blk-throttle.c

/*
 * Interface for controlling IO bandwidth on a request queue
 *
 * Copyright (C) 2010 Vivek Goyal <vgoyal@redhat.com>
 */

#include <linux/module.h>
#include <linux/slab.h>
#include <linux/blkdev.h>
#include <linux/bio.h>
#include <linux/blktrace_api.h>
#include "blk-cgroup.h"
#include "blk.h"

/* Max dispatch from a group in 1 round */
static int throtl_grp_quantum = 8;

/* Total max dispatch from all groups in one round */
static int throtl_quantum = 32;

/* Throttling is performed over 100ms slice and after that slice is renewed */
static unsigned long throtl_slice = HZ/10;	/* 100 ms */

static struct blkcg_policy blkcg_policy_throtl;

/* A workqueue to queue throttle related work */
static struct workqueue_struct *kthrotld_workqueue;
static void throtl_schedule_delayed_work(struct throtl_data *td,
				unsigned long delay);

struct throtl_rb_root {
	struct rb_root rb;
	struct rb_node *left;
	unsigned int count;
	unsigned long min_disptime;
};

#define THROTL_RB_ROOT	(struct throtl_rb_root) { .rb = RB_ROOT, .left = NULL, \
			.count = 0, .min_disptime = 0}

#define rb_entry_tg(node)	rb_entry((node), struct throtl_grp, rb_node)

/* Per-cpu group stats */
struct tg_stats_cpu {
	/* total bytes transferred */
	struct blkg_rwstat		service_bytes;
	/* total IOs serviced, post merge */
	struct blkg_rwstat		serviced;
};

struct throtl_grp {
	/* must be the first member */
	struct blkg_policy_data pd;

	/* active throtl group service_tree member */
	struct rb_node rb_node;

	/*
	 * 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;

	/* Two lists for READ and WRITE */
	struct bio_list bio_lists[2];

	/* Number of queued bios on READ and WRITE lists */
	unsigned int nr_queued[2];

	/* bytes per second rate limits */
	uint64_t bps[2];

	/* IOPS limits */
	unsigned int iops[2];

	/* Number of bytes disptached in current slice */
	uint64_t bytes_disp[2];
	/* Number of bio's dispatched in current slice */
	unsigned int io_disp[2];

	/* When did we start a new slice */
	unsigned long slice_start[2];
	unsigned long slice_end[2];

	/* Some throttle limits got updated for the group */
	int limits_changed;

	/* Per cpu stats pointer */
	struct tg_stats_cpu __percpu *stats_cpu;

	/* List of tgs waiting for per cpu stats memory to be allocated */
	struct list_head stats_alloc_node;
};

struct throtl_data
{
	/* service tree for active throtl groups */
	struct throtl_rb_root tg_service_tree;

	struct request_queue *queue;

	/* Total Number of queued bios on READ and WRITE lists */
	unsigned int nr_queued[2];

	/*
	 * number of total undestroyed groups
	 */
	unsigned int nr_undestroyed_grps;

	/* Work for dispatching throttled bios */
	struct delayed_work throtl_work;

	int limits_changed;
};

/* list and work item to allocate percpu group stats */
static DEFINE_SPINLOCK(tg_stats_alloc_lock);
static LIST_HEAD(tg_stats_alloc_list);

static void tg_stats_alloc_fn(struct work_struct *);
static DECLARE_DELAYED_WORK(tg_stats_alloc_work, tg_stats_alloc_fn);

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

static inline struct throtl_grp *blkg_to_tg(struct blkcg_gq *blkg)
{
	return pd_to_tg(blkg_to_pd(blkg, &blkcg_policy_throtl));
}

static inline struct blkcg_gq *tg_to_blkg(struct throtl_grp *tg)
{
	return pd_to_blkg(&tg->pd);
}

static inline struct throtl_grp *td_root_tg(struct throtl_data *td)
{
	return blkg_to_tg(td->queue->root_blkg);
}

enum tg_state_flags {
	THROTL_TG_FLAG_on_rr = 0,	/* on round-robin busy list */
};

#define THROTL_TG_FNS(name)						\
static inline void throtl_mark_tg_##name(struct throtl_grp *tg)		\
{									\
	(tg)->flags |= (1 << THROTL_TG_FLAG_##name);			\
}									\
static inline void throtl_clear_tg_##name(struct throtl_grp *tg)	\
{									\
	(tg)->flags &= ~(1 << THROTL_TG_FLAG_##name);			\
}									\
static inline int throtl_tg_##name(const struct throtl_grp *tg)		\
{									\
	return ((tg)->flags & (1 << THROTL_TG_FLAG_##name)) != 0;	\
}

