xref: /openbmc/linux/drivers/md/dm-cache-policy-smq.c (revision 66a636356647a9be8885c2ce2948de126577698a)

/*
 * Copyright (C) 2015 Red Hat. All rights reserved.
 *
 * This file is released under the GPL.
 */

#include "dm-cache-policy.h"
#include "dm-cache-policy-internal.h"
#include "dm.h"

#include <linux/hash.h>
#include <linux/jiffies.h>
#include <linux/module.h>
#include <linux/mutex.h>
#include <linux/vmalloc.h>
#include <linux/math64.h>

#define DM_MSG_PREFIX "cache-policy-smq"

/*----------------------------------------------------------------*/

/*
 * Safe division functions that return zero on divide by zero.
 */
static unsigned safe_div(unsigned n, unsigned d)
{
	return d ? n / d : 0u;
}

static unsigned safe_mod(unsigned n, unsigned d)
{
	return d ? n % d : 0u;
}

/*----------------------------------------------------------------*/

struct entry {
	unsigned hash_next:28;
	unsigned prev:28;
	unsigned next:28;
	unsigned level:7;
	bool dirty:1;
	bool allocated:1;
	bool sentinel:1;

	dm_oblock_t oblock;
};

/*----------------------------------------------------------------*/

#define INDEXER_NULL ((1u << 28u) - 1u)

/*
 * An entry_space manages a set of entries that we use for the queues.
 * The clean and dirty queues share entries, so this object is separate
 * from the queue itself.
 */
struct entry_space {
	struct entry *begin;
	struct entry *end;
};

static int space_init(struct entry_space *es, unsigned nr_entries)
{
	if (!nr_entries) {
		es->begin = es->end = NULL;
		return 0;
	}

	es->begin = vzalloc(sizeof(struct entry) * nr_entries);
	if (!es->begin)
		return -ENOMEM;

	es->end = es->begin + nr_entries;
	return 0;
}

static void space_exit(struct entry_space *es)
{
	vfree(es->begin);
}

static struct entry *__get_entry(struct entry_space *es, unsigned block)
{
	struct entry *e;

	e = es->begin + block;
	BUG_ON(e >= es->end);

	return e;
}

static unsigned to_index(struct entry_space *es, struct entry *e)
{
	BUG_ON(e < es->begin || e >= es->end);
	return e - es->begin;
}

static struct entry *to_entry(struct entry_space *es, unsigned block)
{
	if (block == INDEXER_NULL)
		return NULL;

	return __get_entry(es, block);
}

/*----------------------------------------------------------------*/

struct ilist {
	unsigned nr_elts;	/* excluding sentinel entries */
	unsigned head, tail;
};

static void l_init(struct ilist *l)
{
	l->nr_elts = 0;
	l->head = l->tail = INDEXER_NULL;
}

static struct entry *l_head(struct entry_space *es, struct ilist *l)
{
	return to_entry(es, l->head);
}

static struct entry *l_tail(struct entry_space *es, struct ilist *l)
{
	return to_entry(es, l->tail);
}

static struct entry *l_next(struct entry_space *es, struct entry *e)
{
	return to_entry(es, e->next);
}

static struct entry *l_prev(struct entry_space *es, struct entry *e)
{
	return to_entry(es, e->prev);
}

static bool l_empty(struct ilist *l)
{
	return l->head == INDEXER_NULL;
}

static void l_add_head(struct entry_space *es, struct ilist *l, struct entry *e)
{
	struct entry *head = l_head(es, l);

	e->next = l->head;
	e->prev = INDEXER_NULL;

	if (head)
		head->prev = l->head = to_index(es, e);
	else
		l->head = l->tail = to_index(es, e);

	if (!e->sentinel)
		l->nr_elts++;
}

static void l_add_tail(struct entry_space *es, struct ilist *l, struct entry *e)
{
	struct entry *tail = l_tail(es, l);

	e->next = INDEXER_NULL;
	e->prev = l->tail;

