// SPDX-License-Identifier: GPL-2.0

  /*
   * 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 <linux/blk-mq.h>

  #include "blk-rq-qos.h"
  #include "blk-stat.h"

  #include "blk.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 percentile_stats {
  	u64 total;
  	u64 missed;
  };
  
  struct latency_stat {
  	union {
  		struct percentile_stats ps;
  		struct blk_rq_stat rqs;
  	};
  };

  struct iolatency_grp {
  	struct blkg_policy_data pd;

  	struct latency_stat __percpu *stats;

  	struct latency_stat cur_stat;

  	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;

  	bool ssd;

  	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 void latency_stat_init(struct iolatency_grp *iolat,
  				     struct latency_stat *stat)
  {
  	if (iolat->ssd) {
  		stat->ps.total = 0;
  		stat->ps.missed = 0;
  	} else
  		blk_rq_stat_init(&stat->rqs);
  }
  
  static inline void latency_stat_sum(struct iolatency_grp *iolat,
  				    struct latency_stat *sum,
  				    struct latency_stat *stat)
  {
  	if (iolat->ssd) {
  		sum->ps.total += stat->ps.total;
  		sum->ps.missed += stat->ps.missed;
  	} else
  		blk_rq_stat_sum(&sum->rqs, &stat->rqs);
  }
  
  static inline void latency_stat_record_time(struct iolatency_grp *iolat,
  					    u64 req_time)
  {
  	struct latency_stat *stat = get_cpu_ptr(iolat->stats);
  	if (iolat->ssd) {
  		if (req_time >= iolat->min_lat_nsec)
  			stat->ps.missed++;
  		stat->ps.total++;
  	} else
  		blk_rq_stat_add(&stat->rqs, req_time);
  	put_cpu_ptr(stat);
  }
  
  static inline bool latency_sum_ok(struct iolatency_grp *iolat,
  				  struct latency_stat *stat)
  {
  	if (iolat->ssd) {
  		u64 thresh = div64_u64(stat->ps.total, 10);
  		thresh = max(thresh, 1ULL);
  		return stat->ps.missed < thresh;
  	}
  	return stat->rqs.mean <= iolat->min_lat_nsec;
  }
  
  static inline u64 latency_stat_samples(struct iolatency_grp *iolat,
  				       struct latency_stat *stat)
  {
  	if (iolat->ssd)
  		return stat->ps.total;
  	return stat->rqs.nr_samples;
  }
  
  static inline void iolat_update_total_lat_avg(struct iolatency_grp *iolat,
  					      struct latency_stat *stat)
  {
  	int exp_idx;
  
  	if (iolat->ssd)
  		return;
  
  	/*

  	 * 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));

  	iolat->lat_avg = calc_load(iolat->lat_avg,
  				   iolatency_exp_factors[exp_idx],
  				   stat->rqs.mean);

  }

  static void iolat_cleanup_cb(struct rq_wait *rqw, void *private_data)

  {

  	atomic_dec(&rqw->inflight);
  	wake_up(&rqw->wait);
  }

  static bool iolat_acquire_inflight(struct rq_wait *rqw, void *private_data)
  {
  	struct iolatency_grp *iolat = private_data;

  	return rq_wait_inc_below(rqw, iolat->rq_depth.max_depth);
  }
  
  static void __blkcg_iolatency_throttle(struct rq_qos *rqos,
  				       struct iolatency_grp *iolat,

  				       bool issue_as_root,

  				       bool use_memdelay)

  {
  	struct rq_wait *rqw = &iolat->rq_wait;
  	unsigned use_delay = atomic_read(&lat_to_blkg(iolat)->use_delay);

  
  	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;
  	}

  	rq_qos_wait(rqw, iolat, iolat_acquire_inflight, iolat_cleanup_cb);

  }
  
  #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 = blkiolat->rqos.q->nr_requests;

  	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 = iolat->blkiolat->rqos.q->nr_requests;

  	unsigned long scale = scale_amount(qd, up);
  	unsigned long old = iolat->rq_depth.max_depth;