THROTL_TG_FNS(on_rr);

#define throtl_log_tg(td, tg, fmt, args...)	do {			\
	char __pbuf[128];						\
									\
	blkg_path(tg_to_blkg(tg), __pbuf, sizeof(__pbuf));		\
	blk_add_trace_msg((td)->queue, "throtl %s " fmt, __pbuf, ##args); \
} while (0)

#define throtl_log(td, fmt, args...)	\
	blk_add_trace_msg((td)->queue, "throtl " fmt, ##args)

static inline unsigned int total_nr_queued(struct throtl_data *td)
{
	return td->nr_queued[0] + td->nr_queued[1];
}

/*
 * Worker for allocating per cpu stat for tgs. This is scheduled on the
 * system_wq once there are some groups on the alloc_list waiting for
 * allocation.
 */
static void tg_stats_alloc_fn(struct work_struct *work)
{
	static struct tg_stats_cpu *stats_cpu;	/* this fn is non-reentrant */
	struct delayed_work *dwork = to_delayed_work(work);
	bool empty = false;

alloc_stats:
	if (!stats_cpu) {
		stats_cpu = alloc_percpu(struct tg_stats_cpu);
		if (!stats_cpu) {
			/* allocation failed, try again after some time */
			schedule_delayed_work(dwork, msecs_to_jiffies(10));
			return;
		}
	}

	spin_lock_irq(&tg_stats_alloc_lock);

	if (!list_empty(&tg_stats_alloc_list)) {
		struct throtl_grp *tg = list_first_entry(&tg_stats_alloc_list,
							 struct throtl_grp,
							 stats_alloc_node);
		swap(tg->stats_cpu, stats_cpu);
		list_del_init(&tg->stats_alloc_node);
	}

	empty = list_empty(&tg_stats_alloc_list);
	spin_unlock_irq(&tg_stats_alloc_lock);
	if (!empty)
		goto alloc_stats;
}

static void throtl_pd_init(struct blkcg_gq *blkg)
{
	struct throtl_grp *tg = blkg_to_tg(blkg);
	unsigned long flags;

	RB_CLEAR_NODE(&tg->rb_node);
	bio_list_init(&tg->bio_lists[0]);
	bio_list_init(&tg->bio_lists[1]);
	tg->limits_changed = false;

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

	/*
	 * Ugh... We need to perform per-cpu allocation for tg->stats_cpu
	 * but percpu allocator can't be called from IO path.  Queue tg on
	 * tg_stats_alloc_list and allocate from work item.
	 */
	spin_lock_irqsave(&tg_stats_alloc_lock, flags);
	list_add(&tg->stats_alloc_node, &tg_stats_alloc_list);
	schedule_delayed_work(&tg_stats_alloc_work, 0);
	spin_unlock_irqrestore(&tg_stats_alloc_lock, flags);
}

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

	spin_lock_irqsave(&tg_stats_alloc_lock, flags);
	list_del_init(&tg->stats_alloc_node);
	spin_unlock_irqrestore(&tg_stats_alloc_lock, flags);

	free_percpu(tg->stats_cpu);
}

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

	if (tg->stats_cpu == NULL)
		return;

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

		blkg_rwstat_reset(&sc->service_bytes);
		blkg_rwstat_reset(&sc->serviced);
	}
}

static struct throtl_grp *throtl_lookup_tg(struct throtl_data *td,
					   struct blkcg *blkcg)
{
	/*
	 * This is the common case when there are no blkcgs.  Avoid lookup
	 * in this case
	 */
	if (blkcg == &blkcg_root)
		return td_root_tg(td);

	return blkg_to_tg(blkg_lookup(blkcg, td->queue));
}

static struct throtl_grp *throtl_lookup_create_tg(struct throtl_data *td,
						  struct blkcg *blkcg)
{
	struct request_queue *q = td->queue;
	struct throtl_grp *tg = NULL;

	/*
	 * This is the common case when there are no blkcgs.  Avoid lookup
	 * in this case
	 */
	if (blkcg == &blkcg_root) {
		tg = td_root_tg(td);
	} else {
		struct blkcg_gq *blkg;

		blkg = blkg_lookup_create(blkcg, q);

