Linux 4.19-rc7

[linux-2.6/btrfs-unstable.git] / block / blk-iolatency.c

/*
 * Block rq-qos base io controller
 *
 * This works similar to wbt with a few exceptions
 *
 * - It's bio based, so the latency covers the whole block layer in addition to
 *   the actual io.
 * - We will throttle all IO that comes in here if we need to.
 * - We use the mean latency over the 100ms window.  This is because writes can
 *   be particularly fast, which could give us a false sense of the impact of
 *   other workloads on our protected workload.
 * - By default there's no throttling, we set the queue_depth to UINT_MAX so
 *   that we can have as many outstanding bio's as we're allowed to.  Only at
 *   throttle time do we pay attention to the actual queue depth.
 *
 * The hierarchy works like the cpu controller does, we track the latency at
 * every configured node, and each configured node has it's own independent
 * queue depth.  This means that we only care about our latency targets at the
 * peer level.  Some group at the bottom of the hierarchy isn't going to affect
 * a group at the end of some other path if we're only configred at leaf level.
 *
 * Consider the following
 *
 *                   root blkg
 *             /                     \
 *        fast (target=5ms)     slow (target=10ms)
 *         /     \                  /        \
 *       a        b          normal(15ms)   unloved
 *
 * "a" and "b" have no target, but their combined io under "fast" cannot exceed
 * an average latency of 5ms.  If it does then we will throttle the "slow"
 * group.  In the case of "normal", if it exceeds its 15ms target, we will
 * throttle "unloved", but nobody else.
 *
 * In this example "fast", "slow", and "normal" will be the only groups actually
 * accounting their io latencies.  We have to walk up the heirarchy to the root
 * on every submit and complete so we can do the appropriate stat recording and
 * adjust the queue depth of ourselves if needed.
 *
 * There are 2 ways we throttle IO.
 *
 * 1) Queue depth throttling.  As we throttle down we will adjust the maximum
 * number of IO's we're allowed to have in flight.  This starts at (u64)-1 down
 * to 1.  If the group is only ever submitting IO for itself then this is the
 * only way we throttle.
 *
 * 2) Induced delay throttling.  This is for the case that a group is generating
 * IO that has to be issued by the root cg to avoid priority inversion. So think
 * REQ_META or REQ_SWAP.  If we are already at qd == 1 and we're getting a lot
 * of work done for us on behalf of the root cg and are being asked to scale
 * down more then we induce a latency at userspace return.  We accumulate the
 * total amount of time we need to be punished by doing
 *
 * total_time += min_lat_nsec - actual_io_completion
 *
 * and then at throttle time will do
 *
 * throttle_time = min(total_time, NSEC_PER_SEC)
 *
 * This induced delay will throttle back the activity that is generating the
 * root cg issued io's, wethere that's some metadata intensive operation or the
 * group is using so much memory that it is pushing us into swap.
 *
 * Copyright (C) 2018 Josef Bacik
 */
#include <linux/kernel.h>
#include <linux/blk_types.h>
#include <linux/backing-dev.h>
#include <linux/module.h>
#include <linux/timer.h>
#include <linux/memcontrol.h>
#include <linux/sched/loadavg.h>
#include <linux/sched/signal.h>
#include <trace/events/block.h>
#include "blk-rq-qos.h"
#include "blk-stat.h"

#define DEFAULT_SCALE_COOKIE 1000000U

static struct blkcg_policy blkcg_policy_iolatency;
struct iolatency_grp;

struct blk_iolatency {
        struct rq_qos rqos;
        struct timer_list timer;
        atomic_t enabled;
};

static inline struct blk_iolatency *BLKIOLATENCY(struct rq_qos *rqos)
{
        return container_of(rqos, struct blk_iolatency, rqos);
}

static inline bool blk_iolatency_enabled(struct blk_iolatency *blkiolat)
{
        return atomic_read(&blkiolat->enabled) > 0;
}

struct child_latency_info {
        spinlock_t lock;

        /* Last time we adjusted the scale of everybody. */
        u64 last_scale_event;

        /* The latency that we missed. */
        u64 scale_lat;

        /* Total io's from all of our children for the last summation. */
        u64 nr_samples;