	if (tail)
		tail->next = l->tail = to_index(es, e);
	else
		l->head = l->tail = to_index(es, e);

	if (!e->sentinel)
		l->nr_elts++;
}

static void l_add_before(struct entry_space *es, struct ilist *l,
			 struct entry *old, struct entry *e)
{
	struct entry *prev = l_prev(es, old);

	if (!prev)
		l_add_head(es, l, e);

	else {
		e->prev = old->prev;
		e->next = to_index(es, old);
		prev->next = old->prev = to_index(es, e);

		if (!e->sentinel)
			l->nr_elts++;
	}
}

static void l_del(struct entry_space *es, struct ilist *l, struct entry *e)
{
	struct entry *prev = l_prev(es, e);
	struct entry *next = l_next(es, e);

	if (prev)
		prev->next = e->next;
	else
		l->head = e->next;

	if (next)
		next->prev = e->prev;
	else
		l->tail = e->prev;

	if (!e->sentinel)
		l->nr_elts--;
}

static struct entry *l_pop_tail(struct entry_space *es, struct ilist *l)
{
	struct entry *e;

	for (e = l_tail(es, l); e; e = l_prev(es, e))
		if (!e->sentinel) {
			l_del(es, l, e);
			return e;
		}

	return NULL;
}

/*----------------------------------------------------------------*/

/*
 * The stochastic-multi-queue is a set of lru lists stacked into levels.
 * Entries are moved up levels when they are used, which loosely orders the
 * most accessed entries in the top levels and least in the bottom.  This
 * structure is *much* better than a single lru list.
 */
#define MAX_LEVELS 64u

struct queue {
	struct entry_space *es;

	unsigned nr_elts;
	unsigned nr_levels;
	struct ilist qs[MAX_LEVELS];

	/*
	 * We maintain a count of the number of entries we would like in each
	 * level.
	 */
	unsigned last_target_nr_elts;
	unsigned nr_top_levels;
	unsigned nr_in_top_levels;
	unsigned target_count[MAX_LEVELS];
};

static void q_init(struct queue *q, struct entry_space *es, unsigned nr_levels)
{
	unsigned i;

	q->es = es;
	q->nr_elts = 0;
	q->nr_levels = nr_levels;

	for (i = 0; i < q->nr_levels; i++) {
		l_init(q->qs + i);
		q->target_count[i] = 0u;
	}

	q->last_target_nr_elts = 0u;
	q->nr_top_levels = 0u;
	q->nr_in_top_levels = 0u;
}

static unsigned q_size(struct queue *q)
{
	return q->nr_elts;
}

/*
 * Insert an entry to the back of the given level.
 */
static void q_push(struct queue *q, struct entry *e)
{
	if (!e->sentinel)
		q->nr_elts++;

	l_add_tail(q->es, q->qs + e->level, e);
}

static void q_push_before(struct queue *q, struct entry *old, struct entry *e)
{
	if (!e->sentinel)
		q->nr_elts++;

	l_add_before(q->es, q->qs + e->level, old, e);
}

static void q_del(struct queue *q, struct entry *e)
{
	l_del(q->es, q->qs + e->level, e);
	if (!e->sentinel)
		q->nr_elts--;
}

/*
 * Return the oldest entry of the lowest populated level.
 */
static struct entry *q_peek(struct queue *q, unsigned max_level, bool can_cross_sentinel)
{
	unsigned level;
	struct entry *e;

	max_level = min(max_level, q->nr_levels);

	for (level = 0; level < max_level; level++)
		for (e = l_head(q->es, q->qs + level); e; e = l_next(q->es, e)) {
			if (e->sentinel) {
				if (can_cross_sentinel)
					continue;
				else
					break;
			}

			return e;
		}

	return NULL;
}

static struct entry *q_pop(struct queue *q)
{
	struct entry *e = q_peek(q, q->nr_levels, true);

	if (e)
		q_del(q, e);

