blk-throttle: Free up a group only after one rcu grace period

[mirror_ubuntu-artful-kernel.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"

/* 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 */

/* 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)

struct throtl_grp {
        /* List of throtl groups on the request queue*/
        struct hlist_node tg_node;

        /* 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;

        struct blkio_group blkg;
        atomic_t ref;
        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;

        struct rcu_head rcu_head;
};

struct throtl_data
{
        /* List of throtl groups */
        struct hlist_head tg_list;

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

        struct throtl_grp *root_tg;
        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;
};

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...)                             \
        blk_add_trace_msg((td)->queue, "throtl %s " fmt,                \
                                blkg_path(&(tg)->blkg), ##args);        \

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

static inline struct throtl_grp *tg_of_blkg(struct blkio_group *blkg)
{
        if (blkg)
                return container_of(blkg, struct throtl_grp, blkg);

        return NULL;
}

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

static inline struct throtl_grp *throtl_ref_get_tg(struct throtl_grp *tg)
{
        atomic_inc(&tg->ref);
        return tg;
}

static void throtl_free_tg(struct rcu_head *head)
{
        struct throtl_grp *tg;

        tg = container_of(head, struct throtl_grp, rcu_head);
        kfree(tg);
}

static void throtl_put_tg(struct throtl_grp *tg)
{
        BUG_ON(atomic_read(&tg->ref) <= 0);
        if (!atomic_dec_and_test(&tg->ref))
                return;

        /*
         * A group is freed in rcu manner. But having an rcu lock does not
         * mean that one can access all the fields of blkg and assume these
         * are valid. For example, don't try to follow throtl_data and
         * request queue links.
         *
         * Having a reference to blkg under an rcu allows acess to only
         * values local to groups like group stats and group rate limits
         */
        call_rcu(&tg->rcu_head, throtl_free_tg);
}

static void throtl_init_group(struct throtl_grp *tg)
{
        INIT_HLIST_NODE(&tg->tg_node);
        RB_CLEAR_NODE(&tg->rb_node);
        bio_list_init(&tg->bio_lists[0]);
        bio_list_init(&tg->bio_lists[1]);
        tg->limits_changed = false;

        /* Practically unlimited BW */
        tg->bps[0] = tg->bps[1] = -1;
        tg->iops[0] = tg->iops[1] = -1;

        /*
         * Take the initial reference that will be released on destroy
         * This can be thought of a joint reference by cgroup and
         * request queue which will be dropped by either request queue
         * exit or cgroup deletion path depending on who is exiting first.
         */
        atomic_set(&tg->ref, 1);
}

/* Should be called with rcu read lock held (needed for blkcg) */
static void
throtl_add_group_to_td_list(struct throtl_data *td, struct throtl_grp *tg)
{
        hlist_add_head(&tg->tg_node, &td->tg_list);
        td->nr_undestroyed_grps++;
}

static void
__throtl_tg_fill_dev_details(struct throtl_data *td, struct throtl_grp *tg)
{
        struct backing_dev_info *bdi = &td->queue->backing_dev_info;
        unsigned int major, minor;

        if (!tg || tg->blkg.dev)
                return;

        /*
         * Fill in device details for a group which might not have been
         * filled at group creation time as queue was being instantiated
         * and driver had not attached a device yet
         */
        if (bdi->dev && dev_name(bdi->dev)) {
                sscanf(dev_name(bdi->dev), "%u:%u", &major, &minor);
                tg->blkg.dev = MKDEV(major, minor);
        }
}

static void throtl_init_add_tg_lists(struct throtl_data *td,
                        struct throtl_grp *tg, struct blkio_cgroup *blkcg)
{
        __throtl_tg_fill_dev_details(td, tg);

