[linux.git] / block / bfq-cgroup.c

// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * cgroups support for the BFQ I/O scheduler.
 */
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/blkdev.h>
#include <linux/cgroup.h>
#include <linux/elevator.h>
#include <linux/ktime.h>
#include <linux/rbtree.h>
#include <linux/ioprio.h>
#include <linux/sbitmap.h>
#include <linux/delay.h>

#include "bfq-iosched.h"

#if defined(CONFIG_BFQ_GROUP_IOSCHED) &&  defined(CONFIG_DEBUG_BLK_CGROUP)

static int bfq_stat_init(struct bfq_stat *stat, gfp_t gfp)
{
        int ret;

        ret = percpu_counter_init(&stat->cpu_cnt, 0, gfp);
        if (ret)
                return ret;

        atomic64_set(&stat->aux_cnt, 0);
        return 0;
}

static void bfq_stat_exit(struct bfq_stat *stat)
{
        percpu_counter_destroy(&stat->cpu_cnt);
}

/**
 * bfq_stat_add - add a value to a bfq_stat
 * @stat: target bfq_stat
 * @val: value to add
 *
 * Add @val to @stat.  The caller must ensure that IRQ on the same CPU
 * don't re-enter this function for the same counter.
 */
static inline void bfq_stat_add(struct bfq_stat *stat, uint64_t val)
{
        percpu_counter_add_batch(&stat->cpu_cnt, val, BLKG_STAT_CPU_BATCH);
}

/**
 * bfq_stat_read - read the current value of a bfq_stat
 * @stat: bfq_stat to read
 */
static inline uint64_t bfq_stat_read(struct bfq_stat *stat)
{
        return percpu_counter_sum_positive(&stat->cpu_cnt);
}

/**
 * bfq_stat_reset - reset a bfq_stat
 * @stat: bfq_stat to reset
 */
static inline void bfq_stat_reset(struct bfq_stat *stat)
{
        percpu_counter_set(&stat->cpu_cnt, 0);
        atomic64_set(&stat->aux_cnt, 0);
}

/**
 * bfq_stat_add_aux - add a bfq_stat into another's aux count
 * @to: the destination bfq_stat
 * @from: the source
 *
 * Add @from's count including the aux one to @to's aux count.
 */
static inline void bfq_stat_add_aux(struct bfq_stat *to,
                                     struct bfq_stat *from)
{
        atomic64_add(bfq_stat_read(from) + atomic64_read(&from->aux_cnt),
                     &to->aux_cnt);
}

/**
 * bfq_stat_recursive_sum - collect hierarchical bfq_stat
 * @blkg: blkg of interest
 * @pol: blkcg_policy which contains the bfq_stat
 * @off: offset to the bfq_stat in blkg_policy_data or @blkg
 *
 * Collect the bfq_stat specified by @blkg, @pol and @off and all its
 * online descendants and their aux counts.  The caller must be holding the
 * queue lock for online tests.
 *
 * If @pol is NULL, bfq_stat is at @off bytes into @blkg; otherwise, it is
 * at @off bytes into @blkg's blkg_policy_data of the policy.
 */
static u64 bfq_stat_recursive_sum(struct blkcg_gq *blkg,
                            struct blkcg_policy *pol, int off)
{
        struct blkcg_gq *pos_blkg;
        struct cgroup_subsys_state *pos_css;
        u64 sum = 0;

        lockdep_assert_held(&blkg->q->queue_lock);

        rcu_read_lock();
        blkg_for_each_descendant_pre(pos_blkg, pos_css, blkg) {
                struct bfq_stat *stat;

                if (!pos_blkg->online)
                        continue;

                if (pol)
                        stat = (void *)blkg_to_pd(pos_blkg, pol) + off;
                else
                        stat = (void *)blkg + off;

                sum += bfq_stat_read(stat) + atomic64_read(&stat->aux_cnt);
        }
        rcu_read_unlock();

        return sum;
}

/**
 * blkg_prfill_stat - prfill callback for bfq_stat
 * @sf: seq_file to print to
 * @pd: policy private data of interest
 * @off: offset to the bfq_stat in @pd
 *
 * prfill callback for printing a bfq_stat.
 */
static u64 blkg_prfill_stat(struct seq_file *sf, struct blkg_policy_data *pd,
                int off)
{
        return __blkg_prfill_u64(sf, pd, bfq_stat_read((void *)pd + off));
}

/* bfqg stats flags */
enum bfqg_stats_flags {
        BFQG_stats_waiting = 0,
        BFQG_stats_idling,
        BFQG_stats_empty,
};

#define BFQG_FLAG_FNS(name)                                             \
static void bfqg_stats_mark_##name(struct bfqg_stats *stats)    \
{                                                                       \
        stats->flags |= (1 << BFQG_stats_##name);                       \
}                                                                       \
static void bfqg_stats_clear_##name(struct bfqg_stats *stats)   \
{                                                                       \
        stats->flags &= ~(1 << BFQG_stats_##name);                      \
}                                                                       \
static int bfqg_stats_##name(struct bfqg_stats *stats)          \
{                                                                       \
        return (stats->flags & (1 << BFQG_stats_##name)) != 0;          \
}                                                                       \

BFQG_FLAG_FNS(waiting)
BFQG_FLAG_FNS(idling)
BFQG_FLAG_FNS(empty)
#undef BFQG_FLAG_FNS

/* This should be called with the scheduler lock held. */
static void bfqg_stats_update_group_wait_time(struct bfqg_stats *stats)
{
        u64 now;

        if (!bfqg_stats_waiting(stats))
                return;

        now = ktime_get_ns();
        if (now > stats->start_group_wait_time)
                bfq_stat_add(&stats->group_wait_time,
                              now - stats->start_group_wait_time);
        bfqg_stats_clear_waiting(stats);
}