		/* if %NULL and @q is alive, fall back to root_tg */
		if (!IS_ERR(blkg))
			tg = blkg_to_tg(blkg);
		else if (!blk_queue_dying(q))
			tg = td_root_tg(td);
	}

	return tg;
}

static struct throtl_grp *throtl_rb_first(struct throtl_rb_root *root)
{
	/* Service tree is empty */
	if (!root->count)
		return NULL;

	if (!root->left)
		root->left = rb_first(&root->rb);

	if (root->left)
		return rb_entry_tg(root->left);

	return NULL;
}

static void rb_erase_init(struct rb_node *n, struct rb_root *root)
{
	rb_erase(n, root);
	RB_CLEAR_NODE(n);
}

static void throtl_rb_erase(struct rb_node *n, struct throtl_rb_root *root)
{
	if (root->left == n)
		root->left = NULL;
	rb_erase_init(n, &root->rb);
	--root->count;
}

static void update_min_dispatch_time(struct throtl_rb_root *st)
{
	struct throtl_grp *tg;

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

	st->min_disptime = tg->disptime;
}

static void
tg_service_tree_add(struct throtl_rb_root *st, struct throtl_grp *tg)
{
	struct rb_node **node = &st->rb.rb_node;
	struct rb_node *parent = NULL;
	struct throtl_grp *__tg;
	unsigned long key = tg->disptime;
	int left = 1;

	while (*node != NULL) {
		parent = *node;
		__tg = rb_entry_tg(parent);

		if (time_before(key, __tg->disptime))
			node = &parent->rb_left;
		else {
			node = &parent->rb_right;
			left = 0;
		}
	}

	if (left)
		st->left = &tg->rb_node;

	rb_link_node(&tg->rb_node, parent, node);
	rb_insert_color(&tg->rb_node, &st->rb);
}

static void __throtl_enqueue_tg(struct throtl_data *td, struct throtl_grp *tg)
{
	struct throtl_rb_root *st = &td->tg_service_tree;

	tg_service_tree_add(st, tg);
	throtl_mark_tg_on_rr(tg);
	st->count++;
}

static void throtl_enqueue_tg(struct throtl_data *td, struct throtl_grp *tg)
{
	if (!throtl_tg_on_rr(tg))
		__throtl_enqueue_tg(td, tg);
}

static void __throtl_dequeue_tg(struct throtl_data *td, struct throtl_grp *tg)
{
	throtl_rb_erase(&tg->rb_node, &td->tg_service_tree);
	throtl_clear_tg_on_rr(tg);
}

static void throtl_dequeue_tg(struct throtl_data *td, struct throtl_grp *tg)
{
	if (throtl_tg_on_rr(tg))
		__throtl_dequeue_tg(td, tg);
}

static void throtl_schedule_next_dispatch(struct throtl_data *td)
{
	struct throtl_rb_root *st = &td->tg_service_tree;

	/*
	 * If there are more bios pending, schedule more work.
	 */
	if (!total_nr_queued(td))
		return;

	BUG_ON(!st->count);

	update_min_dispatch_time(st);

	if (time_before_eq(st->min_disptime, jiffies))
		throtl_schedule_delayed_work(td, 0);
	else
		throtl_schedule_delayed_work(td, (st->min_disptime - jiffies));
}

static inline void
throtl_start_new_slice(struct throtl_data *td, struct throtl_grp *tg, bool rw)
{
	tg->bytes_disp[rw] = 0;
	tg->io_disp[rw] = 0;
	tg->slice_start[rw] = jiffies;
	tg->slice_end[rw] = jiffies + throtl_slice;
	throtl_log_tg(td, tg, "[%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_data *td,
		struct throtl_grp *tg, bool rw, unsigned long jiffy_end)
{
	tg->slice_end[rw] = roundup(jiffy_end, throtl_slice);
}

static inline void throtl_extend_slice(struct throtl_data *td,
		struct throtl_grp *tg, bool rw, unsigned long jiffy_end)
{
	tg->slice_end[rw] = roundup(jiffy_end, throtl_slice);
	throtl_log_tg(td, tg, "[%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_data *td, struct throtl_grp *tg, bool rw)
{
	if (time_in_range(jiffies, tg->slice_start[rw], tg->slice_end[rw]))
		return 0;

	return 1;
}

/* Trim the used slices and adjust slice start accordingly */
static inline void
throtl_trim_slice(struct throtl_data *td, 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(td, tg, rw))
		return;

