Contributors: 6
Author Tokens Token Proportion Commits Commit Proportion
Omar Sandoval 3263 78.25% 10 52.63%
Jianchao Wang 767 18.39% 1 5.26%
Jens Axboe 90 2.16% 3 15.79%
Christoph Hellwig 42 1.01% 2 10.53%
Ming Lei 5 0.12% 1 5.26%
Ingo Molnar 3 0.07% 2 10.53%
Total 4170 19 


/*
 * The Kyber I/O scheduler. Controls latency by throttling queue depths using
 * scalable techniques.
 *
 * Copyright (C) 2017 Facebook
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public
 * License v2 as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

#include <linux/kernel.h>
#include <linux/blkdev.h>
#include <linux/blk-mq.h>
#include <linux/elevator.h>
#include <linux/module.h>
#include <linux/sbitmap.h>

#include "blk.h"
#include "blk-mq.h"
#include "blk-mq-debugfs.h"
#include "blk-mq-sched.h"
#include "blk-mq-tag.h"

#define CREATE_TRACE_POINTS
#include <trace/events/kyber.h>

/*
 * Scheduling domains: the device is divided into multiple domains based on the
 * request type.
 */
enum {
	KYBER_READ,
	KYBER_WRITE,
	KYBER_DISCARD,
	KYBER_OTHER,
	KYBER_NUM_DOMAINS,
};

static const char *kyber_domain_names[] = {
	[KYBER_READ] = "READ",
	[KYBER_WRITE] = "WRITE",
	[KYBER_DISCARD] = "DISCARD",
	[KYBER_OTHER] = "OTHER",
};

enum {
	/*
	 * In order to prevent starvation of synchronous requests by a flood of
	 * asynchronous requests, we reserve 25% of requests for synchronous
	 * operations.
	 */
	KYBER_ASYNC_PERCENT = 75,
};

/*
 * Maximum device-wide depth for each scheduling domain.
 *
 * Even for fast devices with lots of tags like NVMe, you can saturate the
 * device with only a fraction of the maximum possible queue depth. So, we cap
 * these to a reasonable value.
 */
static const unsigned int kyber_depth[] = {
	[KYBER_READ] = 256,
	[KYBER_WRITE] = 128,
	[KYBER_DISCARD] = 64,
	[KYBER_OTHER] = 16,
};

/*
 * Default latency targets for each scheduling domain.
 */
static const u64 kyber_latency_targets[] = {
	[KYBER_READ] = 2ULL * NSEC_PER_MSEC,
	[KYBER_WRITE] = 10ULL * NSEC_PER_MSEC,
	[KYBER_DISCARD] = 5ULL * NSEC_PER_SEC,
};

/*
 * Batch size (number of requests we'll dispatch in a row) for each scheduling
 * domain.
 */
static const unsigned int kyber_batch_size[] = {
	[KYBER_READ] = 16,
	[KYBER_WRITE] = 8,
	[KYBER_DISCARD] = 1,
	[KYBER_OTHER] = 1,
};

/*
 * Requests latencies are recorded in a histogram with buckets defined relative
 * to the target latency:
 *
 * <= 1/4 * target latency
 * <= 1/2 * target latency
 * <= 3/4 * target latency
 * <= target latency
 * <= 1 1/4 * target latency
 * <= 1 1/2 * target latency
 * <= 1 3/4 * target latency
 * > 1 3/4 * target latency
 */
enum {
	/*
	 * The width of the latency histogram buckets is
	 * 1 / (1 << KYBER_LATENCY_SHIFT) * target latency.
	 */
	KYBER_LATENCY_SHIFT = 2,
	/*
	 * The first (1 << KYBER_LATENCY_SHIFT) buckets are <= target latency,
	 * thus, "good".
	 */
	KYBER_GOOD_BUCKETS = 1 << KYBER_LATENCY_SHIFT,
	/* There are also (1 << KYBER_LATENCY_SHIFT) "bad" buckets. */
	KYBER_LATENCY_BUCKETS = 2 << KYBER_LATENCY_SHIFT,
};

/*
 * We measure both the total latency and the I/O latency (i.e., latency after
 * submitting to the device).
 */
enum {
	KYBER_TOTAL_LATENCY,
	KYBER_IO_LATENCY,
};

static const char *kyber_latency_type_names[] = {
	[KYBER_TOTAL_LATENCY] = "total",
	[KYBER_IO_LATENCY] = "I/O",
};

/*
 * Per-cpu latency histograms: total latency and I/O latency for each scheduling
 * domain except for KYBER_OTHER.
 */
struct kyber_cpu_latency {
	atomic_t buckets[KYBER_OTHER][2][KYBER_LATENCY_BUCKETS];
};

