drm/amdgpu: drop fclk/gfxclk ratio setting

[thirdparty/linux.git] / block / blk-wbt.c

/*
 * buffered writeback throttling. loosely based on CoDel. We can't drop
 * packets for IO scheduling, so the logic is something like this:
 *
 * - Monitor latencies in a defined window of time.
 * - If the minimum latency in the above window exceeds some target, increment
 *   scaling step and scale down queue depth by a factor of 2x. The monitoring
 *   window is then shrunk to 100 / sqrt(scaling step + 1).
 * - For any window where we don't have solid data on what the latencies
 *   look like, retain status quo.
 * - If latencies look good, decrement scaling step.
 * - If we're only doing writes, allow the scaling step to go negative. This
 *   will temporarily boost write performance, snapping back to a stable
 *   scaling step of 0 if reads show up or the heavy writers finish. Unlike
 *   positive scaling steps where we shrink the monitoring window, a negative
 *   scaling step retains the default step==0 window size.
 *
 * Copyright (C) 2016 Jens Axboe
 *
 */
#include <linux/kernel.h>
#include <linux/blk_types.h>
#include <linux/slab.h>
#include <linux/backing-dev.h>
#include <linux/swap.h>

#include "blk-wbt.h"
#include "blk-rq-qos.h"

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

static inline void wbt_clear_state(struct request *rq)
{
        rq->wbt_flags = 0;
}

static inline enum wbt_flags wbt_flags(struct request *rq)
{
        return rq->wbt_flags;
}

static inline bool wbt_is_tracked(struct request *rq)
{
        return rq->wbt_flags & WBT_TRACKED;
}

static inline bool wbt_is_read(struct request *rq)
{
        return rq->wbt_flags & WBT_READ;
}

enum {
        /*
         * Default setting, we'll scale up (to 75% of QD max) or down (min 1)
         * from here depending on device stats
         */
        RWB_DEF_DEPTH   = 16,

        /*
         * 100msec window
         */
        RWB_WINDOW_NSEC         = 100 * 1000 * 1000ULL,

        /*
         * Disregard stats, if we don't meet this minimum
         */
        RWB_MIN_WRITE_SAMPLES   = 3,

        /*
         * If we have this number of consecutive windows with not enough
         * information to scale up or down, scale up.
         */
        RWB_UNKNOWN_BUMP        = 5,
};

static inline bool rwb_enabled(struct rq_wb *rwb)
{
        return rwb && rwb->wb_normal != 0;
}

static void wb_timestamp(struct rq_wb *rwb, unsigned long *var)
{
        if (rwb_enabled(rwb)) {
                const unsigned long cur = jiffies;

                if (cur != *var)
                        *var = cur;
        }
}

/*
 * If a task was rate throttled in balance_dirty_pages() within the last
 * second or so, use that to indicate a higher cleaning rate.
 */
static bool wb_recent_wait(struct rq_wb *rwb)
{
        struct bdi_writeback *wb = &rwb->rqos.q->backing_dev_info->wb;

        return time_before(jiffies, wb->dirty_sleep + HZ);
}

static inline struct rq_wait *get_rq_wait(struct rq_wb *rwb,
                                          enum wbt_flags wb_acct)
{
        if (wb_acct & WBT_KSWAPD)
                return &rwb->rq_wait[WBT_RWQ_KSWAPD];
        else if (wb_acct & WBT_DISCARD)
                return &rwb->rq_wait[WBT_RWQ_DISCARD];

        return &rwb->rq_wait[WBT_RWQ_BG];
}

static void rwb_wake_all(struct rq_wb *rwb)
{
        int i;

        for (i = 0; i < WBT_NUM_RWQ; i++) {
                struct rq_wait *rqw = &rwb->rq_wait[i];

                if (wq_has_sleeper(&rqw->wait))
                        wake_up_all(&rqw->wait);
        }
}

static void wbt_rqw_done(struct rq_wb *rwb, struct rq_wait *rqw,
                         enum wbt_flags wb_acct)
{
        int inflight, limit;

        inflight = atomic_dec_return(&rqw->inflight);

        /*
         * wbt got disabled with IO in flight. Wake up any potential
         * waiters, we don't have to do more than that.
         */
        if (unlikely(!rwb_enabled(rwb))) {
                rwb_wake_all(rwb);
                return;
        }