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

  			old += scale;
  			old = min(old, qd);
  			iolat->rq_depth.max_depth = old;
  			wake_up_all(&iolat->rq_wait.wait);
  		}

  	} else {

  		old >>= 1;

  		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 = max(1ULL, 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)

  {
  	struct blk_iolatency *blkiolat = BLKIOLATENCY(rqos);

  	struct blkcg_gq *blkg = bio->bi_blkg;

  	bool issue_as_root = bio_issue_as_root_blkg(bio);
  
  	if (!blk_iolatency_enabled(blkiolat))
  		return;

  	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, 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)
  {

  	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;
  	}

  	latency_stat_record_time(iolat, req_time);

  }
  
  #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 latency_stat stat;

  	unsigned long flags;

  	int cpu;

  	latency_stat_init(iolat, &stat);

  	preempt_disable();
  	for_each_online_cpu(cpu) {

  		struct latency_stat *s;

  		s = per_cpu_ptr(iolat->stats, cpu);

  		latency_stat_sum(iolat, &stat, s);
  		latency_stat_init(iolat, s);

  	}
  	preempt_enable();

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

  	iolat_update_total_lat_avg(iolat, &stat);

  
  	/* Everything is ok and we don't need to adjust the scale. */

  	if (latency_sum_ok(iolat, &stat) &&

  	    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);

  
  	latency_stat_sum(iolat, &iolat->cur_stat, &stat);

  	lat_info->nr_samples -= iolat->nr_samples;

  	lat_info->nr_samples += latency_stat_samples(iolat, &iolat->cur_stat);
  	iolat->nr_samples = latency_stat_samples(iolat, &iolat->cur_stat);

  
  	if ((lat_info->last_scale_event >= now ||

  	    now - lat_info->last_scale_event < BLKIOLATENCY_MIN_ADJUST_TIME))

  		goto out;

  	if (latency_sum_ok(iolat, &iolat->cur_stat) &&
  	    latency_sum_ok(iolat, &stat)) {
  		if (latency_stat_samples(iolat, &iolat->cur_stat) <

  		    BLKIOLATENCY_MIN_GOOD_SAMPLES)
  			goto out;

  		if (lat_info->scale_grp == iolat) {
  			lat_info->last_scale_event = now;
  			scale_cookie_change(iolat->blkiolat, lat_info, true);
  		}

  	} else if (lat_info->scale_lat == 0 ||
  		   lat_info->scale_lat >= 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);
  	}

  	latency_stat_init(iolat, &iolat->cur_stat);

  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;

  	int inflight = 0;

  
  	blkg = bio->bi_blkg;

  	if (!blkg || !bio_flagged(bio, BIO_TRACKED))

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

  	if (!enabled)
  		return;

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

  		inflight = atomic_dec_return(&rqw->inflight);
  		WARN_ON_ONCE(inflight < 0);

  		/*
  		 * If bi_status is BLK_STS_AGAIN, the bio wasn't actually
  		 * submitted, so do not account for it.
  		 */
  		if (iolat->min_lat_nsec && bio->bi_status != BLK_STS_AGAIN) {
  			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);
  			}

  		}

  		wake_up(&rqw->wait);

  		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,

  	.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_tryget(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_LATENCY;

  	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;
  }

  /*
   * return 1 for enabling iolatency, return -1 for disabling iolatency, otherwise
   * return 0.
   */
  static int iolatency_set_min_lat_nsec(struct blkcg_gq *blkg, u64 val)

  {
  	struct iolatency_grp *iolat = blkg_to_lat(blkg);

  	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)

  		return 1;

  	if (oldval && !val) {
  		blkcg_clear_delay(blkg);

  		return -1;

  	}

  	return 0;

