perf tools: Add FIFO file names as alternative options to --control

[mirror_ubuntu-hirsute-kernel.git] / block / blk-mq-sched.c

// SPDX-License-Identifier: GPL-2.0
/*
 * blk-mq scheduling framework
 *
 * Copyright (C) 2016 Jens Axboe
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/blk-mq.h>
#include <linux/list_sort.h>

#include <trace/events/block.h>

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

void blk_mq_sched_free_hctx_data(struct request_queue *q,
                                 void (*exit)(struct blk_mq_hw_ctx *))
{
        struct blk_mq_hw_ctx *hctx;
        int i;

        queue_for_each_hw_ctx(q, hctx, i) {
                if (exit && hctx->sched_data)
                        exit(hctx);
                kfree(hctx->sched_data);
                hctx->sched_data = NULL;
        }
}
EXPORT_SYMBOL_GPL(blk_mq_sched_free_hctx_data);

void blk_mq_sched_assign_ioc(struct request *rq)
{
        struct request_queue *q = rq->q;
        struct io_context *ioc;
        struct io_cq *icq;

        /*
         * May not have an IO context if it's a passthrough request
         */
        ioc = current->io_context;
        if (!ioc)
                return;

        spin_lock_irq(&q->queue_lock);
        icq = ioc_lookup_icq(ioc, q);
        spin_unlock_irq(&q->queue_lock);

        if (!icq) {
                icq = ioc_create_icq(ioc, q, GFP_ATOMIC);
                if (!icq)
                        return;
        }
        get_io_context(icq->ioc);
        rq->elv.icq = icq;
}

/*
 * Mark a hardware queue as needing a restart. For shared queues, maintain
 * a count of how many hardware queues are marked for restart.
 */
void blk_mq_sched_mark_restart_hctx(struct blk_mq_hw_ctx *hctx)
{
        if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
                return;

        set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
}
EXPORT_SYMBOL_GPL(blk_mq_sched_mark_restart_hctx);

void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
{
        if (!test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
                return;
        clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);

        /*
         * Order clearing SCHED_RESTART and list_empty_careful(&hctx->dispatch)
         * in blk_mq_run_hw_queue(). Its pair is the barrier in
         * blk_mq_dispatch_rq_list(). So dispatch code won't see SCHED_RESTART,
         * meantime new request added to hctx->dispatch is missed to check in
         * blk_mq_run_hw_queue().
         */
        smp_mb();

        blk_mq_run_hw_queue(hctx, true);
}

static int sched_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
{
        struct request *rqa = container_of(a, struct request, queuelist);
        struct request *rqb = container_of(b, struct request, queuelist);

        return rqa->mq_hctx > rqb->mq_hctx;
}

static bool blk_mq_dispatch_hctx_list(struct list_head *rq_list)
{
        struct blk_mq_hw_ctx *hctx =
                list_first_entry(rq_list, struct request, queuelist)->mq_hctx;
        struct request *rq;
        LIST_HEAD(hctx_list);
        unsigned int count = 0;

        list_for_each_entry(rq, rq_list, queuelist) {
                if (rq->mq_hctx != hctx) {
                        list_cut_before(&hctx_list, rq_list, &rq->queuelist);
                        goto dispatch;
                }
                count++;
        }
        list_splice_tail_init(rq_list, &hctx_list);

dispatch:
        return blk_mq_dispatch_rq_list(hctx, &hctx_list, count);
}

#define BLK_MQ_BUDGET_DELAY     3               /* ms units */

/*
 * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
 * its queue by itself in its completion handler, so we don't need to
 * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
 *
 * Returns -EAGAIN if hctx->dispatch was found non-empty and run_work has to
 * be run again.  This is necessary to avoid starving flushes.
 */
static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
{
        struct request_queue *q = hctx->queue;
        struct elevator_queue *e = q->elevator;
        bool multi_hctxs = false, run_queue = false;
        bool dispatched = false, busy = false;
        unsigned int max_dispatch;
        LIST_HEAD(rq_list);
        int count = 0;

