blk-throttle: remove asynchrnous percpu stats allocation mechanism

[firefly-linux-kernel-4.4.55.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 <linux/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;

/*
 * To implement hierarchical throttling, throtl_grps form a tree and bios
 * are dispatched upwards level by level until they reach the top and get
 * issued.  When dispatching bios from the children and local group at each
 * level, if the bios are dispatched into a single bio_list, there's a risk
 * of a local or child group which can queue many bios at once filling up
 * the list starving others.
 *
 * To avoid such starvation, dispatched bios are queued separately
 * according to where they came from.  When they are again dispatched to
 * the parent, they're popped in round-robin order so that no single source
 * hogs the dispatch window.
 *
 * throtl_qnode is used to keep the queued bios separated by their sources.
 * Bios are queued to throtl_qnode which in turn is queued to
 * throtl_service_queue and then dispatched in round-robin order.
 *
 * It's also used to track the reference counts on blkg's.  A qnode always
 * belongs to a throtl_grp and gets queued on itself or the parent, so
 * incrementing the reference of the associated throtl_grp when a qnode is
 * queued and decrementing when dequeued is enough to keep the whole blkg
 * tree pinned while bios are in flight.
 */
struct throtl_qnode {
        struct list_head        node;           /* service_queue->queued[] */
        struct bio_list         bios;           /* queued bios */
        struct throtl_grp       *tg;            /* tg this qnode belongs to */
};

struct throtl_service_queue {
        struct throtl_service_queue *parent_sq; /* the parent service_queue */

        /*
         * Bios queued directly to this service_queue or dispatched from
         * children throtl_grp's.
         */
        struct list_head        queued[2];      /* throtl_qnode [READ/WRITE] */
        unsigned int            nr_queued[2];   /* number of queued bios */

        /*
         * RB tree of active children throtl_grp's, which are sorted by
         * their ->disptime.
         */
        struct rb_root          pending_tree;   /* RB tree of active tgs */
        struct rb_node          *first_pending; /* first node in the tree */
        unsigned int            nr_pending;     /* # queued in the tree */
        unsigned long           first_pending_disptime; /* disptime of the first tg */
        struct timer_list       pending_timer;  /* fires on first_pending_disptime */
};

enum tg_state_flags {
        THROTL_TG_PENDING       = 1 << 0,       /* on parent's pending tree */
        THROTL_TG_WAS_EMPTY     = 1 << 1,       /* bio_lists[] became non-empty */
};

#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_queue member */
        struct rb_node rb_node;

        /* throtl_data this group belongs to */
        struct throtl_data *td;

        /* this group's service queue */
        struct throtl_service_queue service_queue;

        /*
         * qnode_on_self is used when bios are directly queued to this
         * throtl_grp so that local bios compete fairly with bios
         * dispatched from children.  qnode_on_parent is used when bios are
         * dispatched from this throtl_grp into its parent and will compete
         * with the sibling qnode_on_parents and the parent's
         * qnode_on_self.
         */
        struct throtl_qnode qnode_on_self[2];
        struct throtl_qnode qnode_on_parent[2];

        /*
         * 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;

        /* are there any throtl rules between this group and td? */
        bool has_rules[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;
};

struct throtl_data
{
        /* service tree for active throtl groups */
        struct throtl_service_queue service_queue;

        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 work_struct dispatch_work;
};

static void throtl_pending_timer_fn(unsigned long arg);

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);
}

/**
 * sq_to_tg - return the throl_grp the specified service queue belongs to
 * @sq: the throtl_service_queue of interest
 *
 * Return the throtl_grp @sq belongs to.  If @sq is the top-level one
 * embedded in throtl_data, %NULL is returned.
 */
static struct throtl_grp *sq_to_tg(struct throtl_service_queue *sq)
{
        if (sq && sq->parent_sq)
                return container_of(sq, struct throtl_grp, service_queue);
        else
                return NULL;
}

/**
 * sq_to_td - return throtl_data the specified service queue belongs to
 * @sq: the throtl_service_queue of interest
 *
 * A service_queue can be embeded in either a throtl_grp or throtl_data.
 * Determine the associated throtl_data accordingly and return it.
 */
static struct throtl_data *sq_to_td(struct throtl_service_queue *sq)
{
        struct throtl_grp *tg = sq_to_tg(sq);

        if (tg)
                return tg->td;
        else
                return container_of(sq, struct throtl_data, service_queue);
}

