text: // SPDX-License-Identifier: GPL-2.0
  /*
   * Interface for controlling IO bandwidth on a request queue
   *
   * Copyright (C) 2010 Vivek Goyal <vgoyal@redhat.com>
   */
  
  #include <linux/module.h>
  #include <linux/slab.h>
  #include <linux/blkdev.h>
  #include <linux/bio.h>
  #include <linux/blktrace_api.h>
  #include <linux/blk-cgroup.h>
  #include "blk.h"
  #include "blk-cgroup-rwstat.h"
  
  /* Max dispatch from a group in 1 round */
  #define THROTL_GRP_QUANTUM 8
  
  /* Total max dispatch from all groups in one round */
  #define THROTL_QUANTUM 32
  
  /* Throttling is performed over a slice and after that slice is renewed */
  #define DFL_THROTL_SLICE_HD (HZ / 10)
  #define DFL_THROTL_SLICE_SSD (HZ / 50)
  #define MAX_THROTL_SLICE (HZ)
  #define MAX_IDLE_TIME (5L * 1000 * 1000) /* 5 s */
  #define MIN_THROTL_BPS (320 * 1024)
  #define MIN_THROTL_IOPS (10)
  #define DFL_LATENCY_TARGET (-1L)
  #define DFL_IDLE_THRESHOLD (0)
  #define DFL_HD_BASELINE_LATENCY (4000L) /* 4ms */
  #define LATENCY_FILTERED_SSD (0)
  /*
   * For HD, very small latency comes from sequential IO. Such IO is helpless to
   * help determine if its IO is impacted by others, hence we ignore the IO
   */
  #define LATENCY_FILTERED_HD (1000L) /* 1ms */
  
  static struct blkcg_policy blkcg_policy_throtl;
  
  /* A workqueue to queue throttle related work */
  static struct workqueue_struct *kthrotld_workqueue;
  
  /*
   * To implement hierarchical throttling, throtl_grps form a tree and bios
   * are dispatched upwards level by level until they reach the top and get
   * issued.  When dispatching bios from the children and local group at each
   * level, if the bios are dispatched into a single bio_list, there's a risk
   * of a local or child group which can queue many bios at once filling up
   * the list starving others.
   *
   * To avoid such starvation, dispatched bios are queued separately
   * according to where they came from.  When they are again dispatched to
   * the parent, they're popped in round-robin order so that no single source
   * hogs the dispatch window.
   *
   * throtl_qnode is used to keep the queued bios separated by their sources.
   * Bios are queued to throtl_qnode which in turn is queued to
   * throtl_service_queue and then dispatched in round-robin order.
   *
   * It's also used to track the reference counts on blkg's.  A qnode always
   * belongs to a throtl_grp and gets queued on itself or the parent, so
   * incrementing the reference of the associated throtl_grp when a qnode is
   * queued and decrementing when dequeued is enough to keep the whole blkg
   * tree pinned while bios are in flight.
   */
  struct throtl_qnode {
  	struct list_head	node;		/* service_queue->queued[] */
  	struct bio_list		bios;		/* queued bios */
  	struct throtl_grp	*tg;		/* tg this qnode belongs to */
  };
  
  struct throtl_service_queue {
  	struct throtl_service_queue *parent_sq;	/* the parent service_queue */
  
  	/*
  	 * Bios queued directly to this service_queue or dispatched from
  	 * children throtl_grp's.
  	 */
  	struct list_head	queued[2];	/* throtl_qnode [READ/WRITE] */
  	unsigned int		nr_queued[2];	/* number of queued bios */
  
  	/*
  	 * RB tree of active children throtl_grp's, which are sorted by
  	 * their ->disptime.
  	 */
  	struct rb_root_cached	pending_tree;	/* RB tree of active tgs */
  	unsigned int		nr_pending;	/* # queued in the tree */
  	unsigned long		first_pending_disptime;	/* disptime of the first tg */
  	struct timer_list	pending_timer;	/* fires on first_pending_disptime */
  };
  
  enum tg_state_flags {
  	THROTL_TG_PENDING	= 1 << 0,	/* on parent's pending tree */
  	THROTL_TG_WAS_EMPTY	= 1 << 1,	/* bio_lists[] became non-empty */
  };
  
  #define rb_entry_tg(node)	rb_entry((node), struct throtl_grp, rb_node)
  
  enum {
  	LIMIT_LOW,
  	LIMIT_MAX,
  	LIMIT_CNT,
  };
  
  struct throtl_grp {
  	/* must be the first member */
  	struct blkg_policy_data pd;
  
  	/* active throtl group service_queue member */
  	struct rb_node rb_node;
  
  	/* throtl_data this group belongs to */
  	struct throtl_data *td;
  
  	/* this group's service queue */
  	struct throtl_service_queue service_queue;
  
  	/*
  	 * qnode_on_self is used when bios are directly queued to this
  	 * throtl_grp so that local bios compete fairly with bios
  	 * dispatched from children.  qnode_on_parent is used when bios are
  	 * dispatched from this throtl_grp into its parent and will compete
  	 * with the sibling qnode_on_parents and the parent's
  	 * qnode_on_self.
  	 */
  	struct throtl_qnode qnode_on_self[2];
  	struct throtl_qnode qnode_on_parent[2];
  
  	/*
  	 * Dispatch time in jiffies. This is the estimated time when group
  	 * will unthrottle and is ready to dispatch more bio. It is used as
  	 * key to sort active groups in service tree.
  	 */
  	unsigned long disptime;
  
  	unsigned int flags;
  
  	/* are there any throtl rules between this group and td? */
  	bool has_rules[2];
  
  	/* internally used bytes per second rate limits */
  	uint64_t bps[2][LIMIT_CNT];
  	/* user configured bps limits */
  	uint64_t bps_conf[2][LIMIT_CNT];
  
  	/* internally used IOPS limits */
  	unsigned int iops[2][LIMIT_CNT];
  	/* user configured IOPS limits */
  	unsigned int iops_conf[2][LIMIT_CNT];
  
  	/* Number of bytes dispatched in current slice */
  	uint64_t bytes_disp[2];
  	/* Number of bio's dispatched in current slice */
  	unsigned int io_disp[2];
  
  	unsigned long last_low_overflow_time[2];
  
  	uint64_t last_bytes_disp[2];
  	unsigned int last_io_disp[2];
  
  	unsigned long last_check_time;
  
  	unsigned long latency_target; /* us */
  	unsigned long latency_target_conf; /* us */
  	/* When did we start a new slice */
  	unsigned long slice_start[2];
  	unsigned long slice_end[2];
  
  	unsigned long last_finish_time; /* ns / 1024 */
  	unsigned long checked_last_finish_time; /* ns / 1024 */
  	unsigned long avg_idletime; /* ns / 1024 */
  	unsigned long idletime_threshold; /* us */
  	unsigned long idletime_threshold_conf; /* us */
  
  	unsigned int bio_cnt; /* total bios */
  	unsigned int bad_bio_cnt; /* bios exceeding latency threshold */
  	unsigned long bio_cnt_reset_time;
  
  	struct blkg_rwstat stat_bytes;
  	struct blkg_rwstat stat_ios;
  };
  
  /* We measure latency for request size from <= 4k to >= 1M */
  #define LATENCY_BUCKET_SIZE 9
  
  struct latency_bucket {
  	unsigned long total_latency; /* ns / 1024 */
  	int samples;
  };
  
  struct avg_latency_bucket {
  	unsigned long latency; /* ns / 1024 */
  	bool valid;
  };
  
  struct throtl_data
  {
  	/* service tree for active throtl groups */
  	struct throtl_service_queue service_queue;
  
  	struct request_queue *queue;
  
  	/* Total Number of queued bios on READ and WRITE lists */
  	unsigned int nr_queued[2];
  
  	unsigned int throtl_slice;
  
  	/* Work for dispatching throttled bios */
  	struct work_struct dispatch_work;
  	unsigned int limit_index;
  	bool limit_valid[LIMIT_CNT];
  
  	unsigned long low_upgrade_time;
  	unsigned long low_downgrade_time;
  
  	unsigned int scale;
  
  	struct latency_bucket tmp_buckets[2][LATENCY_BUCKET_SIZE];
  	struct avg_latency_bucket avg_buckets[2][LATENCY_BUCKET_SIZE];
  	struct latency_bucket __percpu *latency_buckets[2];
  	unsigned long last_calculate_time;
  	unsigned long filtered_latency;
  
  	bool track_bio_latency;
  };
  
  static void throtl_pending_timer_fn(struct timer_list *t);
  
  static inline struct throtl_grp *pd_to_tg(struct blkg_policy_data *pd)
  {
  	return pd ? container_of(pd, struct throtl_grp, pd) : NULL;
  }
  
  static inline struct throtl_grp *blkg_to_tg(struct blkcg_gq *blkg)
  {
  	return pd_to_tg(blkg_to_pd(blkg, &blkcg_policy_throtl));
  }
  
  static inline struct blkcg_gq *tg_to_blkg(struct throtl_grp *tg)
  {
  	return pd_to_blkg(&tg->pd);
  }
  
  /**
   * sq_to_tg - return the throl_grp the specified service queue belongs to
   * @sq: the throtl_service_queue of interest
   *
   * Return the throtl_grp @sq belongs to.  If @sq is the top-level one
   * embedded in throtl_data, %NULL is returned.
   */
  static struct throtl_grp *sq_to_tg(struct throtl_service_queue *sq)
  {
  	if (sq && sq->parent_sq)
  		return container_of(sq, struct throtl_grp, service_queue);
  	else
  		return NULL;
  }
  
  /**
   * sq_to_td - return throtl_data the specified service queue belongs to
   * @sq: the throtl_service_queue of interest
   *
   * A service_queue can be embedded in either a throtl_grp or throtl_data.
   * Determine the associated throtl_data accordingly and return it.
   */
  static struct throtl_data *sq_to_td(struct throtl_service_queue *sq)
  {
  	struct throtl_grp *tg = sq_to_tg(sq);
  
  	if (tg)
  		return tg->td;
  	else
  		return container_of(sq, struct throtl_data, service_queue);
  }
  