        /*
         * For discards, our limit is always the background. For writes, if
         * the device does write back caching, drop further down before we
         * wake people up.
         */
        if (wb_acct & WBT_DISCARD)
                limit = rwb->wb_background;
        else if (rwb->wc && !wb_recent_wait(rwb))
                limit = 0;
        else
                limit = rwb->wb_normal;

        /*
         * Don't wake anyone up if we are above the normal limit.
         */
        if (inflight && inflight >= limit)
                return;

        if (wq_has_sleeper(&rqw->wait)) {
                int diff = limit - inflight;

                if (!inflight || diff >= rwb->wb_background / 2)
                        wake_up_all(&rqw->wait);
        }
}

static void __wbt_done(struct rq_qos *rqos, enum wbt_flags wb_acct)
{
        struct rq_wb *rwb = RQWB(rqos);
        struct rq_wait *rqw;

        if (!(wb_acct & WBT_TRACKED))
                return;

        rqw = get_rq_wait(rwb, wb_acct);
        wbt_rqw_done(rwb, rqw, wb_acct);
}

/*
 * Called on completion of a request. Note that it's also called when
 * a request is merged, when the request gets freed.
 */
static void wbt_done(struct rq_qos *rqos, struct request *rq)
{
        struct rq_wb *rwb = RQWB(rqos);

        if (!wbt_is_tracked(rq)) {
                if (rwb->sync_cookie == rq) {
                        rwb->sync_issue = 0;
                        rwb->sync_cookie = NULL;
                }

                if (wbt_is_read(rq))
                        wb_timestamp(rwb, &rwb->last_comp);
        } else {
                WARN_ON_ONCE(rq == rwb->sync_cookie);
                __wbt_done(rqos, wbt_flags(rq));
        }
        wbt_clear_state(rq);
}

static inline bool stat_sample_valid(struct blk_rq_stat *stat)
{
        /*
         * We need at least one read sample, and a minimum of
         * RWB_MIN_WRITE_SAMPLES. We require some write samples to know
         * that it's writes impacting us, and not just some sole read on
         * a device that is in a lower power state.
         */
        return (stat[READ].nr_samples >= 1 &&
                stat[WRITE].nr_samples >= RWB_MIN_WRITE_SAMPLES);
}

static u64 rwb_sync_issue_lat(struct rq_wb *rwb)
{
        u64 now, issue = READ_ONCE(rwb->sync_issue);

        if (!issue || !rwb->sync_cookie)
                return 0;

        now = ktime_to_ns(ktime_get());
        return now - issue;
}

enum {
        LAT_OK = 1,
        LAT_UNKNOWN,
        LAT_UNKNOWN_WRITES,
        LAT_EXCEEDED,
};

static int latency_exceeded(struct rq_wb *rwb, struct blk_rq_stat *stat)
{
        struct backing_dev_info *bdi = rwb->rqos.q->backing_dev_info;
        struct rq_depth *rqd = &rwb->rq_depth;
        u64 thislat;

        /*
         * If our stored sync issue exceeds the window size, or it
         * exceeds our min target AND we haven't logged any entries,
         * flag the latency as exceeded. wbt works off completion latencies,
         * but for a flooded device, a single sync IO can take a long time
         * to complete after being issued. If this time exceeds our
         * monitoring window AND we didn't see any other completions in that
         * window, then count that sync IO as a violation of the latency.
         */
        thislat = rwb_sync_issue_lat(rwb);
        if (thislat > rwb->cur_win_nsec ||
            (thislat > rwb->min_lat_nsec && !stat[READ].nr_samples)) {
                trace_wbt_lat(bdi, thislat);
                return LAT_EXCEEDED;
        }

        /*
         * No read/write mix, if stat isn't valid
         */
        if (!stat_sample_valid(stat)) {
                /*
                 * If we had writes in this stat window and the window is
                 * current, we're only doing writes. If a task recently
                 * waited or still has writes in flights, consider us doing
                 * just writes as well.
                 */
                if (stat[WRITE].nr_samples || wb_recent_wait(rwb) ||
                    wbt_inflight(rwb))
                        return LAT_UNKNOWN_WRITES;
                return LAT_UNKNOWN;
        }