        /* The guy who actually changed the latency numbers. */
        struct iolatency_grp *scale_grp;

        /* Cookie to tell if we need to scale up or down. */
        atomic_t scale_cookie;
};

struct iolatency_grp {
        struct blkg_policy_data pd;
        struct blk_rq_stat __percpu *stats;
        struct blk_iolatency *blkiolat;
        struct rq_depth rq_depth;
        struct rq_wait rq_wait;
        atomic64_t window_start;
        atomic_t scale_cookie;
        u64 min_lat_nsec;
        u64 cur_win_nsec;

        /* total running average of our io latency. */
        u64 lat_avg;

        /* Our current number of IO's for the last summation. */
        u64 nr_samples;

        struct child_latency_info child_lat;
};

#define BLKIOLATENCY_MIN_WIN_SIZE (100 * NSEC_PER_MSEC)
#define BLKIOLATENCY_MAX_WIN_SIZE NSEC_PER_SEC
/*
 * These are the constants used to fake the fixed-point moving average
 * calculation just like load average.  The call to CALC_LOAD folds
 * (FIXED_1 (2048) - exp_factor) * new_sample into lat_avg.  The sampling
 * window size is bucketed to try to approximately calculate average
 * latency such that 1/exp (decay rate) is [1 min, 2.5 min) when windows
 * elapse immediately.  Note, windows only elapse with IO activity.  Idle
 * periods extend the most recent window.
 */
#define BLKIOLATENCY_NR_EXP_FACTORS 5
#define BLKIOLATENCY_EXP_BUCKET_SIZE (BLKIOLATENCY_MAX_WIN_SIZE / \
                                      (BLKIOLATENCY_NR_EXP_FACTORS - 1))
static const u64 iolatency_exp_factors[BLKIOLATENCY_NR_EXP_FACTORS] = {
        2045, // exp(1/600) - 600 samples
        2039, // exp(1/240) - 240 samples
        2031, // exp(1/120) - 120 samples
        2023, // exp(1/80)  - 80 samples
        2014, // exp(1/60)  - 60 samples
};

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

static inline struct iolatency_grp *blkg_to_lat(struct blkcg_gq *blkg)
{
        return pd_to_lat(blkg_to_pd(blkg, &blkcg_policy_iolatency));
}

static inline struct blkcg_gq *lat_to_blkg(struct iolatency_grp *iolat)
{
        return pd_to_blkg(&iolat->pd);
}

static inline bool iolatency_may_queue(struct iolatency_grp *iolat,
                                       wait_queue_entry_t *wait,
                                       bool first_block)
{
        struct rq_wait *rqw = &iolat->rq_wait;

        if (first_block && waitqueue_active(&rqw->wait) &&
            rqw->wait.head.next != &wait->entry)
                return false;
        return rq_wait_inc_below(rqw, iolat->rq_depth.max_depth);
}

static void __blkcg_iolatency_throttle(struct rq_qos *rqos,
                                       struct iolatency_grp *iolat,
                                       spinlock_t *lock, bool issue_as_root,
                                       bool use_memdelay)
        __releases(lock)
        __acquires(lock)
{
        struct rq_wait *rqw = &iolat->rq_wait;
        unsigned use_delay = atomic_read(&lat_to_blkg(iolat)->use_delay);
        DEFINE_WAIT(wait);
        bool first_block = true;

        if (use_delay)
                blkcg_schedule_throttle(rqos->q, use_memdelay);

        /*
         * To avoid priority inversions we want to just take a slot if we are
         * issuing as root.  If we're being killed off there's no point in
         * delaying things, we may have been killed by OOM so throttling may
         * make recovery take even longer, so just let the IO's through so the
         * task can go away.
         */
        if (issue_as_root || fatal_signal_pending(current)) {
                atomic_inc(&rqw->inflight);
                return;
        }

        if (iolatency_may_queue(iolat, &wait, first_block))
                return;

        do {
                prepare_to_wait_exclusive(&rqw->wait, &wait,
                                          TASK_UNINTERRUPTIBLE);

                if (iolatency_may_queue(iolat, &wait, first_block))
                        break;
                first_block = false;

                if (lock) {
                        spin_unlock_irq(lock);
                        io_schedule();
                        spin_lock_irq(lock);
                } else {
                        io_schedule();
                }
        } while (1);

        finish_wait(&rqw->wait, &wait);
}

#define SCALE_DOWN_FACTOR 2
#define SCALE_UP_FACTOR 4

static inline unsigned long scale_amount(unsigned long qd, bool up)
{
        return max(up ? qd >> SCALE_UP_FACTOR : qd >> SCALE_DOWN_FACTOR, 1UL);
}