/*
 * There is a same mapping between ctx & hctx and kcq & khd,
 * we use request->mq_ctx->index_hw to index the kcq in khd.
 */
struct kyber_ctx_queue {
	/*
	 * Used to ensure operations on rq_list and kcq_map to be an atmoic one.
	 * Also protect the rqs on rq_list when merge.
	 */
	spinlock_t lock;
	struct list_head rq_list[KYBER_NUM_DOMAINS];
} ____cacheline_aligned_in_smp;

struct kyber_queue_data {
	struct request_queue *q;

	/*
	 * Each scheduling domain has a limited number of in-flight requests
	 * device-wide, limited by these tokens.
	 */
	struct sbitmap_queue domain_tokens[KYBER_NUM_DOMAINS];

	/*
	 * Async request percentage, converted to per-word depth for
	 * sbitmap_get_shallow().
	 */
	unsigned int async_depth;

	struct kyber_cpu_latency __percpu *cpu_latency;

	/* Timer for stats aggregation and adjusting domain tokens. */
	struct timer_list timer;

	unsigned int latency_buckets[KYBER_OTHER][2][KYBER_LATENCY_BUCKETS];

	unsigned long latency_timeout[KYBER_OTHER];

	int domain_p99[KYBER_OTHER];

	/* Target latencies in nanoseconds. */
	u64 latency_targets[KYBER_OTHER];
};

struct kyber_hctx_data {
	spinlock_t lock;
	struct list_head rqs[KYBER_NUM_DOMAINS];
	unsigned int cur_domain;
	unsigned int batching;
	struct kyber_ctx_queue *kcqs;
	struct sbitmap kcq_map[KYBER_NUM_DOMAINS];
	struct sbq_wait domain_wait[KYBER_NUM_DOMAINS];
	struct sbq_wait_state *domain_ws[KYBER_NUM_DOMAINS];
	atomic_t wait_index[KYBER_NUM_DOMAINS];
};

static int kyber_domain_wake(wait_queue_entry_t *wait, unsigned mode, int flags,
			     void *key);

static unsigned int kyber_sched_domain(unsigned int op)
{
	switch (op & REQ_OP_MASK) {
	case REQ_OP_READ:
		return KYBER_READ;
	case REQ_OP_WRITE:
		return KYBER_WRITE;
	case REQ_OP_DISCARD:
		return KYBER_DISCARD;
	default:
		return KYBER_OTHER;
	}
}

static void flush_latency_buckets(struct kyber_queue_data *kqd,
				  struct kyber_cpu_latency *cpu_latency,
				  unsigned int sched_domain, unsigned int type)
{
	unsigned int *buckets = kqd->latency_buckets[sched_domain][type];
	atomic_t *cpu_buckets = cpu_latency->buckets[sched_domain][type];
	unsigned int bucket;

	for (bucket = 0; bucket < KYBER_LATENCY_BUCKETS; bucket++)
		buckets[bucket] += atomic_xchg(&cpu_buckets[bucket], 0);
}

/*
 * Calculate the histogram bucket with the given percentile rank, or -1 if there
 * aren't enough samples yet.
 */
static int calculate_percentile(struct kyber_queue_data *kqd,
				unsigned int sched_domain, unsigned int type,
				unsigned int percentile)
{
	unsigned int *buckets = kqd->latency_buckets[sched_domain][type];
	unsigned int bucket, samples = 0, percentile_samples;

	for (bucket = 0; bucket < KYBER_LATENCY_BUCKETS; bucket++)
		samples += buckets[bucket];

	if (!samples)
		return -1;

	/*
	 * We do the calculation once we have 500 samples or one second passes
	 * since the first sample was recorded, whichever comes first.
	 */
	if (!kqd->latency_timeout[sched_domain])
		kqd->latency_timeout[sched_domain] = max(jiffies + HZ, 1UL);
	if (samples < 500 &&
	    time_is_after_jiffies(kqd->latency_timeout[sched_domain])) {
		return -1;
	}
	kqd->latency_timeout[sched_domain] = 0;

	percentile_samples = DIV_ROUND_UP(samples * percentile, 100);
	for (bucket = 0; bucket < KYBER_LATENCY_BUCKETS - 1; bucket++) {
		if (buckets[bucket] >= percentile_samples)
			break;
		percentile_samples -= buckets[bucket];
	}
	memset(buckets, 0, sizeof(kqd->latency_buckets[sched_domain][type]));

	trace_kyber_latency(kqd->q, kyber_domain_names[sched_domain],
			    kyber_latency_type_names[type], percentile,
			    bucket + 1, 1 << KYBER_LATENCY_SHIFT, samples);

	return bucket;
}

static void kyber_resize_domain(struct kyber_queue_data *kqd,
				unsigned int sched_domain, unsigned int depth)
{
	depth = clamp(depth, 1U, kyber_depth[sched_domain]);
	if (depth != kqd->domain_tokens[sched_domain].sb.depth) {
		sbitmap_queue_resize(&kqd->domain_tokens[sched_domain], depth);
		trace_kyber_adjust(kqd->q, kyber_domain_names[sched_domain],
				   depth);
	}
}

static void kyber_timer_fn(struct timer_list *t)
{
	struct kyber_queue_data *kqd = from_timer(kqd, t, timer);
	unsigned int sched_domain;
	int cpu;
	bool bad = false;