/**
 * throtl_log - log debug message via blktrace
 * @sq: the service_queue being reported
 * @fmt: printf format string
 * @args: printf args
 *
 * The messages are prefixed with "throtl BLKG_NAME" if @sq belongs to a
 * throtl_grp; otherwise, just "throtl".
 *
 * TODO: this should be made a function and name formatting should happen
 * after testing whether blktrace is enabled.
 */
#define throtl_log(sq, fmt, args...)    do {                            \
        struct throtl_grp *__tg = sq_to_tg((sq));                       \
        struct throtl_data *__td = sq_to_td((sq));                      \
                                                                        \
        (void)__td;                                                     \
        if ((__tg)) {                                                   \
                char __pbuf[128];                                       \
                                                                        \
                blkg_path(tg_to_blkg(__tg), __pbuf, sizeof(__pbuf));    \
                blk_add_trace_msg(__td->queue, "throtl %s " fmt, __pbuf, ##args); \
        } else {                                                        \
                blk_add_trace_msg(__td->queue, "throtl " fmt, ##args);  \
        }                                                               \
} while (0)

static void throtl_qnode_init(struct throtl_qnode *qn, struct throtl_grp *tg)
{
        INIT_LIST_HEAD(&qn->node);
        bio_list_init(&qn->bios);
        qn->tg = tg;
}

/**
 * throtl_qnode_add_bio - add a bio to a throtl_qnode and activate it
 * @bio: bio being added
 * @qn: qnode to add bio to
 * @queued: the service_queue->queued[] list @qn belongs to
 *
 * Add @bio to @qn and put @qn on @queued if it's not already on.
 * @qn->tg's reference count is bumped when @qn is activated.  See the
 * comment on top of throtl_qnode definition for details.
 */
static void throtl_qnode_add_bio(struct bio *bio, struct throtl_qnode *qn,
                                 struct list_head *queued)
{
        bio_list_add(&qn->bios, bio);
        if (list_empty(&qn->node)) {
                list_add_tail(&qn->node, queued);
                blkg_get(tg_to_blkg(qn->tg));
        }
}

/**
 * throtl_peek_queued - peek the first bio on a qnode list
 * @queued: the qnode list to peek
 */
static struct bio *throtl_peek_queued(struct list_head *queued)
{
        struct throtl_qnode *qn = list_first_entry(queued, struct throtl_qnode, node);
        struct bio *bio;

        if (list_empty(queued))
                return NULL;

        bio = bio_list_peek(&qn->bios);
        WARN_ON_ONCE(!bio);
        return bio;
}

/**
 * throtl_pop_queued - pop the first bio form a qnode list
 * @queued: the qnode list to pop a bio from
 * @tg_to_put: optional out argument for throtl_grp to put
 *
 * Pop the first bio from the qnode list @queued.  After popping, the first
 * qnode is removed from @queued if empty or moved to the end of @queued so
 * that the popping order is round-robin.
 *
 * When the first qnode is removed, its associated throtl_grp should be put
 * too.  If @tg_to_put is NULL, this function automatically puts it;
 * otherwise, *@tg_to_put is set to the throtl_grp to put and the caller is
 * responsible for putting it.
 */
static struct bio *throtl_pop_queued(struct list_head *queued,
                                     struct throtl_grp **tg_to_put)
{
        struct throtl_qnode *qn = list_first_entry(queued, struct throtl_qnode, node);
        struct bio *bio;

        if (list_empty(queued))
                return NULL;

        bio = bio_list_pop(&qn->bios);
        WARN_ON_ONCE(!bio);

        if (bio_list_empty(&qn->bios)) {
                list_del_init(&qn->node);
                if (tg_to_put)
                        *tg_to_put = qn->tg;
                else
                        blkg_put(tg_to_blkg(qn->tg));
        } else {
                list_move_tail(&qn->node, queued);
        }

        return bio;
}

/* init a service_queue, assumes the caller zeroed it */
static void throtl_service_queue_init(struct throtl_service_queue *sq,
                                      struct throtl_service_queue *parent_sq)
{
        INIT_LIST_HEAD(&sq->queued[0]);
        INIT_LIST_HEAD(&sq->queued[1]);
        sq->pending_tree = RB_ROOT;
        sq->parent_sq = parent_sq;
        setup_timer(&sq->pending_timer, throtl_pending_timer_fn,
                    (unsigned long)sq);
}