        /*
         * If the 'min' latency exceeds our target, step down.
         */
        if (stat[READ].min > rwb->min_lat_nsec) {
                trace_wbt_lat(bdi, stat[READ].min);
                trace_wbt_stat(bdi, stat);
                return LAT_EXCEEDED;
        }

        if (rqd->scale_step)
                trace_wbt_stat(bdi, stat);

        return LAT_OK;
}

static void rwb_trace_step(struct rq_wb *rwb, const char *msg)
{
        struct backing_dev_info *bdi = rwb->rqos.q->backing_dev_info;
        struct rq_depth *rqd = &rwb->rq_depth;

        trace_wbt_step(bdi, msg, rqd->scale_step, rwb->cur_win_nsec,
                        rwb->wb_background, rwb->wb_normal, rqd->max_depth);
}

static void calc_wb_limits(struct rq_wb *rwb)
{
        if (rwb->min_lat_nsec == 0) {
                rwb->wb_normal = rwb->wb_background = 0;
        } else if (rwb->rq_depth.max_depth <= 2) {
                rwb->wb_normal = rwb->rq_depth.max_depth;
                rwb->wb_background = 1;
        } else {
                rwb->wb_normal = (rwb->rq_depth.max_depth + 1) / 2;
                rwb->wb_background = (rwb->rq_depth.max_depth + 3) / 4;
        }
}

static void scale_up(struct rq_wb *rwb)
{
        rq_depth_scale_up(&rwb->rq_depth);
        calc_wb_limits(rwb);
        rwb->unknown_cnt = 0;
        rwb_wake_all(rwb);
        rwb_trace_step(rwb, "scale up");
}

static void scale_down(struct rq_wb *rwb, bool hard_throttle)
{
        rq_depth_scale_down(&rwb->rq_depth, hard_throttle);
        calc_wb_limits(rwb);
        rwb->unknown_cnt = 0;
        rwb_trace_step(rwb, "scale down");
}

static void rwb_arm_timer(struct rq_wb *rwb)
{
        struct rq_depth *rqd = &rwb->rq_depth;

        if (rqd->scale_step > 0) {
                /*
                 * We should speed this up, using some variant of a fast
                 * integer inverse square root calculation. Since we only do
                 * this for every window expiration, it's not a huge deal,
                 * though.
                 */
                rwb->cur_win_nsec = div_u64(rwb->win_nsec << 4,
                                        int_sqrt((rqd->scale_step + 1) << 8));
        } else {
                /*
                 * For step < 0, we don't want to increase/decrease the
                 * window size.
                 */
                rwb->cur_win_nsec = rwb->win_nsec;
        }

        blk_stat_activate_nsecs(rwb->cb, rwb->cur_win_nsec);
}

static void wb_timer_fn(struct blk_stat_callback *cb)
{
        struct rq_wb *rwb = cb->data;
        struct rq_depth *rqd = &rwb->rq_depth;
        unsigned int inflight = wbt_inflight(rwb);
        int status;

        status = latency_exceeded(rwb, cb->stat);

        trace_wbt_timer(rwb->rqos.q->backing_dev_info, status, rqd->scale_step,
                        inflight);

        /*
         * If we exceeded the latency target, step down. If we did not,
         * step one level up. If we don't know enough to say either exceeded
         * or ok, then don't do anything.
         */
        switch (status) {
        case LAT_EXCEEDED:
                scale_down(rwb, true);
                break;
        case LAT_OK:
                scale_up(rwb);
                break;
        case LAT_UNKNOWN_WRITES:
                /*
                 * We started a the center step, but don't have a valid
                 * read/write sample, but we do have writes going on.
                 * Allow step to go negative, to increase write perf.
                 */
                scale_up(rwb);
                break;
        case LAT_UNKNOWN:
                if (++rwb->unknown_cnt < RWB_UNKNOWN_BUMP)
                        break;
                /*
                 * We get here when previously scaled reduced depth, and we
                 * currently don't have a valid read/write sample. For that
                 * case, slowly return to center state (step == 0).
                 */
                if (rqd->scale_step > 0)
                        scale_up(rwb);
                else if (rqd->scale_step < 0)
                        scale_down(rwb, false);
                break;
        default:
                break;
        }