/* This should be called with the scheduler lock held. */
static void bfqg_stats_set_start_group_wait_time(struct bfq_group *bfqg,
                                                 struct bfq_group *curr_bfqg)
{
        struct bfqg_stats *stats = &bfqg->stats;

        if (bfqg_stats_waiting(stats))
                return;
        if (bfqg == curr_bfqg)
                return;
        stats->start_group_wait_time = ktime_get_ns();
        bfqg_stats_mark_waiting(stats);
}

/* This should be called with the scheduler lock held. */
static void bfqg_stats_end_empty_time(struct bfqg_stats *stats)
{
        u64 now;

        if (!bfqg_stats_empty(stats))
                return;

        now = ktime_get_ns();
        if (now > stats->start_empty_time)
                bfq_stat_add(&stats->empty_time,
                              now - stats->start_empty_time);
        bfqg_stats_clear_empty(stats);
}

void bfqg_stats_update_dequeue(struct bfq_group *bfqg)
{
        bfq_stat_add(&bfqg->stats.dequeue, 1);
}

void bfqg_stats_set_start_empty_time(struct bfq_group *bfqg)
{
        struct bfqg_stats *stats = &bfqg->stats;

        if (blkg_rwstat_total(&stats->queued))
                return;

        /*
         * group is already marked empty. This can happen if bfqq got new
         * request in parent group and moved to this group while being added
         * to service tree. Just ignore the event and move on.
         */
        if (bfqg_stats_empty(stats))
                return;

        stats->start_empty_time = ktime_get_ns();
        bfqg_stats_mark_empty(stats);
}

void bfqg_stats_update_idle_time(struct bfq_group *bfqg)
{
        struct bfqg_stats *stats = &bfqg->stats;

        if (bfqg_stats_idling(stats)) {
                u64 now = ktime_get_ns();

                if (now > stats->start_idle_time)
                        bfq_stat_add(&stats->idle_time,
                                      now - stats->start_idle_time);
                bfqg_stats_clear_idling(stats);
        }
}

void bfqg_stats_set_start_idle_time(struct bfq_group *bfqg)
{
        struct bfqg_stats *stats = &bfqg->stats;

        stats->start_idle_time = ktime_get_ns();
        bfqg_stats_mark_idling(stats);
}

void bfqg_stats_update_avg_queue_size(struct bfq_group *bfqg)
{
        struct bfqg_stats *stats = &bfqg->stats;

        bfq_stat_add(&stats->avg_queue_size_sum,
                      blkg_rwstat_total(&stats->queued));
        bfq_stat_add(&stats->avg_queue_size_samples, 1);
        bfqg_stats_update_group_wait_time(stats);
}

void bfqg_stats_update_io_add(struct bfq_group *bfqg, struct bfq_queue *bfqq,
                              unsigned int op)
{
        blkg_rwstat_add(&bfqg->stats.queued, op, 1);
        bfqg_stats_end_empty_time(&bfqg->stats);
        if (!(bfqq == ((struct bfq_data *)bfqg->bfqd)->in_service_queue))
                bfqg_stats_set_start_group_wait_time(bfqg, bfqq_group(bfqq));
}

void bfqg_stats_update_io_remove(struct bfq_group *bfqg, unsigned int op)
{
        blkg_rwstat_add(&bfqg->stats.queued, op, -1);
}

void bfqg_stats_update_io_merged(struct bfq_group *bfqg, unsigned int op)
{
        blkg_rwstat_add(&bfqg->stats.merged, op, 1);
}

void bfqg_stats_update_completion(struct bfq_group *bfqg, u64 start_time_ns,
                                  u64 io_start_time_ns, unsigned int op)
{
        struct bfqg_stats *stats = &bfqg->stats;
        u64 now = ktime_get_ns();

        if (now > io_start_time_ns)
                blkg_rwstat_add(&stats->service_time, op,
                                now - io_start_time_ns);
        if (io_start_time_ns > start_time_ns)
                blkg_rwstat_add(&stats->wait_time, op,
                                io_start_time_ns - start_time_ns);
}

#else /* CONFIG_BFQ_GROUP_IOSCHED && CONFIG_DEBUG_BLK_CGROUP */

void bfqg_stats_update_io_add(struct bfq_group *bfqg, struct bfq_queue *bfqq,
                              unsigned int op) { }
void bfqg_stats_update_io_remove(struct bfq_group *bfqg, unsigned int op) { }
void bfqg_stats_update_io_merged(struct bfq_group *bfqg, unsigned int op) { }
void bfqg_stats_update_completion(struct bfq_group *bfqg, u64 start_time_ns,
                                  u64 io_start_time_ns, unsigned int op) { }
void bfqg_stats_update_dequeue(struct bfq_group *bfqg) { }
void bfqg_stats_set_start_empty_time(struct bfq_group *bfqg) { }
void bfqg_stats_update_idle_time(struct bfq_group *bfqg) { }
void bfqg_stats_set_start_idle_time(struct bfq_group *bfqg) { }
void bfqg_stats_update_avg_queue_size(struct bfq_group *bfqg) { }

#endif /* CONFIG_BFQ_GROUP_IOSCHED && CONFIG_DEBUG_BLK_CGROUP */

#ifdef CONFIG_BFQ_GROUP_IOSCHED

/*
 * blk-cgroup policy-related handlers
 * The following functions help in converting between blk-cgroup
 * internal structures and BFQ-specific structures.
 */

static struct bfq_group *pd_to_bfqg(struct blkg_policy_data *pd)
{
        return pd ? container_of(pd, struct bfq_group, pd) : NULL;
}

struct blkcg_gq *bfqg_to_blkg(struct bfq_group *bfqg)
{
        return pd_to_blkg(&bfqg->pd);
}

static struct bfq_group *blkg_to_bfqg(struct blkcg_gq *blkg)
{
        return pd_to_bfqg(blkg_to_pd(blkg, &blkcg_policy_bfq));
}