static void throtl_service_queue_exit(struct throtl_service_queue *sq)
{
        del_timer_sync(&sq->pending_timer);
}

static struct blkg_policy_data *throtl_pd_alloc(gfp_t gfp, int node)
{
        struct throtl_grp *tg;
        int cpu;

        tg = kzalloc_node(sizeof(*tg), gfp, node);
        if (!tg)
                return NULL;

        tg->stats_cpu = alloc_percpu_gfp(struct tg_stats_cpu, gfp);
        if (!tg->stats_cpu) {
                kfree(tg);
                return NULL;
        }

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

                blkg_rwstat_init(&stats_cpu->service_bytes);
                blkg_rwstat_init(&stats_cpu->serviced);
        }

        return &tg->pd;
}

static void throtl_pd_init(struct blkcg_gq *blkg)
{
        struct throtl_grp *tg = blkg_to_tg(blkg);
        struct throtl_data *td = blkg->q->td;
        struct throtl_service_queue *parent_sq;
        int rw;

        /*
         * If on the default hierarchy, we switch to properly hierarchical
         * behavior where limits on a given throtl_grp are applied to the
         * whole subtree rather than just the group itself.  e.g. If 16M
         * read_bps limit is set on the root group, the whole system can't
         * exceed 16M for the device.
         *
         * If not on the default hierarchy, the broken flat hierarchy
         * behavior is retained where all throtl_grps are treated as if
         * they're all separate root groups right below throtl_data.
         * Limits of a group don't interact with limits of other groups
         * regardless of the position of the group in the hierarchy.
         */
        parent_sq = &td->service_queue;

        if (cgroup_on_dfl(blkg->blkcg->css.cgroup) && blkg->parent)
                parent_sq = &blkg_to_tg(blkg->parent)->service_queue;

        throtl_service_queue_init(&tg->service_queue, parent_sq);

        for (rw = READ; rw <= WRITE; rw++) {
                throtl_qnode_init(&tg->qnode_on_self[rw], tg);
                throtl_qnode_init(&tg->qnode_on_parent[rw], tg);
        }

        RB_CLEAR_NODE(&tg->rb_node);
        tg->td = td;

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

/*
 * Set has_rules[] if @tg or any of its parents have limits configured.
 * This doesn't require walking up to the top of the hierarchy as the
 * parent's has_rules[] is guaranteed to be correct.
 */
static void tg_update_has_rules(struct throtl_grp *tg)
{
        struct throtl_grp *parent_tg = sq_to_tg(tg->service_queue.parent_sq);
        int rw;

        for (rw = READ; rw <= WRITE; rw++)
                tg->has_rules[rw] = (parent_tg && parent_tg->has_rules[rw]) ||
                                    (tg->bps[rw] != -1 || tg->iops[rw] != -1);
}

static void throtl_pd_online(struct blkcg_gq *blkg)
{
        /*
         * We don't want new groups to escape the limits of its ancestors.
         * Update has_rules[] after a new group is brought online.
         */
        tg_update_has_rules(blkg_to_tg(blkg));
}

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

        throtl_service_queue_exit(&tg->service_queue);
}

static void throtl_pd_free(struct blkg_policy_data *pd)
{
        struct throtl_grp *tg = pd_to_tg(pd);

        free_percpu(tg->stats_cpu);
        kfree(tg);
}

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

        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_service_queue *parent_sq)
{
        /* Service tree is empty */
        if (!parent_sq->nr_pending)
                return NULL;

        if (!parent_sq->first_pending)
                parent_sq->first_pending = rb_first(&parent_sq->pending_tree);

        if (parent_sq->first_pending)
                return rb_entry_tg(parent_sq->first_pending);

        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_service_queue *parent_sq)
{
        if (parent_sq->first_pending == n)
                parent_sq->first_pending = NULL;
        rb_erase_init(n, &parent_sq->pending_tree);
        --parent_sq->nr_pending;
}

static void update_min_dispatch_time(struct throtl_service_queue *parent_sq)
{
        struct throtl_grp *tg;