        if (hctx->dispatch_busy)
                max_dispatch = 1;
        else
                max_dispatch = hctx->queue->nr_requests;

        do {
                struct request *rq;

                if (e->type->ops.has_work && !e->type->ops.has_work(hctx))
                        break;

                if (!list_empty_careful(&hctx->dispatch)) {
                        busy = true;
                        break;
                }

                if (!blk_mq_get_dispatch_budget(q))
                        break;

                rq = e->type->ops.dispatch_request(hctx);
                if (!rq) {
                        blk_mq_put_dispatch_budget(q);
                        /*
                         * We're releasing without dispatching. Holding the
                         * budget could have blocked any "hctx"s with the
                         * same queue and if we didn't dispatch then there's
                         * no guarantee anyone will kick the queue.  Kick it
                         * ourselves.
                         */
                        run_queue = true;
                        break;
                }

                /*
                 * Now this rq owns the budget which has to be released
                 * if this rq won't be queued to driver via .queue_rq()
                 * in blk_mq_dispatch_rq_list().
                 */
                list_add_tail(&rq->queuelist, &rq_list);
                if (rq->mq_hctx != hctx)
                        multi_hctxs = true;
        } while (++count < max_dispatch);

        if (!count) {
                if (run_queue)
                        blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);
        } else if (multi_hctxs) {
                /*
                 * Requests from different hctx may be dequeued from some
                 * schedulers, such as bfq and deadline.
                 *
                 * Sort the requests in the list according to their hctx,
                 * dispatch batching requests from same hctx at a time.
                 */
                list_sort(NULL, &rq_list, sched_rq_cmp);
                do {
                        dispatched |= blk_mq_dispatch_hctx_list(&rq_list);
                } while (!list_empty(&rq_list));
        } else {
                dispatched = blk_mq_dispatch_rq_list(hctx, &rq_list, count);
        }

        if (busy)
                return -EAGAIN;
        return !!dispatched;
}

static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
{
        int ret;

        do {
                ret = __blk_mq_do_dispatch_sched(hctx);
        } while (ret == 1);

        return ret;
}

static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
                                          struct blk_mq_ctx *ctx)
{
        unsigned short idx = ctx->index_hw[hctx->type];

        if (++idx == hctx->nr_ctx)
                idx = 0;

        return hctx->ctxs[idx];
}

/*
 * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
 * its queue by itself in its completion handler, so we don't need to
 * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
 *
 * Returns -EAGAIN if hctx->dispatch was found non-empty and run_work has to
 * be run again.  This is necessary to avoid starving flushes.
 */
static int blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
{
        struct request_queue *q = hctx->queue;
        LIST_HEAD(rq_list);
        struct blk_mq_ctx *ctx = READ_ONCE(hctx->dispatch_from);
        int ret = 0;
        struct request *rq;

        do {
                if (!list_empty_careful(&hctx->dispatch)) {
                        ret = -EAGAIN;
                        break;
                }

                if (!sbitmap_any_bit_set(&hctx->ctx_map))
                        break;

                if (!blk_mq_get_dispatch_budget(q))
                        break;

                rq = blk_mq_dequeue_from_ctx(hctx, ctx);
                if (!rq) {
                        blk_mq_put_dispatch_budget(q);
                        /*
                         * We're releasing without dispatching. Holding the
                         * budget could have blocked any "hctx"s with the
                         * same queue and if we didn't dispatch then there's
                         * no guarantee anyone will kick the queue.  Kick it
                         * ourselves.
                         */
                        blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);
                        break;
                }

                /*
                 * Now this rq owns the budget which has to be released
                 * if this rq won't be queued to driver via .queue_rq()
                 * in blk_mq_dispatch_rq_list().
                 */
                list_add(&rq->queuelist, &rq_list);