  /*
   * cgroup's limit in LIMIT_MAX is scaled if low limit is set. This scale is to
   * make the IO dispatch more smooth.
   * Scale up: linearly scale up according to lapsed time since upgrade. For
   *           every throtl_slice, the limit scales up 1/2 .low limit till the
   *           limit hits .max limit
   * Scale down: exponentially scale down if a cgroup doesn't hit its .low limit
   */
  static uint64_t throtl_adjusted_limit(uint64_t low, struct throtl_data *td)
  {
  	/* arbitrary value to avoid too big scale */
  	if (td->scale < 4096 && time_after_eq(jiffies,
  	    td->low_upgrade_time + td->scale * td->throtl_slice))
  		td->scale = (jiffies - td->low_upgrade_time) / td->throtl_slice;
  
  	return low + (low >> 1) * td->scale;
  }
  
  static uint64_t tg_bps_limit(struct throtl_grp *tg, int rw)
  {
  	struct blkcg_gq *blkg = tg_to_blkg(tg);
  	struct throtl_data *td;
  	uint64_t ret;
  
  	if (cgroup_subsys_on_dfl(io_cgrp_subsys) && !blkg->parent)
  		return U64_MAX;
  
  	td = tg->td;
  	ret = tg->bps[rw][td->limit_index];
  	if (ret == 0 && td->limit_index == LIMIT_LOW) {
  		/* intermediate node or iops isn't 0 */
  		if (!list_empty(&blkg->blkcg->css.children) ||
  		    tg->iops[rw][td->limit_index])
  			return U64_MAX;
  		else
  			return MIN_THROTL_BPS;
  	}
  
  	if (td->limit_index == LIMIT_MAX && tg->bps[rw][LIMIT_LOW] &&
  	    tg->bps[rw][LIMIT_LOW] != tg->bps[rw][LIMIT_MAX]) {
  		uint64_t adjusted;
  
  		adjusted = throtl_adjusted_limit(tg->bps[rw][LIMIT_LOW], td);
  		ret = min(tg->bps[rw][LIMIT_MAX], adjusted);
  	}
  	return ret;
  }
  
  static unsigned int tg_iops_limit(struct throtl_grp *tg, int rw)
  {
  	struct blkcg_gq *blkg = tg_to_blkg(tg);
  	struct throtl_data *td;
  	unsigned int ret;
  
  	if (cgroup_subsys_on_dfl(io_cgrp_subsys) && !blkg->parent)
  		return UINT_MAX;
  
  	td = tg->td;
  	ret = tg->iops[rw][td->limit_index];
  	if (ret == 0 && tg->td->limit_index == LIMIT_LOW) {
  		/* intermediate node or bps isn't 0 */
  		if (!list_empty(&blkg->blkcg->css.children) ||
  		    tg->bps[rw][td->limit_index])
  			return UINT_MAX;
  		else
  			return MIN_THROTL_IOPS;
  	}
  
  	if (td->limit_index == LIMIT_MAX && tg->iops[rw][LIMIT_LOW] &&
  	    tg->iops[rw][LIMIT_LOW] != tg->iops[rw][LIMIT_MAX]) {
  		uint64_t adjusted;
  
  		adjusted = throtl_adjusted_limit(tg->iops[rw][LIMIT_LOW], td);
  		if (adjusted > UINT_MAX)
  			adjusted = UINT_MAX;
  		ret = min_t(unsigned int, tg->iops[rw][LIMIT_MAX], adjusted);
  	}
  	return ret;
  }
  
  #define request_bucket_index(sectors) \
  	clamp_t(int, order_base_2(sectors) - 3, 0, LATENCY_BUCKET_SIZE - 1)
  
  /**
   * throtl_log - log debug message via blktrace
   * @sq: the service_queue being reported
   * @fmt: printf format string
   * @args: printf args
   *
   * The messages are prefixed with "throtl BLKG_NAME" if @sq belongs to a
   * throtl_grp; otherwise, just "throtl".
   */
  #define throtl_log(sq, fmt, args...)	do {				\
  	struct throtl_grp *__tg = sq_to_tg((sq));			\
  	struct throtl_data *__td = sq_to_td((sq));			\
  									\
  	(void)__td;							\
  	if (likely(!blk_trace_note_message_enabled(__td->queue)))	\
  		break;							\
  	if ((__tg)) {							\
  		blk_add_cgroup_trace_msg(__td->queue,			\
  			tg_to_blkg(__tg)->blkcg, "throtl " fmt, ##args);\
  	} else {							\
  		blk_add_trace_msg(__td->queue, "throtl " fmt, ##args);	\
  	}								\
  } while (0)
  
  static inline unsigned int throtl_bio_data_size(struct bio *bio)
  {
  	/* assume it's one sector */
  	if (unlikely(bio_op(bio) == REQ_OP_DISCARD))
  		return 512;
  	return bio->bi_iter.bi_size;
  }
  
  static void throtl_qnode_init(struct throtl_qnode *qn, struct throtl_grp *tg)
  {
  	INIT_LIST_HEAD(&qn->node);
  	bio_list_init(&qn->bios);
  	qn->tg = tg;
  }
  
  /**
   * throtl_qnode_add_bio - add a bio to a throtl_qnode and activate it
   * @bio: bio being added
   * @qn: qnode to add bio to
   * @queued: the service_queue->queued[] list @qn belongs to
   *
   * Add @bio to @qn and put @qn on @queued if it's not already on.
   * @qn->tg's reference count is bumped when @qn is activated.  See the
   * comment on top of throtl_qnode definition for details.
   */
  static void throtl_qnode_add_bio(struct bio *bio, struct throtl_qnode *qn,
  				 struct list_head *queued)
  {
  	bio_list_add(&qn->bios, bio);
  	if (list_empty(&qn->node)) {
  		list_add_tail(&qn->node, queued);
  		blkg_get(tg_to_blkg(qn->tg));
  	}
  }
  
  /**
   * throtl_peek_queued - peek the first bio on a qnode list
   * @queued: the qnode list to peek
   */
  static struct bio *throtl_peek_queued(struct list_head *queued)
  {
  	struct throtl_qnode *qn;
  	struct bio *bio;
  
  	if (list_empty(queued))
  		return NULL;
  
  	qn = list_first_entry(queued, struct throtl_qnode, node);
  	bio = bio_list_peek(&qn->bios);
  	WARN_ON_ONCE(!bio);
  	return bio;
  }
  
  /**
   * throtl_pop_queued - pop the first bio form a qnode list
   * @queued: the qnode list to pop a bio from
   * @tg_to_put: optional out argument for throtl_grp to put
   *
   * Pop the first bio from the qnode list @queued.  After popping, the first
   * qnode is removed from @queued if empty or moved to the end of @queued so
   * that the popping order is round-robin.
   *
   * When the first qnode is removed, its associated throtl_grp should be put
   * too.  If @tg_to_put is NULL, this function automatically puts it;
   * otherwise, *@tg_to_put is set to the throtl_grp to put and the caller is
   * responsible for putting it.
   */
  static struct bio *throtl_pop_queued(struct list_head *queued,
  				     struct throtl_grp **tg_to_put)
  {
  	struct throtl_qnode *qn;
  	struct bio *bio;
  
  	if (list_empty(queued))
  		return NULL;
  
  	qn = list_first_entry(queued, struct throtl_qnode, node);
  	bio = bio_list_pop(&qn->bios);
  	WARN_ON_ONCE(!bio);
  
  	if (bio_list_empty(&qn->bios)) {
  		list_del_init(&qn->node);
  		if (tg_to_put)
  			*tg_to_put = qn->tg;
  		else
  			blkg_put(tg_to_blkg(qn->tg));
  	} else {
  		list_move_tail(&qn->node, queued);
  	}
  
  	return bio;
  }
  
  /* init a service_queue, assumes the caller zeroed it */
  static void throtl_service_queue_init(struct throtl_service_queue *sq)
  {
  	INIT_LIST_HEAD(&sq->queued[0]);
  	INIT_LIST_HEAD(&sq->queued[1]);
  	sq->pending_tree = RB_ROOT_CACHED;
  	timer_setup(&sq->pending_timer, throtl_pending_timer_fn, 0);
  }
  
  static struct blkg_policy_data *throtl_pd_alloc(gfp_t gfp,
  						struct request_queue *q,
  						struct blkcg *blkcg)
  {
  	struct throtl_grp *tg;
  	int rw;
  
  	tg = kzalloc_node(sizeof(*tg), gfp, q->node);
  	if (!tg)
  		return NULL;
  
  	if (blkg_rwstat_init(&tg->stat_bytes, gfp))
  		goto err_free_tg;
  
  	if (blkg_rwstat_init(&tg->stat_ios, gfp))
  		goto err_exit_stat_bytes;
  
  	throtl_service_queue_init(&tg->service_queue);
  
  	for (rw = READ; rw <= WRITE; rw++) {
  		throtl_qnode_init(&tg->qnode_on_self[rw], tg);
  		throtl_qnode_init(&tg->qnode_on_parent[rw], tg);
  	}
  
  	RB_CLEAR_NODE(&tg->rb_node);
  	tg->bps[READ][LIMIT_MAX] = U64_MAX;
  	tg->bps[WRITE][LIMIT_MAX] = U64_MAX;
  	tg->iops[READ][LIMIT_MAX] = UINT_MAX;
  	tg->iops[WRITE][LIMIT_MAX] = UINT_MAX;
  	tg->bps_conf[READ][LIMIT_MAX] = U64_MAX;
  	tg->bps_conf[WRITE][LIMIT_MAX] = U64_MAX;
  	tg->iops_conf[READ][LIMIT_MAX] = UINT_MAX;
  	tg->iops_conf[WRITE][LIMIT_MAX] = UINT_MAX;
  	/* LIMIT_LOW will have default value 0 */
  
  	tg->latency_target = DFL_LATENCY_TARGET;
  	tg->latency_target_conf = DFL_LATENCY_TARGET;
  	tg->idletime_threshold = DFL_IDLE_THRESHOLD;
  	tg->idletime_threshold_conf = DFL_IDLE_THRESHOLD;
  
  	return &tg->pd;
  
  err_exit_stat_bytes:
  	blkg_rwstat_exit(&tg->stat_bytes);
  err_free_tg:
  	kfree(tg);
  	return NULL;
  }
  
  static void throtl_pd_init(struct blkg_policy_data *pd)
  {
  	struct throtl_grp *tg = pd_to_tg(pd);
  	struct blkcg_gq *blkg = tg_to_blkg(tg);
  	struct throtl_data *td = blkg->q->td;
  	struct throtl_service_queue *sq = &tg->service_queue;
  
  	/*
  	 * If on the default hierarchy, we switch to properly hierarchical
  	 * behavior where limits on a given throtl_grp are applied to the
  	 * whole subtree rather than just the group itself.  e.g. If 16M
  	 * read_bps limit is set on the root group, the whole system can't
  	 * exceed 16M for the device.
  	 *
  	 * If not on the default hierarchy, the broken flat hierarchy
  	 * behavior is retained where all throtl_grps are treated as if
  	 * they're all separate root groups right below throtl_data.
  	 * Limits of a group don't interact with limits of other groups
  	 * regardless of the position of the group in the hierarchy.
  	 */
  	sq->parent_sq = &td->service_queue;
  	if (cgroup_subsys_on_dfl(io_cgrp_subsys) && blkg->parent)
  		sq->parent_sq = &blkg_to_tg(blkg->parent)->service_queue;
  	tg->td = td;
  }
  
  /*
   * Set has_rules[] if @tg or any of its parents have limits configured.
   * This doesn't require walking up to the top of the hierarchy as the
   * parent's has_rules[] is guaranteed to be correct.
   */
  static void tg_update_has_rules(struct throtl_grp *tg)
  {
  	struct throtl_grp *parent_tg = sq_to_tg(tg->service_queue.parent_sq);
  	struct throtl_data *td = tg->td;
  	int rw;
  
  	for (rw = READ; rw <= WRITE; rw++)
  		tg->has_rules[rw] = (parent_tg && parent_tg->has_rules[rw]) ||
  			(td->limit_valid[td->limit_index] &&
  			 (tg_bps_limit(tg, rw) != U64_MAX ||
  			  tg_iops_limit(tg, rw) != UINT_MAX));
  }
  
  static void throtl_pd_online(struct blkg_policy_data *pd)
  {
  	struct throtl_grp *tg = pd_to_tg(pd);
  	/*
  	 * We don't want new groups to escape the limits of its ancestors.
  	 * Update has_rules[] after a new group is brought online.
  	 */
  	tg_update_has_rules(tg);
  }
  
  static void blk_throtl_update_limit_valid(struct throtl_data *td)
  {
  	struct cgroup_subsys_state *pos_css;
  	struct blkcg_gq *blkg;
  	bool low_valid = false;
  
  	rcu_read_lock();
  	blkg_for_each_descendant_post(blkg, pos_css, td->queue->root_blkg) {
  		struct throtl_grp *tg = blkg_to_tg(blkg);
  
  		if (tg->bps[READ][LIMIT_LOW] || tg->bps[WRITE][LIMIT_LOW] ||
  		    tg->iops[READ][LIMIT_LOW] || tg->iops[WRITE][LIMIT_LOW]) {
  			low_valid = true;
  			break;
  		}
  	}
  	rcu_read_unlock();
  