	/* Sum all of the per-cpu latency histograms. */
	for_each_online_cpu(cpu) {
		struct kyber_cpu_latency *cpu_latency;

		cpu_latency = per_cpu_ptr(kqd->cpu_latency, cpu);
		for (sched_domain = 0; sched_domain < KYBER_OTHER; sched_domain++) {
			flush_latency_buckets(kqd, cpu_latency, sched_domain,
					      KYBER_TOTAL_LATENCY);
			flush_latency_buckets(kqd, cpu_latency, sched_domain,
					      KYBER_IO_LATENCY);
		}
	}

	/*
	 * Check if any domains have a high I/O latency, which might indicate
	 * congestion in the device. Note that we use the p90; we don't want to
	 * be too sensitive to outliers here.
	 */
	for (sched_domain = 0; sched_domain < KYBER_OTHER; sched_domain++) {
		int p90;

		p90 = calculate_percentile(kqd, sched_domain, KYBER_IO_LATENCY,
					   90);
		if (p90 >= KYBER_GOOD_BUCKETS)
			bad = true;
	}

	/*
	 * Adjust the scheduling domain depths. If we determined that there was
	 * congestion, we throttle all domains with good latencies. Either way,
	 * we ease up on throttling domains with bad latencies.
	 */
	for (sched_domain = 0; sched_domain < KYBER_OTHER; sched_domain++) {
		unsigned int orig_depth, depth;
		int p99;

		p99 = calculate_percentile(kqd, sched_domain,
					   KYBER_TOTAL_LATENCY, 99);
		/*
		 * This is kind of subtle: different domains will not
		 * necessarily have enough samples to calculate the latency
		 * percentiles during the same window, so we have to remember
		 * the p99 for the next time we observe congestion; once we do,
		 * we don't want to throttle again until we get more data, so we
		 * reset it to -1.
		 */
		if (bad) {
			if (p99 < 0)
				p99 = kqd->domain_p99[sched_domain];
			kqd->domain_p99[sched_domain] = -1;
		} else if (p99 >= 0) {
			kqd->domain_p99[sched_domain] = p99;
		}
		if (p99 < 0)
			continue;

		/*
		 * If this domain has bad latency, throttle less. Otherwise,
		 * throttle more iff we determined that there is congestion.
		 *
		 * The new depth is scaled linearly with the p99 latency vs the
		 * latency target. E.g., if the p99 is 3/4 of the target, then
		 * we throttle down to 3/4 of the current depth, and if the p99
		 * is 2x the target, then we double the depth.
		 */
		if (bad || p99 >= KYBER_GOOD_BUCKETS) {
			orig_depth = kqd->domain_tokens[sched_domain].sb.depth;
			depth = (orig_depth * (p99 + 1)) >> KYBER_LATENCY_SHIFT;
			kyber_resize_domain(kqd, sched_domain, depth);
		}
	}
}

static unsigned int kyber_sched_tags_shift(struct request_queue *q)
{
	/*
	 * All of the hardware queues have the same depth, so we can just grab
	 * the shift of the first one.
	 */
	return q->queue_hw_ctx[0]->sched_tags->bitmap_tags.sb.shift;
}

static struct kyber_queue_data *kyber_queue_data_alloc(struct request_queue *q)
{
	struct kyber_queue_data *kqd;
	unsigned int shift;
	int ret = -ENOMEM;
	int i;

	kqd = kzalloc_node(sizeof(*kqd), GFP_KERNEL, q->node);
	if (!kqd)
		goto err;

	kqd->q = q;

	kqd->cpu_latency = alloc_percpu_gfp(struct kyber_cpu_latency,
					    GFP_KERNEL | __GFP_ZERO);
	if (!kqd->cpu_latency)
		goto err_kqd;

	timer_setup(&kqd->timer, kyber_timer_fn, 0);

	for (i = 0; i < KYBER_NUM_DOMAINS; i++) {
		WARN_ON(!kyber_depth[i]);
		WARN_ON(!kyber_batch_size[i]);
		ret = sbitmap_queue_init_node(&kqd->domain_tokens[i],
					      kyber_depth[i], -1, false,
					      GFP_KERNEL, q->node);
		if (ret) {
			while (--i >= 0)
				sbitmap_queue_free(&kqd->domain_tokens[i]);
			goto err_buckets;
		}
	}