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

        parent_sq->first_pending_disptime = tg->disptime;
}

static void tg_service_queue_add(struct throtl_grp *tg)
{
        struct throtl_service_queue *parent_sq = tg->service_queue.parent_sq;
        struct rb_node **node = &parent_sq->pending_tree.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)
                parent_sq->first_pending = &tg->rb_node;

        rb_link_node(&tg->rb_node, parent, node);
        rb_insert_color(&tg->rb_node, &parent_sq->pending_tree);
}

static void __throtl_enqueue_tg(struct throtl_grp *tg)
{
        tg_service_queue_add(tg);
        tg->flags |= THROTL_TG_PENDING;
        tg->service_queue.parent_sq->nr_pending++;
}

static void throtl_enqueue_tg(struct throtl_grp *tg)
{
        if (!(tg->flags & THROTL_TG_PENDING))
                __throtl_enqueue_tg(tg);
}

static void __throtl_dequeue_tg(struct throtl_grp *tg)
{
        throtl_rb_erase(&tg->rb_node, tg->service_queue.parent_sq);
        tg->flags &= ~THROTL_TG_PENDING;
}

static void throtl_dequeue_tg(struct throtl_grp *tg)
{
        if (tg->flags & THROTL_TG_PENDING)
                __throtl_dequeue_tg(tg);
}

/* Call with queue lock held */
static void throtl_schedule_pending_timer(struct throtl_service_queue *sq,
                                          unsigned long expires)
{
        mod_timer(&sq->pending_timer, expires);
        throtl_log(sq, "schedule timer. delay=%lu jiffies=%lu",
                   expires - jiffies, jiffies);
}

/**
 * throtl_schedule_next_dispatch - schedule the next dispatch cycle
 * @sq: the service_queue to schedule dispatch for
 * @force: force scheduling
 *
 * Arm @sq->pending_timer so that the next dispatch cycle starts on the
 * dispatch time of the first pending child.  Returns %true if either timer
 * is armed or there's no pending child left.  %false if the current
 * dispatch window is still open and the caller should continue
 * dispatching.
 *
 * If @force is %true, the dispatch timer is always scheduled and this
 * function is guaranteed to return %true.  This is to be used when the
 * caller can't dispatch itself and needs to invoke pending_timer
 * unconditionally.  Note that forced scheduling is likely to induce short
 * delay before dispatch starts even if @sq->first_pending_disptime is not
 * in the future and thus shouldn't be used in hot paths.
 */
static bool throtl_schedule_next_dispatch(struct throtl_service_queue *sq,
                                          bool force)
{
        /* any pending children left? */
        if (!sq->nr_pending)
                return true;

        update_min_dispatch_time(sq);

        /* is the next dispatch time in the future? */
        if (force || time_after(sq->first_pending_disptime, jiffies)) {
                throtl_schedule_pending_timer(sq, sq->first_pending_disptime);
                return true;
        }

        /* tell the caller to continue dispatching */
        return false;
}

static inline void throtl_start_new_slice_with_credit(struct throtl_grp *tg,
                bool rw, unsigned long start)
{
        tg->bytes_disp[rw] = 0;
        tg->io_disp[rw] = 0;

        /*
         * Previous slice has expired. We must have trimmed it after last
         * bio dispatch. That means since start of last slice, we never used
         * that bandwidth. Do try to make use of that bandwidth while giving
         * credit.
         */
        if (time_after_eq(start, tg->slice_start[rw]))
                tg->slice_start[rw] = start;

        tg->slice_end[rw] = jiffies + throtl_slice;
        throtl_log(&tg->service_queue,
                   "[%c] new slice with credit start=%lu end=%lu jiffies=%lu",
                   rw == READ ? 'R' : 'W', tg->slice_start[rw],
                   tg->slice_end[rw], jiffies);
}

static inline void throtl_start_new_slice(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->service_queue,
                   "[%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_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_grp *tg, bool rw,
                                       unsigned long jiffy_end)
{
        tg->slice_end[rw] = roundup(jiffy_end, throtl_slice);
        throtl_log(&tg->service_queue,
                   "[%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_grp *tg, bool rw)
{
        if (time_in_range(jiffies, tg->slice_start[rw], tg->slice_end[rw]))
                return false;

        return 1;
}

/* Trim the used slices and adjust slice start accordingly */
static inline void throtl_trim_slice(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(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(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->service_queue,
                   "[%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_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 true;
        }

        /* 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_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_iter.bi_size <= bytes_allowed) {
                if (wait)
                        *wait = 0;
                return true;
        }

