blk-throttle: collapse throtl_dispatch() into the work function

[deliverable/linux.git] / block / blk-throttle.c

/*
 * Interface for controlling IO bandwidth on a request queue
 *
 * Copyright (C) 2010 Vivek Goyal <vgoyal@redhat.com>
 */

#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];

        /* 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 dispatch_work;
};

/* 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->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_dying(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);

                if (nr_disp >= throtl_quantum)
                        break;
        }

        return nr_disp;
}

/* work function to dispatch throttled bios */
void blk_throtl_dispatch_work_fn(struct work_struct *work)
{
        struct throtl_data *td = container_of(to_delayed_work(work),
                                              struct throtl_data, dispatch_work);
        struct request_queue *q = td->queue;
        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);

        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);
        }
}

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

        struct delayed_work *dwork = &td->dispatch_work;

        if (total_nr_queued(td)) {
                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;

        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]);

        /*
         * We're already holding queue_lock and know @tg is valid.  Let's
         * apply the new config directly.
         *
         * 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);
                throtl_schedule_next_dispatch(td);
        }

        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->dispatch_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;
}

/**
 * blk_throtl_drain - drain throttled bios
 * @q: request_queue to drain throttled bios for
 *
 * Dispatch all currently throttled bios on @q through ->make_request_fn().
 */
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;
        INIT_DELAYED_WORK(&td->dispatch_work, blk_throtl_dispatch_work_fn);

        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);