	return e;
}

/*
 * Pops an entry from a level that is not past a sentinel.
 */
static struct entry *q_pop_old(struct queue *q, unsigned max_level)
{
	struct entry *e = q_peek(q, max_level, false);

	if (e)
		q_del(q, e);

	return e;
}

/*
 * This function assumes there is a non-sentinel entry to pop.  It's only
 * used by redistribute, so we know this is true.  It also doesn't adjust
 * the q->nr_elts count.
 */
static struct entry *__redist_pop_from(struct queue *q, unsigned level)
{
	struct entry *e;

	for (; level < q->nr_levels; level++)
		for (e = l_head(q->es, q->qs + level); e; e = l_next(q->es, e))
			if (!e->sentinel) {
				l_del(q->es, q->qs + e->level, e);
				return e;
			}

	return NULL;
}

static void q_set_targets_subrange_(struct queue *q, unsigned nr_elts, unsigned lbegin, unsigned lend)
{
	unsigned level, nr_levels, entries_per_level, remainder;

	BUG_ON(lbegin > lend);
	BUG_ON(lend > q->nr_levels);
	nr_levels = lend - lbegin;
	entries_per_level = safe_div(nr_elts, nr_levels);
	remainder = safe_mod(nr_elts, nr_levels);

	for (level = lbegin; level < lend; level++)
		q->target_count[level] =
			(level < (lbegin + remainder)) ? entries_per_level + 1u : entries_per_level;
}

/*
 * Typically we have fewer elements in the top few levels which allows us
 * to adjust the promote threshold nicely.
 */
static void q_set_targets(struct queue *q)
{
	if (q->last_target_nr_elts == q->nr_elts)
		return;

	q->last_target_nr_elts = q->nr_elts;

	if (q->nr_top_levels > q->nr_levels)
		q_set_targets_subrange_(q, q->nr_elts, 0, q->nr_levels);

	else {
		q_set_targets_subrange_(q, q->nr_in_top_levels,
					q->nr_levels - q->nr_top_levels, q->nr_levels);

		if (q->nr_in_top_levels < q->nr_elts)
			q_set_targets_subrange_(q, q->nr_elts - q->nr_in_top_levels,
						0, q->nr_levels - q->nr_top_levels);
		else
			q_set_targets_subrange_(q, 0, 0, q->nr_levels - q->nr_top_levels);
	}
}

static void q_redistribute(struct queue *q)
{
	unsigned target, level;
	struct ilist *l, *l_above;
	struct entry *e;

	q_set_targets(q);

	for (level = 0u; level < q->nr_levels - 1u; level++) {
		l = q->qs + level;
		target = q->target_count[level];

		/*
		 * Pull down some entries from the level above.
		 */
		while (l->nr_elts < target) {
			e = __redist_pop_from(q, level + 1u);
			if (!e) {
				/* bug in nr_elts */
				break;
			}

			e->level = level;
			l_add_tail(q->es, l, e);
		}

		/*
		 * Push some entries up.
		 */
		l_above = q->qs + level + 1u;
		while (l->nr_elts > target) {
			e = l_pop_tail(q->es, l);

			if (!e)
				/* bug in nr_elts */
				break;

			e->level = level + 1u;
			l_add_head(q->es, l_above, e);
		}
	}
}

static void q_requeue_before(struct queue *q, struct entry *dest, struct entry *e, unsigned extra_levels)
{
	struct entry *de;
	unsigned new_level;

	q_del(q, e);

	if (extra_levels && (e->level < q->nr_levels - 1u)) {
		new_level = min(q->nr_levels - 1u, e->level + extra_levels);
		for (de = l_head(q->es, q->qs + new_level); de; de = l_next(q->es, de)) {
			if (de->sentinel)
				continue;

			q_del(q, de);
			de->level = e->level;

			if (dest)
				q_push_before(q, dest, de);
			else
				q_push(q, de);
			break;
		}

		e->level = new_level;
	}

	q_push(q, e);
}

static void q_requeue(struct queue *q, struct entry *e, unsigned extra_levels)
{
	q_requeue_before(q, NULL, e, extra_levels);
}