	/*
	 * A bio has been dispatched. Also adjust slice_end. It might happen
	 * that initially cgroup limit was very low resulting in high
	 * slice_end, but later limit was bumped up and bio was dispached
	 * sooner, then we need to reduce slice_end. A high bogus slice_end
	 * is bad because it does not allow new slice to start.
	 */

	throtl_set_slice_end(td, tg, rw, jiffies + throtl_slice);

	time_elapsed = jiffies - tg->slice_start[rw];

	nr_slices = time_elapsed / throtl_slice;

	if (!nr_slices)
		return;
	tmp = tg->bps[rw] * throtl_slice * nr_slices;
	do_div(tmp, HZ);
	bytes_trim = tmp;

	io_trim = (tg->iops[rw] * throtl_slice * nr_slices)/HZ;

	if (!bytes_trim && !io_trim)
		return;

	if (tg->bytes_disp[rw] >= bytes_trim)
		tg->bytes_disp[rw] -= bytes_trim;
	else
		tg->bytes_disp[rw] = 0;

	if (tg->io_disp[rw] >= io_trim)
		tg->io_disp[rw] -= io_trim;
	else
		tg->io_disp[rw] = 0;

	tg->slice_start[rw] += nr_slices * throtl_slice;

	throtl_log_tg(td, tg, "[%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_data *td, struct throtl_grp *tg,
		struct bio *bio, unsigned long *wait)
{
	bool rw = bio_data_dir(bio);
	unsigned int io_allowed;
	unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
	u64 tmp;

	jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];

	/* Slice has just started. Consider one slice interval */
	if (!jiffy_elapsed)
		jiffy_elapsed_rnd = throtl_slice;

	jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, throtl_slice);

	/*
	 * jiffy_elapsed_rnd should not be a big value as minimum iops can be
	 * 1 then at max jiffy elapsed should be equivalent of 1 second as we
	 * will allow dispatch after 1 second and after that slice should
	 * have been trimmed.
	 */

	tmp = (u64)tg->iops[rw] * jiffy_elapsed_rnd;
	do_div(tmp, HZ);

	if (tmp > UINT_MAX)
		io_allowed = UINT_MAX;
	else
		io_allowed = tmp;

	if (tg->io_disp[rw] + 1 <= io_allowed) {
		if (wait)
			*wait = 0;
		return 1;
	}

	/* Calc approx time to dispatch */
	jiffy_wait = ((tg->io_disp[rw] + 1) * HZ)/tg->iops[rw] + 1;

	if (jiffy_wait > jiffy_elapsed)
		jiffy_wait = jiffy_wait - jiffy_elapsed;
	else
		jiffy_wait = 1;

	if (wait)
		*wait = jiffy_wait;
	return 0;
}

static bool tg_with_in_bps_limit(struct throtl_data *td, struct throtl_grp *tg,
		struct bio *bio, unsigned long *wait)
{
	bool rw = bio_data_dir(bio);
	u64 bytes_allowed, extra_bytes, tmp;
	unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;

	jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];

	/* Slice has just started. Consider one slice interval */
	if (!jiffy_elapsed)
		jiffy_elapsed_rnd = throtl_slice;

	jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, throtl_slice);

	tmp = tg->bps[rw] * jiffy_elapsed_rnd;
	do_div(tmp, HZ);
	bytes_allowed = tmp;

	if (tg->bytes_disp[rw] + bio->bi_size <= bytes_allowed) {
		if (wait)
			*wait = 0;
		return 1;
	}

	/* Calc approx time to dispatch */
	extra_bytes = tg->bytes_disp[rw] + bio->bi_size - bytes_allowed;
	jiffy_wait = div64_u64(extra_bytes * HZ, tg->bps[rw]);

	if (!jiffy_wait)
		jiffy_wait = 1;

	/*
	 * This wait time is without taking into consideration the rounding
	 * up we did. Add that time also.
	 */
	jiffy_wait = jiffy_wait + (jiffy_elapsed_rnd - jiffy_elapsed);
	if (wait)
		*wait = jiffy_wait;
	return 0;
}

static bool tg_no_rule_group(struct throtl_grp *tg, bool rw) {
	if (tg->bps[rw] == -1 && tg->iops[rw] == -1)
		return 1;
	return 0;
}

/*
 * Returns whether one can dispatch a bio or not. Also returns approx number
 * of jiffies to wait before this bio is with-in IO rate and can be dispatched
 */
static bool tg_may_dispatch(struct throtl_data *td, struct throtl_grp *tg,
				struct bio *bio, unsigned long *wait)
{
	bool rw = bio_data_dir(bio);
	unsigned long bps_wait = 0, iops_wait = 0, max_wait = 0;