  	td->limit_valid[LIMIT_LOW] = low_valid;
  }
  
  static void throtl_upgrade_state(struct throtl_data *td);
  static void throtl_pd_offline(struct blkg_policy_data *pd)
  {
  	struct throtl_grp *tg = pd_to_tg(pd);
  
  	tg->bps[READ][LIMIT_LOW] = 0;
  	tg->bps[WRITE][LIMIT_LOW] = 0;
  	tg->iops[READ][LIMIT_LOW] = 0;
  	tg->iops[WRITE][LIMIT_LOW] = 0;
  
  	blk_throtl_update_limit_valid(tg->td);
  
  	if (!tg->td->limit_valid[tg->td->limit_index])
  		throtl_upgrade_state(tg->td);
  }
  
  static void throtl_pd_free(struct blkg_policy_data *pd)
  {
  	struct throtl_grp *tg = pd_to_tg(pd);
  
  	del_timer_sync(&tg->service_queue.pending_timer);
  	blkg_rwstat_exit(&tg->stat_bytes);
  	blkg_rwstat_exit(&tg->stat_ios);
  	kfree(tg);
  }
  
  static struct throtl_grp *
  throtl_rb_first(struct throtl_service_queue *parent_sq)
  {
  	struct rb_node *n;
  
  	n = rb_first_cached(&parent_sq->pending_tree);
  	WARN_ON_ONCE(!n);
  	if (!n)
  		return NULL;
  	return rb_entry_tg(n);
  }
  
  static void throtl_rb_erase(struct rb_node *n,
  			    struct throtl_service_queue *parent_sq)
  {
  	rb_erase_cached(n, &parent_sq->pending_tree);
  	RB_CLEAR_NODE(n);
  	--parent_sq->nr_pending;
  }
  
  static void update_min_dispatch_time(struct throtl_service_queue *parent_sq)
  {
  	struct throtl_grp *tg;
  
  	tg = throtl_rb_first(parent_sq);
  	if (!tg)
  		return;
  
  	parent_sq->first_pending_disptime = tg->disptime;
  }
  
  static void tg_service_queue_add(struct throtl_grp *tg)
  {
  	struct throtl_service_queue *parent_sq = tg->service_queue.parent_sq;
  	struct rb_node **node = &parent_sq->pending_tree.rb_root.rb_node;
  	struct rb_node *parent = NULL;
  	struct throtl_grp *__tg;
  	unsigned long key = tg->disptime;
  	bool leftmost = true;
  
  	while (*node != NULL) {
  		parent = *node;
  		__tg = rb_entry_tg(parent);
  
  		if (time_before(key, __tg->disptime))
  			node = &parent->rb_left;
  		else {
  			node = &parent->rb_right;
  			leftmost = false;
  		}
  	}
  
  	rb_link_node(&tg->rb_node, parent, node);
  	rb_insert_color_cached(&tg->rb_node, &parent_sq->pending_tree,
  			       leftmost);
  }
  
  static void throtl_enqueue_tg(struct throtl_grp *tg)
  {
  	if (!(tg->flags & THROTL_TG_PENDING)) {
  		tg_service_queue_add(tg);
  		tg->flags |= THROTL_TG_PENDING;
  		tg->service_queue.parent_sq->nr_pending++;
  	}
  }
  
  static void throtl_dequeue_tg(struct throtl_grp *tg)
  {
  	if (tg->flags & THROTL_TG_PENDING) {
  		throtl_rb_erase(&tg->rb_node, tg->service_queue.parent_sq);
  		tg->flags &= ~THROTL_TG_PENDING;
  	}
  }
  
  /* Call with queue lock held */
  static void throtl_schedule_pending_timer(struct throtl_service_queue *sq,
  					  unsigned long expires)
  {
  	unsigned long max_expire = jiffies + 8 * sq_to_td(sq)->throtl_slice;
  
  	/*
  	 * Since we are adjusting the throttle limit dynamically, the sleep
  	 * time calculated according to previous limit might be invalid. It's
  	 * possible the cgroup sleep time is very long and no other cgroups
  	 * have IO running so notify the limit changes. Make sure the cgroup
  	 * doesn't sleep too long to avoid the missed notification.
  	 */
  	if (time_after(expires, max_expire))
  		expires = max_expire;
  	mod_timer(&sq->pending_timer, expires);
  	throtl_log(sq, "schedule timer. delay=%lu jiffies=%lu",
  		   expires - jiffies, jiffies);
  }
  
  /**
   * throtl_schedule_next_dispatch - schedule the next dispatch cycle
   * @sq: the service_queue to schedule dispatch for
   * @force: force scheduling
   *
   * Arm @sq->pending_timer so that the next dispatch cycle starts on the
   * dispatch time of the first pending child.  Returns %true if either timer
   * is armed or there's no pending child left.  %false if the current
   * dispatch window is still open and the caller should continue
   * dispatching.
   *
   * If @force is %true, the dispatch timer is always scheduled and this
   * function is guaranteed to return %true.  This is to be used when the
   * caller can't dispatch itself and needs to invoke pending_timer
   * unconditionally.  Note that forced scheduling is likely to induce short
   * delay before dispatch starts even if @sq->first_pending_disptime is not
   * in the future and thus shouldn't be used in hot paths.
   */
  static bool throtl_schedule_next_dispatch(struct throtl_service_queue *sq,
  					  bool force)
  {
  	/* any pending children left? */
  	if (!sq->nr_pending)
  		return true;
  
  	update_min_dispatch_time(sq);
  
  	/* is the next dispatch time in the future? */
  	if (force || time_after(sq->first_pending_disptime, jiffies)) {
  		throtl_schedule_pending_timer(sq, sq->first_pending_disptime);
  		return true;
  	}
  
  	/* tell the caller to continue dispatching */
  	return false;
  }
  
  static inline void throtl_start_new_slice_with_credit(struct throtl_grp *tg,
  		bool rw, unsigned long start)
  {
  	tg->bytes_disp[rw] = 0;
  	tg->io_disp[rw] = 0;
  
  	/*
  	 * Previous slice has expired. We must have trimmed it after last
  	 * bio dispatch. That means since start of last slice, we never used
  	 * that bandwidth. Do try to make use of that bandwidth while giving
  	 * credit.
  	 */
  	if (time_after_eq(start, tg->slice_start[rw]))
  		tg->slice_start[rw] = start;
  
  	tg->slice_end[rw] = jiffies + tg->td->throtl_slice;
  	throtl_log(&tg->service_queue,
  		   "[%c] new slice with credit start=%lu end=%lu jiffies=%lu",
  		   rw == READ ? 'R' : 'W', tg->slice_start[rw],
  		   tg->slice_end[rw], jiffies);
  }
  
  static inline void throtl_start_new_slice(struct throtl_grp *tg, bool rw)
  {
  	tg->bytes_disp[rw] = 0;
  	tg->io_disp[rw] = 0;
  	tg->slice_start[rw] = jiffies;
  	tg->slice_end[rw] = jiffies + tg->td->throtl_slice;
  	throtl_log(&tg->service_queue,
  		   "[%c] new slice start=%lu end=%lu jiffies=%lu",
  		   rw == READ ? 'R' : 'W', tg->slice_start[rw],
  		   tg->slice_end[rw], jiffies);
  }
  
  static inline void throtl_set_slice_end(struct throtl_grp *tg, bool rw,
  					unsigned long jiffy_end)
  {
  	tg->slice_end[rw] = roundup(jiffy_end, tg->td->throtl_slice);
  }
  
  static inline void throtl_extend_slice(struct throtl_grp *tg, bool rw,
  				       unsigned long jiffy_end)
  {
  	throtl_set_slice_end(tg, rw, jiffy_end);
  	throtl_log(&tg->service_queue,
  		   "[%c] extend slice start=%lu end=%lu jiffies=%lu",
  		   rw == READ ? 'R' : 'W', tg->slice_start[rw],
  		   tg->slice_end[rw], jiffies);
  }
  
  /* Determine if previously allocated or extended slice is complete or not */
  static bool throtl_slice_used(struct throtl_grp *tg, bool rw)
  {
  	if (time_in_range(jiffies, tg->slice_start[rw], tg->slice_end[rw]))
  		return false;
  
  	return true;
  }
  
  /* Trim the used slices and adjust slice start accordingly */
  static inline void throtl_trim_slice(struct throtl_grp *tg, bool rw)
  {
  	unsigned long nr_slices, time_elapsed, io_trim;
  	u64 bytes_trim, tmp;
  
  	BUG_ON(time_before(tg->slice_end[rw], tg->slice_start[rw]));
  
  	/*
  	 * If bps are unlimited (-1), then time slice don't get
  	 * renewed. Don't try to trim the slice if slice is used. A new
  	 * slice will start when appropriate.
  	 */
  	if (throtl_slice_used(tg, rw))
  		return;
  
  	/*
  	 * A bio has been dispatched. Also adjust slice_end. It might happen
  	 * that initially cgroup limit was very low resulting in high
  	 * slice_end, but later limit was bumped up and bio was dispatched
  	 * sooner, then we need to reduce slice_end. A high bogus slice_end
  	 * is bad because it does not allow new slice to start.
  	 */
  
  	throtl_set_slice_end(tg, rw, jiffies + tg->td->throtl_slice);
  
  	time_elapsed = jiffies - tg->slice_start[rw];
  
  	nr_slices = time_elapsed / tg->td->throtl_slice;
  
  	if (!nr_slices)
  		return;
  	tmp = tg_bps_limit(tg, rw) * tg->td->throtl_slice * nr_slices;
  	do_div(tmp, HZ);
  	bytes_trim = tmp;
  
  	io_trim = (tg_iops_limit(tg, rw) * tg->td->throtl_slice * nr_slices) /
  		HZ;
  
  	if (!bytes_trim && !io_trim)
  		return;
  
  	if (tg->bytes_disp[rw] >= bytes_trim)
  		tg->bytes_disp[rw] -= bytes_trim;
  	else
  		tg->bytes_disp[rw] = 0;
  
  	if (tg->io_disp[rw] >= io_trim)
  		tg->io_disp[rw] -= io_trim;
  	else
  		tg->io_disp[rw] = 0;
  
  	tg->slice_start[rw] += nr_slices * tg->td->throtl_slice;
  
  	throtl_log(&tg->service_queue,
  		   "[%c] trim slice nr=%lu bytes=%llu io=%lu start=%lu end=%lu jiffies=%lu",
  		   rw == READ ? 'R' : 'W', nr_slices, bytes_trim, io_trim,
  		   tg->slice_start[rw], tg->slice_end[rw], jiffies);
  }
  
  static bool tg_with_in_iops_limit(struct throtl_grp *tg, struct bio *bio,
  				  u32 iops_limit, unsigned long *wait)
  {
  	bool rw = bio_data_dir(bio);
  	unsigned int io_allowed;
  	unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
  	u64 tmp;
  
  	if (iops_limit == UINT_MAX) {
  		if (wait)
  			*wait = 0;
  		return true;
  	}
  
  	jiffy_elapsed = jiffies - tg->slice_start[rw];
  
  	/* Round up to the next throttle slice, wait time must be nonzero */
  	jiffy_elapsed_rnd = roundup(jiffy_elapsed + 1, tg->td->throtl_slice);
  
  	/*
  	 * jiffy_elapsed_rnd should not be a big value as minimum iops can be
  	 * 1 then at max jiffy elapsed should be equivalent of 1 second as we
  	 * will allow dispatch after 1 second and after that slice should
  	 * have been trimmed.
  	 */
  
  	tmp = (u64)iops_limit * jiffy_elapsed_rnd;
  	do_div(tmp, HZ);
  
  	if (tmp > UINT_MAX)
  		io_allowed = UINT_MAX;
  	else
  		io_allowed = tmp;
  
  	if (tg->io_disp[rw] + 1 <= io_allowed) {
  		if (wait)
  			*wait = 0;
  		return true;
  	}
  