/*----------------------------------------------------------------*/

#define FP_SHIFT 8
#define SIXTEENTH (1u << (FP_SHIFT - 4u))
#define EIGHTH (1u << (FP_SHIFT - 3u))

struct stats {
	unsigned hit_threshold;
	unsigned hits;
	unsigned misses;
};

enum performance {
	Q_POOR,
	Q_FAIR,
	Q_WELL
};

static void stats_init(struct stats *s, unsigned nr_levels)
{
	s->hit_threshold = (nr_levels * 3u) / 4u;
	s->hits = 0u;
	s->misses = 0u;
}

static void stats_reset(struct stats *s)
{
	s->hits = s->misses = 0u;
}

static void stats_level_accessed(struct stats *s, unsigned level)
{
	if (level >= s->hit_threshold)
		s->hits++;
	else
		s->misses++;
}

static void stats_miss(struct stats *s)
{
	s->misses++;
}

/*
 * There are times when we don't have any confidence in the hotspot queue.
 * Such as when a fresh cache is created and the blocks have been spread
 * out across the levels, or if an io load changes.  We detect this by
 * seeing how often a lookup is in the top levels of the hotspot queue.
 */
static enum performance stats_assess(struct stats *s)
{
	unsigned confidence = safe_div(s->hits << FP_SHIFT, s->hits + s->misses);

	if (confidence < SIXTEENTH)
		return Q_POOR;

	else if (confidence < EIGHTH)
		return Q_FAIR;

	else
		return Q_WELL;
}

/*----------------------------------------------------------------*/

struct hash_table {
	struct entry_space *es;
	unsigned long long hash_bits;
	unsigned *buckets;
};

/*
 * All cache entries are stored in a chained hash table.  To save space we
 * use indexing again, and only store indexes to the next entry.
 */
static int h_init(struct hash_table *ht, struct entry_space *es, unsigned nr_entries)
{
	unsigned i, nr_buckets;

	ht->es = es;
	nr_buckets = roundup_pow_of_two(max(nr_entries / 4u, 16u));
	ht->hash_bits = ffs(nr_buckets) - 1;

	ht->buckets = vmalloc(sizeof(*ht->buckets) * nr_buckets);
	if (!ht->buckets)
		return -ENOMEM;

	for (i = 0; i < nr_buckets; i++)
		ht->buckets[i] = INDEXER_NULL;

	return 0;
}

static void h_exit(struct hash_table *ht)
{
	vfree(ht->buckets);
}

static struct entry *h_head(struct hash_table *ht, unsigned bucket)
{
	return to_entry(ht->es, ht->buckets[bucket]);
}

static struct entry *h_next(struct hash_table *ht, struct entry *e)
{
	return to_entry(ht->es, e->hash_next);
}

static void __h_insert(struct hash_table *ht, unsigned bucket, struct entry *e)
{
	e->hash_next = ht->buckets[bucket];
	ht->buckets[bucket] = to_index(ht->es, e);
}

static void h_insert(struct hash_table *ht, struct entry *e)
{
	unsigned h = hash_64(from_oblock(e->oblock), ht->hash_bits);
	__h_insert(ht, h, e);
}

static struct entry *__h_lookup(struct hash_table *ht, unsigned h, dm_oblock_t oblock,
				struct entry **prev)
{
	struct entry *e;

	*prev = NULL;
	for (e = h_head(ht, h); e; e = h_next(ht, e)) {
		if (e->oblock == oblock)
			return e;

		*prev = e;
	}

	return NULL;
}

static void __h_unlink(struct hash_table *ht, unsigned h,
		       struct entry *e, struct entry *prev)
{
	if (prev)
		prev->hash_next = e->hash_next;
	else
		ht->buckets[h] = e->hash_next;
}