        /* Add group onto cgroup list */
        blkiocg_add_blkio_group(blkcg, &tg->blkg, (void *)td,
                                tg->blkg.dev, BLKIO_POLICY_THROTL);

        tg->bps[READ] = blkcg_get_read_bps(blkcg, tg->blkg.dev);
        tg->bps[WRITE] = blkcg_get_write_bps(blkcg, tg->blkg.dev);
        tg->iops[READ] = blkcg_get_read_iops(blkcg, tg->blkg.dev);
        tg->iops[WRITE] = blkcg_get_write_iops(blkcg, tg->blkg.dev);

        throtl_add_group_to_td_list(td, tg);
}

/* Should be called without queue lock and outside of rcu period */
static struct throtl_grp *throtl_alloc_tg(struct throtl_data *td)
{
        struct throtl_grp *tg = NULL;

        tg = kzalloc_node(sizeof(*tg), GFP_ATOMIC, td->queue->node);
        if (!tg)
                return NULL;

        throtl_init_group(tg);
        return tg;
}

static struct
throtl_grp *throtl_find_tg(struct throtl_data *td, struct blkio_cgroup *blkcg)
{
        struct throtl_grp *tg = NULL;
        void *key = td;

        /*
         * This is the common case when there are no blkio cgroups.
         * Avoid lookup in this case
         */
        if (blkcg == &blkio_root_cgroup)
                tg = td->root_tg;
        else
                tg = tg_of_blkg(blkiocg_lookup_group(blkcg, key));

        __throtl_tg_fill_dev_details(td, tg);
        return tg;
}

/*
 * This function returns with queue lock unlocked in case of error, like
 * request queue is no more
 */
static struct throtl_grp * throtl_get_tg(struct throtl_data *td)
{
        struct throtl_grp *tg = NULL, *__tg = NULL;
        struct blkio_cgroup *blkcg;
        struct request_queue *q = td->queue;

        rcu_read_lock();
        blkcg = task_blkio_cgroup(current);
        tg = throtl_find_tg(td, blkcg);
        if (tg) {
                rcu_read_unlock();
                return tg;
        }

        /*
         * Need to allocate a group. Allocation of group also needs allocation
         * of per cpu stats which in-turn takes a mutex() and can block. Hence
         * we need to drop rcu lock and queue_lock before we call alloc
         *
         * Take the request queue reference to make sure queue does not
         * go away once we return from allocation.
         */
        blk_get_queue(q);
        rcu_read_unlock();
        spin_unlock_irq(q->queue_lock);

        tg = throtl_alloc_tg(td);
        /*
         * We might have slept in group allocation. Make sure queue is not
         * dead
         */
        if (unlikely(test_bit(QUEUE_FLAG_DEAD, &q->queue_flags))) {
                blk_put_queue(q);
                if (tg)
                        kfree(tg);

                return ERR_PTR(-ENODEV);
        }
        blk_put_queue(q);

        /* Group allocated and queue is still alive. take the lock */
        spin_lock_irq(q->queue_lock);

        /*
         * Initialize the new group. After sleeping, read the blkcg again.
         */
        rcu_read_lock();
        blkcg = task_blkio_cgroup(current);

        /*
         * If some other thread already allocated the group while we were
         * not holding queue lock, free up the group
         */
        __tg = throtl_find_tg(td, blkcg);

        if (__tg) {
                kfree(tg);
                rcu_read_unlock();
                return __tg;
        }

        /* Group allocation failed. Account the IO to root group */
        if (!tg) {
                tg = td->root_tg;
                return tg;
        }

        throtl_init_add_tg_lists(td, tg, blkcg);
        rcu_read_unlock();
        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;
}

/*
 * 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_charge_bio(struct throtl_grp *tg, struct bio *bio)
{
        bool rw = bio_data_dir(bio);
        bool sync = bio->bi_rw & REQ_SYNC;

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

        /*
         * TODO: This will take blkg->stats_lock. Figure out a way
         * to avoid this cost.
         */
        blkiocg_update_dispatch_stats(&tg->blkg, bio->bi_size, rw, sync);
}