	for (i = 0; i < KYBER_OTHER; i++) {
		kqd->domain_p99[i] = -1;
		kqd->latency_targets[i] = kyber_latency_targets[i];
	}

	shift = kyber_sched_tags_shift(q);
	kqd->async_depth = (1U << shift) * KYBER_ASYNC_PERCENT / 100U;

	return kqd;

err_buckets:
	free_percpu(kqd->cpu_latency);
err_kqd:
	kfree(kqd);
err:
	return ERR_PTR(ret);
}

static int kyber_init_sched(struct request_queue *q, struct elevator_type *e)
{
	struct kyber_queue_data *kqd;
	struct elevator_queue *eq;

	eq = elevator_alloc(q, e);
	if (!eq)
		return -ENOMEM;

	kqd = kyber_queue_data_alloc(q);
	if (IS_ERR(kqd)) {
		kobject_put(&eq->kobj);
		return PTR_ERR(kqd);
	}

	blk_stat_enable_accounting(q);

	eq->elevator_data = kqd;
	q->elevator = eq;

	return 0;
}

static void kyber_exit_sched(struct elevator_queue *e)
{
	struct kyber_queue_data *kqd = e->elevator_data;
	int i;

	del_timer_sync(&kqd->timer);

	for (i = 0; i < KYBER_NUM_DOMAINS; i++)
		sbitmap_queue_free(&kqd->domain_tokens[i]);
	free_percpu(kqd->cpu_latency);
	kfree(kqd);
}

static void kyber_ctx_queue_init(struct kyber_ctx_queue *kcq)
{
	unsigned int i;

	spin_lock_init(&kcq->lock);
	for (i = 0; i < KYBER_NUM_DOMAINS; i++)
		INIT_LIST_HEAD(&kcq->rq_list[i]);
}

static int kyber_init_hctx(struct blk_mq_hw_ctx *hctx, unsigned int hctx_idx)
{
	struct kyber_queue_data *kqd = hctx->queue->elevator->elevator_data;
	struct kyber_hctx_data *khd;
	int i;

	khd = kmalloc_node(sizeof(*khd), GFP_KERNEL, hctx->numa_node);
	if (!khd)
		return -ENOMEM;

	khd->kcqs = kmalloc_array_node(hctx->nr_ctx,
				       sizeof(struct kyber_ctx_queue),
				       GFP_KERNEL, hctx->numa_node);
	if (!khd->kcqs)
		goto err_khd;

	for (i = 0; i < hctx->nr_ctx; i++)
		kyber_ctx_queue_init(&khd->kcqs[i]);

	for (i = 0; i < KYBER_NUM_DOMAINS; i++) {
		if (sbitmap_init_node(&khd->kcq_map[i], hctx->nr_ctx,
				      ilog2(8), GFP_KERNEL, hctx->numa_node)) {
			while (--i >= 0)
				sbitmap_free(&khd->kcq_map[i]);
			goto err_kcqs;
		}
	}

	spin_lock_init(&khd->lock);

	for (i = 0; i < KYBER_NUM_DOMAINS; i++) {
		INIT_LIST_HEAD(&khd->rqs[i]);
		khd->domain_wait[i].sbq = NULL;
		init_waitqueue_func_entry(&khd->domain_wait[i].wait,
					  kyber_domain_wake);
		khd->domain_wait[i].wait.private = hctx;
		INIT_LIST_HEAD(&khd->domain_wait[i].wait.entry);
		atomic_set(&khd->wait_index[i], 0);
	}

	khd->cur_domain = 0;
	khd->batching = 0;

	hctx->sched_data = khd;
	sbitmap_queue_min_shallow_depth(&hctx->sched_tags->bitmap_tags,
					kqd->async_depth);

	return 0;

err_kcqs:
	kfree(khd->kcqs);
err_khd:
	kfree(khd);
	return -ENOMEM;
}

static void kyber_exit_hctx(struct blk_mq_hw_ctx *hctx, unsigned int hctx_idx)
{
	struct kyber_hctx_data *khd = hctx->sched_data;
	int i;

	for (i = 0; i < KYBER_NUM_DOMAINS; i++)
		sbitmap_free(&khd->kcq_map[i]);
	kfree(khd->kcqs);
	kfree(hctx->sched_data);
}

static int rq_get_domain_token(struct request *rq)
{
	return (long)rq->elv.priv[0];
}

static void rq_set_domain_token(struct request *rq, int token)
{
	rq->elv.priv[0] = (void *)(long)token;
}

static void rq_clear_domain_token(struct kyber_queue_data *kqd,
				  struct request *rq)
{
	unsigned int sched_domain;
	int nr;