/*
 * We scale the qd down faster than we scale up, so we need to use this helper
 * to adjust the scale_cookie accordingly so we don't prematurely get
 * scale_cookie at DEFAULT_SCALE_COOKIE and unthrottle too much.
 *
 * Each group has their own local copy of the last scale cookie they saw, so if
 * the global scale cookie goes up or down they know which way they need to go
 * based on their last knowledge of it.
 */
static void scale_cookie_change(struct blk_iolatency *blkiolat,
                                struct child_latency_info *lat_info,
                                bool up)
{
        unsigned long qd = blk_queue_depth(blkiolat->rqos.q);
        unsigned long scale = scale_amount(qd, up);
        unsigned long old = atomic_read(&lat_info->scale_cookie);
        unsigned long max_scale = qd << 1;
        unsigned long diff = 0;

        if (old < DEFAULT_SCALE_COOKIE)
                diff = DEFAULT_SCALE_COOKIE - old;

        if (up) {
                if (scale + old > DEFAULT_SCALE_COOKIE)
                        atomic_set(&lat_info->scale_cookie,
                                   DEFAULT_SCALE_COOKIE);
                else if (diff > qd)
                        atomic_inc(&lat_info->scale_cookie);
                else
                        atomic_add(scale, &lat_info->scale_cookie);
        } else {
                /*
                 * We don't want to dig a hole so deep that it takes us hours to
                 * dig out of it.  Just enough that we don't throttle/unthrottle
                 * with jagged workloads but can still unthrottle once pressure
                 * has sufficiently dissipated.
                 */
                if (diff > qd) {
                        if (diff < max_scale)
                                atomic_dec(&lat_info->scale_cookie);
                } else {
                        atomic_sub(scale, &lat_info->scale_cookie);
                }
        }
}

/*
 * Change the queue depth of the iolatency_grp.  We add/subtract 1/16th of the
 * queue depth at a time so we don't get wild swings and hopefully dial in to
 * fairer distribution of the overall queue depth.
 */
static void scale_change(struct iolatency_grp *iolat, bool up)
{
        unsigned long qd = blk_queue_depth(iolat->blkiolat->rqos.q);
        unsigned long scale = scale_amount(qd, up);
        unsigned long old = iolat->rq_depth.max_depth;
        bool changed = false;

        if (old > qd)
                old = qd;

        if (up) {
                if (old == 1 && blkcg_unuse_delay(lat_to_blkg(iolat)))
                        return;

                if (old < qd) {
                        changed = true;
                        old += scale;
                        old = min(old, qd);
                        iolat->rq_depth.max_depth = old;
                        wake_up_all(&iolat->rq_wait.wait);
                }
        } else if (old > 1) {
                old >>= 1;
                changed = true;
                iolat->rq_depth.max_depth = max(old, 1UL);
        }
}

/* Check our parent and see if the scale cookie has changed. */
static void check_scale_change(struct iolatency_grp *iolat)
{
        struct iolatency_grp *parent;
        struct child_latency_info *lat_info;
        unsigned int cur_cookie;
        unsigned int our_cookie = atomic_read(&iolat->scale_cookie);
        u64 scale_lat;
        unsigned int old;
        int direction = 0;

        if (lat_to_blkg(iolat)->parent == NULL)
                return;

        parent = blkg_to_lat(lat_to_blkg(iolat)->parent);
        if (!parent)
                return;

        lat_info = &parent->child_lat;
        cur_cookie = atomic_read(&lat_info->scale_cookie);
        scale_lat = READ_ONCE(lat_info->scale_lat);

        if (cur_cookie < our_cookie)
                direction = -1;
        else if (cur_cookie > our_cookie)
                direction = 1;
        else
                return;

        old = atomic_cmpxchg(&iolat->scale_cookie, our_cookie, cur_cookie);