                /* round robin for fair dispatch */
                ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);

        } while (blk_mq_dispatch_rq_list(rq->mq_hctx, &rq_list, 1));

        WRITE_ONCE(hctx->dispatch_from, ctx);
        return ret;
}

static int __blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
{
        struct request_queue *q = hctx->queue;
        struct elevator_queue *e = q->elevator;
        const bool has_sched_dispatch = e && e->type->ops.dispatch_request;
        int ret = 0;
        LIST_HEAD(rq_list);

        /*
         * If we have previous entries on our dispatch list, grab them first for
         * more fair dispatch.
         */
        if (!list_empty_careful(&hctx->dispatch)) {
                spin_lock(&hctx->lock);
                if (!list_empty(&hctx->dispatch))
                        list_splice_init(&hctx->dispatch, &rq_list);
                spin_unlock(&hctx->lock);
        }

        /*
         * Only ask the scheduler for requests, if we didn't have residual
         * requests from the dispatch list. This is to avoid the case where
         * we only ever dispatch a fraction of the requests available because
         * of low device queue depth. Once we pull requests out of the IO
         * scheduler, we can no longer merge or sort them. So it's best to
         * leave them there for as long as we can. Mark the hw queue as
         * needing a restart in that case.
         *
         * We want to dispatch from the scheduler if there was nothing
         * on the dispatch list or we were able to dispatch from the
         * dispatch list.
         */
        if (!list_empty(&rq_list)) {
                blk_mq_sched_mark_restart_hctx(hctx);
                if (blk_mq_dispatch_rq_list(hctx, &rq_list, 0)) {
                        if (has_sched_dispatch)
                                ret = blk_mq_do_dispatch_sched(hctx);
                        else
                                ret = blk_mq_do_dispatch_ctx(hctx);
                }
        } else if (has_sched_dispatch) {
                ret = blk_mq_do_dispatch_sched(hctx);
        } else if (hctx->dispatch_busy) {
                /* dequeue request one by one from sw queue if queue is busy */
                ret = blk_mq_do_dispatch_ctx(hctx);
        } else {
                blk_mq_flush_busy_ctxs(hctx, &rq_list);
                blk_mq_dispatch_rq_list(hctx, &rq_list, 0);
        }

        return ret;
}

void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
{
        struct request_queue *q = hctx->queue;

        /* RCU or SRCU read lock is needed before checking quiesced flag */
        if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
                return;

        hctx->run++;

        /*
         * A return of -EAGAIN is an indication that hctx->dispatch is not
         * empty and we must run again in order to avoid starving flushes.
         */
        if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN) {
                if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN)
                        blk_mq_run_hw_queue(hctx, true);
        }
}

bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
                unsigned int nr_segs, struct request **merged_request)
{
        struct request *rq;

        switch (elv_merge(q, &rq, bio)) {
        case ELEVATOR_BACK_MERGE:
                if (!blk_mq_sched_allow_merge(q, rq, bio))
                        return false;
                if (!bio_attempt_back_merge(rq, bio, nr_segs))
                        return false;
                *merged_request = attempt_back_merge(q, rq);
                if (!*merged_request)
                        elv_merged_request(q, rq, ELEVATOR_BACK_MERGE);
                return true;
        case ELEVATOR_FRONT_MERGE:
                if (!blk_mq_sched_allow_merge(q, rq, bio))
                        return false;
                if (!bio_attempt_front_merge(rq, bio, nr_segs))
                        return false;
                *merged_request = attempt_front_merge(q, rq);
                if (!*merged_request)
                        elv_merged_request(q, rq, ELEVATOR_FRONT_MERGE);
                return true;
        case ELEVATOR_DISCARD_MERGE:
                return bio_attempt_discard_merge(q, rq, bio);
        default:
                return false;
        }
}
EXPORT_SYMBOL_GPL(blk_mq_sched_try_merge);