/*
 * Also moves each entry to the front of the bucket.
 */
static struct entry *h_lookup(struct hash_table *ht, dm_oblock_t oblock)
{
	struct entry *e, *prev;
	unsigned h = hash_64(from_oblock(oblock), ht->hash_bits);

	e = __h_lookup(ht, h, oblock, &prev);
	if (e && prev) {
		/*
		 * Move to the front because this entry is likely
		 * to be hit again.
		 */
		__h_unlink(ht, h, e, prev);
		__h_insert(ht, h, e);
	}

	return e;
}

static void h_remove(struct hash_table *ht, struct entry *e)
{
	unsigned h = hash_64(from_oblock(e->oblock), ht->hash_bits);
	struct entry *prev;

	/*
	 * The down side of using a singly linked list is we have to
	 * iterate the bucket to remove an item.
	 */
	e = __h_lookup(ht, h, e->oblock, &prev);
	if (e)
		__h_unlink(ht, h, e, prev);
}

/*----------------------------------------------------------------*/

struct entry_alloc {
	struct entry_space *es;
	unsigned begin;

	unsigned nr_allocated;
	struct ilist free;
};

static void init_allocator(struct entry_alloc *ea, struct entry_space *es,
			   unsigned begin, unsigned end)
{
	unsigned i;

	ea->es = es;
	ea->nr_allocated = 0u;
	ea->begin = begin;

	l_init(&ea->free);
	for (i = begin; i != end; i++)
		l_add_tail(ea->es, &ea->free, __get_entry(ea->es, i));
}

static void init_entry(struct entry *e)
{
	/*
	 * We can't memset because that would clear the hotspot and
	 * sentinel bits which remain constant.
	 */
	e->hash_next = INDEXER_NULL;
	e->next = INDEXER_NULL;
	e->prev = INDEXER_NULL;
	e->level = 0u;
	e->allocated = true;
}

static struct entry *alloc_entry(struct entry_alloc *ea)
{
	struct entry *e;

	if (l_empty(&ea->free))
		return NULL;

	e = l_pop_tail(ea->es, &ea->free);
	init_entry(e);
	ea->nr_allocated++;

	return e;
}

/*
 * This assumes the cblock hasn't already been allocated.
 */
static struct entry *alloc_particular_entry(struct entry_alloc *ea, unsigned i)
{
	struct entry *e = __get_entry(ea->es, ea->begin + i);

	BUG_ON(e->allocated);

	l_del(ea->es, &ea->free, e);
	init_entry(e);
	ea->nr_allocated++;

	return e;
}

static void free_entry(struct entry_alloc *ea, struct entry *e)
{
	BUG_ON(!ea->nr_allocated);
	BUG_ON(!e->allocated);

	ea->nr_allocated--;
	e->allocated = false;
	l_add_tail(ea->es, &ea->free, e);
}

static bool allocator_empty(struct entry_alloc *ea)
{
	return l_empty(&ea->free);
}

static unsigned get_index(struct entry_alloc *ea, struct entry *e)
{
	return to_index(ea->es, e) - ea->begin;
}

static struct entry *get_entry(struct entry_alloc *ea, unsigned index)
{
	return __get_entry(ea->es, ea->begin + index);
}

/*----------------------------------------------------------------*/

#define NR_HOTSPOT_LEVELS 64u
#define NR_CACHE_LEVELS 64u

#define WRITEBACK_PERIOD (10 * HZ)
#define DEMOTE_PERIOD (60 * HZ)

#define HOTSPOT_UPDATE_PERIOD (HZ)
#define CACHE_UPDATE_PERIOD (10u * HZ)

struct smq_policy {
	struct dm_cache_policy policy;

	/* protects everything */
	struct mutex lock;
	dm_cblock_t cache_size;
	sector_t cache_block_size;

	sector_t hotspot_block_size;
	unsigned nr_hotspot_blocks;
	unsigned cache_blocks_per_hotspot_block;
	unsigned hotspot_level_jump;

	struct entry_space es;
	struct entry_alloc writeback_sentinel_alloc;
	struct entry_alloc demote_sentinel_alloc;
	struct entry_alloc hotspot_alloc;
	struct entry_alloc cache_alloc;

	unsigned long *hotspot_hit_bits;
	unsigned long *cache_hit_bits;