	nr = rq_get_domain_token(rq);
	if (nr != -1) {
		sched_domain = kyber_sched_domain(rq->cmd_flags);
		sbitmap_queue_clear(&kqd->domain_tokens[sched_domain], nr,
				    rq->mq_ctx->cpu);
	}
}

static void kyber_limit_depth(unsigned int op, struct blk_mq_alloc_data *data)
{
	/*
	 * We use the scheduler tags as per-hardware queue queueing tokens.
	 * Async requests can be limited at this stage.
	 */
	if (!op_is_sync(op)) {
		struct kyber_queue_data *kqd = data->q->elevator->elevator_data;

		data->shallow_depth = kqd->async_depth;
	}
}

static bool kyber_bio_merge(struct blk_mq_hw_ctx *hctx, struct bio *bio)
{
	struct kyber_hctx_data *khd = hctx->sched_data;
	struct blk_mq_ctx *ctx = blk_mq_get_ctx(hctx->queue);
	struct kyber_ctx_queue *kcq = &khd->kcqs[ctx->index_hw[hctx->type]];
	unsigned int sched_domain = kyber_sched_domain(bio->bi_opf);
	struct list_head *rq_list = &kcq->rq_list[sched_domain];
	bool merged;

	spin_lock(&kcq->lock);
	merged = blk_mq_bio_list_merge(hctx->queue, rq_list, bio);
	spin_unlock(&kcq->lock);
	blk_mq_put_ctx(ctx);

	return merged;
}

static void kyber_prepare_request(struct request *rq, struct bio *bio)
{
	rq_set_domain_token(rq, -1);
}

static void kyber_insert_requests(struct blk_mq_hw_ctx *hctx,
				  struct list_head *rq_list, bool at_head)
{
	struct kyber_hctx_data *khd = hctx->sched_data;
	struct request *rq, *next;

	list_for_each_entry_safe(rq, next, rq_list, queuelist) {
		unsigned int sched_domain = kyber_sched_domain(rq->cmd_flags);
		struct kyber_ctx_queue *kcq = &khd->kcqs[rq->mq_ctx->index_hw[hctx->type]];
		struct list_head *head = &kcq->rq_list[sched_domain];

		spin_lock(&kcq->lock);
		if (at_head)
			list_move(&rq->queuelist, head);
		else
			list_move_tail(&rq->queuelist, head);
		sbitmap_set_bit(&khd->kcq_map[sched_domain],
				rq->mq_ctx->index_hw[hctx->type]);
		blk_mq_sched_request_inserted(rq);
		spin_unlock(&kcq->lock);
	}
}

static void kyber_finish_request(struct request *rq)
{
	struct kyber_queue_data *kqd = rq->q->elevator->elevator_data;

	rq_clear_domain_token(kqd, rq);
}

static void add_latency_sample(struct kyber_cpu_latency *cpu_latency,
			       unsigned int sched_domain, unsigned int type,
			       u64 target, u64 latency)
{
	unsigned int bucket;
	u64 divisor;

	if (latency > 0) {
		divisor = max_t(u64, target >> KYBER_LATENCY_SHIFT, 1);
		bucket = min_t(unsigned int, div64_u64(latency - 1, divisor),
			       KYBER_LATENCY_BUCKETS - 1);
	} else {
		bucket = 0;
	}

	atomic_inc(&cpu_latency->buckets[sched_domain][type][bucket]);
}

static void kyber_completed_request(struct request *rq, u64 now)
{
	struct kyber_queue_data *kqd = rq->q->elevator->elevator_data;
	struct kyber_cpu_latency *cpu_latency;
	unsigned int sched_domain;
	u64 target;

	sched_domain = kyber_sched_domain(rq->cmd_flags);
	if (sched_domain == KYBER_OTHER)
		return;

	cpu_latency = get_cpu_ptr(kqd->cpu_latency);
	target = kqd->latency_targets[sched_domain];
	add_latency_sample(cpu_latency, sched_domain, KYBER_TOTAL_LATENCY,
			   target, now - rq->start_time_ns);
	add_latency_sample(cpu_latency, sched_domain, KYBER_IO_LATENCY, target,
			   now - rq->io_start_time_ns);
	put_cpu_ptr(kqd->cpu_latency);

	timer_reduce(&kqd->timer, jiffies + HZ / 10);
}

struct flush_kcq_data {
	struct kyber_hctx_data *khd;
	unsigned int sched_domain;
	struct list_head *list;
};

static bool flush_busy_kcq(struct sbitmap *sb, unsigned int bitnr, void *data)
{
	struct flush_kcq_data *flush_data = data;
	struct kyber_ctx_queue *kcq = &flush_data->khd->kcqs[bitnr];

	spin_lock(&kcq->lock);
	list_splice_tail_init(&kcq->rq_list[flush_data->sched_domain],
			      flush_data->list);
	sbitmap_clear_bit(sb, bitnr);
	spin_unlock(&kcq->lock);