  }
  
  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 blkg_conf_ctx ctx;
  	struct iolatency_grp *iolat;
  	char *p, *tok;
  	u64 lat_val = 0;
  	u64 oldval;
  	int ret;

  	int enable = 0;

  
  	ret = blkg_conf_prep(blkcg, &blkcg_policy_iolatency, buf, &ctx);
  	if (ret)
  		return ret;
  
  	iolat = blkg_to_lat(ctx.blkg);

  	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;

  	enable = iolatency_set_min_lat_nsec(blkg, lat_val);
  	if (enable) {
  		WARN_ON_ONCE(!blk_get_queue(blkg->q));
  		blkg_get(blkg);
  	}

  	if (oldval != iolat->min_lat_nsec) {
  		iolatency_clear_scaling(blkg);
  	}
  
  	ret = 0;
  out:
  	blkg_conf_finish(&ctx);

  	if (ret == 0 && enable) {
  		struct iolatency_grp *tmp = blkg_to_lat(blkg);
  		struct blk_iolatency *blkiolat = tmp->blkiolat;
  
  		blk_mq_freeze_queue(blkg->q);
  
  		if (enable == 1)
  			atomic_inc(&blkiolat->enabled);
  		else if (enable == -1)
  			atomic_dec(&blkiolat->enabled);
  		else
  			WARN_ON_ONCE(1);
  
  		blk_mq_unfreeze_queue(blkg->q);
  
  		blkg_put(blkg);
  		blk_put_queue(blkg->q);
  	}

  	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
  ",

  		   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_ssd_stat(struct iolatency_grp *iolat, char *buf,
  				 size_t size)
  {
  	struct latency_stat stat;
  	int cpu;
  
  	latency_stat_init(iolat, &stat);
  	preempt_disable();
  	for_each_online_cpu(cpu) {
  		struct latency_stat *s;
  		s = per_cpu_ptr(iolat->stats, cpu);
  		latency_stat_sum(iolat, &stat, s);
  	}
  	preempt_enable();
  
  	if (iolat->rq_depth.max_depth == UINT_MAX)
  		return scnprintf(buf, size, " missed=%llu total=%llu depth=max",
  				 (unsigned long long)stat.ps.missed,
  				 (unsigned long long)stat.ps.total);
  	return scnprintf(buf, size, " missed=%llu total=%llu depth=%u",
  			 (unsigned long long)stat.ps.missed,
  			 (unsigned long long)stat.ps.total,
  			 iolat->rq_depth.max_depth);
  }

  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;
  	unsigned long long cur_win;

  	if (!blkcg_debug_stats)
  		return 0;

  	if (iolat->ssd)
  		return iolatency_ssd_stat(iolat, buf, size);

  	avg_lat = div64_u64(iolat->lat_avg, NSEC_PER_USEC);
  	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,
  						   struct request_queue *q,
  						   struct blkcg *blkcg)

  {
  	struct iolatency_grp *iolat;

  	iolat = kzalloc_node(sizeof(*iolat), gfp, q->node);

  	if (!iolat)
  		return NULL;

  	iolat->stats = __alloc_percpu_gfp(sizeof(struct latency_stat),
  				       __alignof__(struct latency_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;

  	if (blk_queue_nonrot(blkg->q))
  		iolat->ssd = true;
  	else
  		iolat->ssd = false;

  	for_each_possible_cpu(cpu) {

  		struct latency_stat *stat;

  		stat = per_cpu_ptr(iolat->stats, cpu);

  		latency_stat_init(iolat, stat);

  	}

  	latency_stat_init(iolat, &iolat->cur_stat);

  	rq_wait_init(&iolat->rq_wait);
  	spin_lock_init(&iolat->child_lat.lock);

  	iolat->rq_depth.queue_depth = blkg->q->nr_requests;

  	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);

  	struct blk_iolatency *blkiolat = iolat->blkiolat;
  	int ret;

  	ret = iolatency_set_min_lat_nsec(blkg, 0);
  	if (ret == 1)
  		atomic_inc(&blkiolat->enabled);
  	if (ret == -1)
  		atomic_dec(&blkiolat->enabled);

  	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);