        /* Somebody beat us to the punch, just bail. */
        if (old != our_cookie)
                return;

        if (direction < 0 && iolat->min_lat_nsec) {
                u64 samples_thresh;

                if (!scale_lat || iolat->min_lat_nsec <= scale_lat)
                        return;

                /*
                 * Sometimes high priority groups are their own worst enemy, so
                 * instead of taking it out on some poor other group that did 5%
                 * or less of the IO's for the last summation just skip this
                 * scale down event.
                 */
                samples_thresh = lat_info->nr_samples * 5;
                samples_thresh = div64_u64(samples_thresh, 100);
                if (iolat->nr_samples <= samples_thresh)
                        return;
        }

        /* We're as low as we can go. */
        if (iolat->rq_depth.max_depth == 1 && direction < 0) {
                blkcg_use_delay(lat_to_blkg(iolat));
                return;
        }

        /* We're back to the default cookie, unthrottle all the things. */
        if (cur_cookie == DEFAULT_SCALE_COOKIE) {
                blkcg_clear_delay(lat_to_blkg(iolat));
                iolat->rq_depth.max_depth = UINT_MAX;
                wake_up_all(&iolat->rq_wait.wait);
                return;
        }

        scale_change(iolat, direction > 0);
}

static void blkcg_iolatency_throttle(struct rq_qos *rqos, struct bio *bio,
                                     spinlock_t *lock)
{
        struct blk_iolatency *blkiolat = BLKIOLATENCY(rqos);
        struct blkcg *blkcg;
        struct blkcg_gq *blkg;
        struct request_queue *q = rqos->q;
        bool issue_as_root = bio_issue_as_root_blkg(bio);

        if (!blk_iolatency_enabled(blkiolat))
                return;

        rcu_read_lock();
        blkcg = bio_blkcg(bio);
        bio_associate_blkcg(bio, &blkcg->css);
        blkg = blkg_lookup(blkcg, q);
        if (unlikely(!blkg)) {
                if (!lock)
                        spin_lock_irq(q->queue_lock);
                blkg = blkg_lookup_create(blkcg, q);
                if (IS_ERR(blkg))
                        blkg = NULL;
                if (!lock)
                        spin_unlock_irq(q->queue_lock);
        }
        if (!blkg)
                goto out;

        bio_issue_init(&bio->bi_issue, bio_sectors(bio));
        bio_associate_blkg(bio, blkg);
out:
        rcu_read_unlock();
        while (blkg && blkg->parent) {
                struct iolatency_grp *iolat = blkg_to_lat(blkg);
                if (!iolat) {
                        blkg = blkg->parent;
                        continue;
                }

                check_scale_change(iolat);
                __blkcg_iolatency_throttle(rqos, iolat, lock, issue_as_root,
                                     (bio->bi_opf & REQ_SWAP) == REQ_SWAP);
                blkg = blkg->parent;
        }
        if (!timer_pending(&blkiolat->timer))
                mod_timer(&blkiolat->timer, jiffies + HZ);
}

static void iolatency_record_time(struct iolatency_grp *iolat,
                                  struct bio_issue *issue, u64 now,
                                  bool issue_as_root)
{
        struct blk_rq_stat *rq_stat;
        u64 start = bio_issue_time(issue);
        u64 req_time;

        /*
         * Have to do this so we are truncated to the correct time that our
         * issue is truncated to.
         */
        now = __bio_issue_time(now);

        if (now <= start)
                return;

        req_time = now - start;

        /*
         * We don't want to count issue_as_root bio's in the cgroups latency
         * statistics as it could skew the numbers downwards.
         */
        if (unlikely(issue_as_root && iolat->rq_depth.max_depth != UINT_MAX)) {
                u64 sub = iolat->min_lat_nsec;
                if (req_time < sub)
                        blkcg_add_delay(lat_to_blkg(iolat), now, sub - req_time);
                return;
        }

        rq_stat = get_cpu_ptr(iolat->stats);
        blk_rq_stat_add(rq_stat, req_time);
        put_cpu_ptr(rq_stat);
}