/*
 * Iterate list of requests and see if we can merge this bio with any
 * of them.
 */
bool blk_mq_bio_list_merge(struct request_queue *q, struct list_head *list,
                           struct bio *bio, unsigned int nr_segs)
{
        struct request *rq;
        int checked = 8;

        list_for_each_entry_reverse(rq, list, queuelist) {
                bool merged = false;

                if (!checked--)
                        break;

                if (!blk_rq_merge_ok(rq, bio))
                        continue;

                switch (blk_try_merge(rq, bio)) {
                case ELEVATOR_BACK_MERGE:
                        if (blk_mq_sched_allow_merge(q, rq, bio))
                                merged = bio_attempt_back_merge(rq, bio,
                                                nr_segs);
                        break;
                case ELEVATOR_FRONT_MERGE:
                        if (blk_mq_sched_allow_merge(q, rq, bio))
                                merged = bio_attempt_front_merge(rq, bio,
                                                nr_segs);
                        break;
                case ELEVATOR_DISCARD_MERGE:
                        merged = bio_attempt_discard_merge(q, rq, bio);
                        break;
                default:
                        continue;
                }

                return merged;
        }

        return false;
}
EXPORT_SYMBOL_GPL(blk_mq_bio_list_merge);

/*
 * Reverse check our software queue for entries that we could potentially
 * merge with. Currently includes a hand-wavy stop count of 8, to not spend
 * too much time checking for merges.
 */
static bool blk_mq_attempt_merge(struct request_queue *q,
                                 struct blk_mq_hw_ctx *hctx,
                                 struct blk_mq_ctx *ctx, struct bio *bio,
                                 unsigned int nr_segs)
{
        enum hctx_type type = hctx->type;

        lockdep_assert_held(&ctx->lock);

        if (blk_mq_bio_list_merge(q, &ctx->rq_lists[type], bio, nr_segs)) {
                ctx->rq_merged++;
                return true;
        }

        return false;
}

bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio,
                unsigned int nr_segs)
{
        struct elevator_queue *e = q->elevator;
        struct blk_mq_ctx *ctx = blk_mq_get_ctx(q);
        struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, bio->bi_opf, ctx);
        bool ret = false;
        enum hctx_type type;

        if (e && e->type->ops.bio_merge)
                return e->type->ops.bio_merge(hctx, bio, nr_segs);

        type = hctx->type;
        if ((hctx->flags & BLK_MQ_F_SHOULD_MERGE) &&
                        !list_empty_careful(&ctx->rq_lists[type])) {
                /* default per sw-queue merge */
                spin_lock(&ctx->lock);
                ret = blk_mq_attempt_merge(q, hctx, ctx, bio, nr_segs);
                spin_unlock(&ctx->lock);
        }

        return ret;
}

bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq)
{
        return rq_mergeable(rq) && elv_attempt_insert_merge(q, rq);
}
EXPORT_SYMBOL_GPL(blk_mq_sched_try_insert_merge);

void blk_mq_sched_request_inserted(struct request *rq)
{
        trace_block_rq_insert(rq->q, rq);
}
EXPORT_SYMBOL_GPL(blk_mq_sched_request_inserted);

static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx,
                                       bool has_sched,
                                       struct request *rq)
{
        /*
         * dispatch flush and passthrough rq directly
         *
         * passthrough request has to be added to hctx->dispatch directly.
         * For some reason, device may be in one situation which can't
         * handle FS request, so STS_RESOURCE is always returned and the
         * FS request will be added to hctx->dispatch. However passthrough
         * request may be required at that time for fixing the problem. If
         * passthrough request is added to scheduler queue, there isn't any
         * chance to dispatch it given we prioritize requests in hctx->dispatch.
         */
        if ((rq->rq_flags & RQF_FLUSH_SEQ) || blk_rq_is_passthrough(rq))
                return true;

        if (has_sched)
                rq->rq_flags |= RQF_SORTED;

        return false;
}

void blk_mq_sched_insert_request(struct request *rq, bool at_head,
                                 bool run_queue, bool async)
{
        struct request_queue *q = rq->q;
        struct elevator_queue *e = q->elevator;
        struct blk_mq_ctx *ctx = rq->mq_ctx;
        struct blk_mq_hw_ctx *hctx = rq->mq_hctx;