	return true;
}

static void kyber_flush_busy_kcqs(struct kyber_hctx_data *khd,
				  unsigned int sched_domain,
				  struct list_head *list)
{
	struct flush_kcq_data data = {
		.khd = khd,
		.sched_domain = sched_domain,
		.list = list,
	};

	sbitmap_for_each_set(&khd->kcq_map[sched_domain],
			     flush_busy_kcq, &data);
}

static int kyber_domain_wake(wait_queue_entry_t *wqe, unsigned mode, int flags,
			     void *key)
{
	struct blk_mq_hw_ctx *hctx = READ_ONCE(wqe->private);
	struct sbq_wait *wait = container_of(wqe, struct sbq_wait, wait);

	sbitmap_del_wait_queue(wait);
	blk_mq_run_hw_queue(hctx, true);
	return 1;
}

static int kyber_get_domain_token(struct kyber_queue_data *kqd,
				  struct kyber_hctx_data *khd,
				  struct blk_mq_hw_ctx *hctx)
{
	unsigned int sched_domain = khd->cur_domain;
	struct sbitmap_queue *domain_tokens = &kqd->domain_tokens[sched_domain];
	struct sbq_wait *wait = &khd->domain_wait[sched_domain];
	struct sbq_wait_state *ws;
	int nr;

	nr = __sbitmap_queue_get(domain_tokens);

	/*
	 * If we failed to get a domain token, make sure the hardware queue is
	 * run when one becomes available. Note that this is serialized on
	 * khd->lock, but we still need to be careful about the waker.
	 */
	if (nr < 0 && list_empty_careful(&wait->wait.entry)) {
		ws = sbq_wait_ptr(domain_tokens,
				  &khd->wait_index[sched_domain]);
		khd->domain_ws[sched_domain] = ws;
		sbitmap_add_wait_queue(domain_tokens, ws, wait);

		/*
		 * Try again in case a token was freed before we got on the wait
		 * queue.
		 */
		nr = __sbitmap_queue_get(domain_tokens);
	}

	/*
	 * If we got a token while we were on the wait queue, remove ourselves
	 * from the wait queue to ensure that all wake ups make forward
	 * progress. It's possible that the waker already deleted the entry
	 * between the !list_empty_careful() check and us grabbing the lock, but
	 * list_del_init() is okay with that.
	 */
	if (nr >= 0 && !list_empty_careful(&wait->wait.entry)) {
		ws = khd->domain_ws[sched_domain];
		spin_lock_irq(&ws->wait.lock);
		sbitmap_del_wait_queue(wait);
		spin_unlock_irq(&ws->wait.lock);
	}

	return nr;
}

static struct request *
kyber_dispatch_cur_domain(struct kyber_queue_data *kqd,
			  struct kyber_hctx_data *khd,
			  struct blk_mq_hw_ctx *hctx)
{
	struct list_head *rqs;
	struct request *rq;
	int nr;

	rqs = &khd->rqs[khd->cur_domain];

	/*
	 * If we already have a flushed request, then we just need to get a
	 * token for it. Otherwise, if there are pending requests in the kcqs,
	 * flush the kcqs, but only if we can get a token. If not, we should
	 * leave the requests in the kcqs so that they can be merged. Note that
	 * khd->lock serializes the flushes, so if we observed any bit set in
	 * the kcq_map, we will always get a request.
	 */
	rq = list_first_entry_or_null(rqs, struct request, queuelist);
	if (rq) {
		nr = kyber_get_domain_token(kqd, khd, hctx);
		if (nr >= 0) {
			khd->batching++;
			rq_set_domain_token(rq, nr);
			list_del_init(&rq->queuelist);
			return rq;
		} else {
			trace_kyber_throttled(kqd->q,
					      kyber_domain_names[khd->cur_domain]);
		}
	} else if (sbitmap_any_bit_set(&khd->kcq_map[khd->cur_domain])) {
		nr = kyber_get_domain_token(kqd, khd, hctx);
		if (nr >= 0) {
			kyber_flush_busy_kcqs(khd, khd->cur_domain, rqs);
			rq = list_first_entry(rqs, struct request, queuelist);
			khd->batching++;
			rq_set_domain_token(rq, nr);
			list_del_init(&rq->queuelist);
			return rq;
		} else {
			trace_kyber_throttled(kqd->q,
					      kyber_domain_names[khd->cur_domain]);
		}
	}

	/* There were either no pending requests or no tokens. */
	return NULL;
}

static struct request *kyber_dispatch_request(struct blk_mq_hw_ctx *hctx)
{
	struct kyber_queue_data *kqd = hctx->queue->elevator->elevator_data;
	struct kyber_hctx_data *khd = hctx->sched_data;
	struct request *rq;
	int i;

	spin_lock(&khd->lock);

