
    __releases(q->queue_lock) __acquires(q->queue_lock)
{
    struct throtl_data *td = q->td;
    struct throtl_rb_root *st = &td->tg_service_tree;
    struct throtl_grp *tg;
    struct bio_list bl;
    struct bio *bio;

    queue_lockdep_assert_held(q);

    bio_list_init(&bl);

    while ((tg = throtl_rb_first(st))) {
        throtl_dequeue_tg(td, tg);

        while ((bio = bio_list_peek(&tg->bio_lists[READ])))
            tg_dispatch_one_bio(td, tg, bio_data_dir(bio), &bl);
        while ((bio = bio_list_peek(&tg->bio_lists[WRITE])))
            tg_dispatch_one_bio(td, tg, bio_data_dir(bio), &bl);
    }
    spin_unlock_irq(q->queue_lock);

    while ((bio = bio_list_pop(&bl)))
        generic_make_request(bio);

    spin_lock_irq(q->queue_lock);
}

int blk_throtl_init(struct request_queue *q)
{
    struct throtl_data *td;
    int ret;

    td = kzalloc_node(sizeof(*td), GFP_KERNEL, q->node);
    if (!td)
        return -ENOMEM;

    td->tg_service_tree = THROTL_RB_ROOT;
    td->limits_changed = false;
    INIT_DELAYED_WORK(&td->throtl_work, blk_throtl_work);

    q->td = td;
    td->queue = q;

    /* activate policy */
    ret = blkcg_activate_policy(q, &blkcg_policy_throtl);
    if (ret)
        kfree(td);
    return ret;
}

void blk_throtl_exit(struct request_queue *q)
{
    BUG_ON(!q->td);
    throtl_shutdown_wq(q);
    blkcg_deactivate_policy(q, &blkcg_policy_throtl);
    kfree(q->td);
}

static int __init throtl_init(void)
{
    kthrotld_workqueue = alloc_workqueue("kthrotld", WQ_MEM_RECLAIM, 0);
    if (!kthrotld_workqueue)
        panic("Failed to create kthrotld\n");

    return blkcg_policy_register(&blkcg_policy_throtl);
}

module_init(throtl_init);