	/*
 	 * Currently whole state machine of group depends on first bio
	 * queued in the group bio list. So one should not be calling
	 * this function with a different bio if there are other bios
	 * queued.
	 */
	BUG_ON(tg->nr_queued[rw] && bio != bio_list_peek(&tg->bio_lists[rw]));

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

	/*
	 * If previous slice expired, start a new one otherwise renew/extend
	 * existing slice to make sure it is at least throtl_slice interval
	 * long since now.
	 */
	if (throtl_slice_used(td, tg, rw))
		throtl_start_new_slice(td, tg, rw);
	else {
		if (time_before(tg->slice_end[rw], jiffies + throtl_slice))
			throtl_extend_slice(td, tg, rw, jiffies + throtl_slice);
	}

	if (tg_with_in_bps_limit(td, tg, bio, &bps_wait)
	    && tg_with_in_iops_limit(td, tg, bio, &iops_wait)) {
		if (wait)
			*wait = 0;
		return 1;
	}

	max_wait = max(bps_wait, iops_wait);

	if (wait)
		*wait = max_wait;

	if (time_before(tg->slice_end[rw], jiffies + max_wait))
		throtl_extend_slice(td, tg, rw, jiffies + max_wait);

	return 0;
}

static void throtl_update_dispatch_stats(struct blkcg_gq *blkg, u64 bytes,
					 int rw)
{
	struct throtl_grp *tg = blkg_to_tg(blkg);
	struct tg_stats_cpu *stats_cpu;
	unsigned long flags;

	/* If per cpu stats are not allocated yet, don't do any accounting. */
	if (tg->stats_cpu == NULL)
		return;

	/*
	 * Disabling interrupts to provide mutual exclusion between two
	 * writes on same cpu. It probably is not needed for 64bit. Not
	 * optimizing that case yet.
	 */
	local_irq_save(flags);

	stats_cpu = this_cpu_ptr(tg->stats_cpu);

	blkg_rwstat_add(&stats_cpu->serviced, rw, 1);
	blkg_rwstat_add(&stats_cpu->service_bytes, rw, bytes);

	local_irq_restore(flags);
}

static void throtl_charge_bio(struct throtl_grp *tg, struct bio *bio)
{
	bool rw = bio_data_dir(bio);

	/* Charge the bio to the group */
	tg->bytes_disp[rw] += bio->bi_size;
	tg->io_disp[rw]++;

	throtl_update_dispatch_stats(tg_to_blkg(tg), bio->bi_size, bio->bi_rw);
}

static void throtl_add_bio_tg(struct throtl_data *td, struct throtl_grp *tg,
			struct bio *bio)
{
	bool rw = bio_data_dir(bio);

	bio_list_add(&tg->bio_lists[rw], bio);
	/* Take a bio reference on tg */
	blkg_get(tg_to_blkg(tg));
	tg->nr_queued[rw]++;
	td->nr_queued[rw]++;
	throtl_enqueue_tg(td, tg);
}

static void tg_update_disptime(struct throtl_data *td, struct throtl_grp *tg)
{
	unsigned long read_wait = -1, write_wait = -1, min_wait = -1, disptime;
	struct bio *bio;

	if ((bio = bio_list_peek(&tg->bio_lists[READ])))
		tg_may_dispatch(td, tg, bio, &read_wait);

	if ((bio = bio_list_peek(&tg->bio_lists[WRITE])))
		tg_may_dispatch(td, tg, bio, &write_wait);

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

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

static void tg_dispatch_one_bio(struct throtl_data *td, struct throtl_grp *tg,
				bool rw, struct bio_list *bl)
{
	struct bio *bio;

	bio = bio_list_pop(&tg->bio_lists[rw]);
	tg->nr_queued[rw]--;
	/* Drop bio reference on blkg */
	blkg_put(tg_to_blkg(tg));

	BUG_ON(td->nr_queued[rw] <= 0);
	td->nr_queued[rw]--;

	throtl_charge_bio(tg, bio);
	bio_list_add(bl, bio);
	bio->bi_rw |= REQ_THROTTLED;

	throtl_trim_slice(td, tg, rw);
}

static int throtl_dispatch_tg(struct throtl_data *td, struct throtl_grp *tg,
				struct bio_list *bl)
{
	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 = bio_list_peek(&tg->bio_lists[READ]))
		&& tg_may_dispatch(td, tg, bio, NULL)) {