#define BLKIOLATENCY_MIN_ADJUST_TIME (500 * NSEC_PER_MSEC)
#define BLKIOLATENCY_MIN_GOOD_SAMPLES 5

static void iolatency_check_latencies(struct iolatency_grp *iolat, u64 now)
{
        struct blkcg_gq *blkg = lat_to_blkg(iolat);
        struct iolatency_grp *parent;
        struct child_latency_info *lat_info;
        struct blk_rq_stat stat;
        unsigned long flags;
        int cpu, exp_idx;

        blk_rq_stat_init(&stat);
        preempt_disable();
        for_each_online_cpu(cpu) {
                struct blk_rq_stat *s;
                s = per_cpu_ptr(iolat->stats, cpu);
                blk_rq_stat_sum(&stat, s);
                blk_rq_stat_init(s);
        }
        preempt_enable();

        parent = blkg_to_lat(blkg->parent);
        if (!parent)
                return;

        lat_info = &parent->child_lat;

        /*
         * CALC_LOAD takes in a number stored in fixed point representation.
         * Because we are using this for IO time in ns, the values stored
         * are significantly larger than the FIXED_1 denominator (2048).
         * Therefore, rounding errors in the calculation are negligible and
         * can be ignored.
         */
        exp_idx = min_t(int, BLKIOLATENCY_NR_EXP_FACTORS - 1,
                        div64_u64(iolat->cur_win_nsec,
                                  BLKIOLATENCY_EXP_BUCKET_SIZE));
        CALC_LOAD(iolat->lat_avg, iolatency_exp_factors[exp_idx], stat.mean);

        /* Everything is ok and we don't need to adjust the scale. */
        if (stat.mean <= iolat->min_lat_nsec &&
            atomic_read(&lat_info->scale_cookie) == DEFAULT_SCALE_COOKIE)
                return;

        /* Somebody beat us to the punch, just bail. */
        spin_lock_irqsave(&lat_info->lock, flags);
        lat_info->nr_samples -= iolat->nr_samples;
        lat_info->nr_samples += stat.nr_samples;
        iolat->nr_samples = stat.nr_samples;

        if ((lat_info->last_scale_event >= now ||
            now - lat_info->last_scale_event < BLKIOLATENCY_MIN_ADJUST_TIME) &&
            lat_info->scale_lat <= iolat->min_lat_nsec)
                goto out;

        if (stat.mean <= iolat->min_lat_nsec &&
            stat.nr_samples >= BLKIOLATENCY_MIN_GOOD_SAMPLES) {
                if (lat_info->scale_grp == iolat) {
                        lat_info->last_scale_event = now;
                        scale_cookie_change(iolat->blkiolat, lat_info, true);
                }
        } else if (stat.mean > iolat->min_lat_nsec) {
                lat_info->last_scale_event = now;
                if (!lat_info->scale_grp ||
                    lat_info->scale_lat > iolat->min_lat_nsec) {
                        WRITE_ONCE(lat_info->scale_lat, iolat->min_lat_nsec);
                        lat_info->scale_grp = iolat;
                }
                scale_cookie_change(iolat->blkiolat, lat_info, false);
        }
out:
        spin_unlock_irqrestore(&lat_info->lock, flags);
}

static void blkcg_iolatency_done_bio(struct rq_qos *rqos, struct bio *bio)
{
        struct blkcg_gq *blkg;
        struct rq_wait *rqw;
        struct iolatency_grp *iolat;
        u64 window_start;
        u64 now = ktime_to_ns(ktime_get());
        bool issue_as_root = bio_issue_as_root_blkg(bio);
        bool enabled = false;

        blkg = bio->bi_blkg;
        if (!blkg)
                return;

        iolat = blkg_to_lat(bio->bi_blkg);
        if (!iolat)
                return;

        enabled = blk_iolatency_enabled(iolat->blkiolat);
        while (blkg && blkg->parent) {
                iolat = blkg_to_lat(blkg);
                if (!iolat) {
                        blkg = blkg->parent;
                        continue;
                }
                rqw = &iolat->rq_wait;