/*
 * bfq_group handlers
 * The following functions help in navigating the bfq_group hierarchy
 * by allowing to find the parent of a bfq_group or the bfq_group
 * associated to a bfq_queue.
 */

static struct bfq_group *bfqg_parent(struct bfq_group *bfqg)
{
        struct blkcg_gq *pblkg = bfqg_to_blkg(bfqg)->parent;

        return pblkg ? blkg_to_bfqg(pblkg) : NULL;
}

struct bfq_group *bfqq_group(struct bfq_queue *bfqq)
{
        struct bfq_entity *group_entity = bfqq->entity.parent;

        return group_entity ? container_of(group_entity, struct bfq_group,
                                           entity) :
                              bfqq->bfqd->root_group;
}

/*
 * The following two functions handle get and put of a bfq_group by
 * wrapping the related blk-cgroup hooks.
 */

static void bfqg_get(struct bfq_group *bfqg)
{
        bfqg->ref++;
}

static void bfqg_put(struct bfq_group *bfqg)
{
        bfqg->ref--;

        if (bfqg->ref == 0)
                kfree(bfqg);
}

static void bfqg_and_blkg_get(struct bfq_group *bfqg)
{
        /* see comments in bfq_bic_update_cgroup for why refcounting bfqg */
        bfqg_get(bfqg);

        blkg_get(bfqg_to_blkg(bfqg));
}

void bfqg_and_blkg_put(struct bfq_group *bfqg)
{
        blkg_put(bfqg_to_blkg(bfqg));

        bfqg_put(bfqg);
}

/* @stats = 0 */
static void bfqg_stats_reset(struct bfqg_stats *stats)
{
#ifdef CONFIG_DEBUG_BLK_CGROUP
        /* queued stats shouldn't be cleared */
        blkg_rwstat_reset(&stats->merged);
        blkg_rwstat_reset(&stats->service_time);
        blkg_rwstat_reset(&stats->wait_time);
        bfq_stat_reset(&stats->time);
        bfq_stat_reset(&stats->avg_queue_size_sum);
        bfq_stat_reset(&stats->avg_queue_size_samples);
        bfq_stat_reset(&stats->dequeue);
        bfq_stat_reset(&stats->group_wait_time);
        bfq_stat_reset(&stats->idle_time);
        bfq_stat_reset(&stats->empty_time);
#endif
}

/* @to += @from */
static void bfqg_stats_add_aux(struct bfqg_stats *to, struct bfqg_stats *from)
{
        if (!to || !from)
                return;

#ifdef CONFIG_DEBUG_BLK_CGROUP
        /* queued stats shouldn't be cleared */
        blkg_rwstat_add_aux(&to->merged, &from->merged);
        blkg_rwstat_add_aux(&to->service_time, &from->service_time);
        blkg_rwstat_add_aux(&to->wait_time, &from->wait_time);
        bfq_stat_add_aux(&from->time, &from->time);
        bfq_stat_add_aux(&to->avg_queue_size_sum, &from->avg_queue_size_sum);
        bfq_stat_add_aux(&to->avg_queue_size_samples,
                          &from->avg_queue_size_samples);
        bfq_stat_add_aux(&to->dequeue, &from->dequeue);
        bfq_stat_add_aux(&to->group_wait_time, &from->group_wait_time);
        bfq_stat_add_aux(&to->idle_time, &from->idle_time);
        bfq_stat_add_aux(&to->empty_time, &from->empty_time);
#endif
}

/*
 * Transfer @bfqg's stats to its parent's aux counts so that the ancestors'
 * recursive stats can still account for the amount used by this bfqg after
 * it's gone.
 */
static void bfqg_stats_xfer_dead(struct bfq_group *bfqg)
{
        struct bfq_group *parent;

        if (!bfqg) /* root_group */
                return;

        parent = bfqg_parent(bfqg);

        lockdep_assert_held(&bfqg_to_blkg(bfqg)->q->queue_lock);

        if (unlikely(!parent))
                return;

        bfqg_stats_add_aux(&parent->stats, &bfqg->stats);
        bfqg_stats_reset(&bfqg->stats);
}

void bfq_init_entity(struct bfq_entity *entity, struct bfq_group *bfqg)
{
        struct bfq_queue *bfqq = bfq_entity_to_bfqq(entity);

        entity->weight = entity->new_weight;
        entity->orig_weight = entity->new_weight;
        if (bfqq) {
                bfqq->ioprio = bfqq->new_ioprio;
                bfqq->ioprio_class = bfqq->new_ioprio_class;
                /*
                 * Make sure that bfqg and its associated blkg do not
                 * disappear before entity.
                 */
                bfqg_and_blkg_get(bfqg);
        }
        entity->parent = bfqg->my_entity; /* NULL for root group */
        entity->sched_data = &bfqg->sched_data;
}

static void bfqg_stats_exit(struct bfqg_stats *stats)
{
#ifdef CONFIG_DEBUG_BLK_CGROUP
        blkg_rwstat_exit(&stats->merged);
        blkg_rwstat_exit(&stats->service_time);
        blkg_rwstat_exit(&stats->wait_time);
        blkg_rwstat_exit(&stats->queued);
        bfq_stat_exit(&stats->time);
        bfq_stat_exit(&stats->avg_queue_size_sum);
        bfq_stat_exit(&stats->avg_queue_size_samples);
        bfq_stat_exit(&stats->dequeue);
        bfq_stat_exit(&stats->group_wait_time);
        bfq_stat_exit(&stats->idle_time);
        bfq_stat_exit(&stats->empty_time);
#endif
}