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

		if (nr_reads >= max_nr_reads)
			break;
	}

	while ((bio = bio_list_peek(&tg->bio_lists[WRITE]))
		&& tg_may_dispatch(td, tg, bio, NULL)) {

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

		if (nr_writes >= max_nr_writes)
			break;
	}

	return nr_reads + nr_writes;
}

static int throtl_select_dispatch(struct throtl_data *td, struct bio_list *bl)
{
	unsigned int nr_disp = 0;
	struct throtl_grp *tg;
	struct throtl_rb_root *st = &td->tg_service_tree;

	while (1) {
		tg = throtl_rb_first(st);

		if (!tg)
			break;

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

		throtl_dequeue_tg(td, tg);

		nr_disp += throtl_dispatch_tg(td, tg, bl);

		if (tg->nr_queued[0] || tg->nr_queued[1]) {
			tg_update_disptime(td, tg);
			throtl_enqueue_tg(td, tg);
		}

		if (nr_disp >= throtl_quantum)
			break;
	}

	return nr_disp;
}

static void throtl_process_limit_change(struct throtl_data *td)
{
	struct request_queue *q = td->queue;
	struct blkcg_gq *blkg, *n;

	if (!td->limits_changed)
		return;

	xchg(&td->limits_changed, false);

	throtl_log(td, "limits changed");

	list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
		struct throtl_grp *tg = blkg_to_tg(blkg);

		if (!tg->limits_changed)
			continue;

		if (!xchg(&tg->limits_changed, false))
			continue;

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

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

		if (throtl_tg_on_rr(tg))
			tg_update_disptime(td, tg);
	}
}

/* Dispatch throttled bios. Should be called without queue lock held. */
static int throtl_dispatch(struct request_queue *q)
{
	struct throtl_data *td = q->td;
	unsigned int nr_disp = 0;
	struct bio_list bio_list_on_stack;
	struct bio *bio;
	struct blk_plug plug;

	spin_lock_irq(q->queue_lock);

	throtl_process_limit_change(td);

	if (!total_nr_queued(td))
		goto out;

	bio_list_init(&bio_list_on_stack);

	throtl_log(td, "dispatch nr_queued=%u read=%u write=%u",
			total_nr_queued(td), td->nr_queued[READ],
			td->nr_queued[WRITE]);

	nr_disp = throtl_select_dispatch(td, &bio_list_on_stack);

	if (nr_disp)
		throtl_log(td, "bios disp=%u", nr_disp);

	throtl_schedule_next_dispatch(td);
out:
	spin_unlock_irq(q->queue_lock);

	/*
	 * If we dispatched some requests, unplug the queue to make sure
	 * immediate dispatch
	 */
	if (nr_disp) {
		blk_start_plug(&plug);
		while((bio = bio_list_pop(&bio_list_on_stack)))
			generic_make_request(bio);
		blk_finish_plug(&plug);
	}
	return nr_disp;
}

void blk_throtl_work(struct work_struct *work)
{
	struct throtl_data *td = container_of(work, struct throtl_data,
					throtl_work.work);
	struct request_queue *q = td->queue;

	throtl_dispatch(q);
}

/* Call with queue lock held */
static void
throtl_schedule_delayed_work(struct throtl_data *td, unsigned long delay)
{

	struct delayed_work *dwork = &td->throtl_work;

	/* schedule work if limits changed even if no bio is queued */
	if (total_nr_queued(td) || td->limits_changed) {
		mod_delayed_work(kthrotld_workqueue, dwork, delay);
		throtl_log(td, "schedule work. delay=%lu jiffies=%lu",
				delay, jiffies);
	}
}

static u64 tg_prfill_cpu_rwstat(struct seq_file *sf,
				struct blkg_policy_data *pd, int off)
{
	struct throtl_grp *tg = pd_to_tg(pd);
	struct blkg_rwstat rwstat = { }, tmp;
	int i, cpu;