        /* Calc approx time to dispatch */
        extra_bytes = tg->bytes_disp[rw] + bio->bi_iter.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_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->service_queue.nr_queued[rw] &&
               bio != throtl_peek_queued(&tg->service_queue.queued[rw]));

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

        /*
         * 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(tg, rw))
                throtl_start_new_slice(tg, rw);
        else {
                if (time_before(tg->slice_end[rw], jiffies + throtl_slice))
                        throtl_extend_slice(tg, rw, jiffies + throtl_slice);
        }

        if (tg_with_in_bps_limit(tg, bio, &bps_wait) &&
            tg_with_in_iops_limit(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(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;

        /*
         * 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_iter.bi_size;
        tg->io_disp[rw]++;

        /*
         * REQ_THROTTLED is used to prevent the same bio to be throttled
         * more than once as a throttled bio will go through blk-throtl the
         * second time when it eventually gets issued.  Set it when a bio
         * is being charged to a tg.
         *
         * Dispatch stats aren't recursive and each @bio should only be
         * accounted by the @tg it was originally associated with.  Let's
         * update the stats when setting REQ_THROTTLED for the first time
         * which is guaranteed to be for the @bio's original tg.
         */
        if (!(bio->bi_rw & REQ_THROTTLED)) {
                bio->bi_rw |= REQ_THROTTLED;
                throtl_update_dispatch_stats(tg_to_blkg(tg),
                                             bio->bi_iter.bi_size, bio->bi_rw);
        }
}

/**
 * throtl_add_bio_tg - add a bio to the specified throtl_grp
 * @bio: bio to add
 * @qn: qnode to use
 * @tg: the target throtl_grp
 *
 * Add @bio to @tg's service_queue using @qn.  If @qn is not specified,
 * tg->qnode_on_self[] is used.
 */
static void throtl_add_bio_tg(struct bio *bio, struct throtl_qnode *qn,
                              struct throtl_grp *tg)
{
        struct throtl_service_queue *sq = &tg->service_queue;
        bool rw = bio_data_dir(bio);

        if (!qn)
                qn = &tg->qnode_on_self[rw];

        /*
         * If @tg doesn't currently have any bios queued in the same
         * direction, queueing @bio can change when @tg should be
         * dispatched.  Mark that @tg was empty.  This is automatically
         * cleaered on the next tg_update_disptime().
         */
        if (!sq->nr_queued[rw])
                tg->flags |= THROTL_TG_WAS_EMPTY;

        throtl_qnode_add_bio(bio, qn, &sq->queued[rw]);

        sq->nr_queued[rw]++;
        throtl_enqueue_tg(tg);
}

static void tg_update_disptime(struct throtl_grp *tg)
{
        struct throtl_service_queue *sq = &tg->service_queue;
        unsigned long read_wait = -1, write_wait = -1, min_wait = -1, disptime;
        struct bio *bio;

        if ((bio = throtl_peek_queued(&sq->queued[READ])))
                tg_may_dispatch(tg, bio, &read_wait);

        if ((bio = throtl_peek_queued(&sq->queued[WRITE])))
                tg_may_dispatch(tg, bio, &write_wait);

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

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

        /* see throtl_add_bio_tg() */
        tg->flags &= ~THROTL_TG_WAS_EMPTY;
}

static void start_parent_slice_with_credit(struct throtl_grp *child_tg,
                                        struct throtl_grp *parent_tg, bool rw)
{
        if (throtl_slice_used(parent_tg, rw)) {
                throtl_start_new_slice_with_credit(parent_tg, rw,
                                child_tg->slice_start[rw]);
        }

}

static void tg_dispatch_one_bio(struct throtl_grp *tg, bool rw)
{
        struct throtl_service_queue *sq = &tg->service_queue;
        struct throtl_service_queue *parent_sq = sq->parent_sq;
        struct throtl_grp *parent_tg = sq_to_tg(parent_sq);
        struct throtl_grp *tg_to_put = NULL;
        struct bio *bio;

        /*
         * @bio is being transferred from @tg to @parent_sq.  Popping a bio
         * from @tg may put its reference and @parent_sq might end up
         * getting released prematurely.  Remember the tg to put and put it
         * after @bio is transferred to @parent_sq.
         */
        bio = throtl_pop_queued(&sq->queued[rw], &tg_to_put);
        sq->nr_queued[rw]--;

        throtl_charge_bio(tg, bio);