static int bfqg_stats_init(struct bfqg_stats *stats, gfp_t gfp)
{
#ifdef CONFIG_DEBUG_BLK_CGROUP
        if (blkg_rwstat_init(&stats->merged, gfp) ||
            blkg_rwstat_init(&stats->service_time, gfp) ||
            blkg_rwstat_init(&stats->wait_time, gfp) ||
            blkg_rwstat_init(&stats->queued, gfp) ||
            bfq_stat_init(&stats->time, gfp) ||
            bfq_stat_init(&stats->avg_queue_size_sum, gfp) ||
            bfq_stat_init(&stats->avg_queue_size_samples, gfp) ||
            bfq_stat_init(&stats->dequeue, gfp) ||
            bfq_stat_init(&stats->group_wait_time, gfp) ||
            bfq_stat_init(&stats->idle_time, gfp) ||
            bfq_stat_init(&stats->empty_time, gfp)) {
                bfqg_stats_exit(stats);
                return -ENOMEM;
        }
#endif

        return 0;
}

static struct bfq_group_data *cpd_to_bfqgd(struct blkcg_policy_data *cpd)
{
        return cpd ? container_of(cpd, struct bfq_group_data, pd) : NULL;
}

static struct bfq_group_data *blkcg_to_bfqgd(struct blkcg *blkcg)
{
        return cpd_to_bfqgd(blkcg_to_cpd(blkcg, &blkcg_policy_bfq));
}

static struct blkcg_policy_data *bfq_cpd_alloc(gfp_t gfp)
{
        struct bfq_group_data *bgd;

        bgd = kzalloc(sizeof(*bgd), gfp);
        if (!bgd)
                return NULL;
        return &bgd->pd;
}

static void bfq_cpd_init(struct blkcg_policy_data *cpd)
{
        struct bfq_group_data *d = cpd_to_bfqgd(cpd);

        d->weight = cgroup_subsys_on_dfl(io_cgrp_subsys) ?
                CGROUP_WEIGHT_DFL : BFQ_WEIGHT_LEGACY_DFL;
}

static void bfq_cpd_free(struct blkcg_policy_data *cpd)
{
        kfree(cpd_to_bfqgd(cpd));
}

static struct blkg_policy_data *bfq_pd_alloc(gfp_t gfp, int node)
{
        struct bfq_group *bfqg;

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

        if (bfqg_stats_init(&bfqg->stats, gfp)) {
                kfree(bfqg);
                return NULL;
        }

        /* see comments in bfq_bic_update_cgroup for why refcounting */
        bfqg_get(bfqg);
        return &bfqg->pd;
}

static void bfq_pd_init(struct blkg_policy_data *pd)
{
        struct blkcg_gq *blkg = pd_to_blkg(pd);
        struct bfq_group *bfqg = blkg_to_bfqg(blkg);
        struct bfq_data *bfqd = blkg->q->elevator->elevator_data;
        struct bfq_entity *entity = &bfqg->entity;
        struct bfq_group_data *d = blkcg_to_bfqgd(blkg->blkcg);

        entity->orig_weight = entity->weight = entity->new_weight = d->weight;
        entity->my_sched_data = &bfqg->sched_data;
        bfqg->my_entity = entity; /*
                                   * the root_group's will be set to NULL
                                   * in bfq_init_queue()
                                   */
        bfqg->bfqd = bfqd;
        bfqg->active_entities = 0;
        bfqg->rq_pos_tree = RB_ROOT;
}

static void bfq_pd_free(struct blkg_policy_data *pd)
{
        struct bfq_group *bfqg = pd_to_bfqg(pd);

        bfqg_stats_exit(&bfqg->stats);
        bfqg_put(bfqg);
}

static void bfq_pd_reset_stats(struct blkg_policy_data *pd)
{
        struct bfq_group *bfqg = pd_to_bfqg(pd);

        bfqg_stats_reset(&bfqg->stats);
}

static void bfq_group_set_parent(struct bfq_group *bfqg,
                                        struct bfq_group *parent)
{
        struct bfq_entity *entity;

        entity = &bfqg->entity;
        entity->parent = parent->my_entity;
        entity->sched_data = &parent->sched_data;
}

static struct bfq_group *bfq_lookup_bfqg(struct bfq_data *bfqd,
                                         struct blkcg *blkcg)
{
        struct blkcg_gq *blkg;

        blkg = blkg_lookup(blkcg, bfqd->queue);
        if (likely(blkg))
                return blkg_to_bfqg(blkg);
        return NULL;
}

struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd,
                                     struct blkcg *blkcg)
{
        struct bfq_group *bfqg, *parent;
        struct bfq_entity *entity;

        bfqg = bfq_lookup_bfqg(bfqd, blkcg);

        if (unlikely(!bfqg))
                return NULL;

        /*
         * Update chain of bfq_groups as we might be handling a leaf group
         * which, along with some of its relatives, has not been hooked yet
         * to the private hierarchy of BFQ.
         */
        entity = &bfqg->entity;
        for_each_entity(entity) {
                bfqg = container_of(entity, struct bfq_group, entity);
                if (bfqg != bfqd->root_group) {
                        parent = bfqg_parent(bfqg);
                        if (!parent)
                                parent = bfqd->root_group;
                        bfq_group_set_parent(bfqg, parent);
                }
        }

        return bfqg;
}

/**
 * bfq_bfqq_move - migrate @bfqq to @bfqg.
 * @bfqd: queue descriptor.
 * @bfqq: the queue to move.
 * @bfqg: the group to move to.
 *
 * Move @bfqq to @bfqg, deactivating it from its old group and reactivating
 * it on the new one.  Avoid putting the entity on the old group idle tree.
 *
 * Must be called under the scheduler lock, to make sure that the blkg
 * owning @bfqg does not disappear (see comments in
 * bfq_bic_update_cgroup on guaranteeing the consistency of blkg
 * objects).
 */
void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
                   struct bfq_group *bfqg)
{
        struct bfq_entity *entity = &bfqq->entity;