  	/* Calc approx time to dispatch */
  	jiffy_wait = jiffy_elapsed_rnd - jiffy_elapsed;
  
  	if (wait)
  		*wait = jiffy_wait;
  	return false;
  }
  
  static bool tg_with_in_bps_limit(struct throtl_grp *tg, struct bio *bio,
  				 u64 bps_limit, unsigned long *wait)
  {
  	bool rw = bio_data_dir(bio);
  	u64 bytes_allowed, extra_bytes, tmp;
  	unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
  	unsigned int bio_size = throtl_bio_data_size(bio);
  
  	if (bps_limit == U64_MAX) {
  		if (wait)
  			*wait = 0;
  		return true;
  	}
  
  	jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];
  
  	/* Slice has just started. Consider one slice interval */
  	if (!jiffy_elapsed)
  		jiffy_elapsed_rnd = tg->td->throtl_slice;
  
  	jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, tg->td->throtl_slice);
  
  	tmp = bps_limit * jiffy_elapsed_rnd;
  	do_div(tmp, HZ);
  	bytes_allowed = tmp;
  
  	if (tg->bytes_disp[rw] + bio_size <= bytes_allowed) {
  		if (wait)
  			*wait = 0;
  		return true;
  	}
  
  	/* Calc approx time to dispatch */
  	extra_bytes = tg->bytes_disp[rw] + bio_size - bytes_allowed;
  	jiffy_wait = div64_u64(extra_bytes * HZ, bps_limit);
  
  	if (!jiffy_wait)
  		jiffy_wait = 1;
  
  	/*
  	 * This wait time is without taking into consideration the rounding
  	 * up we did. Add that time also.
  	 */
  	jiffy_wait = jiffy_wait + (jiffy_elapsed_rnd - jiffy_elapsed);
  	if (wait)
  		*wait = jiffy_wait;
  	return false;
  }
  
  /*
   * Returns whether one can dispatch a bio or not. Also returns approx number
   * of jiffies to wait before this bio is with-in IO rate and can be dispatched
   */
  static bool tg_may_dispatch(struct throtl_grp *tg, struct bio *bio,
  			    unsigned long *wait)
  {
  	bool rw = bio_data_dir(bio);
  	unsigned long bps_wait = 0, iops_wait = 0, max_wait = 0;
  	u64 bps_limit = tg_bps_limit(tg, rw);
  	u32 iops_limit = tg_iops_limit(tg, rw);
  
  	/*
   	 * Currently whole state machine of group depends on first bio
  	 * queued in the group bio list. So one should not be calling
  	 * this function with a different bio if there are other bios
  	 * queued.
  	 */
  	BUG_ON(tg->service_queue.nr_queued[rw] &&
  	       bio != throtl_peek_queued(&tg->service_queue.queued[rw]));
  
  	/* If tg->bps = -1, then BW is unlimited */
  	if (bps_limit == U64_MAX && iops_limit == UINT_MAX) {
  		if (wait)
  			*wait = 0;
  		return true;
  	}
  
  	/*
  	 * If previous slice expired, start a new one otherwise renew/extend
  	 * existing slice to make sure it is at least throtl_slice interval
  	 * long since now. New slice is started only for empty throttle group.
  	 * If there is queued bio, that means there should be an active
  	 * slice and it should be extended instead.
  	 */
  	if (throtl_slice_used(tg, rw) && !(tg->service_queue.nr_queued[rw]))
  		throtl_start_new_slice(tg, rw);
  	else {
  		if (time_before(tg->slice_end[rw],
  		    jiffies + tg->td->throtl_slice))
  			throtl_extend_slice(tg, rw,
  				jiffies + tg->td->throtl_slice);
  	}
  
  	if (tg_with_in_bps_limit(tg, bio, bps_limit, &bps_wait) &&
  	    tg_with_in_iops_limit(tg, bio, iops_limit, &iops_wait)) {
  		if (wait)
  			*wait = 0;
  		return true;
  	}
  
  	max_wait = max(bps_wait, iops_wait);
  
  	if (wait)
  		*wait = max_wait;
  
  	if (time_before(tg->slice_end[rw], jiffies + max_wait))
  		throtl_extend_slice(tg, rw, jiffies + max_wait);
  
  	return false;
  }
  
  static void throtl_charge_bio(struct throtl_grp *tg, struct bio *bio)
  {
  	bool rw = bio_data_dir(bio);
  	unsigned int bio_size = throtl_bio_data_size(bio);
  
  	/* Charge the bio to the group */
  	tg->bytes_disp[rw] += bio_size;
  	tg->io_disp[rw]++;
  	tg->last_bytes_disp[rw] += bio_size;
  	tg->last_io_disp[rw]++;
  
  	/*
  	 * BIO_THROTTLED is used to prevent the same bio to be throttled
  	 * more than once as a throttled bio will go through blk-throtl the
  	 * second time when it eventually gets issued.  Set it when a bio
  	 * is being charged to a tg.
  	 */
  	if (!bio_flagged(bio, BIO_THROTTLED))
  		bio_set_flag(bio, BIO_THROTTLED);
  }
  
  /**
   * throtl_add_bio_tg - add a bio to the specified throtl_grp
   * @bio: bio to add
   * @qn: qnode to use
   * @tg: the target throtl_grp
   *
   * Add @bio to @tg's service_queue using @qn.  If @qn is not specified,
   * tg->qnode_on_self[] is used.
   */
  static void throtl_add_bio_tg(struct bio *bio, struct throtl_qnode *qn,
  			      struct throtl_grp *tg)
  {
  	struct throtl_service_queue *sq = &tg->service_queue;
  	bool rw = bio_data_dir(bio);
  
  	if (!qn)
  		qn = &tg->qnode_on_self[rw];
  
  	/*
  	 * If @tg doesn't currently have any bios queued in the same
  	 * direction, queueing @bio can change when @tg should be
  	 * dispatched.  Mark that @tg was empty.  This is automatically
  	 * cleared on the next tg_update_disptime().
  	 */
  	if (!sq->nr_queued[rw])
  		tg->flags |= THROTL_TG_WAS_EMPTY;
  
  	throtl_qnode_add_bio(bio, qn, &sq->queued[rw]);
  
  	sq->nr_queued[rw]++;
  	throtl_enqueue_tg(tg);
  }
  
  static void tg_update_disptime(struct throtl_grp *tg)
  {
  	struct throtl_service_queue *sq = &tg->service_queue;
  	unsigned long read_wait = -1, write_wait = -1, min_wait = -1, disptime;
  	struct bio *bio;
  
  	bio = throtl_peek_queued(&sq->queued[READ]);
  	if (bio)
  		tg_may_dispatch(tg, bio, &read_wait);
  
  	bio = throtl_peek_queued(&sq->queued[WRITE]);
  	if (bio)
  		tg_may_dispatch(tg, bio, &write_wait);
  
  	min_wait = min(read_wait, write_wait);
  	disptime = jiffies + min_wait;
  
  	/* Update dispatch time */
  	throtl_dequeue_tg(tg);
  	tg->disptime = disptime;
  	throtl_enqueue_tg(tg);
  
  	/* see throtl_add_bio_tg() */
  	tg->flags &= ~THROTL_TG_WAS_EMPTY;
  }
  
  static void start_parent_slice_with_credit(struct throtl_grp *child_tg,
  					struct throtl_grp *parent_tg, bool rw)
  {
  	if (throtl_slice_used(parent_tg, rw)) {
  		throtl_start_new_slice_with_credit(parent_tg, rw,
  				child_tg->slice_start[rw]);
  	}
  
  }
  
  static void tg_dispatch_one_bio(struct throtl_grp *tg, bool rw)
  {
  	struct throtl_service_queue *sq = &tg->service_queue;
  	struct throtl_service_queue *parent_sq = sq->parent_sq;
  	struct throtl_grp *parent_tg = sq_to_tg(parent_sq);
  	struct throtl_grp *tg_to_put = NULL;
  	struct bio *bio;
  
  	/*
  	 * @bio is being transferred from @tg to @parent_sq.  Popping a bio
  	 * from @tg may put its reference and @parent_sq might end up
  	 * getting released prematurely.  Remember the tg to put and put it
  	 * after @bio is transferred to @parent_sq.
  	 */
  	bio = throtl_pop_queued(&sq->queued[rw], &tg_to_put);
  	sq->nr_queued[rw]--;
  
  	throtl_charge_bio(tg, bio);
  
  	/*
  	 * If our parent is another tg, we just need to transfer @bio to
  	 * the parent using throtl_add_bio_tg().  If our parent is
  	 * @td->service_queue, @bio is ready to be issued.  Put it on its
  	 * bio_lists[] and decrease total number queued.  The caller is
  	 * responsible for issuing these bios.
  	 */
  	if (parent_tg) {
  		throtl_add_bio_tg(bio, &tg->qnode_on_parent[rw], parent_tg);
  		start_parent_slice_with_credit(tg, parent_tg, rw);
  	} else {
  		throtl_qnode_add_bio(bio, &tg->qnode_on_parent[rw],
  				     &parent_sq->queued[rw]);
  		BUG_ON(tg->td->nr_queued[rw] <= 0);
  		tg->td->nr_queued[rw]--;
  	}
  
  	throtl_trim_slice(tg, rw);
  
  	if (tg_to_put)
  		blkg_put(tg_to_blkg(tg_to_put));
  }
  
  static int throtl_dispatch_tg(struct throtl_grp *tg)
  {
  	struct throtl_service_queue *sq = &tg->service_queue;
  	unsigned int nr_reads = 0, nr_writes = 0;
  	unsigned int max_nr_reads = THROTL_GRP_QUANTUM * 3 / 4;
  	unsigned int max_nr_writes = THROTL_GRP_QUANTUM - max_nr_reads;
  	struct bio *bio;
  
  	/* Try to dispatch 75% READS and 25% WRITES */
  
  	while ((bio = throtl_peek_queued(&sq->queued[READ])) &&
  	       tg_may_dispatch(tg, bio, NULL)) {
  
  		tg_dispatch_one_bio(tg, bio_data_dir(bio));
  		nr_reads++;
  
  		if (nr_reads >= max_nr_reads)
  			break;
  	}
  
  	while ((bio = throtl_peek_queued(&sq->queued[WRITE])) &&
  	       tg_may_dispatch(tg, bio, NULL)) {
  
  		tg_dispatch_one_bio(tg, bio_data_dir(bio));
  		nr_writes++;
  
  		if (nr_writes >= max_nr_writes)
  			break;
  	}
  
  	return nr_reads + nr_writes;
  }
  
  static int throtl_select_dispatch(struct throtl_service_queue *parent_sq)
  {
  	unsigned int nr_disp = 0;
  
  	while (1) {
  		struct throtl_grp *tg;
  		struct throtl_service_queue *sq;
  
  		if (!parent_sq->nr_pending)
  			break;
  
  		tg = throtl_rb_first(parent_sq);
  		if (!tg)
  			break;
  
  		if (time_before(jiffies, tg->disptime))
  			break;
  
  		throtl_dequeue_tg(tg);
  
  		nr_disp += throtl_dispatch_tg(tg);
  
  		sq = &tg->service_queue;
  		if (sq->nr_queued[0] || sq->nr_queued[1])
  			tg_update_disptime(tg);
  
  		if (nr_disp >= THROTL_QUANTUM)
  			break;
  	}
  
  	return nr_disp;
  }
  
  static bool throtl_can_upgrade(struct throtl_data *td,
  	struct throtl_grp *this_tg);
  /**
   * throtl_pending_timer_fn - timer function for service_queue->pending_timer
   * @t: the pending_timer member of the throtl_service_queue being serviced
   *
   * This timer is armed when a child throtl_grp with active bio's become
   * pending and queued on the service_queue's pending_tree and expires when
   * the first child throtl_grp should be dispatched.  This function
   * dispatches bio's from the children throtl_grps to the parent
   * service_queue.
   *
   * If the parent's parent is another throtl_grp, dispatching is propagated
   * by either arming its pending_timer or repeating dispatch directly.  If
   * the top-level service_tree is reached, throtl_data->dispatch_work is
   * kicked so that the ready bio's are issued.
   */
  static void throtl_pending_timer_fn(struct timer_list *t)
  {
  	struct throtl_service_queue *sq = from_timer(sq, t, pending_timer);
  	struct throtl_grp *tg = sq_to_tg(sq);
  	struct throtl_data *td = sq_to_td(sq);
  	struct request_queue *q = td->queue;
  	struct throtl_service_queue *parent_sq;
  	bool dispatched;
  	int ret;
  
  	spin_lock_irq(&q->queue_lock);
  	if (throtl_can_upgrade(td, NULL))
  		throtl_upgrade_state(td);
  
  again:
  	parent_sq = sq->parent_sq;
  	dispatched = false;
  
  	while (true) {
  		throtl_log(sq, "dispatch nr_queued=%u read=%u write=%u",
  			   sq->nr_queued[READ] + sq->nr_queued[WRITE],
  			   sq->nr_queued[READ], sq->nr_queued[WRITE]);
  
  		ret = throtl_select_dispatch(sq);
  		if (ret) {
  			throtl_log(sq, "bios disp=%u", ret);
  			dispatched = true;
  		}
  
  		if (throtl_schedule_next_dispatch(sq, false))
  			break;
  