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 */
        throtl_ref_get_tg(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 tg */
        throtl_put_tg(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 throtl_grp *tg;
        struct hlist_node *pos, *n;

        if (!td->limits_changed)
                return;

        xchg(&td->limits_changed, false);

        throtl_log(td, "limits changed");

        hlist_for_each_entry_safe(tg, pos, n, &td->tg_list, tg_node) {
                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=%lu 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) > 0 || td->limits_changed) {
                /*
                 * We might have a work scheduled to be executed in future.
                 * Cancel that and schedule a new one.
                 */
                __cancel_delayed_work(dwork);
                queue_delayed_work(kthrotld_workqueue, dwork, delay);
                throtl_log(td, "schedule work. delay=%lu jiffies=%lu",
                                delay, jiffies);
        }
}

static void
throtl_destroy_tg(struct throtl_data *td, struct throtl_grp *tg)
{
        /* Something wrong if we are trying to remove same group twice */
        BUG_ON(hlist_unhashed(&tg->tg_node));

        hlist_del_init(&tg->tg_node);

        /*
         * Put the reference taken at the time of creation so that when all
         * queues are gone, group can be destroyed.
         */
        throtl_put_tg(tg);
        td->nr_undestroyed_grps--;
}

static void throtl_release_tgs(struct throtl_data *td)
{
        struct hlist_node *pos, *n;
        struct throtl_grp *tg;

        hlist_for_each_entry_safe(tg, pos, n, &td->tg_list, tg_node) {
                /*
                 * If cgroup removal path got to blk_group first and removed
                 * it from cgroup list, then it will take care of destroying
                 * cfqg also.
                 */
                if (!blkiocg_del_blkio_group(&tg->blkg))
                        throtl_destroy_tg(td, tg);
        }
}

static void throtl_td_free(struct throtl_data *td)
{
        kfree(td);
}

/*
 * Blk cgroup controller notification saying that blkio_group object is being
 * delinked as associated cgroup object is going away. That also means that
 * no new IO will come in this group. So get rid of this group as soon as
 * any pending IO in the group is finished.
 *
 * This function is called under rcu_read_lock(). key is the rcu protected
 * pointer. That means "key" is a valid throtl_data pointer as long as we are
 * rcu read lock.
 *
 * "key" was fetched from blkio_group under blkio_cgroup->lock. That means
 * it should not be NULL as even if queue was going away, cgroup deltion
 * path got to it first.
 */
void throtl_unlink_blkio_group(void *key, struct blkio_group *blkg)
{
        unsigned long flags;
        struct throtl_data *td = key;

        spin_lock_irqsave(td->queue->queue_lock, flags);
        throtl_destroy_tg(td, tg_of_blkg(blkg));
        spin_unlock_irqrestore(td->queue->queue_lock, flags);
}

static void throtl_update_blkio_group_common(struct throtl_data *td,
                                struct throtl_grp *tg)
{
        xchg(&tg->limits_changed, true);
        xchg(&td->limits_changed, true);
        /* Schedule a work now to process the limit change */
        throtl_schedule_delayed_work(td, 0);
}

/*
 * For all update functions, key should be a valid pointer because these
 * update functions are called under blkcg_lock, that means, blkg is
 * valid and in turn key is valid. queue exit path can not race because
 * of blkcg_lock
 *
 * Can not take queue lock in update functions as queue lock under blkcg_lock
 * is not allowed. Under other paths we take blkcg_lock under queue_lock.
 */
static void throtl_update_blkio_group_read_bps(void *key,
                                struct blkio_group *blkg, u64 read_bps)
{
        struct throtl_data *td = key;
        struct throtl_grp *tg = tg_of_blkg(blkg);

        tg->bps[READ] = read_bps;
        throtl_update_blkio_group_common(td, tg);
}

static void throtl_update_blkio_group_write_bps(void *key,
                                struct blkio_group *blkg, u64 write_bps)
{
        struct throtl_data *td = key;
        struct throtl_grp *tg = tg_of_blkg(blkg);