        /* If bfqq is empty, then bfq_bfqq_expire also invokes
         * bfq_del_bfqq_busy, thereby removing bfqq and its entity
         * from data structures related to current group. Otherwise we
         * need to remove bfqq explicitly with bfq_deactivate_bfqq, as
         * we do below.
         */
        if (bfqq == bfqd->in_service_queue)
                bfq_bfqq_expire(bfqd, bfqd->in_service_queue,
                                false, BFQQE_PREEMPTED);

        if (bfq_bfqq_busy(bfqq))
                bfq_deactivate_bfqq(bfqd, bfqq, false, false);
        else if (entity->on_st)
                bfq_put_idle_entity(bfq_entity_service_tree(entity), entity);
        bfqg_and_blkg_put(bfqq_group(bfqq));

        entity->parent = bfqg->my_entity;
        entity->sched_data = &bfqg->sched_data;
        /* pin down bfqg and its associated blkg  */
        bfqg_and_blkg_get(bfqg);

        if (bfq_bfqq_busy(bfqq)) {
                if (unlikely(!bfqd->nonrot_with_queueing))
                        bfq_pos_tree_add_move(bfqd, bfqq);
                bfq_activate_bfqq(bfqd, bfqq);
        }

        if (!bfqd->in_service_queue && !bfqd->rq_in_driver)
                bfq_schedule_dispatch(bfqd);
}

/**
 * __bfq_bic_change_cgroup - move @bic to @cgroup.
 * @bfqd: the queue descriptor.
 * @bic: the bic to move.
 * @blkcg: the blk-cgroup to move to.
 *
 * Move bic to blkcg, assuming that bfqd->lock is held; which makes
 * sure that the reference to cgroup is valid across the call (see
 * comments in bfq_bic_update_cgroup on this issue)
 *
 * NOTE: an alternative approach might have been to store the current
 * cgroup in bfqq and getting a reference to it, reducing the lookup
 * time here, at the price of slightly more complex code.
 */
static struct bfq_group *__bfq_bic_change_cgroup(struct bfq_data *bfqd,
                                                struct bfq_io_cq *bic,
                                                struct blkcg *blkcg)
{
        struct bfq_queue *async_bfqq = bic_to_bfqq(bic, 0);
        struct bfq_queue *sync_bfqq = bic_to_bfqq(bic, 1);
        struct bfq_group *bfqg;
        struct bfq_entity *entity;

        bfqg = bfq_find_set_group(bfqd, blkcg);

        if (unlikely(!bfqg))
                bfqg = bfqd->root_group;

        if (async_bfqq) {
                entity = &async_bfqq->entity;

                if (entity->sched_data != &bfqg->sched_data) {
                        bic_set_bfqq(bic, NULL, 0);
                        bfq_log_bfqq(bfqd, async_bfqq,
                                     "bic_change_group: %p %d",
                                     async_bfqq, async_bfqq->ref);
                        bfq_put_queue(async_bfqq);
                }
        }

        if (sync_bfqq) {
                entity = &sync_bfqq->entity;
                if (entity->sched_data != &bfqg->sched_data)
                        bfq_bfqq_move(bfqd, sync_bfqq, bfqg);
        }

        return bfqg;
}

void bfq_bic_update_cgroup(struct bfq_io_cq *bic, struct bio *bio)
{
        struct bfq_data *bfqd = bic_to_bfqd(bic);
        struct bfq_group *bfqg = NULL;
        uint64_t serial_nr;

        rcu_read_lock();
        serial_nr = __bio_blkcg(bio)->css.serial_nr;

        /*
         * Check whether blkcg has changed.  The condition may trigger
         * spuriously on a newly created cic but there's no harm.
         */
        if (unlikely(!bfqd) || likely(bic->blkcg_serial_nr == serial_nr))
                goto out;

        bfqg = __bfq_bic_change_cgroup(bfqd, bic, __bio_blkcg(bio));
        /*
         * Update blkg_path for bfq_log_* functions. We cache this
         * path, and update it here, for the following
         * reasons. Operations on blkg objects in blk-cgroup are
         * protected with the request_queue lock, and not with the
         * lock that protects the instances of this scheduler
         * (bfqd->lock). This exposes BFQ to the following sort of
         * race.
         *
         * The blkg_lookup performed in bfq_get_queue, protected
         * through rcu, may happen to return the address of a copy of
         * the original blkg. If this is the case, then the
         * bfqg_and_blkg_get performed in bfq_get_queue, to pin down
         * the blkg, is useless: it does not prevent blk-cgroup code
         * from destroying both the original blkg and all objects
         * directly or indirectly referred by the copy of the
         * blkg.
         *
         * On the bright side, destroy operations on a blkg invoke, as
         * a first step, hooks of the scheduler associated with the
         * blkg. And these hooks are executed with bfqd->lock held for
         * BFQ. As a consequence, for any blkg associated with the
         * request queue this instance of the scheduler is attached
         * to, we are guaranteed that such a blkg is not destroyed, and
         * that all the pointers it contains are consistent, while we
         * are holding bfqd->lock. A blkg_lookup performed with
         * bfqd->lock held then returns a fully consistent blkg, which
         * remains consistent until this lock is held.
         *
         * Thanks to the last fact, and to the fact that: (1) bfqg has
         * been obtained through a blkg_lookup in the above
         * assignment, and (2) bfqd->lock is being held, here we can
         * safely use the policy data for the involved blkg (i.e., the
         * field bfqg->pd) to get to the blkg associated with bfqg,
         * and then we can safely use any field of blkg. After we
         * release bfqd->lock, even just getting blkg through this
         * bfqg may cause dangling references to be traversed, as
         * bfqg->pd may not exist any more.
         *
         * In view of the above facts, here we cache, in the bfqg, any
         * blkg data we may need for this bic, and for its associated
         * bfq_queue. As of now, we need to cache only the path of the
         * blkg, which is used in the bfq_log_* functions.
         *
         * Finally, note that bfqg itself needs to be protected from
         * destruction on the blkg_free of the original blkg (which
         * invokes bfq_pd_free). We use an additional private
         * refcounter for bfqg, to let it disappear only after no
         * bfq_queue refers to it any longer.
         */
        blkg_path(bfqg_to_blkg(bfqg), bfqg->blkg_path, sizeof(bfqg->blkg_path));
        bic->blkcg_serial_nr = serial_nr;
out:
        rcu_read_unlock();
}