	/*
	 * First, if we are still entitled to batch, try to dispatch a request
	 * from the batch.
	 */
	if (khd->batching < kyber_batch_size[khd->cur_domain]) {
		rq = kyber_dispatch_cur_domain(kqd, khd, hctx);
		if (rq)
			goto out;
	}

	/*
	 * Either,
	 * 1. We were no longer entitled to a batch.
	 * 2. The domain we were batching didn't have any requests.
	 * 3. The domain we were batching was out of tokens.
	 *
	 * Start another batch. Note that this wraps back around to the original
	 * domain if no other domains have requests or tokens.
	 */
	khd->batching = 0;
	for (i = 0; i < KYBER_NUM_DOMAINS; i++) {
		if (khd->cur_domain == KYBER_NUM_DOMAINS - 1)
			khd->cur_domain = 0;
		else
			khd->cur_domain++;

		rq = kyber_dispatch_cur_domain(kqd, khd, hctx);
		if (rq)
			goto out;
	}

	rq = NULL;
out:
	spin_unlock(&khd->lock);
	return rq;
}

static bool kyber_has_work(struct blk_mq_hw_ctx *hctx)
{
	struct kyber_hctx_data *khd = hctx->sched_data;
	int i;

	for (i = 0; i < KYBER_NUM_DOMAINS; i++) {
		if (!list_empty_careful(&khd->rqs[i]) ||
		    sbitmap_any_bit_set(&khd->kcq_map[i]))
			return true;
	}

	return false;
}

#define KYBER_LAT_SHOW_STORE(domain, name)				\
static ssize_t kyber_##name##_lat_show(struct elevator_queue *e,	\
				       char *page)			\
{									\
	struct kyber_queue_data *kqd = e->elevator_data;		\
									\
	return sprintf(page, "%llu\n", kqd->latency_targets[domain]);	\
}									\
									\
static ssize_t kyber_##name##_lat_store(struct elevator_queue *e,	\
					const char *page, size_t count)	\
{									\
	struct kyber_queue_data *kqd = e->elevator_data;		\
	unsigned long long nsec;					\
	int ret;							\
									\
	ret = kstrtoull(page, 10, &nsec);				\
	if (ret)							\
		return ret;						\
									\
	kqd->latency_targets[domain] = nsec;				\
									\
	return count;							\
}
KYBER_LAT_SHOW_STORE(KYBER_READ, read);
KYBER_LAT_SHOW_STORE(KYBER_WRITE, write);
#undef KYBER_LAT_SHOW_STORE

#define KYBER_LAT_ATTR(op) __ATTR(op##_lat_nsec, 0644, kyber_##op##_lat_show, kyber_##op##_lat_store)
static struct elv_fs_entry kyber_sched_attrs[] = {
	KYBER_LAT_ATTR(read),
	KYBER_LAT_ATTR(write),
	__ATTR_NULL
};
#undef KYBER_LAT_ATTR

#ifdef CONFIG_BLK_DEBUG_FS
#define KYBER_DEBUGFS_DOMAIN_ATTRS(domain, name)			\
static int kyber_##name##_tokens_show(void *data, struct seq_file *m)	\
{									\
	struct request_queue *q = data;					\
	struct kyber_queue_data *kqd = q->elevator->elevator_data;	\
									\
	sbitmap_queue_show(&kqd->domain_tokens[domain], m);		\
	return 0;							\
}									\
									\
static void *kyber_##name##_rqs_start(struct seq_file *m, loff_t *pos)	\
	__acquires(&khd->lock)						\
{									\
	struct blk_mq_hw_ctx *hctx = m->private;			\
	struct kyber_hctx_data *khd = hctx->sched_data;			\
									\
	spin_lock(&khd->lock);						\
	return seq_list_start(&khd->rqs[domain], *pos);			\
}									\
									\
static void *kyber_##name##_rqs_next(struct seq_file *m, void *v,	\
				     loff_t *pos)			\
{									\
	struct blk_mq_hw_ctx *hctx = m->private;			\
	struct kyber_hctx_data *khd = hctx->sched_data;			\
									\
	return seq_list_next(v, &khd->rqs[domain], pos);		\
}									\
									\
static void kyber_##name##_rqs_stop(struct seq_file *m, void *v)	\
	__releases(&khd->lock)						\
{									\
	struct blk_mq_hw_ctx *hctx = m->private;			\
	struct kyber_hctx_data *khd = hctx->sched_data;			\
									\
	spin_unlock(&khd->lock);					\
}									\
									\
static const struct seq_operations kyber_##name##_rqs_seq_ops = {	\
	.start	= kyber_##name##_rqs_start,				\
	.next	= kyber_##name##_rqs_next,				\
	.stop	= kyber_##name##_rqs_stop,				\
	.show	= blk_mq_debugfs_rq_show,				\
};									\
									\
static int kyber_##name##_waiting_show(void *data, struct seq_file *m)	\
{									\
	struct blk_mq_hw_ctx *hctx = data;				\
	struct kyber_hctx_data *khd = hctx->sched_data;			\
	wait_queue_entry_t *wait = &khd->domain_wait[domain].wait;	\
									\
	seq_printf(m, "%d\n", !list_empty_careful(&wait->entry));	\
	return 0;							\
}
KYBER_DEBUGFS_DOMAIN_ATTRS(KYBER_READ, read)
KYBER_DEBUGFS_DOMAIN_ATTRS(KYBER_WRITE, write)
KYBER_DEBUGFS_DOMAIN_ATTRS(KYBER_DISCARD, discard)
KYBER_DEBUGFS_DOMAIN_ATTRS(KYBER_OTHER, other)
#undef KYBER_DEBUGFS_DOMAIN_ATTRS