        /*
         * Re-arm timer, if we have IO in flight
         */
        if (rqd->scale_step || inflight)
                rwb_arm_timer(rwb);
}

static void __wbt_update_limits(struct rq_wb *rwb)
{
        struct rq_depth *rqd = &rwb->rq_depth;

        rqd->scale_step = 0;
        rqd->scaled_max = false;

        rq_depth_calc_max_depth(rqd);
        calc_wb_limits(rwb);

        rwb_wake_all(rwb);
}

void wbt_update_limits(struct request_queue *q)
{
        struct rq_qos *rqos = wbt_rq_qos(q);
        if (!rqos)
                return;
        __wbt_update_limits(RQWB(rqos));
}

u64 wbt_get_min_lat(struct request_queue *q)
{
        struct rq_qos *rqos = wbt_rq_qos(q);
        if (!rqos)
                return 0;
        return RQWB(rqos)->min_lat_nsec;
}

void wbt_set_min_lat(struct request_queue *q, u64 val)
{
        struct rq_qos *rqos = wbt_rq_qos(q);
        if (!rqos)
                return;
        RQWB(rqos)->min_lat_nsec = val;
        RQWB(rqos)->enable_state = WBT_STATE_ON_MANUAL;
        __wbt_update_limits(RQWB(rqos));
}


static bool close_io(struct rq_wb *rwb)
{
        const unsigned long now = jiffies;

        return time_before(now, rwb->last_issue + HZ / 10) ||
                time_before(now, rwb->last_comp + HZ / 10);
}

#define REQ_HIPRIO      (REQ_SYNC | REQ_META | REQ_PRIO)

static inline unsigned int get_limit(struct rq_wb *rwb, unsigned long rw)
{
        unsigned int limit;

        /*
         * If we got disabled, just return UINT_MAX. This ensures that
         * we'll properly inc a new IO, and dec+wakeup at the end.
         */
        if (!rwb_enabled(rwb))
                return UINT_MAX;

        if ((rw & REQ_OP_MASK) == REQ_OP_DISCARD)
                return rwb->wb_background;

        /*
         * At this point we know it's a buffered write. If this is
         * kswapd trying to free memory, or REQ_SYNC is set, then
         * it's WB_SYNC_ALL writeback, and we'll use the max limit for
         * that. If the write is marked as a background write, then use
         * the idle limit, or go to normal if we haven't had competing
         * IO for a bit.
         */
        if ((rw & REQ_HIPRIO) || wb_recent_wait(rwb) || current_is_kswapd())
                limit = rwb->rq_depth.max_depth;
        else if ((rw & REQ_BACKGROUND) || close_io(rwb)) {
                /*
                 * If less than 100ms since we completed unrelated IO,
                 * limit us to half the depth for background writeback.
                 */
                limit = rwb->wb_background;
        } else
                limit = rwb->wb_normal;

        return limit;
}

struct wbt_wait_data {
        struct wait_queue_entry wq;
        struct task_struct *task;
        struct rq_wb *rwb;
        struct rq_wait *rqw;
        unsigned long rw;
        bool got_token;
};

static int wbt_wake_function(struct wait_queue_entry *curr, unsigned int mode,
                             int wake_flags, void *key)
{
        struct wbt_wait_data *data = container_of(curr, struct wbt_wait_data,
                                                        wq);

        /*
         * If we fail to get a budget, return -1 to interrupt the wake up
         * loop in __wake_up_common.
         */
        if (!rq_wait_inc_below(data->rqw, get_limit(data->rwb, data->rw)))
                return -1;

        data->got_token = true;
        list_del_init(&curr->entry);
        wake_up_process(data->task);
        return 1;
}

/*
 * Block if we will exceed our limit, or if we are currently waiting for
 * the timer to kick off queuing again.
 */
static void __wbt_wait(struct rq_wb *rwb, enum wbt_flags wb_acct,
                       unsigned long rw, spinlock_t *lock)
        __releases(lock)
        __acquires(lock)
{
        struct rq_wait *rqw = get_rq_wait(rwb, wb_acct);
        struct wbt_wait_data data = {
                .wq = {
                        .func   = wbt_wake_function,
                        .entry  = LIST_HEAD_INIT(data.wq.entry),
                },
                .task = current,
                .rwb = rwb,
                .rqw = rqw,
                .rw = rw,
        };
        bool has_sleeper;

        has_sleeper = wq_has_sleeper(&rqw->wait);
        if (!has_sleeper && rq_wait_inc_below(rqw, get_limit(rwb, rw)))
                return;

        prepare_to_wait_exclusive(&rqw->wait, &data.wq, TASK_UNINTERRUPTIBLE);
        do {
                if (data.got_token)
                        break;

                if (!has_sleeper &&
                    rq_wait_inc_below(rqw, get_limit(rwb, rw))) {
                        finish_wait(&rqw->wait, &data.wq);