/**
 * bfq_flush_idle_tree - deactivate any entity on the idle tree of @st.
 * @st: the service tree being flushed.
 */
static void bfq_flush_idle_tree(struct bfq_service_tree *st)
{
        struct bfq_entity *entity = st->first_idle;

        for (; entity ; entity = st->first_idle)
                __bfq_deactivate_entity(entity, false);
}

/**
 * bfq_reparent_leaf_entity - move leaf entity to the root_group.
 * @bfqd: the device data structure with the root group.
 * @entity: the entity to move.
 */
static void bfq_reparent_leaf_entity(struct bfq_data *bfqd,
                                     struct bfq_entity *entity)
{
        struct bfq_queue *bfqq = bfq_entity_to_bfqq(entity);

        bfq_bfqq_move(bfqd, bfqq, bfqd->root_group);
}

/**
 * bfq_reparent_active_entities - move to the root group all active
 *                                entities.
 * @bfqd: the device data structure with the root group.
 * @bfqg: the group to move from.
 * @st: the service tree with the entities.
 */
static void bfq_reparent_active_entities(struct bfq_data *bfqd,
                                         struct bfq_group *bfqg,
                                         struct bfq_service_tree *st)
{
        struct rb_root *active = &st->active;
        struct bfq_entity *entity = NULL;

        if (!RB_EMPTY_ROOT(&st->active))
                entity = bfq_entity_of(rb_first(active));

        for (; entity ; entity = bfq_entity_of(rb_first(active)))
                bfq_reparent_leaf_entity(bfqd, entity);

        if (bfqg->sched_data.in_service_entity)
                bfq_reparent_leaf_entity(bfqd,
                        bfqg->sched_data.in_service_entity);
}

/**
 * bfq_pd_offline - deactivate the entity associated with @pd,
 *                  and reparent its children entities.
 * @pd: descriptor of the policy going offline.
 *
 * blkio already grabs the queue_lock for us, so no need to use
 * RCU-based magic
 */
static void bfq_pd_offline(struct blkg_policy_data *pd)
{
        struct bfq_service_tree *st;
        struct bfq_group *bfqg = pd_to_bfqg(pd);
        struct bfq_data *bfqd = bfqg->bfqd;
        struct bfq_entity *entity = bfqg->my_entity;
        unsigned long flags;
        int i;

        spin_lock_irqsave(&bfqd->lock, flags);

        if (!entity) /* root group */
                goto put_async_queues;

        /*
         * Empty all service_trees belonging to this group before
         * deactivating the group itself.
         */
        for (i = 0; i < BFQ_IOPRIO_CLASSES; i++) {
                st = bfqg->sched_data.service_tree + i;

                /*
                 * The idle tree may still contain bfq_queues belonging
                 * to exited task because they never migrated to a different
                 * cgroup from the one being destroyed now.
                 */
                bfq_flush_idle_tree(st);

                /*
                 * It may happen that some queues are still active
                 * (busy) upon group destruction (if the corresponding
                 * processes have been forced to terminate). We move
                 * all the leaf entities corresponding to these queues
                 * to the root_group.
                 * Also, it may happen that the group has an entity
                 * in service, which is disconnected from the active
                 * tree: it must be moved, too.
                 * There is no need to put the sync queues, as the
                 * scheduler has taken no reference.
                 */
                bfq_reparent_active_entities(bfqd, bfqg, st);
        }

        __bfq_deactivate_entity(entity, false);

put_async_queues:
        bfq_put_async_queues(bfqd, bfqg);

        spin_unlock_irqrestore(&bfqd->lock, flags);
        /*
         * @blkg is going offline and will be ignored by
         * blkg_[rw]stat_recursive_sum().  Transfer stats to the parent so
         * that they don't get lost.  If IOs complete after this point, the
         * stats for them will be lost.  Oh well...
         */
        bfqg_stats_xfer_dead(bfqg);
}

void bfq_end_wr_async(struct bfq_data *bfqd)
{
        struct blkcg_gq *blkg;

        list_for_each_entry(blkg, &bfqd->queue->blkg_list, q_node) {
                struct bfq_group *bfqg = blkg_to_bfqg(blkg);

                bfq_end_wr_async_queues(bfqd, bfqg);
        }
        bfq_end_wr_async_queues(bfqd, bfqd->root_group);
}

static int bfq_io_show_weight(struct seq_file *sf, void *v)
{
        struct blkcg *blkcg = css_to_blkcg(seq_css(sf));
        struct bfq_group_data *bfqgd = blkcg_to_bfqgd(blkcg);
        unsigned int val = 0;

        if (bfqgd)
                val = bfqgd->weight;

        seq_printf(sf, "%u\n", val);

        return 0;
}

static int bfq_io_set_weight_legacy(struct cgroup_subsys_state *css,
                                    struct cftype *cftype,
                                    u64 val)
{
        struct blkcg *blkcg = css_to_blkcg(css);
        struct bfq_group_data *bfqgd = blkcg_to_bfqgd(blkcg);
        struct blkcg_gq *blkg;
        int ret = -ERANGE;

        if (val < BFQ_MIN_WEIGHT || val > BFQ_MAX_WEIGHT)
                return ret;

        ret = 0;
        spin_lock_irq(&blkcg->lock);
        bfqgd->weight = (unsigned short)val;
        hlist_for_each_entry(blkg, &blkcg->blkg_list, blkcg_node) {
                struct bfq_group *bfqg = blkg_to_bfqg(blkg);