                atomic_dec(&rqw->inflight);
                if (!enabled || iolat->min_lat_nsec == 0)
                        goto next;
                iolatency_record_time(iolat, &bio->bi_issue, now,
                                      issue_as_root);
                window_start = atomic64_read(&iolat->window_start);
                if (now > window_start &&
                    (now - window_start) >= iolat->cur_win_nsec) {
                        if (atomic64_cmpxchg(&iolat->window_start,
                                        window_start, now) == window_start)
                                iolatency_check_latencies(iolat, now);
                }
next:
                wake_up(&rqw->wait);
                blkg = blkg->parent;
        }
}

static void blkcg_iolatency_cleanup(struct rq_qos *rqos, struct bio *bio)
{
        struct blkcg_gq *blkg;

        blkg = bio->bi_blkg;
        while (blkg && blkg->parent) {
                struct rq_wait *rqw;
                struct iolatency_grp *iolat;

                iolat = blkg_to_lat(blkg);
                if (!iolat)
                        goto next;

                rqw = &iolat->rq_wait;
                atomic_dec(&rqw->inflight);
                wake_up(&rqw->wait);
next:
                blkg = blkg->parent;
        }
}

static void blkcg_iolatency_exit(struct rq_qos *rqos)
{
        struct blk_iolatency *blkiolat = BLKIOLATENCY(rqos);

        del_timer_sync(&blkiolat->timer);
        blkcg_deactivate_policy(rqos->q, &blkcg_policy_iolatency);
        kfree(blkiolat);
}

static struct rq_qos_ops blkcg_iolatency_ops = {
        .throttle = blkcg_iolatency_throttle,
        .cleanup = blkcg_iolatency_cleanup,
        .done_bio = blkcg_iolatency_done_bio,
        .exit = blkcg_iolatency_exit,
};

static void blkiolatency_timer_fn(struct timer_list *t)
{
        struct blk_iolatency *blkiolat = from_timer(blkiolat, t, timer);
        struct blkcg_gq *blkg;
        struct cgroup_subsys_state *pos_css;
        u64 now = ktime_to_ns(ktime_get());

        rcu_read_lock();
        blkg_for_each_descendant_pre(blkg, pos_css,
                                     blkiolat->rqos.q->root_blkg) {
                struct iolatency_grp *iolat;
                struct child_latency_info *lat_info;
                unsigned long flags;
                u64 cookie;

                /*
                 * We could be exiting, don't access the pd unless we have a
                 * ref on the blkg.
                 */
                if (!blkg_try_get(blkg))
                        continue;

                iolat = blkg_to_lat(blkg);
                if (!iolat)
                        goto next;

                lat_info = &iolat->child_lat;
                cookie = atomic_read(&lat_info->scale_cookie);

                if (cookie >= DEFAULT_SCALE_COOKIE)
                        goto next;

                spin_lock_irqsave(&lat_info->lock, flags);
                if (lat_info->last_scale_event >= now)
                        goto next_lock;

                /*
                 * We scaled down but don't have a scale_grp, scale up and carry
                 * on.
                 */
                if (lat_info->scale_grp == NULL) {
                        scale_cookie_change(iolat->blkiolat, lat_info, true);
                        goto next_lock;
                }

                /*
                 * It's been 5 seconds since our last scale event, clear the
                 * scale grp in case the group that needed the scale down isn't
                 * doing any IO currently.
                 */
                if (now - lat_info->last_scale_event >=
                    ((u64)NSEC_PER_SEC * 5))
                        lat_info->scale_grp = NULL;
next_lock:
                spin_unlock_irqrestore(&lat_info->lock, flags);
next:
                blkg_put(blkg);
        }
        rcu_read_unlock();
}

int blk_iolatency_init(struct request_queue *q)
{
        struct blk_iolatency *blkiolat;
        struct rq_qos *rqos;
        int ret;

        blkiolat = kzalloc(sizeof(*blkiolat), GFP_KERNEL);
        if (!blkiolat)
                return -ENOMEM;

        rqos = &blkiolat->rqos;
        rqos->id = RQ_QOS_CGROUP;
        rqos->ops = &blkcg_iolatency_ops;
        rqos->q = q;

        rq_qos_add(q, rqos);