        /*
         * If our parent is another tg, we just need to transfer @bio to
         * the parent using throtl_add_bio_tg().  If our parent is
         * @td->service_queue, @bio is ready to be issued.  Put it on its
         * bio_lists[] and decrease total number queued.  The caller is
         * responsible for issuing these bios.
         */
        if (parent_tg) {
                throtl_add_bio_tg(bio, &tg->qnode_on_parent[rw], parent_tg);
                start_parent_slice_with_credit(tg, parent_tg, rw);
        } else {
                throtl_qnode_add_bio(bio, &tg->qnode_on_parent[rw],
                                     &parent_sq->queued[rw]);
                BUG_ON(tg->td->nr_queued[rw] <= 0);
                tg->td->nr_queued[rw]--;
        }

        throtl_trim_slice(tg, rw);

        if (tg_to_put)
                blkg_put(tg_to_blkg(tg_to_put));
}

static int throtl_dispatch_tg(struct throtl_grp *tg)
{
        struct throtl_service_queue *sq = &tg->service_queue;
        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 = throtl_peek_queued(&sq->queued[READ])) &&
               tg_may_dispatch(tg, bio, NULL)) {

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

                if (nr_reads >= max_nr_reads)
                        break;
        }

        while ((bio = throtl_peek_queued(&sq->queued[WRITE])) &&
               tg_may_dispatch(tg, bio, NULL)) {

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

                if (nr_writes >= max_nr_writes)
                        break;
        }

        return nr_reads + nr_writes;
}

static int throtl_select_dispatch(struct throtl_service_queue *parent_sq)
{
        unsigned int nr_disp = 0;

        while (1) {
                struct throtl_grp *tg = throtl_rb_first(parent_sq);
                struct throtl_service_queue *sq = &tg->service_queue;

                if (!tg)
                        break;

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

                throtl_dequeue_tg(tg);

                nr_disp += throtl_dispatch_tg(tg);

                if (sq->nr_queued[0] || sq->nr_queued[1])
                        tg_update_disptime(tg);

                if (nr_disp >= throtl_quantum)
                        break;
        }

        return nr_disp;
}

/**
 * throtl_pending_timer_fn - timer function for service_queue->pending_timer
 * @arg: the throtl_service_queue being serviced
 *
 * This timer is armed when a child throtl_grp with active bio's become
 * pending and queued on the service_queue's pending_tree and expires when
 * the first child throtl_grp should be dispatched.  This function
 * dispatches bio's from the children throtl_grps to the parent
 * service_queue.
 *
 * If the parent's parent is another throtl_grp, dispatching is propagated
 * by either arming its pending_timer or repeating dispatch directly.  If
 * the top-level service_tree is reached, throtl_data->dispatch_work is
 * kicked so that the ready bio's are issued.
 */
static void throtl_pending_timer_fn(unsigned long arg)
{
        struct throtl_service_queue *sq = (void *)arg;
        struct throtl_grp *tg = sq_to_tg(sq);
        struct throtl_data *td = sq_to_td(sq);
        struct request_queue *q = td->queue;
        struct throtl_service_queue *parent_sq;
        bool dispatched;
        int ret;

        spin_lock_irq(q->queue_lock);
again:
        parent_sq = sq->parent_sq;
        dispatched = false;

        while (true) {
                throtl_log(sq, "dispatch nr_queued=%u read=%u write=%u",
                           sq->nr_queued[READ] + sq->nr_queued[WRITE],
                           sq->nr_queued[READ], sq->nr_queued[WRITE]);

                ret = throtl_select_dispatch(sq);
                if (ret) {
                        throtl_log(sq, "bios disp=%u", ret);
                        dispatched = true;
                }

                if (throtl_schedule_next_dispatch(sq, false))
                        break;

                /* this dispatch windows is still open, relax and repeat */
                spin_unlock_irq(q->queue_lock);
                cpu_relax();
                spin_lock_irq(q->queue_lock);
        }

        if (!dispatched)
                goto out_unlock;

        if (parent_sq) {
                /* @parent_sq is another throl_grp, propagate dispatch */
                if (tg->flags & THROTL_TG_WAS_EMPTY) {
                        tg_update_disptime(tg);
                        if (!throtl_schedule_next_dispatch(parent_sq, false)) {
                                /* window is already open, repeat dispatching */
                                sq = parent_sq;
                                tg = sq_to_tg(sq);
                                goto again;
                        }
                }
        } else {
                /* reached the top-level, queue issueing */
                queue_work(kthrotld_workqueue, &td->dispatch_work);
        }
out_unlock:
        spin_unlock_irq(q->queue_lock);
}