  		/* this dispatch windows is still open, relax and repeat */
  		spin_unlock_irq(&q->queue_lock);
  		cpu_relax();
  		spin_lock_irq(&q->queue_lock);
  	}
  
  	if (!dispatched)
  		goto out_unlock;
  
  	if (parent_sq) {
  		/* @parent_sq is another throl_grp, propagate dispatch */
  		if (tg->flags & THROTL_TG_WAS_EMPTY) {
  			tg_update_disptime(tg);
  			if (!throtl_schedule_next_dispatch(parent_sq, false)) {
  				/* window is already open, repeat dispatching */
  				sq = parent_sq;
  				tg = sq_to_tg(sq);
  				goto again;
  			}
  		}
  	} else {
  		/* reached the top-level, queue issuing */
  		queue_work(kthrotld_workqueue, &td->dispatch_work);
  	}
  out_unlock:
  	spin_unlock_irq(&q->queue_lock);
  }
  
  /**
   * blk_throtl_dispatch_work_fn - work function for throtl_data->dispatch_work
   * @work: work item being executed
   *
   * This function is queued for execution when bios reach the bio_lists[]
   * of throtl_data->service_queue.  Those bios are ready and issued by this
   * function.
   */
  static void blk_throtl_dispatch_work_fn(struct work_struct *work)
  {
  	struct throtl_data *td = container_of(work, struct throtl_data,
  					      dispatch_work);
  	struct throtl_service_queue *td_sq = &td->service_queue;
  	struct request_queue *q = td->queue;
  	struct bio_list bio_list_on_stack;
  	struct bio *bio;
  	struct blk_plug plug;
  	int rw;
  
  	bio_list_init(&bio_list_on_stack);
  
  	spin_lock_irq(&q->queue_lock);
  	for (rw = READ; rw <= WRITE; rw++)
  		while ((bio = throtl_pop_queued(&td_sq->queued[rw], NULL)))
  			bio_list_add(&bio_list_on_stack, bio);
  	spin_unlock_irq(&q->queue_lock);
  
  	if (!bio_list_empty(&bio_list_on_stack)) {
  		blk_start_plug(&plug);
  		while ((bio = bio_list_pop(&bio_list_on_stack)))
  			submit_bio_noacct(bio);
  		blk_finish_plug(&plug);
  	}
  }
  
  static u64 tg_prfill_conf_u64(struct seq_file *sf, struct blkg_policy_data *pd,
  			      int off)
  {
  	struct throtl_grp *tg = pd_to_tg(pd);
  	u64 v = *(u64 *)((void *)tg + off);
  
  	if (v == U64_MAX)
  		return 0;
  	return __blkg_prfill_u64(sf, pd, v);
  }
  
  static u64 tg_prfill_conf_uint(struct seq_file *sf, struct blkg_policy_data *pd,
  			       int off)
  {
  	struct throtl_grp *tg = pd_to_tg(pd);
  	unsigned int v = *(unsigned int *)((void *)tg + off);
  
  	if (v == UINT_MAX)
  		return 0;
  	return __blkg_prfill_u64(sf, pd, v);
  }
  
  static int tg_print_conf_u64(struct seq_file *sf, void *v)
  {
  	blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)), tg_prfill_conf_u64,
  			  &blkcg_policy_throtl, seq_cft(sf)->private, false);
  	return 0;
  }
  
  static int tg_print_conf_uint(struct seq_file *sf, void *v)
  {
  	blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)), tg_prfill_conf_uint,
  			  &blkcg_policy_throtl, seq_cft(sf)->private, false);
  	return 0;
  }
  
  static void tg_conf_updated(struct throtl_grp *tg, bool global)
  {
  	struct throtl_service_queue *sq = &tg->service_queue;
  	struct cgroup_subsys_state *pos_css;
  	struct blkcg_gq *blkg;
  
  	throtl_log(&tg->service_queue,
  		   "limit change rbps=%llu wbps=%llu riops=%u wiops=%u",
  		   tg_bps_limit(tg, READ), tg_bps_limit(tg, WRITE),
  		   tg_iops_limit(tg, READ), tg_iops_limit(tg, WRITE));
  
  	/*
  	 * Update has_rules[] flags for the updated tg's subtree.  A tg is
  	 * considered to have rules if either the tg itself or any of its
  	 * ancestors has rules.  This identifies groups without any
  	 * restrictions in the whole hierarchy and allows them to bypass
  	 * blk-throttle.
  	 */
  	blkg_for_each_descendant_pre(blkg, pos_css,
  			global ? tg->td->queue->root_blkg : tg_to_blkg(tg)) {
  		struct throtl_grp *this_tg = blkg_to_tg(blkg);
  		struct throtl_grp *parent_tg;
  
  		tg_update_has_rules(this_tg);
  		/* ignore root/second level */
  		if (!cgroup_subsys_on_dfl(io_cgrp_subsys) || !blkg->parent ||
  		    !blkg->parent->parent)
  			continue;
  		parent_tg = blkg_to_tg(blkg->parent);
  		/*
  		 * make sure all children has lower idle time threshold and
  		 * higher latency target
  		 */
  		this_tg->idletime_threshold = min(this_tg->idletime_threshold,
  				parent_tg->idletime_threshold);
  		this_tg->latency_target = max(this_tg->latency_target,
  				parent_tg->latency_target);
  	}
  
  	/*
  	 * We're already holding queue_lock and know @tg is valid.  Let's
  	 * apply the new config directly.
  	 *
  	 * Restart the slices for both READ and WRITES. It might happen
  	 * that a group's limit are dropped suddenly and we don't want to
  	 * account recently dispatched IO with new low rate.
  	 */
  	throtl_start_new_slice(tg, READ);
  	throtl_start_new_slice(tg, WRITE);
  
  	if (tg->flags & THROTL_TG_PENDING) {
  		tg_update_disptime(tg);
  		throtl_schedule_next_dispatch(sq->parent_sq, true);
  	}
  }
  
  static ssize_t tg_set_conf(struct kernfs_open_file *of,
  			   char *buf, size_t nbytes, loff_t off, bool is_u64)
  {
  	struct blkcg *blkcg = css_to_blkcg(of_css(of));
  	struct blkg_conf_ctx ctx;
  	struct throtl_grp *tg;
  	int ret;
  	u64 v;
  
  	ret = blkg_conf_prep(blkcg, &blkcg_policy_throtl, buf, &ctx);
  	if (ret)
  		return ret;
  
  	ret = -EINVAL;
  	if (sscanf(ctx.body, "%llu", &v) != 1)
  		goto out_finish;
  	if (!v)
  		v = U64_MAX;
  
  	tg = blkg_to_tg(ctx.blkg);
  
  	if (is_u64)
  		*(u64 *)((void *)tg + of_cft(of)->private) = v;
  	else
  		*(unsigned int *)((void *)tg + of_cft(of)->private) = v;
  
  	tg_conf_updated(tg, false);
  	ret = 0;
  out_finish:
  	blkg_conf_finish(&ctx);
  	return ret ?: nbytes;
  }
  
  static ssize_t tg_set_conf_u64(struct kernfs_open_file *of,
  			       char *buf, size_t nbytes, loff_t off)
  {
  	return tg_set_conf(of, buf, nbytes, off, true);
  }
  
  static ssize_t tg_set_conf_uint(struct kernfs_open_file *of,
  				char *buf, size_t nbytes, loff_t off)
  {
  	return tg_set_conf(of, buf, nbytes, off, false);
  }
  
  static int tg_print_rwstat(struct seq_file *sf, void *v)
  {
  	blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)),
  			  blkg_prfill_rwstat, &blkcg_policy_throtl,
  			  seq_cft(sf)->private, true);
  	return 0;
  }
  
  static u64 tg_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_throtl, off,
  				  &sum);
  	return __blkg_prfill_rwstat(sf, pd, &sum);
  }
  
  static int tg_print_rwstat_recursive(struct seq_file *sf, void *v)
  {
  	blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)),
  			  tg_prfill_rwstat_recursive, &blkcg_policy_throtl,
  			  seq_cft(sf)->private, true);
  	return 0;
  }
  
  static struct cftype throtl_legacy_files[] = {
  	{
  		.name = "throttle.read_bps_device",
  		.private = offsetof(struct throtl_grp, bps[READ][LIMIT_MAX]),
  		.seq_show = tg_print_conf_u64,
  		.write = tg_set_conf_u64,
  	},
  	{
  		.name = "throttle.write_bps_device",
  		.private = offsetof(struct throtl_grp, bps[WRITE][LIMIT_MAX]),
  		.seq_show = tg_print_conf_u64,
  		.write = tg_set_conf_u64,
  	},
  	{
  		.name = "throttle.read_iops_device",
  		.private = offsetof(struct throtl_grp, iops[READ][LIMIT_MAX]),
  		.seq_show = tg_print_conf_uint,
  		.write = tg_set_conf_uint,
  	},
  	{
  		.name = "throttle.write_iops_device",
  		.private = offsetof(struct throtl_grp, iops[WRITE][LIMIT_MAX]),
  		.seq_show = tg_print_conf_uint,
  		.write = tg_set_conf_uint,
  	},
  	{
  		.name = "throttle.io_service_bytes",
  		.private = offsetof(struct throtl_grp, stat_bytes),
  		.seq_show = tg_print_rwstat,
  	},
  	{
  		.name = "throttle.io_service_bytes_recursive",
  		.private = offsetof(struct throtl_grp, stat_bytes),
  		.seq_show = tg_print_rwstat_recursive,
  	},
  	{
  		.name = "throttle.io_serviced",
  		.private = offsetof(struct throtl_grp, stat_ios),
  		.seq_show = tg_print_rwstat,
  	},
  	{
  		.name = "throttle.io_serviced_recursive",
  		.private = offsetof(struct throtl_grp, stat_ios),
  		.seq_show = tg_print_rwstat_recursive,
  	},
  	{ }	/* terminate */
  };
  
  static u64 tg_prfill_limit(struct seq_file *sf, struct blkg_policy_data *pd,
  			 int off)
  {
  	struct throtl_grp *tg = pd_to_tg(pd);
  	const char *dname = blkg_dev_name(pd->blkg);
  	char bufs[4][21] = { "max", "max", "max", "max" };
  	u64 bps_dft;
  	unsigned int iops_dft;
  	char idle_time[26] = "";
  	char latency_time[26] = "";
  