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

		tmp = blkg_rwstat_read((void *)sc + off);
		for (i = 0; i < BLKG_RWSTAT_NR; i++)
			rwstat.cnt[i] += tmp.cnt[i];
	}

	return __blkg_prfill_rwstat(sf, pd, &rwstat);
}

static int tg_print_cpu_rwstat(struct cgroup *cgrp, struct cftype *cft,
			       struct seq_file *sf)
{
	struct blkcg *blkcg = cgroup_to_blkcg(cgrp);

	blkcg_print_blkgs(sf, blkcg, tg_prfill_cpu_rwstat, &blkcg_policy_throtl,
			  cft->private, true);
	return 0;
}

static u64 tg_prfill_conf_u64(struct seq_file *sf, struct blkg_policy_data *pd,
			      int off)
{
	struct throtl_grp *tg = pd_to_tg(pd);
	u64 v = *(u64 *)((void *)tg + off);

	if (v == -1)
		return 0;
	return __blkg_prfill_u64(sf, pd, v);
}

static u64 tg_prfill_conf_uint(struct seq_file *sf, struct blkg_policy_data *pd,
			       int off)
{
	struct throtl_grp *tg = pd_to_tg(pd);
	unsigned int v = *(unsigned int *)((void *)tg + off);

	if (v == -1)
		return 0;
	return __blkg_prfill_u64(sf, pd, v);
}

static int tg_print_conf_u64(struct cgroup *cgrp, struct cftype *cft,
			     struct seq_file *sf)
{
	blkcg_print_blkgs(sf, cgroup_to_blkcg(cgrp), tg_prfill_conf_u64,
			  &blkcg_policy_throtl, cft->private, false);
	return 0;
}

static int tg_print_conf_uint(struct cgroup *cgrp, struct cftype *cft,
			      struct seq_file *sf)
{
	blkcg_print_blkgs(sf, cgroup_to_blkcg(cgrp), tg_prfill_conf_uint,
			  &blkcg_policy_throtl, cft->private, false);
	return 0;
}

static int tg_set_conf(struct cgroup *cgrp, struct cftype *cft, const char *buf,
		       bool is_u64)
{
	struct blkcg *blkcg = cgroup_to_blkcg(cgrp);
	struct blkg_conf_ctx ctx;
	struct throtl_grp *tg;
	struct throtl_data *td;
	int ret;

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

	tg = blkg_to_tg(ctx.blkg);
	td = ctx.blkg->q->td;

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

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

	/* XXX: we don't need the following deferred processing */
	xchg(&tg->limits_changed, true);
	xchg(&td->limits_changed, true);
	throtl_schedule_delayed_work(td, 0);

	blkg_conf_finish(&ctx);
	return 0;
}

static int tg_set_conf_u64(struct cgroup *cgrp, struct cftype *cft,
			   const char *buf)
{
	return tg_set_conf(cgrp, cft, buf, true);
}

static int tg_set_conf_uint(struct cgroup *cgrp, struct cftype *cft,
			    const char *buf)
{
	return tg_set_conf(cgrp, cft, buf, false);
}

static struct cftype throtl_files[] = {
	{
		.name = "throttle.read_bps_device",
		.private = offsetof(struct throtl_grp, bps[READ]),
		.read_seq_string = tg_print_conf_u64,
		.write_string = tg_set_conf_u64,
		.max_write_len = 256,
	},
	{
		.name = "throttle.write_bps_device",
		.private = offsetof(struct throtl_grp, bps[WRITE]),
		.read_seq_string = tg_print_conf_u64,
		.write_string = tg_set_conf_u64,
		.max_write_len = 256,
	},
	{
		.name = "throttle.read_iops_device",
		.private = offsetof(struct throtl_grp, iops[READ]),
		.read_seq_string = tg_print_conf_uint,
		.write_string = tg_set_conf_uint,
		.max_write_len = 256,
	},
	{
		.name = "throttle.write_iops_device",
		.private = offsetof(struct throtl_grp, iops[WRITE]),
		.read_seq_string = tg_print_conf_uint,
		.write_string = tg_set_conf_uint,
		.max_write_len = 256,
	},
	{
		.name = "throttle.io_service_bytes",
		.private = offsetof(struct tg_stats_cpu, service_bytes),
		.read_seq_string = tg_print_cpu_rwstat,
	},
	{
		.name = "throttle.io_serviced",
		.private = offsetof(struct tg_stats_cpu, serviced),
		.read_seq_string = tg_print_cpu_rwstat,
	},
	{ }	/* terminate */
};

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

	cancel_delayed_work_sync(&td->throtl_work);
}

static struct blkcg_policy blkcg_policy_throtl = {
	.pd_size		= sizeof(struct throtl_grp),
	.cftypes		= throtl_files,