                        /*
                         * We raced with wbt_wake_function() getting a token,
                         * which means we now have two. Put our local token
                         * and wake anyone else potentially waiting for one.
                         */
                        if (data.got_token)
                                wbt_rqw_done(rwb, rqw, wb_acct);
                        break;
                }

                if (lock) {
                        spin_unlock_irq(lock);
                        io_schedule();
                        spin_lock_irq(lock);
                } else
                        io_schedule();

                has_sleeper = false;
        } while (1);

        finish_wait(&rqw->wait, &data.wq);
}

static inline bool wbt_should_throttle(struct rq_wb *rwb, struct bio *bio)
{
        switch (bio_op(bio)) {
        case REQ_OP_WRITE:
                /*
                 * Don't throttle WRITE_ODIRECT
                 */
                if ((bio->bi_opf & (REQ_SYNC | REQ_IDLE)) ==
                    (REQ_SYNC | REQ_IDLE))
                        return false;
                /* fallthrough */
        case REQ_OP_DISCARD:
                return true;
        default:
                return false;
        }
}

static enum wbt_flags bio_to_wbt_flags(struct rq_wb *rwb, struct bio *bio)
{
        enum wbt_flags flags = 0;

        if (!rwb_enabled(rwb))
                return 0;

        if (bio_op(bio) == REQ_OP_READ) {
                flags = WBT_READ;
        } else if (wbt_should_throttle(rwb, bio)) {
                if (current_is_kswapd())
                        flags |= WBT_KSWAPD;
                if (bio_op(bio) == REQ_OP_DISCARD)
                        flags |= WBT_DISCARD;
                flags |= WBT_TRACKED;
        }
        return flags;
}

static void wbt_cleanup(struct rq_qos *rqos, struct bio *bio)
{
        struct rq_wb *rwb = RQWB(rqos);
        enum wbt_flags flags = bio_to_wbt_flags(rwb, bio);
        __wbt_done(rqos, flags);
}

/*
 * Returns true if the IO request should be accounted, false if not.
 * May sleep, if we have exceeded the writeback limits. Caller can pass
 * in an irq held spinlock, if it holds one when calling this function.
 * If we do sleep, we'll release and re-grab it.
 */
static void wbt_wait(struct rq_qos *rqos, struct bio *bio, spinlock_t *lock)
{
        struct rq_wb *rwb = RQWB(rqos);
        enum wbt_flags flags;

        flags = bio_to_wbt_flags(rwb, bio);
        if (!(flags & WBT_TRACKED)) {
                if (flags & WBT_READ)
                        wb_timestamp(rwb, &rwb->last_issue);
                return;
        }

        __wbt_wait(rwb, flags, bio->bi_opf, lock);

        if (!blk_stat_is_active(rwb->cb))
                rwb_arm_timer(rwb);
}

static void wbt_track(struct rq_qos *rqos, struct request *rq, struct bio *bio)
{
        struct rq_wb *rwb = RQWB(rqos);
        rq->wbt_flags |= bio_to_wbt_flags(rwb, bio);
}

void wbt_issue(struct rq_qos *rqos, struct request *rq)
{
        struct rq_wb *rwb = RQWB(rqos);

        if (!rwb_enabled(rwb))
                return;

        /*
         * Track sync issue, in case it takes a long time to complete. Allows us
         * to react quicker, if a sync IO takes a long time to complete. Note
         * that this is just a hint. The request can go away when it completes,
         * so it's important we never dereference it. We only use the address to
         * compare with, which is why we store the sync_issue time locally.
         */
        if (wbt_is_read(rq) && !rwb->sync_issue) {
                rwb->sync_cookie = rq;
                rwb->sync_issue = rq->io_start_time_ns;
        }
}

void wbt_requeue(struct rq_qos *rqos, struct request *rq)
{
        struct rq_wb *rwb = RQWB(rqos);
        if (!rwb_enabled(rwb))
                return;
        if (rq == rwb->sync_cookie) {
                rwb->sync_issue = 0;
                rwb->sync_cookie = NULL;
        }
}

void wbt_set_queue_depth(struct request_queue *q, unsigned int depth)
{
        struct rq_qos *rqos = wbt_rq_qos(q);
        if (rqos) {
                RQWB(rqos)->rq_depth.queue_depth = depth;
                __wbt_update_limits(RQWB(rqos));
        }
}

void wbt_set_write_cache(struct request_queue *q, bool write_cache_on)
{
        struct rq_qos *rqos = wbt_rq_qos(q);
        if (rqos)
                RQWB(rqos)->wc = write_cache_on;
}