        /* flush rq in flush machinery need to be dispatched directly */
        if (!(rq->rq_flags & RQF_FLUSH_SEQ) && op_is_flush(rq->cmd_flags)) {
                blk_insert_flush(rq);
                goto run;
        }

        WARN_ON(e && (rq->tag != -1));

        if (blk_mq_sched_bypass_insert(hctx, !!e, rq)) {
                /*
                 * Firstly normal IO request is inserted to scheduler queue or
                 * sw queue, meantime we add flush request to dispatch queue(
                 * hctx->dispatch) directly and there is at most one in-flight
                 * flush request for each hw queue, so it doesn't matter to add
                 * flush request to tail or front of the dispatch queue.
                 *
                 * Secondly in case of NCQ, flush request belongs to non-NCQ
                 * command, and queueing it will fail when there is any
                 * in-flight normal IO request(NCQ command). When adding flush
                 * rq to the front of hctx->dispatch, it is easier to introduce
                 * extra time to flush rq's latency because of S_SCHED_RESTART
                 * compared with adding to the tail of dispatch queue, then
                 * chance of flush merge is increased, and less flush requests
                 * will be issued to controller. It is observed that ~10% time
                 * is saved in blktests block/004 on disk attached to AHCI/NCQ
                 * drive when adding flush rq to the front of hctx->dispatch.
                 *
                 * Simply queue flush rq to the front of hctx->dispatch so that
                 * intensive flush workloads can benefit in case of NCQ HW.
                 */
                at_head = (rq->rq_flags & RQF_FLUSH_SEQ) ? true : at_head;
                blk_mq_request_bypass_insert(rq, at_head, false);
                goto run;
        }

        if (e && e->type->ops.insert_requests) {
                LIST_HEAD(list);

                list_add(&rq->queuelist, &list);
                e->type->ops.insert_requests(hctx, &list, at_head);
        } else {
                spin_lock(&ctx->lock);
                __blk_mq_insert_request(hctx, rq, at_head);
                spin_unlock(&ctx->lock);
        }

run:
        if (run_queue)
                blk_mq_run_hw_queue(hctx, async);
}

void blk_mq_sched_insert_requests(struct blk_mq_hw_ctx *hctx,
                                  struct blk_mq_ctx *ctx,
                                  struct list_head *list, bool run_queue_async)
{
        struct elevator_queue *e;
        struct request_queue *q = hctx->queue;

        /*
         * blk_mq_sched_insert_requests() is called from flush plug
         * context only, and hold one usage counter to prevent queue
         * from being released.
         */
        percpu_ref_get(&q->q_usage_counter);

        e = hctx->queue->elevator;
        if (e && e->type->ops.insert_requests)
                e->type->ops.insert_requests(hctx, list, false);
        else {
                /*
                 * try to issue requests directly if the hw queue isn't
                 * busy in case of 'none' scheduler, and this way may save
                 * us one extra enqueue & dequeue to sw queue.
                 */
                if (!hctx->dispatch_busy && !e && !run_queue_async) {
                        blk_mq_try_issue_list_directly(hctx, list);
                        if (list_empty(list))
                                goto out;
                }
                blk_mq_insert_requests(hctx, ctx, list);
        }

        blk_mq_run_hw_queue(hctx, run_queue_async);
 out:
        percpu_ref_put(&q->q_usage_counter);
}