	/*
	 * We maintain three queues of entries.  The cache proper,
	 * consisting of a clean and dirty queue, containing the currently
	 * active mappings.  The hotspot queue uses a larger block size to
	 * track blocks that are being hit frequently and potential
	 * candidates for promotion to the cache.
	 */
	struct queue hotspot;
	struct queue clean;
	struct queue dirty;

	struct stats hotspot_stats;
	struct stats cache_stats;

	/*
	 * Keeps track of time, incremented by the core.  We use this to
	 * avoid attributing multiple hits within the same tick.
	 *
	 * Access to tick_protected should be done with the spin lock held.
	 * It's copied to tick at the start of the map function (within the
	 * mutex).
	 */
	spinlock_t tick_lock;
	unsigned tick_protected;
	unsigned tick;

	/*
	 * The hash tables allows us to quickly find an entry by origin
	 * block.
	 */
	struct hash_table table;
	struct hash_table hotspot_table;

	bool current_writeback_sentinels;
	unsigned long next_writeback_period;

	bool current_demote_sentinels;
	unsigned long next_demote_period;

	unsigned write_promote_level;
	unsigned read_promote_level;

	unsigned long next_hotspot_period;
	unsigned long next_cache_period;
};

/*----------------------------------------------------------------*/

static struct entry *get_sentinel(struct entry_alloc *ea, unsigned level, bool which)
{
	return get_entry(ea, which ? level : NR_CACHE_LEVELS + level);
}

static struct entry *writeback_sentinel(struct smq_policy *mq, unsigned level)
{
	return get_sentinel(&mq->writeback_sentinel_alloc, level, mq->current_writeback_sentinels);
}

static struct entry *demote_sentinel(struct smq_policy *mq, unsigned level)
{
	return get_sentinel(&mq->demote_sentinel_alloc, level, mq->current_demote_sentinels);
}

static void __update_writeback_sentinels(struct smq_policy *mq)
{
	unsigned level;
	struct queue *q = &mq->dirty;
	struct entry *sentinel;

	for (level = 0; level < q->nr_levels; level++) {
		sentinel = writeback_sentinel(mq, level);
		q_del(q, sentinel);
		q_push(q, sentinel);
	}
}

static void __update_demote_sentinels(struct smq_policy *mq)
{
	unsigned level;
	struct queue *q = &mq->clean;
	struct entry *sentinel;

	for (level = 0; level < q->nr_levels; level++) {
		sentinel = demote_sentinel(mq, level);
		q_del(q, sentinel);
		q_push(q, sentinel);
	}
}

static void update_sentinels(struct smq_policy *mq)
{
	if (time_after(jiffies, mq->next_writeback_period)) {
		__update_writeback_sentinels(mq);
		mq->next_writeback_period = jiffies + WRITEBACK_PERIOD;
		mq->current_writeback_sentinels = !mq->current_writeback_sentinels;
	}

	if (time_after(jiffies, mq->next_demote_period)) {
		__update_demote_sentinels(mq);
		mq->next_demote_period = jiffies + DEMOTE_PERIOD;
		mq->current_demote_sentinels = !mq->current_demote_sentinels;
	}
}

static void __sentinels_init(struct smq_policy *mq)
{
	unsigned level;
	struct entry *sentinel;

	for (level = 0; level < NR_CACHE_LEVELS; level++) {
		sentinel = writeback_sentinel(mq, level);
		sentinel->level = level;
		q_push(&mq->dirty, sentinel);

		sentinel = demote_sentinel(mq, level);
		sentinel->level = level;
		q_push(&mq->clean, sentinel);
	}
}

static void sentinels_init(struct smq_policy *mq)
{
	mq->next_writeback_period = jiffies + WRITEBACK_PERIOD;
	mq->next_demote_period = jiffies + DEMOTE_PERIOD;

	mq->current_writeback_sentinels = false;
	mq->current_demote_sentinels = false;
	__sentinels_init(mq);

	mq->current_writeback_sentinels = !mq->current_writeback_sentinels;
	mq->current_demote_sentinels = !mq->current_demote_sentinels;
	__sentinels_init(mq);
}