        ret = blkcg_activate_policy(q, &blkcg_policy_iolatency);
        if (ret) {
                rq_qos_del(q, rqos);
                kfree(blkiolat);
                return ret;
        }

        timer_setup(&blkiolat->timer, blkiolatency_timer_fn, 0);

        return 0;
}

static void iolatency_set_min_lat_nsec(struct blkcg_gq *blkg, u64 val)
{
        struct iolatency_grp *iolat = blkg_to_lat(blkg);
        struct blk_iolatency *blkiolat = iolat->blkiolat;
        u64 oldval = iolat->min_lat_nsec;

        iolat->min_lat_nsec = val;
        iolat->cur_win_nsec = max_t(u64, val << 4, BLKIOLATENCY_MIN_WIN_SIZE);
        iolat->cur_win_nsec = min_t(u64, iolat->cur_win_nsec,
                                    BLKIOLATENCY_MAX_WIN_SIZE);

        if (!oldval && val)
                atomic_inc(&blkiolat->enabled);
        if (oldval && !val)
                atomic_dec(&blkiolat->enabled);
}

static void iolatency_clear_scaling(struct blkcg_gq *blkg)
{
        if (blkg->parent) {
                struct iolatency_grp *iolat = blkg_to_lat(blkg->parent);
                struct child_latency_info *lat_info;
                if (!iolat)
                        return;

                lat_info = &iolat->child_lat;
                spin_lock(&lat_info->lock);
                atomic_set(&lat_info->scale_cookie, DEFAULT_SCALE_COOKIE);
                lat_info->last_scale_event = 0;
                lat_info->scale_grp = NULL;
                lat_info->scale_lat = 0;
                spin_unlock(&lat_info->lock);
        }
}

static ssize_t iolatency_set_limit(struct kernfs_open_file *of, char *buf,
                             size_t nbytes, loff_t off)
{
        struct blkcg *blkcg = css_to_blkcg(of_css(of));
        struct blkcg_gq *blkg;
        struct blk_iolatency *blkiolat;
        struct blkg_conf_ctx ctx;
        struct iolatency_grp *iolat;
        char *p, *tok;
        u64 lat_val = 0;
        u64 oldval;
        int ret;

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

        iolat = blkg_to_lat(ctx.blkg);
        blkiolat = iolat->blkiolat;
        p = ctx.body;

        ret = -EINVAL;
        while ((tok = strsep(&p, " "))) {
                char key[16];
                char val[21];   /* 18446744073709551616 */

                if (sscanf(tok, "%15[^=]=%20s", key, val) != 2)
                        goto out;

                if (!strcmp(key, "target")) {
                        u64 v;

                        if (!strcmp(val, "max"))
                                lat_val = 0;
                        else if (sscanf(val, "%llu", &v) == 1)
                                lat_val = v * NSEC_PER_USEC;
                        else
                                goto out;
                } else {
                        goto out;
                }
        }

        /* Walk up the tree to see if our new val is lower than it should be. */
        blkg = ctx.blkg;
        oldval = iolat->min_lat_nsec;

        iolatency_set_min_lat_nsec(blkg, lat_val);
        if (oldval != iolat->min_lat_nsec) {
                iolatency_clear_scaling(blkg);
        }

        ret = 0;
out:
        blkg_conf_finish(&ctx);
        return ret ?: nbytes;
}

static u64 iolatency_prfill_limit(struct seq_file *sf,
                                  struct blkg_policy_data *pd, int off)
{
        struct iolatency_grp *iolat = pd_to_lat(pd);
        const char *dname = blkg_dev_name(pd->blkg);

        if (!dname || !iolat->min_lat_nsec)
                return 0;
        seq_printf(sf, "%s target=%llu\n",
                   dname, div_u64(iolat->min_lat_nsec, NSEC_PER_USEC));
        return 0;
}