  	if (!dname)
  		return 0;
  
  	if (off == LIMIT_LOW) {
  		bps_dft = 0;
  		iops_dft = 0;
  	} else {
  		bps_dft = U64_MAX;
  		iops_dft = UINT_MAX;
  	}
  
  	if (tg->bps_conf[READ][off] == bps_dft &&
  	    tg->bps_conf[WRITE][off] == bps_dft &&
  	    tg->iops_conf[READ][off] == iops_dft &&
  	    tg->iops_conf[WRITE][off] == iops_dft &&
  	    (off != LIMIT_LOW ||
  	     (tg->idletime_threshold_conf == DFL_IDLE_THRESHOLD &&
  	      tg->latency_target_conf == DFL_LATENCY_TARGET)))
  		return 0;
  
  	if (tg->bps_conf[READ][off] != U64_MAX)
  		snprintf(bufs[0], sizeof(bufs[0]), "%llu",
  			tg->bps_conf[READ][off]);
  	if (tg->bps_conf[WRITE][off] != U64_MAX)
  		snprintf(bufs[1], sizeof(bufs[1]), "%llu",
  			tg->bps_conf[WRITE][off]);
  	if (tg->iops_conf[READ][off] != UINT_MAX)
  		snprintf(bufs[2], sizeof(bufs[2]), "%u",
  			tg->iops_conf[READ][off]);
  	if (tg->iops_conf[WRITE][off] != UINT_MAX)
  		snprintf(bufs[3], sizeof(bufs[3]), "%u",
  			tg->iops_conf[WRITE][off]);
  	if (off == LIMIT_LOW) {
  		if (tg->idletime_threshold_conf == ULONG_MAX)
  			strcpy(idle_time, " idle=max");
  		else
  			snprintf(idle_time, sizeof(idle_time), " idle=%lu",
  				tg->idletime_threshold_conf);
  
  		if (tg->latency_target_conf == ULONG_MAX)
  			strcpy(latency_time, " latency=max");
  		else
  			snprintf(latency_time, sizeof(latency_time),
  				" latency=%lu", tg->latency_target_conf);
  	}
  
  	seq_printf(sf, "%s rbps=%s wbps=%s riops=%s wiops=%s%s%s\n",
  		   dname, bufs[0], bufs[1], bufs[2], bufs[3], idle_time,
  		   latency_time);
  	return 0;
  }
  
  static int tg_print_limit(struct seq_file *sf, void *v)
  {
  	blkcg_print_blkgs(sf, css_to_blkcg(seq_css(sf)), tg_prfill_limit,
  			  &blkcg_policy_throtl, seq_cft(sf)->private, false);
  	return 0;
  }
  
  static ssize_t tg_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 blkg_conf_ctx ctx;
  	struct throtl_grp *tg;
  	u64 v[4];
  	unsigned long idle_time;
  	unsigned long latency_time;
  	int ret;
  	int index = of_cft(of)->private;
  
  	ret = blkg_conf_prep(blkcg, &blkcg_policy_throtl, buf, &ctx);
  	if (ret)
  		return ret;
  
  	tg = blkg_to_tg(ctx.blkg);
  
  	v[0] = tg->bps_conf[READ][index];
  	v[1] = tg->bps_conf[WRITE][index];
  	v[2] = tg->iops_conf[READ][index];
  	v[3] = tg->iops_conf[WRITE][index];
  
  	idle_time = tg->idletime_threshold_conf;
  	latency_time = tg->latency_target_conf;
  	while (true) {
  		char tok[27];	/* wiops=18446744073709551616 */
  		char *p;
  		u64 val = U64_MAX;
  		int len;
  
  		if (sscanf(ctx.body, "%26s%n", tok, &len) != 1)
  			break;
  		if (tok[0] == '\0')
  			break;
  		ctx.body += len;
  
  		ret = -EINVAL;
  		p = tok;
  		strsep(&p, "=");
  		if (!p || (sscanf(p, "%llu", &val) != 1 && strcmp(p, "max")))
  			goto out_finish;
  
  		ret = -ERANGE;
  		if (!val)
  			goto out_finish;
  
  		ret = -EINVAL;
  		if (!strcmp(tok, "rbps") && val > 1)
  			v[0] = val;
  		else if (!strcmp(tok, "wbps") && val > 1)
  			v[1] = val;
  		else if (!strcmp(tok, "riops") && val > 1)
  			v[2] = min_t(u64, val, UINT_MAX);
  		else if (!strcmp(tok, "wiops") && val > 1)
  			v[3] = min_t(u64, val, UINT_MAX);
  		else if (off == LIMIT_LOW && !strcmp(tok, "idle"))
  			idle_time = val;
  		else if (off == LIMIT_LOW && !strcmp(tok, "latency"))
  			latency_time = val;
  		else
  			goto out_finish;
  	}
  
  	tg->bps_conf[READ][index] = v[0];
  	tg->bps_conf[WRITE][index] = v[1];
  	tg->iops_conf[READ][index] = v[2];
  	tg->iops_conf[WRITE][index] = v[3];
  
  	if (index == LIMIT_MAX) {
  		tg->bps[READ][index] = v[0];
  		tg->bps[WRITE][index] = v[1];
  		tg->iops[READ][index] = v[2];
  		tg->iops[WRITE][index] = v[3];
  	}
  	tg->bps[READ][LIMIT_LOW] = min(tg->bps_conf[READ][LIMIT_LOW],
  		tg->bps_conf[READ][LIMIT_MAX]);
  	tg->bps[WRITE][LIMIT_LOW] = min(tg->bps_conf[WRITE][LIMIT_LOW],
  		tg->bps_conf[WRITE][LIMIT_MAX]);
  	tg->iops[READ][LIMIT_LOW] = min(tg->iops_conf[READ][LIMIT_LOW],
  		tg->iops_conf[READ][LIMIT_MAX]);
  	tg->iops[WRITE][LIMIT_LOW] = min(tg->iops_conf[WRITE][LIMIT_LOW],
  		tg->iops_conf[WRITE][LIMIT_MAX]);
  	tg->idletime_threshold_conf = idle_time;
  	tg->latency_target_conf = latency_time;
  
  	/* force user to configure all settings for low limit  */
  	if (!(tg->bps[READ][LIMIT_LOW] || tg->iops[READ][LIMIT_LOW] ||
  	      tg->bps[WRITE][LIMIT_LOW] || tg->iops[WRITE][LIMIT_LOW]) ||
  	    tg->idletime_threshold_conf == DFL_IDLE_THRESHOLD ||
  	    tg->latency_target_conf == DFL_LATENCY_TARGET) {
  		tg->bps[READ][LIMIT_LOW] = 0;
  		tg->bps[WRITE][LIMIT_LOW] = 0;
  		tg->iops[READ][LIMIT_LOW] = 0;
  		tg->iops[WRITE][LIMIT_LOW] = 0;
  		tg->idletime_threshold = DFL_IDLE_THRESHOLD;
  		tg->latency_target = DFL_LATENCY_TARGET;
  	} else if (index == LIMIT_LOW) {
  		tg->idletime_threshold = tg->idletime_threshold_conf;
  		tg->latency_target = tg->latency_target_conf;
  	}
  
  	blk_throtl_update_limit_valid(tg->td);
  	if (tg->td->limit_valid[LIMIT_LOW]) {
  		if (index == LIMIT_LOW)
  			tg->td->limit_index = LIMIT_LOW;
  	} else
  		tg->td->limit_index = LIMIT_MAX;
  	tg_conf_updated(tg, index == LIMIT_LOW &&
  		tg->td->limit_valid[LIMIT_LOW]);
  	ret = 0;
  out_finish:
  	blkg_conf_finish(&ctx);
  	return ret ?: nbytes;
  }
  
  static struct cftype throtl_files[] = {
  #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
  	{
  		.name = "low",
  		.flags = CFTYPE_NOT_ON_ROOT,
  		.seq_show = tg_print_limit,
  		.write = tg_set_limit,
  		.private = LIMIT_LOW,
  	},
  #endif
  	{
  		.name = "max",
  		.flags = CFTYPE_NOT_ON_ROOT,
  		.seq_show = tg_print_limit,
  		.write = tg_set_limit,
  		.private = LIMIT_MAX,
  	},
  	{ }	/* terminate */
  };
  
  static void throtl_shutdown_wq(struct request_queue *q)
  {
  	struct throtl_data *td = q->td;
  
  	cancel_work_sync(&td->dispatch_work);
  }
  
  static struct blkcg_policy blkcg_policy_throtl = {
  	.dfl_cftypes		= throtl_files,
  	.legacy_cftypes		= throtl_legacy_files,
  
  	.pd_alloc_fn		= throtl_pd_alloc,
  	.pd_init_fn		= throtl_pd_init,
  	.pd_online_fn		= throtl_pd_online,
  	.pd_offline_fn		= throtl_pd_offline,
  	.pd_free_fn		= throtl_pd_free,
  };
  
  static unsigned long __tg_last_low_overflow_time(struct throtl_grp *tg)
  {
  	unsigned long rtime = jiffies, wtime = jiffies;
  
  	if (tg->bps[READ][LIMIT_LOW] || tg->iops[READ][LIMIT_LOW])
  		rtime = tg->last_low_overflow_time[READ];
  	if (tg->bps[WRITE][LIMIT_LOW] || tg->iops[WRITE][LIMIT_LOW])
  		wtime = tg->last_low_overflow_time[WRITE];
  	return min(rtime, wtime);
  }
  
  /* tg should not be an intermediate node */
  static unsigned long tg_last_low_overflow_time(struct throtl_grp *tg)
  {
  	struct throtl_service_queue *parent_sq;
  	struct throtl_grp *parent = tg;
  	unsigned long ret = __tg_last_low_overflow_time(tg);
  
  	while (true) {
  		parent_sq = parent->service_queue.parent_sq;
  		parent = sq_to_tg(parent_sq);
  		if (!parent)
  			break;
  
  		/*
  		 * The parent doesn't have low limit, it always reaches low
  		 * limit. Its overflow time is useless for children
  		 */
  		if (!parent->bps[READ][LIMIT_LOW] &&
  		    !parent->iops[READ][LIMIT_LOW] &&
  		    !parent->bps[WRITE][LIMIT_LOW] &&
  		    !parent->iops[WRITE][LIMIT_LOW])
  			continue;
  		if (time_after(__tg_last_low_overflow_time(parent), ret))
  			ret = __tg_last_low_overflow_time(parent);
  	}
  	return ret;
  }
  
  static bool throtl_tg_is_idle(struct throtl_grp *tg)
  {
  	/*
  	 * cgroup is idle if:
  	 * - single idle is too long, longer than a fixed value (in case user
  	 *   configure a too big threshold) or 4 times of idletime threshold
  	 * - average think time is more than threshold
  	 * - IO latency is largely below threshold
  	 */
  	unsigned long time;
  	bool ret;
  
  	time = min_t(unsigned long, MAX_IDLE_TIME, 4 * tg->idletime_threshold);
  	ret = tg->latency_target == DFL_LATENCY_TARGET ||
  	      tg->idletime_threshold == DFL_IDLE_THRESHOLD ||
  	      (ktime_get_ns() >> 10) - tg->last_finish_time > time ||
  	      tg->avg_idletime > tg->idletime_threshold ||
  	      (tg->latency_target && tg->bio_cnt &&
  		tg->bad_bio_cnt * 5 < tg->bio_cnt);
  	throtl_log(&tg->service_queue,
  		"avg_idle=%ld, idle_threshold=%ld, bad_bio=%d, total_bio=%d, is_idle=%d, scale=%d",
  		tg->avg_idletime, tg->idletime_threshold, tg->bad_bio_cnt,
  		tg->bio_cnt, ret, tg->td->scale);
  	return ret;
  }
  
  static bool throtl_tg_can_upgrade(struct throtl_grp *tg)
  {
  	struct throtl_service_queue *sq = &tg->service_queue;
  	bool read_limit, write_limit;
  