/**
 * blk_throtl_dispatch_work_fn - work function for throtl_data->dispatch_work
 * @work: work item being executed
 *
 * This function is queued for execution when bio's reach the bio_lists[]
 * of throtl_data->service_queue.  Those bio's are ready and issued by this
 * function.
 */
static void blk_throtl_dispatch_work_fn(struct work_struct *work)
{
        struct throtl_data *td = container_of(work, struct throtl_data,
                                              dispatch_work);
        struct throtl_service_queue *td_sq = &td->service_queue;
        struct request_queue *q = td->queue;
        struct bio_list bio_list_on_stack;
        struct bio *bio;
        struct blk_plug plug;
        int rw;

        bio_list_init(&bio_list_on_stack);

        spin_lock_irq(q->queue_lock);
        for (rw = READ; rw <= WRITE; rw++)
                while ((bio = throtl_pop_queued(&td_sq->queued[rw], NULL)))
                        bio_list_add(&bio_list_on_stack, bio);
        spin_unlock_irq(q->queue_lock);

        if (!bio_list_empty(&bio_list_on_stack)) {
                blk_start_plug(&plug);
                while((bio = bio_list_pop(&bio_list_on_stack)))
                        generic_make_request(bio);
                blk_finish_plug(&plug);
        }
}

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 seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)), tg_prfill_cpu_rwstat,
                          &blkcg_policy_throtl, seq_cft(sf)->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 seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)), tg_prfill_conf_u64,
                          &blkcg_policy_throtl, seq_cft(sf)->private, false);
        return 0;
}

static int tg_print_conf_uint(struct seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)), tg_prfill_conf_uint,
                          &blkcg_policy_throtl, seq_cft(sf)->private, false);
        return 0;
}

static ssize_t tg_set_conf(struct kernfs_open_file *of,
                           char *buf, size_t nbytes, loff_t off, bool is_u64)
{
        struct blkcg *blkcg = css_to_blkcg(of_css(of));
        struct blkg_conf_ctx ctx;
        struct throtl_grp *tg;
        struct throtl_service_queue *sq;
        struct blkcg_gq *blkg;
        struct cgroup_subsys_state *pos_css;
        int ret;

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

        tg = blkg_to_tg(ctx.blkg);
        sq = &tg->service_queue;

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

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

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

        /*
         * Update has_rules[] flags for the updated tg's subtree.  A tg is
         * considered to have rules if either the tg itself or any of its
         * ancestors has rules.  This identifies groups without any
         * restrictions in the whole hierarchy and allows them to bypass
         * blk-throttle.
         */
        blkg_for_each_descendant_pre(blkg, pos_css, ctx.blkg)
                tg_update_has_rules(blkg_to_tg(blkg));

        /*
         * 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(tg, 0);
        throtl_start_new_slice(tg, 1);

        if (tg->flags & THROTL_TG_PENDING) {
                tg_update_disptime(tg);
                throtl_schedule_next_dispatch(sq->parent_sq, true);
        }

        blkg_conf_finish(&ctx);
        return nbytes;
}

static ssize_t tg_set_conf_u64(struct kernfs_open_file *of,
                               char *buf, size_t nbytes, loff_t off)
{
        return tg_set_conf(of, buf, nbytes, off, true);
}

static ssize_t tg_set_conf_uint(struct kernfs_open_file *of,
                                char *buf, size_t nbytes, loff_t off)
{
        return tg_set_conf(of, buf, nbytes, off, false);
}

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

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

        cancel_work_sync(&td->dispatch_work);
}

static struct blkcg_policy blkcg_policy_throtl = {
        .cftypes                = throtl_files,

        .pd_alloc_fn            = throtl_pd_alloc,
        .pd_init_fn             = throtl_pd_init,
        .pd_online_fn           = throtl_pd_online,
        .pd_exit_fn             = throtl_pd_exit,
        .pd_free_fn             = throtl_pd_free,
        .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_qnode *qn = NULL;
        struct throtl_grp *tg;
        struct throtl_service_queue *sq;
        bool rw = bio_data_dir(bio);
        struct blkcg *blkcg;
        bool throttled = false;