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

static size_t iolatency_pd_stat(struct blkg_policy_data *pd, char *buf,
                                size_t size)
{
        struct iolatency_grp *iolat = pd_to_lat(pd);
        unsigned long long avg_lat = div64_u64(iolat->lat_avg, NSEC_PER_USEC);
        unsigned long long cur_win = div64_u64(iolat->cur_win_nsec, NSEC_PER_MSEC);

        if (iolat->rq_depth.max_depth == UINT_MAX)
                return scnprintf(buf, size, " depth=max avg_lat=%llu win=%llu",
                                 avg_lat, cur_win);

        return scnprintf(buf, size, " depth=%u avg_lat=%llu win=%llu",
                         iolat->rq_depth.max_depth, avg_lat, cur_win);
}


static struct blkg_policy_data *iolatency_pd_alloc(gfp_t gfp, int node)
{
        struct iolatency_grp *iolat;

        iolat = kzalloc_node(sizeof(*iolat), gfp, node);
        if (!iolat)
                return NULL;
        iolat->stats = __alloc_percpu_gfp(sizeof(struct blk_rq_stat),
                                       __alignof__(struct blk_rq_stat), gfp);
        if (!iolat->stats) {
                kfree(iolat);
                return NULL;
        }
        return &iolat->pd;
}

static void iolatency_pd_init(struct blkg_policy_data *pd)
{
        struct iolatency_grp *iolat = pd_to_lat(pd);
        struct blkcg_gq *blkg = lat_to_blkg(iolat);
        struct rq_qos *rqos = blkcg_rq_qos(blkg->q);
        struct blk_iolatency *blkiolat = BLKIOLATENCY(rqos);
        u64 now = ktime_to_ns(ktime_get());
        int cpu;

        for_each_possible_cpu(cpu) {
                struct blk_rq_stat *stat;
                stat = per_cpu_ptr(iolat->stats, cpu);
                blk_rq_stat_init(stat);
        }

        rq_wait_init(&iolat->rq_wait);
        spin_lock_init(&iolat->child_lat.lock);
        iolat->rq_depth.queue_depth = blk_queue_depth(blkg->q);
        iolat->rq_depth.max_depth = UINT_MAX;
        iolat->rq_depth.default_depth = iolat->rq_depth.queue_depth;
        iolat->blkiolat = blkiolat;
        iolat->cur_win_nsec = 100 * NSEC_PER_MSEC;
        atomic64_set(&iolat->window_start, now);

        /*
         * We init things in list order, so the pd for the parent may not be
         * init'ed yet for whatever reason.
         */
        if (blkg->parent && blkg_to_pd(blkg->parent, &blkcg_policy_iolatency)) {
                struct iolatency_grp *parent = blkg_to_lat(blkg->parent);
                atomic_set(&iolat->scale_cookie,
                           atomic_read(&parent->child_lat.scale_cookie));
        } else {
                atomic_set(&iolat->scale_cookie, DEFAULT_SCALE_COOKIE);
        }

        atomic_set(&iolat->child_lat.scale_cookie, DEFAULT_SCALE_COOKIE);
}

static void iolatency_pd_offline(struct blkg_policy_data *pd)
{
        struct iolatency_grp *iolat = pd_to_lat(pd);
        struct blkcg_gq *blkg = lat_to_blkg(iolat);

        iolatency_set_min_lat_nsec(blkg, 0);
        iolatency_clear_scaling(blkg);
}

static void iolatency_pd_free(struct blkg_policy_data *pd)
{
        struct iolatency_grp *iolat = pd_to_lat(pd);
        free_percpu(iolat->stats);
        kfree(iolat);
}

static struct cftype iolatency_files[] = {
        {
                .name = "latency",
                .flags = CFTYPE_NOT_ON_ROOT,
                .seq_show = iolatency_print_limit,
                .write = iolatency_set_limit,
        },
        {}
};

static struct blkcg_policy blkcg_policy_iolatency = {
        .dfl_cftypes    = iolatency_files,
        .pd_alloc_fn    = iolatency_pd_alloc,
        .pd_init_fn     = iolatency_pd_init,
        .pd_offline_fn  = iolatency_pd_offline,
        .pd_free_fn     = iolatency_pd_free,
        .pd_stat_fn     = iolatency_pd_stat,
};

static int __init iolatency_init(void)
{
        return blkcg_policy_register(&blkcg_policy_iolatency);
}

static void __exit iolatency_exit(void)
{
        return blkcg_policy_unregister(&blkcg_policy_iolatency);
}

module_init(iolatency_init);
module_exit(iolatency_exit);