  	/*
  	 * if cgroup reaches low limit (if low limit is 0, the cgroup always
  	 * reaches), it's ok to upgrade to next limit
  	 */
  	read_limit = tg->bps[READ][LIMIT_LOW] || tg->iops[READ][LIMIT_LOW];
  	write_limit = tg->bps[WRITE][LIMIT_LOW] || tg->iops[WRITE][LIMIT_LOW];
  	if (!read_limit && !write_limit)
  		return true;
  	if (read_limit && sq->nr_queued[READ] &&
  	    (!write_limit || sq->nr_queued[WRITE]))
  		return true;
  	if (write_limit && sq->nr_queued[WRITE] &&
  	    (!read_limit || sq->nr_queued[READ]))
  		return true;
  
  	if (time_after_eq(jiffies,
  		tg_last_low_overflow_time(tg) + tg->td->throtl_slice) &&
  	    throtl_tg_is_idle(tg))
  		return true;
  	return false;
  }
  
  static bool throtl_hierarchy_can_upgrade(struct throtl_grp *tg)
  {
  	while (true) {
  		if (throtl_tg_can_upgrade(tg))
  			return true;
  		tg = sq_to_tg(tg->service_queue.parent_sq);
  		if (!tg || !tg_to_blkg(tg)->parent)
  			return false;
  	}
  	return false;
  }
  
  static bool throtl_can_upgrade(struct throtl_data *td,
  	struct throtl_grp *this_tg)
  {
  	struct cgroup_subsys_state *pos_css;
  	struct blkcg_gq *blkg;
  
  	if (td->limit_index != LIMIT_LOW)
  		return false;
  
  	if (time_before(jiffies, td->low_downgrade_time + td->throtl_slice))
  		return false;
  
  	rcu_read_lock();
  	blkg_for_each_descendant_post(blkg, pos_css, td->queue->root_blkg) {
  		struct throtl_grp *tg = blkg_to_tg(blkg);
  
  		if (tg == this_tg)
  			continue;
  		if (!list_empty(&tg_to_blkg(tg)->blkcg->css.children))
  			continue;
  		if (!throtl_hierarchy_can_upgrade(tg)) {
  			rcu_read_unlock();
  			return false;
  		}
  	}
  	rcu_read_unlock();
  	return true;
  }
  
  static void throtl_upgrade_check(struct throtl_grp *tg)
  {
  	unsigned long now = jiffies;
  
  	if (tg->td->limit_index != LIMIT_LOW)
  		return;
  
  	if (time_after(tg->last_check_time + tg->td->throtl_slice, now))
  		return;
  
  	tg->last_check_time = now;
  
  	if (!time_after_eq(now,
  	     __tg_last_low_overflow_time(tg) + tg->td->throtl_slice))
  		return;
  
  	if (throtl_can_upgrade(tg->td, NULL))
  		throtl_upgrade_state(tg->td);
  }
  
  static void throtl_upgrade_state(struct throtl_data *td)
  {
  	struct cgroup_subsys_state *pos_css;
  	struct blkcg_gq *blkg;
  
  	throtl_log(&td->service_queue, "upgrade to max");
  	td->limit_index = LIMIT_MAX;
  	td->low_upgrade_time = jiffies;
  	td->scale = 0;
  	rcu_read_lock();
  	blkg_for_each_descendant_post(blkg, pos_css, td->queue->root_blkg) {
  		struct throtl_grp *tg = blkg_to_tg(blkg);
  		struct throtl_service_queue *sq = &tg->service_queue;
  
  		tg->disptime = jiffies - 1;
  		throtl_select_dispatch(sq);
  		throtl_schedule_next_dispatch(sq, true);
  	}
  	rcu_read_unlock();
  	throtl_select_dispatch(&td->service_queue);
  	throtl_schedule_next_dispatch(&td->service_queue, true);
  	queue_work(kthrotld_workqueue, &td->dispatch_work);
  }
  
  static void throtl_downgrade_state(struct throtl_data *td)
  {
  	td->scale /= 2;
  
  	throtl_log(&td->service_queue, "downgrade, scale %d", td->scale);
  	if (td->scale) {
  		td->low_upgrade_time = jiffies - td->scale * td->throtl_slice;
  		return;
  	}
  
  	td->limit_index = LIMIT_LOW;
  	td->low_downgrade_time = jiffies;
  }
  
  static bool throtl_tg_can_downgrade(struct throtl_grp *tg)
  {
  	struct throtl_data *td = tg->td;
  	unsigned long now = jiffies;
  
  	/*
  	 * If cgroup is below low limit, consider downgrade and throttle other
  	 * cgroups
  	 */
  	if (time_after_eq(now, td->low_upgrade_time + td->throtl_slice) &&
  	    time_after_eq(now, tg_last_low_overflow_time(tg) +
  					td->throtl_slice) &&
  	    (!throtl_tg_is_idle(tg) ||
  	     !list_empty(&tg_to_blkg(tg)->blkcg->css.children)))
  		return true;
  	return false;
  }
  
  static bool throtl_hierarchy_can_downgrade(struct throtl_grp *tg)
  {
  	while (true) {
  		if (!throtl_tg_can_downgrade(tg))
  			return false;
  		tg = sq_to_tg(tg->service_queue.parent_sq);
  		if (!tg || !tg_to_blkg(tg)->parent)
  			break;
  	}
  	return true;
  }
  
  static void throtl_downgrade_check(struct throtl_grp *tg)
  {
  	uint64_t bps;
  	unsigned int iops;
  	unsigned long elapsed_time;
  	unsigned long now = jiffies;
  
  	if (tg->td->limit_index != LIMIT_MAX ||
  	    !tg->td->limit_valid[LIMIT_LOW])
  		return;
  	if (!list_empty(&tg_to_blkg(tg)->blkcg->css.children))
  		return;
  	if (time_after(tg->last_check_time + tg->td->throtl_slice, now))
  		return;
  
  	elapsed_time = now - tg->last_check_time;
  	tg->last_check_time = now;
  
  	if (time_before(now, tg_last_low_overflow_time(tg) +
  			tg->td->throtl_slice))
  		return;
  
  	if (tg->bps[READ][LIMIT_LOW]) {
  		bps = tg->last_bytes_disp[READ] * HZ;
  		do_div(bps, elapsed_time);
  		if (bps >= tg->bps[READ][LIMIT_LOW])
  			tg->last_low_overflow_time[READ] = now;
  	}
  
  	if (tg->bps[WRITE][LIMIT_LOW]) {
  		bps = tg->last_bytes_disp[WRITE] * HZ;
  		do_div(bps, elapsed_time);
  		if (bps >= tg->bps[WRITE][LIMIT_LOW])
  			tg->last_low_overflow_time[WRITE] = now;
  	}
  
  	if (tg->iops[READ][LIMIT_LOW]) {
  		iops = tg->last_io_disp[READ] * HZ / elapsed_time;
  		if (iops >= tg->iops[READ][LIMIT_LOW])
  			tg->last_low_overflow_time[READ] = now;
  	}
  
  	if (tg->iops[WRITE][LIMIT_LOW]) {
  		iops = tg->last_io_disp[WRITE] * HZ / elapsed_time;
  		if (iops >= tg->iops[WRITE][LIMIT_LOW])
  			tg->last_low_overflow_time[WRITE] = now;
  	}
  
  	/*
  	 * If cgroup is below low limit, consider downgrade and throttle other
  	 * cgroups
  	 */
  	if (throtl_hierarchy_can_downgrade(tg))
  		throtl_downgrade_state(tg->td);
  
  	tg->last_bytes_disp[READ] = 0;
  	tg->last_bytes_disp[WRITE] = 0;
  	tg->last_io_disp[READ] = 0;
  	tg->last_io_disp[WRITE] = 0;
  }
  
  static void blk_throtl_update_idletime(struct throtl_grp *tg)
  {
  	unsigned long now;
  	unsigned long last_finish_time = tg->last_finish_time;
  
  	if (last_finish_time == 0)
  		return;
  
  	now = ktime_get_ns() >> 10;
  	if (now <= last_finish_time ||
  	    last_finish_time == tg->checked_last_finish_time)
  		return;
  
  	tg->avg_idletime = (tg->avg_idletime * 7 + now - last_finish_time) >> 3;
  	tg->checked_last_finish_time = last_finish_time;
  }
  
  #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
  static void throtl_update_latency_buckets(struct throtl_data *td)
  {
  	struct avg_latency_bucket avg_latency[2][LATENCY_BUCKET_SIZE];
  	int i, cpu, rw;
  	unsigned long last_latency[2] = { 0 };
  	unsigned long latency[2];
  
  	if (!blk_queue_nonrot(td->queue) || !td->limit_valid[LIMIT_LOW])
  		return;
  	if (time_before(jiffies, td->last_calculate_time + HZ))
  		return;
  	td->last_calculate_time = jiffies;
  
  	memset(avg_latency, 0, sizeof(avg_latency));
  	for (rw = READ; rw <= WRITE; rw++) {
  		for (i = 0; i < LATENCY_BUCKET_SIZE; i++) {
  			struct latency_bucket *tmp = &td->tmp_buckets[rw][i];
  
  			for_each_possible_cpu(cpu) {
  				struct latency_bucket *bucket;
  
  				/* this isn't race free, but ok in practice */
  				bucket = per_cpu_ptr(td->latency_buckets[rw],
  					cpu);
  				tmp->total_latency += bucket[i].total_latency;
  				tmp->samples += bucket[i].samples;
  				bucket[i].total_latency = 0;
  				bucket[i].samples = 0;
  			}
  
  			if (tmp->samples >= 32) {
  				int samples = tmp->samples;
  
  				latency[rw] = tmp->total_latency;
  
  				tmp->total_latency = 0;
  				tmp->samples = 0;
  				latency[rw] /= samples;
  				if (latency[rw] == 0)
  					continue;
  				avg_latency[rw][i].latency = latency[rw];
  			}
  		}
  	}
  
  	for (rw = READ; rw <= WRITE; rw++) {
  		for (i = 0; i < LATENCY_BUCKET_SIZE; i++) {
  			if (!avg_latency[rw][i].latency) {
  				if (td->avg_buckets[rw][i].latency < last_latency[rw])
  					td->avg_buckets[rw][i].latency =
  						last_latency[rw];
  				continue;
  			}
  
  			if (!td->avg_buckets[rw][i].valid)
  				latency[rw] = avg_latency[rw][i].latency;
  			else
  				latency[rw] = (td->avg_buckets[rw][i].latency * 7 +
  					avg_latency[rw][i].latency) >> 3;
  
  			td->avg_buckets[rw][i].latency = max(latency[rw],
  				last_latency[rw]);
  			td->avg_buckets[rw][i].valid = true;
  			last_latency[rw] = td->avg_buckets[rw][i].latency;
  		}
  	}
  
  	for (i = 0; i < LATENCY_BUCKET_SIZE; i++)
  		throtl_log(&td->service_queue,
  			"Latency bucket %d: read latency=%ld, read valid=%d, "
  			"write latency=%ld, write valid=%d", i,
  			td->avg_buckets[READ][i].latency,
  			td->avg_buckets[READ][i].valid,
  			td->avg_buckets[WRITE][i].latency,
  			td->avg_buckets[WRITE][i].valid);
  }
  #else
  static inline void throtl_update_latency_buckets(struct throtl_data *td)
  {
  }
  #endif
  
  bool blk_throtl_bio(struct bio *bio)
  {
  	struct request_queue *q = bio->bi_disk->queue;
  	struct blkcg_gq *blkg = bio->bi_blkg;
  	struct throtl_qnode *qn = NULL;
  	struct throtl_grp *tg = blkg_to_tg(blkg);
  	struct throtl_service_queue *sq;
  	bool rw = bio_data_dir(bio);
  	bool throttled = false;
  	struct throtl_data *td = tg->td;
  
  	rcu_read_lock();
  