/*
 * Enable wbt if defaults are configured that way
 */
void wbt_enable_default(struct request_queue *q)
{
        struct rq_qos *rqos = wbt_rq_qos(q);
        /* Throttling already enabled? */
        if (rqos)
                return;

        /* Queue not registered? Maybe shutting down... */
        if (!test_bit(QUEUE_FLAG_REGISTERED, &q->queue_flags))
                return;

        if ((q->mq_ops && IS_ENABLED(CONFIG_BLK_WBT_MQ)) ||
            (q->request_fn && IS_ENABLED(CONFIG_BLK_WBT_SQ)))
                wbt_init(q);
}
EXPORT_SYMBOL_GPL(wbt_enable_default);

u64 wbt_default_latency_nsec(struct request_queue *q)
{
        /*
         * We default to 2msec for non-rotational storage, and 75msec
         * for rotational storage.
         */
        if (blk_queue_nonrot(q))
                return 2000000ULL;
        else
                return 75000000ULL;
}

static int wbt_data_dir(const struct request *rq)
{
        const int op = req_op(rq);

        if (op == REQ_OP_READ)
                return READ;
        else if (op_is_write(op))
                return WRITE;

        /* don't account */
        return -1;
}

static void wbt_exit(struct rq_qos *rqos)
{
        struct rq_wb *rwb = RQWB(rqos);
        struct request_queue *q = rqos->q;

        blk_stat_remove_callback(q, rwb->cb);
        blk_stat_free_callback(rwb->cb);
        kfree(rwb);
}

/*
 * Disable wbt, if enabled by default.
 */
void wbt_disable_default(struct request_queue *q)
{
        struct rq_qos *rqos = wbt_rq_qos(q);
        struct rq_wb *rwb;
        if (!rqos)
                return;
        rwb = RQWB(rqos);
        if (rwb->enable_state == WBT_STATE_ON_DEFAULT)
                rwb->wb_normal = 0;
}
EXPORT_SYMBOL_GPL(wbt_disable_default);


static struct rq_qos_ops wbt_rqos_ops = {
        .throttle = wbt_wait,
        .issue = wbt_issue,
        .track = wbt_track,
        .requeue = wbt_requeue,
        .done = wbt_done,
        .cleanup = wbt_cleanup,
        .exit = wbt_exit,
};

int wbt_init(struct request_queue *q)
{
        struct rq_wb *rwb;
        int i;

        rwb = kzalloc(sizeof(*rwb), GFP_KERNEL);
        if (!rwb)
                return -ENOMEM;

        rwb->cb = blk_stat_alloc_callback(wb_timer_fn, wbt_data_dir, 2, rwb);
        if (!rwb->cb) {
                kfree(rwb);
                return -ENOMEM;
        }

        for (i = 0; i < WBT_NUM_RWQ; i++)
                rq_wait_init(&rwb->rq_wait[i]);

        rwb->rqos.id = RQ_QOS_WBT;
        rwb->rqos.ops = &wbt_rqos_ops;
        rwb->rqos.q = q;
        rwb->last_comp = rwb->last_issue = jiffies;
        rwb->win_nsec = RWB_WINDOW_NSEC;
        rwb->enable_state = WBT_STATE_ON_DEFAULT;
        rwb->wc = 1;
        rwb->rq_depth.default_depth = RWB_DEF_DEPTH;
        __wbt_update_limits(rwb);

        /*
         * Assign rwb and add the stats callback.
         */
        rq_qos_add(q, &rwb->rqos);
        blk_stat_add_callback(q, rwb->cb);

        rwb->min_lat_nsec = wbt_default_latency_nsec(q);

        wbt_set_queue_depth(q, blk_queue_depth(q));
        wbt_set_write_cache(q, test_bit(QUEUE_FLAG_WC, &q->queue_flags));

        return 0;
}