        tg->bps[WRITE] = write_bps;
        throtl_update_blkio_group_common(td, tg);
}

static void throtl_update_blkio_group_read_iops(void *key,
                        struct blkio_group *blkg, unsigned int read_iops)
{
        struct throtl_data *td = key;
        struct throtl_grp *tg = tg_of_blkg(blkg);

        tg->iops[READ] = read_iops;
        throtl_update_blkio_group_common(td, tg);
}

static void throtl_update_blkio_group_write_iops(void *key,
                        struct blkio_group *blkg, unsigned int write_iops)
{
        struct throtl_data *td = key;
        struct throtl_grp *tg = tg_of_blkg(blkg);

        tg->iops[WRITE] = write_iops;
        throtl_update_blkio_group_common(td, tg);
}

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

        cancel_delayed_work_sync(&td->throtl_work);
}

static struct blkio_policy_type blkio_policy_throtl = {
        .ops = {
                .blkio_unlink_group_fn = throtl_unlink_blkio_group,
                .blkio_update_group_read_bps_fn =
                                        throtl_update_blkio_group_read_bps,
                .blkio_update_group_write_bps_fn =
                                        throtl_update_blkio_group_write_bps,
                .blkio_update_group_read_iops_fn =
                                        throtl_update_blkio_group_read_iops,
                .blkio_update_group_write_iops_fn =
                                        throtl_update_blkio_group_write_iops,
        },
        .plid = BLKIO_POLICY_THROTL,
};

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

        if (bio->bi_rw & REQ_THROTTLED) {
                bio->bi_rw &= ~REQ_THROTTLED;
                return 0;
        }

        spin_lock_irq(q->queue_lock);
        tg = throtl_get_tg(td);

        if (IS_ERR(tg)) {
                if (PTR_ERR(tg) == -ENODEV) {
                        /*
                         * Queue is gone. No queue lock held here.
                         */
                        return -ENODEV;
                }
        }

        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;
        }

queue_bio:
        throtl_log_tg(td, tg, "[%c] bio. bdisp=%u 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]);

        throtl_add_bio_tg(q->td, tg, bio);
        *biop = NULL;

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

out:
        spin_unlock_irq(q->queue_lock);
        return 0;
}

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

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

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

        /* alloc and Init root group. */
        td->queue = q;
        tg = throtl_alloc_tg(td);

        if (!tg) {
                kfree(td);
                return -ENOMEM;
        }

        td->root_tg = tg;

        rcu_read_lock();
        throtl_init_add_tg_lists(td, tg, &blkio_root_cgroup);
        rcu_read_unlock();

        /* Attach throtl data to request queue */
        q->td = td;
        return 0;
}

void blk_throtl_exit(struct request_queue *q)
{
        struct throtl_data *td = q->td;
        bool wait = false;

        BUG_ON(!td);

        throtl_shutdown_wq(q);

        spin_lock_irq(q->queue_lock);
        throtl_release_tgs(td);

        /* If there are other groups */
        if (td->nr_undestroyed_grps > 0)
                wait = true;

        spin_unlock_irq(q->queue_lock);

        /*
         * Wait for tg->blkg->key accessors to exit their grace periods.
         * Do this wait only if there are other undestroyed groups out
         * there (other than root group). This can happen if cgroup deletion
         * path claimed the responsibility of cleaning up a group before
         * queue cleanup code get to the group.
         *
         * Do not call synchronize_rcu() unconditionally as there are drivers
         * which create/delete request queue hundreds of times during scan/boot
         * and synchronize_rcu() can take significant time and slow down boot.
         */
        if (wait)
                synchronize_rcu();

        /*
         * Just being safe to make sure after previous flush if some body did
         * update limits through cgroup and another work got queued, cancel
         * it.
         */
        throtl_shutdown_wq(q);
        throtl_td_free(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");

        blkio_policy_register(&blkio_policy_throtl);
        return 0;
}

module_init(throtl_init);