                if (!bfqg)
                        continue;
                /*
                 * Setting the prio_changed flag of the entity
                 * to 1 with new_weight == weight would re-set
                 * the value of the weight to its ioprio mapping.
                 * Set the flag only if necessary.
                 */
                if ((unsigned short)val != bfqg->entity.new_weight) {
                        bfqg->entity.new_weight = (unsigned short)val;
                        /*
                         * Make sure that the above new value has been
                         * stored in bfqg->entity.new_weight before
                         * setting the prio_changed flag. In fact,
                         * this flag may be read asynchronously (in
                         * critical sections protected by a different
                         * lock than that held here), and finding this
                         * flag set may cause the execution of the code
                         * for updating parameters whose value may
                         * depend also on bfqg->entity.new_weight (in
                         * __bfq_entity_update_weight_prio).
                         * This barrier makes sure that the new value
                         * of bfqg->entity.new_weight is correctly
                         * seen in that code.
                         */
                        smp_wmb();
                        bfqg->entity.prio_changed = 1;
                }
        }
        spin_unlock_irq(&blkcg->lock);

        return ret;
}

static ssize_t bfq_io_set_weight(struct kernfs_open_file *of,
                                 char *buf, size_t nbytes,
                                 loff_t off)
{
        u64 weight;
        /* First unsigned long found in the file is used */
        int ret = kstrtoull(strim(buf), 0, &weight);

        if (ret)
                return ret;

        ret = bfq_io_set_weight_legacy(of_css(of), NULL, weight);
        return ret ?: nbytes;
}

#ifdef CONFIG_DEBUG_BLK_CGROUP
static int bfqg_print_stat(struct seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)), blkg_prfill_stat,
                          &blkcg_policy_bfq, seq_cft(sf)->private, false);
        return 0;
}

static int bfqg_print_rwstat(struct seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)), blkg_prfill_rwstat,
                          &blkcg_policy_bfq, seq_cft(sf)->private, true);
        return 0;
}

static u64 bfqg_prfill_stat_recursive(struct seq_file *sf,
                                      struct blkg_policy_data *pd, int off)
{
        u64 sum = bfq_stat_recursive_sum(pd_to_blkg(pd),
                                          &blkcg_policy_bfq, off);
        return __blkg_prfill_u64(sf, pd, sum);
}

static u64 bfqg_prfill_rwstat_recursive(struct seq_file *sf,
                                        struct blkg_policy_data *pd, int off)
{
        struct blkg_rwstat_sample sum;

        blkg_rwstat_recursive_sum(pd_to_blkg(pd), &blkcg_policy_bfq, off, &sum);
        return __blkg_prfill_rwstat(sf, pd, &sum);
}

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

static int bfqg_print_rwstat_recursive(struct seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)),
                          bfqg_prfill_rwstat_recursive, &blkcg_policy_bfq,
                          seq_cft(sf)->private, true);
        return 0;
}

static u64 bfqg_prfill_sectors(struct seq_file *sf, struct blkg_policy_data *pd,
                               int off)
{
        u64 sum = blkg_rwstat_total(&pd->blkg->stat_bytes);

        return __blkg_prfill_u64(sf, pd, sum >> 9);
}

static int bfqg_print_stat_sectors(struct seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)),
                          bfqg_prfill_sectors, &blkcg_policy_bfq, 0, false);
        return 0;
}

static u64 bfqg_prfill_sectors_recursive(struct seq_file *sf,
                                         struct blkg_policy_data *pd, int off)
{
        struct blkg_rwstat_sample tmp;

        blkg_rwstat_recursive_sum(pd->blkg, NULL,
                        offsetof(struct blkcg_gq, stat_bytes), &tmp);

        return __blkg_prfill_u64(sf, pd,
                (tmp.cnt[BLKG_RWSTAT_READ] + tmp.cnt[BLKG_RWSTAT_WRITE]) >> 9);
}

static int bfqg_print_stat_sectors_recursive(struct seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)),
                          bfqg_prfill_sectors_recursive, &blkcg_policy_bfq, 0,
                          false);
        return 0;
}

static u64 bfqg_prfill_avg_queue_size(struct seq_file *sf,
                                      struct blkg_policy_data *pd, int off)
{
        struct bfq_group *bfqg = pd_to_bfqg(pd);
        u64 samples = bfq_stat_read(&bfqg->stats.avg_queue_size_samples);
        u64 v = 0;

        if (samples) {
                v = bfq_stat_read(&bfqg->stats.avg_queue_size_sum);
                v = div64_u64(v, samples);
        }
        __blkg_prfill_u64(sf, pd, v);
        return 0;
}

/* print avg_queue_size */
static int bfqg_print_avg_queue_size(struct seq_file *sf, void *v)
{
        blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)),
                          bfqg_prfill_avg_queue_size, &blkcg_policy_bfq,
                          0, false);
        return 0;
}
#endif /* CONFIG_DEBUG_BLK_CGROUP */

struct bfq_group *bfq_create_group_hierarchy(struct bfq_data *bfqd, int node)
{
        int ret;

        ret = blkcg_activate_policy(bfqd->queue, &blkcg_policy_bfq);
        if (ret)
                return NULL;

        return blkg_to_bfqg(bfqd->queue->root_blkg);
}

struct blkcg_policy blkcg_policy_bfq = {
        .dfl_cftypes            = bfq_blkg_files,
        .legacy_cftypes         = bfq_blkcg_legacy_files,

        .cpd_alloc_fn           = bfq_cpd_alloc,
        .cpd_init_fn            = bfq_cpd_init,
        .cpd_bind_fn            = bfq_cpd_init,
        .cpd_free_fn            = bfq_cpd_free,

        .pd_alloc_fn            = bfq_pd_alloc,
        .pd_init_fn             = bfq_pd_init,
        .pd_offline_fn          = bfq_pd_offline,
        .pd_free_fn             = bfq_pd_free,
        .pd_reset_stats_fn      = bfq_pd_reset_stats,
};

struct cftype bfq_blkcg_legacy_files[] = {
        {
                .name = "bfq.weight",
                .flags = CFTYPE_NOT_ON_ROOT,
                .seq_show = bfq_io_show_weight,
                .write_u64 = bfq_io_set_weight_legacy,
        },