        /* see throtl_charge_bio() */
        if (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->has_rules[rw]) {
                        throtl_update_dispatch_stats(tg_to_blkg(tg),
                                        bio->bi_iter.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;

        sq = &tg->service_queue;

        while (true) {
                /* throtl is FIFO - if bios are already queued, should queue */
                if (sq->nr_queued[rw])
                        break;

                /* if above limits, break to queue */
                if (!tg_may_dispatch(tg, bio, NULL))
                        break;

                /* within limits, let's charge and dispatch directly */
                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(tg, rw);

                /*
                 * @bio passed through this layer without being throttled.
                 * Climb up the ladder.  If we''re already at the top, it
                 * can be executed directly.
                 */
                qn = &tg->qnode_on_parent[rw];
                sq = sq->parent_sq;
                tg = sq_to_tg(sq);
                if (!tg)
                        goto out_unlock;
        }

        /* out-of-limit, queue to @tg */
        throtl_log(sq, "[%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_iter.bi_size, tg->bps[rw],
                   tg->io_disp[rw], tg->iops[rw],
                   sq->nr_queued[READ], sq->nr_queued[WRITE]);

        bio_associate_current(bio);
        tg->td->nr_queued[rw]++;
        throtl_add_bio_tg(bio, qn, tg);
        throttled = true;

        /*
         * Update @tg's dispatch time and force schedule dispatch if @tg
         * was empty before @bio.  The forced scheduling isn't likely to
         * cause undue delay as @bio is likely to be dispatched directly if
         * its @tg's disptime is not in the future.
         */
        if (tg->flags & THROTL_TG_WAS_EMPTY) {
                tg_update_disptime(tg);
                throtl_schedule_next_dispatch(tg->service_queue.parent_sq, true);
        }

out_unlock:
        spin_unlock_irq(q->queue_lock);
out_unlock_rcu:
        rcu_read_unlock();
out:
        /*
         * As multiple blk-throtls may stack in the same issue path, we
         * don't want bios to leave with the flag set.  Clear the flag if
         * being issued.
         */
        if (!throttled)
                bio->bi_rw &= ~REQ_THROTTLED;
        return throttled;
}

/*
 * Dispatch all bios from all children tg's queued on @parent_sq.  On
 * return, @parent_sq is guaranteed to not have any active children tg's
 * and all bios from previously active tg's are on @parent_sq->bio_lists[].
 */
static void tg_drain_bios(struct throtl_service_queue *parent_sq)
{
        struct throtl_grp *tg;

        while ((tg = throtl_rb_first(parent_sq))) {
                struct throtl_service_queue *sq = &tg->service_queue;
                struct bio *bio;

                throtl_dequeue_tg(tg);

                while ((bio = throtl_peek_queued(&sq->queued[READ])))
                        tg_dispatch_one_bio(tg, bio_data_dir(bio));
                while ((bio = throtl_peek_queued(&sq->queued[WRITE])))
                        tg_dispatch_one_bio(tg, bio_data_dir(bio));
        }
}

/**
 * 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 blkcg_gq *blkg;
        struct cgroup_subsys_state *pos_css;
        struct bio *bio;
        int rw;

        queue_lockdep_assert_held(q);
        rcu_read_lock();

        /*
         * Drain each tg while doing post-order walk on the blkg tree, so
         * that all bios are propagated to td->service_queue.  It'd be
         * better to walk service_queue tree directly but blkg walk is
         * easier.
         */
        blkg_for_each_descendant_post(blkg, pos_css, td->queue->root_blkg)
                tg_drain_bios(&blkg_to_tg(blkg)->service_queue);

        /* finally, transfer bios from top-level tg's into the td */
        tg_drain_bios(&td->service_queue);

        rcu_read_unlock();
        spin_unlock_irq(q->queue_lock);

        /* all bios now should be in td->service_queue, issue them */
        for (rw = READ; rw <= WRITE; rw++)
                while ((bio = throtl_pop_queued(&td->service_queue.queued[rw],
                                                NULL)))
                        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;

        INIT_WORK(&td->dispatch_work, blk_throtl_dispatch_work_fn);
        throtl_service_queue_init(&td->service_queue, NULL);

        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);