  	/* see throtl_charge_bio() */
  	if (bio_flagged(bio, BIO_THROTTLED))
  		goto out;
  
  	if (!cgroup_subsys_on_dfl(io_cgrp_subsys)) {
  		blkg_rwstat_add(&tg->stat_bytes, bio->bi_opf,
  				bio->bi_iter.bi_size);
  		blkg_rwstat_add(&tg->stat_ios, bio->bi_opf, 1);
  	}
  
  	if (!tg->has_rules[rw])
  		goto out;
  
  	spin_lock_irq(&q->queue_lock);
  
  	throtl_update_latency_buckets(td);
  
  	blk_throtl_update_idletime(tg);
  
  	sq = &tg->service_queue;
  
  again:
  	while (true) {
  		if (tg->last_low_overflow_time[rw] == 0)
  			tg->last_low_overflow_time[rw] = jiffies;
  		throtl_downgrade_check(tg);
  		throtl_upgrade_check(tg);
  		/* throtl is FIFO - if bios are already queued, should queue */
  		if (sq->nr_queued[rw])
  			break;
  
  		/* if above limits, break to queue */
  		if (!tg_may_dispatch(tg, bio, NULL)) {
  			tg->last_low_overflow_time[rw] = jiffies;
  			if (throtl_can_upgrade(td, tg)) {
  				throtl_upgrade_state(td);
  				goto again;
  			}
  			break;
  		}
  
  		/* within limits, let's charge and dispatch directly */
  		throtl_charge_bio(tg, bio);
  
  		/*
  		 * We need to trim slice even when bios are not being queued
  		 * otherwise it might happen that a bio is not queued for
  		 * a long time and slice keeps on extending and trim is not
  		 * called for a long time. Now if limits are reduced suddenly
  		 * we take into account all the IO dispatched so far at new
  		 * low rate and * newly queued IO gets a really long dispatch
  		 * time.
  		 *
  		 * So keep on trimming slice even if bio is not queued.
  		 */
  		throtl_trim_slice(tg, rw);
  
  		/*
  		 * @bio passed through this layer without being throttled.
  		 * Climb up the ladder.  If we're already at the top, it
  		 * can be executed directly.
  		 */
  		qn = &tg->qnode_on_parent[rw];
  		sq = sq->parent_sq;
  		tg = sq_to_tg(sq);
  		if (!tg)
  			goto out_unlock;
  	}
  
  	/* out-of-limit, queue to @tg */
  	throtl_log(sq, "[%c] bio. bdisp=%llu sz=%u bps=%llu iodisp=%u iops=%u queued=%d/%d",
  		   rw == READ ? 'R' : 'W',
  		   tg->bytes_disp[rw], bio->bi_iter.bi_size,
  		   tg_bps_limit(tg, rw),
  		   tg->io_disp[rw], tg_iops_limit(tg, rw),
  		   sq->nr_queued[READ], sq->nr_queued[WRITE]);
  
  	tg->last_low_overflow_time[rw] = jiffies;
  
  	td->nr_queued[rw]++;
  	throtl_add_bio_tg(bio, qn, tg);
  	throttled = true;
  
  	/*
  	 * Update @tg's dispatch time and force schedule dispatch if @tg
  	 * was empty before @bio.  The forced scheduling isn't likely to
  	 * cause undue delay as @bio is likely to be dispatched directly if
  	 * its @tg's disptime is not in the future.
  	 */
  	if (tg->flags & THROTL_TG_WAS_EMPTY) {
  		tg_update_disptime(tg);
  		throtl_schedule_next_dispatch(tg->service_queue.parent_sq, true);
  	}
  
  out_unlock:
  	spin_unlock_irq(&q->queue_lock);
  out:
  	bio_set_flag(bio, BIO_THROTTLED);
  
  #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
  	if (throttled || !td->track_bio_latency)
  		bio->bi_issue.value |= BIO_ISSUE_THROTL_SKIP_LATENCY;
  #endif
  	rcu_read_unlock();
  	return throttled;
  }
  
  #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
  static void throtl_track_latency(struct throtl_data *td, sector_t size,
  	int op, unsigned long time)
  {
  	struct latency_bucket *latency;
  	int index;
  
  	if (!td || td->limit_index != LIMIT_LOW ||
  	    !(op == REQ_OP_READ || op == REQ_OP_WRITE) ||
  	    !blk_queue_nonrot(td->queue))
  		return;
  
  	index = request_bucket_index(size);
  
  	latency = get_cpu_ptr(td->latency_buckets[op]);
  	latency[index].total_latency += time;
  	latency[index].samples++;
  	put_cpu_ptr(td->latency_buckets[op]);
  }
  
  void blk_throtl_stat_add(struct request *rq, u64 time_ns)
  {
  	struct request_queue *q = rq->q;
  	struct throtl_data *td = q->td;
  
  	throtl_track_latency(td, blk_rq_stats_sectors(rq), req_op(rq),
  			     time_ns >> 10);
  }
  
  void blk_throtl_bio_endio(struct bio *bio)
  {
  	struct blkcg_gq *blkg;
  	struct throtl_grp *tg;
  	u64 finish_time_ns;
  	unsigned long finish_time;
  	unsigned long start_time;
  	unsigned long lat;
  	int rw = bio_data_dir(bio);
  
  	blkg = bio->bi_blkg;
  	if (!blkg)
  		return;
  	tg = blkg_to_tg(blkg);
  	if (!tg->td->limit_valid[LIMIT_LOW])
  		return;
  
  	finish_time_ns = ktime_get_ns();
  	tg->last_finish_time = finish_time_ns >> 10;
  
  	start_time = bio_issue_time(&bio->bi_issue) >> 10;
  	finish_time = __bio_issue_time(finish_time_ns) >> 10;
  	if (!start_time || finish_time <= start_time)
  		return;
  
  	lat = finish_time - start_time;
  	/* this is only for bio based driver */
  	if (!(bio->bi_issue.value & BIO_ISSUE_THROTL_SKIP_LATENCY))
  		throtl_track_latency(tg->td, bio_issue_size(&bio->bi_issue),
  				     bio_op(bio), lat);
  
  	if (tg->latency_target && lat >= tg->td->filtered_latency) {
  		int bucket;
  		unsigned int threshold;
  
  		bucket = request_bucket_index(bio_issue_size(&bio->bi_issue));
  		threshold = tg->td->avg_buckets[rw][bucket].latency +
  			tg->latency_target;
  		if (lat > threshold)
  			tg->bad_bio_cnt++;
  		/*
  		 * Not race free, could get wrong count, which means cgroups
  		 * will be throttled
  		 */
  		tg->bio_cnt++;
  	}
  
  	if (time_after(jiffies, tg->bio_cnt_reset_time) || tg->bio_cnt > 1024) {
  		tg->bio_cnt_reset_time = tg->td->throtl_slice + jiffies;
  		tg->bio_cnt /= 2;
  		tg->bad_bio_cnt /= 2;
  	}
  }
  #endif
  
  int blk_throtl_init(struct request_queue *q)
  {
  	struct throtl_data *td;
  	int ret;
  
  	td = kzalloc_node(sizeof(*td), GFP_KERNEL, q->node);
  	if (!td)
  		return -ENOMEM;
  	td->latency_buckets[READ] = __alloc_percpu(sizeof(struct latency_bucket) *
  		LATENCY_BUCKET_SIZE, __alignof__(u64));
  	if (!td->latency_buckets[READ]) {
  		kfree(td);
  		return -ENOMEM;
  	}
  	td->latency_buckets[WRITE] = __alloc_percpu(sizeof(struct latency_bucket) *
  		LATENCY_BUCKET_SIZE, __alignof__(u64));
  	if (!td->latency_buckets[WRITE]) {
  		free_percpu(td->latency_buckets[READ]);
  		kfree(td);
  		return -ENOMEM;
  	}
  
  	INIT_WORK(&td->dispatch_work, blk_throtl_dispatch_work_fn);
  	throtl_service_queue_init(&td->service_queue);
  
  	q->td = td;
  	td->queue = q;
  
  	td->limit_valid[LIMIT_MAX] = true;
  	td->limit_index = LIMIT_MAX;
  	td->low_upgrade_time = jiffies;
  	td->low_downgrade_time = jiffies;
  
  	/* activate policy */
  	ret = blkcg_activate_policy(q, &blkcg_policy_throtl);
  	if (ret) {
  		free_percpu(td->latency_buckets[READ]);
  		free_percpu(td->latency_buckets[WRITE]);
  		kfree(td);
  	}
  	return ret;
  }
  
  void blk_throtl_exit(struct request_queue *q)
  {
  	BUG_ON(!q->td);
  	throtl_shutdown_wq(q);
  	blkcg_deactivate_policy(q, &blkcg_policy_throtl);
  	free_percpu(q->td->latency_buckets[READ]);
  	free_percpu(q->td->latency_buckets[WRITE]);
  	kfree(q->td);
  }
  
  void blk_throtl_register_queue(struct request_queue *q)
  {
  	struct throtl_data *td;
  	int i;
  
  	td = q->td;
  	BUG_ON(!td);
  
  	if (blk_queue_nonrot(q)) {
  		td->throtl_slice = DFL_THROTL_SLICE_SSD;
  		td->filtered_latency = LATENCY_FILTERED_SSD;
  	} else {
  		td->throtl_slice = DFL_THROTL_SLICE_HD;
  		td->filtered_latency = LATENCY_FILTERED_HD;
  		for (i = 0; i < LATENCY_BUCKET_SIZE; i++) {
  			td->avg_buckets[READ][i].latency = DFL_HD_BASELINE_LATENCY;
  			td->avg_buckets[WRITE][i].latency = DFL_HD_BASELINE_LATENCY;
  		}
  	}
  #ifndef CONFIG_BLK_DEV_THROTTLING_LOW
  	/* if no low limit, use previous default */
  	td->throtl_slice = DFL_THROTL_SLICE_HD;
  #endif
  
  	td->track_bio_latency = !queue_is_mq(q);
  	if (!td->track_bio_latency)
  		blk_stat_enable_accounting(q);
  }
  
  #ifdef CONFIG_BLK_DEV_THROTTLING_LOW
  ssize_t blk_throtl_sample_time_show(struct request_queue *q, char *page)
  {
  	if (!q->td)
  		return -EINVAL;
  	return sprintf(page, "%u\n", jiffies_to_msecs(q->td->throtl_slice));
  }
  
  ssize_t blk_throtl_sample_time_store(struct request_queue *q,
  	const char *page, size_t count)
  {
  	unsigned long v;
  	unsigned long t;
  
  	if (!q->td)
  		return -EINVAL;
  	if (kstrtoul(page, 10, &v))
  		return -EINVAL;
  	t = msecs_to_jiffies(v);
  	if (t == 0 || t > MAX_THROTL_SLICE)
  		return -EINVAL;
  	q->td->throtl_slice = t;
  	return count;
  }
  #endif
  
  static int __init throtl_init(void)
  {
  	kthrotld_workqueue = alloc_workqueue("kthrotld", WQ_MEM_RECLAIM, 0);
  	if (!kthrotld_workqueue)
  		panic("Failed to create kthrotld\n");
  
  	return blkcg_policy_register(&blkcg_policy_throtl);
  }
  
  module_init(throtl_init);
  