        /* statistics, covers only the tasks in the bfqg */
        {
                .name = "bfq.io_service_bytes",
                .private = (unsigned long)&blkcg_policy_bfq,
                .seq_show = blkg_print_stat_bytes,
        },
        {
                .name = "bfq.io_serviced",
                .private = (unsigned long)&blkcg_policy_bfq,
                .seq_show = blkg_print_stat_ios,
        },
#ifdef CONFIG_DEBUG_BLK_CGROUP
        {
                .name = "bfq.time",
                .private = offsetof(struct bfq_group, stats.time),
                .seq_show = bfqg_print_stat,
        },
        {
                .name = "bfq.sectors",
                .seq_show = bfqg_print_stat_sectors,
        },
        {
                .name = "bfq.io_service_time",
                .private = offsetof(struct bfq_group, stats.service_time),
                .seq_show = bfqg_print_rwstat,
        },
        {
                .name = "bfq.io_wait_time",
                .private = offsetof(struct bfq_group, stats.wait_time),
                .seq_show = bfqg_print_rwstat,
        },
        {
                .name = "bfq.io_merged",
                .private = offsetof(struct bfq_group, stats.merged),
                .seq_show = bfqg_print_rwstat,
        },
        {
                .name = "bfq.io_queued",
                .private = offsetof(struct bfq_group, stats.queued),
                .seq_show = bfqg_print_rwstat,
        },
#endif /* CONFIG_DEBUG_BLK_CGROUP */

        /* the same statistics which cover the bfqg and its descendants */
        {
                .name = "bfq.io_service_bytes_recursive",
                .private = (unsigned long)&blkcg_policy_bfq,
                .seq_show = blkg_print_stat_bytes_recursive,
        },
        {
                .name = "bfq.io_serviced_recursive",
                .private = (unsigned long)&blkcg_policy_bfq,
                .seq_show = blkg_print_stat_ios_recursive,
        },
#ifdef CONFIG_DEBUG_BLK_CGROUP
        {
                .name = "bfq.time_recursive",
                .private = offsetof(struct bfq_group, stats.time),
                .seq_show = bfqg_print_stat_recursive,
        },
        {
                .name = "bfq.sectors_recursive",
                .seq_show = bfqg_print_stat_sectors_recursive,
        },
        {
                .name = "bfq.io_service_time_recursive",
                .private = offsetof(struct bfq_group, stats.service_time),
                .seq_show = bfqg_print_rwstat_recursive,
        },
        {
                .name = "bfq.io_wait_time_recursive",
                .private = offsetof(struct bfq_group, stats.wait_time),
                .seq_show = bfqg_print_rwstat_recursive,
        },
        {
                .name = "bfq.io_merged_recursive",
                .private = offsetof(struct bfq_group, stats.merged),
                .seq_show = bfqg_print_rwstat_recursive,
        },
        {
                .name = "bfq.io_queued_recursive",
                .private = offsetof(struct bfq_group, stats.queued),
                .seq_show = bfqg_print_rwstat_recursive,
        },
        {
                .name = "bfq.avg_queue_size",
                .seq_show = bfqg_print_avg_queue_size,
        },
        {
                .name = "bfq.group_wait_time",
                .private = offsetof(struct bfq_group, stats.group_wait_time),
                .seq_show = bfqg_print_stat,
        },
        {
                .name = "bfq.idle_time",
                .private = offsetof(struct bfq_group, stats.idle_time),
                .seq_show = bfqg_print_stat,
        },
        {
                .name = "bfq.empty_time",
                .private = offsetof(struct bfq_group, stats.empty_time),
                .seq_show = bfqg_print_stat,
        },
        {
                .name = "bfq.dequeue",
                .private = offsetof(struct bfq_group, stats.dequeue),
                .seq_show = bfqg_print_stat,
        },
#endif  /* CONFIG_DEBUG_BLK_CGROUP */
        { }     /* terminate */
};

struct cftype bfq_blkg_files[] = {
        {
                .name = "bfq.weight",
                .flags = CFTYPE_NOT_ON_ROOT,
                .seq_show = bfq_io_show_weight,
                .write = bfq_io_set_weight,
        },
        {} /* terminate */
};

#else   /* CONFIG_BFQ_GROUP_IOSCHED */

void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
                   struct bfq_group *bfqg) {}

void bfq_init_entity(struct bfq_entity *entity, struct bfq_group *bfqg)
{
        struct bfq_queue *bfqq = bfq_entity_to_bfqq(entity);

        entity->weight = entity->new_weight;
        entity->orig_weight = entity->new_weight;
        if (bfqq) {
                bfqq->ioprio = bfqq->new_ioprio;
                bfqq->ioprio_class = bfqq->new_ioprio_class;
        }
        entity->sched_data = &bfqg->sched_data;
}

void bfq_bic_update_cgroup(struct bfq_io_cq *bic, struct bio *bio) {}

void bfq_end_wr_async(struct bfq_data *bfqd)
{
        bfq_end_wr_async_queues(bfqd, bfqd->root_group);
}

struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd, struct blkcg *blkcg)
{
        return bfqd->root_group;
}

struct bfq_group *bfqq_group(struct bfq_queue *bfqq)
{
        return bfqq->bfqd->root_group;
}

struct bfq_group *bfq_create_group_hierarchy(struct bfq_data *bfqd, int node)
{
        struct bfq_group *bfqg;
        int i;

        bfqg = kmalloc_node(sizeof(*bfqg), GFP_KERNEL | __GFP_ZERO, node);
        if (!bfqg)
                return NULL;

        for (i = 0; i < BFQ_IOPRIO_CLASSES; i++)
                bfqg->sched_data.service_tree[i] = BFQ_SERVICE_TREE_INIT;

        return bfqg;
}
#endif  /* CONFIG_BFQ_GROUP_IOSCHED */