	.pd_init_fn		= throtl_pd_init,
	.pd_exit_fn		= throtl_pd_exit,
	.pd_reset_stats_fn	= throtl_pd_reset_stats,
};

bool blk_throtl_bio(struct request_queue *q, struct bio *bio)
{
	struct throtl_data *td = q->td;
	struct throtl_grp *tg;
	bool rw = bio_data_dir(bio), update_disptime = true;
	struct blkcg *blkcg;
	bool throttled = false;

	if (bio->bi_rw & REQ_THROTTLED) {
		bio->bi_rw &= ~REQ_THROTTLED;
		goto out;
	}

	/*
	 * A throtl_grp pointer retrieved under rcu can be used to access
	 * basic fields like stats and io rates. If a group has no rules,
	 * just update the dispatch stats in lockless manner and return.
	 */
	rcu_read_lock();
	blkcg = bio_blkcg(bio);
	tg = throtl_lookup_tg(td, blkcg);
	if (tg) {
		if (tg_no_rule_group(tg, rw)) {
			throtl_update_dispatch_stats(tg_to_blkg(tg),
						     bio->bi_size, bio->bi_rw);
			goto out_unlock_rcu;
		}
	}

	/*
	 * Either group has not been allocated yet or it is not an unlimited
	 * IO group
	 */
	spin_lock_irq(q->queue_lock);
	tg = throtl_lookup_create_tg(td, blkcg);
	if (unlikely(!tg))
		goto out_unlock;

	if (tg->nr_queued[rw]) {
		/*
		 * There is already another bio queued in same dir. No
		 * need to update dispatch time.
		 */
		update_disptime = false;
		goto queue_bio;

	}

	/* Bio is with-in rate limit of group */
	if (tg_may_dispatch(td, tg, bio, NULL)) {
		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(td, tg, rw);
		goto out_unlock;
	}

queue_bio:
	throtl_log_tg(td, tg, "[%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_size, tg->bps[rw],
			tg->io_disp[rw], tg->iops[rw],
			tg->nr_queued[READ], tg->nr_queued[WRITE]);

	bio_associate_current(bio);
	throtl_add_bio_tg(q->td, tg, bio);
	throttled = true;

	if (update_disptime) {
		tg_update_disptime(td, tg);
		throtl_schedule_next_dispatch(td);
	}

out_unlock:
	spin_unlock_irq(q->queue_lock);
out_unlock_rcu:
	rcu_read_unlock();
out:
	return throttled;
}

/**
 * blk_throtl_drain - drain throttled bios
 * @q: request_queue to drain throttled bios for
 *
 * Dispatch all currently throttled bios on @q through ->make_request_fn().
 */
void blk_throtl_drain(struct request_queue *q)
	__releases(q->queue_lock) __acquires(q->queue_lock)
{
	struct throtl_data *td = q->td;
	struct throtl_rb_root *st = &td->tg_service_tree;
	struct throtl_grp *tg;
	struct bio_list bl;
	struct bio *bio;

	queue_lockdep_assert_held(q);

	bio_list_init(&bl);

	while ((tg = throtl_rb_first(st))) {
		throtl_dequeue_tg(td, tg);

		while ((bio = bio_list_peek(&tg->bio_lists[READ])))
			tg_dispatch_one_bio(td, tg, bio_data_dir(bio), &bl);
		while ((bio = bio_list_peek(&tg->bio_lists[WRITE])))
			tg_dispatch_one_bio(td, tg, bio_data_dir(bio), &bl);
	}
	spin_unlock_irq(q->queue_lock);

	while ((bio = bio_list_pop(&bl)))
		generic_make_request(bio);

	spin_lock_irq(q->queue_lock);
}

int blk_throtl_init(struct request_queue *q)
{
	struct throtl_data *td;
	int ret;

	td = kzalloc_node(sizeof(*td), GFP_KERNEL, q->node);
	if (!td)
		return -ENOMEM;

	td->tg_service_tree = THROTL_RB_ROOT;
	td->limits_changed = false;
	INIT_DELAYED_WORK(&td->throtl_work, blk_throtl_work);

	q->td = td;
	td->queue = q;

	/* activate policy */
	ret = blkcg_activate_policy(q, &blkcg_policy_throtl);
	if (ret)
		kfree(td);
	return ret;
}

void blk_throtl_exit(struct request_queue *q)
{
	BUG_ON(!q->td);
	throtl_shutdown_wq(q);
	blkcg_deactivate_policy(q, &blkcg_policy_throtl);
	kfree(q->td);
}

static int __init throtl_init(void)
{
	kthrotld_workqueue = alloc_workqueue("kthrotld", WQ_MEM_RECLAIM, 0);
	if (!kthrotld_workqueue)
		panic("Failed to create kthrotld\n");

	return blkcg_policy_register(&blkcg_policy_throtl);
}

module_init(throtl_init);