static void blk_mq_sched_free_tags(struct blk_mq_tag_set *set,
                                   struct blk_mq_hw_ctx *hctx,
                                   unsigned int hctx_idx)
{
        if (hctx->sched_tags) {
                blk_mq_free_rqs(set, hctx->sched_tags, hctx_idx);
                blk_mq_free_rq_map(hctx->sched_tags);
                hctx->sched_tags = NULL;
        }
}

static int blk_mq_sched_alloc_tags(struct request_queue *q,
                                   struct blk_mq_hw_ctx *hctx,
                                   unsigned int hctx_idx)
{
        struct blk_mq_tag_set *set = q->tag_set;
        int ret;

        hctx->sched_tags = blk_mq_alloc_rq_map(set, hctx_idx, q->nr_requests,
                                               set->reserved_tags);
        if (!hctx->sched_tags)
                return -ENOMEM;

        ret = blk_mq_alloc_rqs(set, hctx->sched_tags, hctx_idx, q->nr_requests);
        if (ret)
                blk_mq_sched_free_tags(set, hctx, hctx_idx);

        return ret;
}

/* called in queue's release handler, tagset has gone away */
static void blk_mq_sched_tags_teardown(struct request_queue *q)
{
        struct blk_mq_hw_ctx *hctx;
        int i;

        queue_for_each_hw_ctx(q, hctx, i) {
                if (hctx->sched_tags) {
                        blk_mq_free_rq_map(hctx->sched_tags);
                        hctx->sched_tags = NULL;
                }
        }
}

int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
{
        struct blk_mq_hw_ctx *hctx;
        struct elevator_queue *eq;
        unsigned int i;
        int ret;

        if (!e) {
                q->elevator = NULL;
                q->nr_requests = q->tag_set->queue_depth;
                return 0;
        }

        /*
         * Default to double of smaller one between hw queue_depth and 128,
         * since we don't split into sync/async like the old code did.
         * Additionally, this is a per-hw queue depth.
         */
        q->nr_requests = 2 * min_t(unsigned int, q->tag_set->queue_depth,
                                   BLKDEV_MAX_RQ);

        queue_for_each_hw_ctx(q, hctx, i) {
                ret = blk_mq_sched_alloc_tags(q, hctx, i);
                if (ret)
                        goto err;
        }

        ret = e->ops.init_sched(q, e);
        if (ret)
                goto err;

        blk_mq_debugfs_register_sched(q);

        queue_for_each_hw_ctx(q, hctx, i) {
                if (e->ops.init_hctx) {
                        ret = e->ops.init_hctx(hctx, i);
                        if (ret) {
                                eq = q->elevator;
                                blk_mq_sched_free_requests(q);
                                blk_mq_exit_sched(q, eq);
                                kobject_put(&eq->kobj);
                                return ret;
                        }
                }
                blk_mq_debugfs_register_sched_hctx(q, hctx);
        }

        return 0;

err:
        blk_mq_sched_free_requests(q);
        blk_mq_sched_tags_teardown(q);
        q->elevator = NULL;
        return ret;
}

/*
 * called in either blk_queue_cleanup or elevator_switch, tagset
 * is required for freeing requests
 */
void blk_mq_sched_free_requests(struct request_queue *q)
{
        struct blk_mq_hw_ctx *hctx;
        int i;

        queue_for_each_hw_ctx(q, hctx, i) {
                if (hctx->sched_tags)
                        blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i);
        }
}

void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
{
        struct blk_mq_hw_ctx *hctx;
        unsigned int i;

        queue_for_each_hw_ctx(q, hctx, i) {
                blk_mq_debugfs_unregister_sched_hctx(hctx);
                if (e->type->ops.exit_hctx && hctx->sched_data) {
                        e->type->ops.exit_hctx(hctx, i);
                        hctx->sched_data = NULL;
                }
        }
        blk_mq_debugfs_unregister_sched(q);
        if (e->type->ops.exit_sched)
                e->type->ops.exit_sched(e);
        blk_mq_sched_tags_teardown(q);
        q->elevator = NULL;
}