static int kyber_async_depth_show(void *data, struct seq_file *m)
{
	struct request_queue *q = data;
	struct kyber_queue_data *kqd = q->elevator->elevator_data;

	seq_printf(m, "%u\n", kqd->async_depth);
	return 0;
}

static int kyber_cur_domain_show(void *data, struct seq_file *m)
{
	struct blk_mq_hw_ctx *hctx = data;
	struct kyber_hctx_data *khd = hctx->sched_data;

	seq_printf(m, "%s\n", kyber_domain_names[khd->cur_domain]);
	return 0;
}

static int kyber_batching_show(void *data, struct seq_file *m)
{
	struct blk_mq_hw_ctx *hctx = data;
	struct kyber_hctx_data *khd = hctx->sched_data;

	seq_printf(m, "%u\n", khd->batching);
	return 0;
}

#define KYBER_QUEUE_DOMAIN_ATTRS(name)	\
	{#name "_tokens", 0400, kyber_##name##_tokens_show}
static const struct blk_mq_debugfs_attr kyber_queue_debugfs_attrs[] = {
	KYBER_QUEUE_DOMAIN_ATTRS(read),
	KYBER_QUEUE_DOMAIN_ATTRS(write),
	KYBER_QUEUE_DOMAIN_ATTRS(discard),
	KYBER_QUEUE_DOMAIN_ATTRS(other),
	{"async_depth", 0400, kyber_async_depth_show},
	{},
};
#undef KYBER_QUEUE_DOMAIN_ATTRS

#define KYBER_HCTX_DOMAIN_ATTRS(name)					\
	{#name "_rqs", 0400, .seq_ops = &kyber_##name##_rqs_seq_ops},	\
	{#name "_waiting", 0400, kyber_##name##_waiting_show}
static const struct blk_mq_debugfs_attr kyber_hctx_debugfs_attrs[] = {
	KYBER_HCTX_DOMAIN_ATTRS(read),
	KYBER_HCTX_DOMAIN_ATTRS(write),
	KYBER_HCTX_DOMAIN_ATTRS(discard),
	KYBER_HCTX_DOMAIN_ATTRS(other),
	{"cur_domain", 0400, kyber_cur_domain_show},
	{"batching", 0400, kyber_batching_show},
	{},
};
#undef KYBER_HCTX_DOMAIN_ATTRS
#endif

static struct elevator_type kyber_sched = {
	.ops = {
		.init_sched = kyber_init_sched,
		.exit_sched = kyber_exit_sched,
		.init_hctx = kyber_init_hctx,
		.exit_hctx = kyber_exit_hctx,
		.limit_depth = kyber_limit_depth,
		.bio_merge = kyber_bio_merge,
		.prepare_request = kyber_prepare_request,
		.insert_requests = kyber_insert_requests,
		.finish_request = kyber_finish_request,
		.requeue_request = kyber_finish_request,
		.completed_request = kyber_completed_request,
		.dispatch_request = kyber_dispatch_request,
		.has_work = kyber_has_work,
	},
#ifdef CONFIG_BLK_DEBUG_FS
	.queue_debugfs_attrs = kyber_queue_debugfs_attrs,
	.hctx_debugfs_attrs = kyber_hctx_debugfs_attrs,
#endif
	.elevator_attrs = kyber_sched_attrs,
	.elevator_name = "kyber",
	.elevator_owner = THIS_MODULE,
};

static int __init kyber_init(void)
{
	return elv_register(&kyber_sched);
}

static void __exit kyber_exit(void)
{
	elv_unregister(&kyber_sched);
}

module_init(kyber_init);
module_exit(kyber_exit);

MODULE_AUTHOR("Omar Sandoval");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Kyber I/O scheduler");