Adding upstream version 4.19.249.upstream/4.19.249upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 828 insertions, 0 deletions

diff --git a/block/blk-wbt.c b/block/blk-wbt.c
new file mode 100644
index 000000000..880a41add
--- /dev/null
+++ b/block/blk-wbt.c
@@ -0,0 +1,828 @@
+/*
+ * 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->enable_state != WBT_STATE_OFF_DEFAULT &&
+		      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)
+{
+	if (!rq_depth_scale_up(&rwb->rq_depth))
+		return;
+	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)
+{
+	if (!rq_depth_scale_down(&rwb->rq_depth, hard_throttle))
+		return;
+	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) {
+		if (RQWB(rqos)->enable_state == WBT_STATE_OFF_DEFAULT)
+			RQWB(rqos)->enable_state = WBT_STATE_ON_DEFAULT;
+		return;
+	}
+
+	/* Queue not registered? Maybe shutting down... */
+	if (!blk_queue_registered(q))
+		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) {
+		blk_stat_deactivate(rwb->cb);
+		rwb->enable_state = WBT_STATE_OFF_DEFAULT;
+	}
+}
+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;
+}