blk-mq: allow software queue to map to multiple hardware queues

[thirdparty/kernel/stable.git] / block / kyber-iosched.c

/*
 * 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];
        wait_queue_entry_t 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]);
                init_waitqueue_func_entry(&khd->domain_wait[i],
                                          kyber_domain_wake);
                khd->domain_wait[i].private = hctx;
                INIT_LIST_HEAD(&khd->domain_wait[i].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 *wait, unsigned mode, int flags,
                             void *key)
{
        struct blk_mq_hw_ctx *hctx = READ_ONCE(wait->private);

        list_del_init(&wait->entry);
        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];
        wait_queue_entry_t *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->entry)) {
                ws = sbq_wait_ptr(domain_tokens,
                                  &khd->wait_index[sched_domain]);
                khd->domain_ws[sched_domain] = ws;
                add_wait_queue(&ws->wait, 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->entry)) {
                ws = khd->domain_ws[sched_domain];
                spin_lock_irq(&ws->wait.lock);
                list_del_init(&wait->entry);
                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];           \
                                                                        \
        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");