/*----------------------------------------------------------------*/

/*
 * These methods tie together the dirty queue, clean queue and hash table.
 */
static void push_new(struct smq_policy *mq, struct entry *e)
{
	struct queue *q = e->dirty ? &mq->dirty : &mq->clean;
	h_insert(&mq->table, e);
	q_push(q, e);
}

static void push(struct smq_policy *mq, struct entry *e)
{
	struct entry *sentinel;

	h_insert(&mq->table, e);

	/*
	 * Punch this into the queue just in front of the sentinel, to
	 * ensure it's cleaned straight away.
	 */
	if (e->dirty) {
		sentinel = writeback_sentinel(mq, e->level);
		q_push_before(&mq->dirty, sentinel, e);
	} else {
		sentinel = demote_sentinel(mq, e->level);
		q_push_before(&mq->clean, sentinel, e);
	}
}

/*
 * Removes an entry from cache.  Removes from the hash table.
 */
static void __del(struct smq_policy *mq, struct queue *q, struct entry *e)
{
	q_del(q, e);
	h_remove(&mq->table, e);
}

static void del(struct smq_policy *mq, struct entry *e)
{
	__del(mq, e->dirty ? &mq->dirty : &mq->clean, e);
}

static struct entry *pop_old(struct smq_policy *mq, struct queue *q, unsigned max_level)
{
	struct entry *e = q_pop_old(q, max_level);
	if (e)
		h_remove(&mq->table, e);
	return e;
}

static dm_cblock_t infer_cblock(struct smq_policy *mq, struct entry *e)
{
	return to_cblock(get_index(&mq->cache_alloc, e));
}

static void requeue(struct smq_policy *mq, struct entry *e)
{
	struct entry *sentinel;

	if (!test_and_set_bit(from_cblock(infer_cblock(mq, e)), mq->cache_hit_bits)) {
		if (e->dirty) {
			sentinel = writeback_sentinel(mq, e->level);
			q_requeue_before(&mq->dirty, sentinel, e, 1u);
		} else {
			sentinel = demote_sentinel(mq, e->level);
			q_requeue_before(&mq->clean, sentinel, e, 1u);
		}
	}
}

static unsigned default_promote_level(struct smq_policy *mq)
{
	/*
	 * The promote level depends on the current performance of the
	 * cache.
	 *
	 * If the cache is performing badly, then we can't afford
	 * to promote much without causing performance to drop below that
	 * of the origin device.
	 *
	 * If the cache is performing well, then we don't need to promote
	 * much.  If it isn't broken, don't fix it.
	 *
	 * If the cache is middling then we promote more.
	 *
	 * This scheme reminds me of a graph of entropy vs probability of a
	 * binary variable.
	 */
	static unsigned table[] = {1, 1, 1, 2, 4, 6, 7, 8, 7, 6, 4, 4, 3, 3, 2, 2, 1};

	unsigned hits = mq->cache_stats.hits;
	unsigned misses = mq->cache_stats.misses;
	unsigned index = safe_div(hits << 4u, hits + misses);
	return table[index];
}

static void update_promote_levels(struct smq_policy *mq)
{
	/*
	 * If there are unused cache entries then we want to be really
	 * eager to promote.
	 */
	unsigned threshold_level = allocator_empty(&mq->cache_alloc) ?
		default_promote_level(mq) : (NR_HOTSPOT_LEVELS / 2u);

	/*
	 * If the hotspot queue is performing badly then we have little
	 * confidence that we know which blocks to promote.  So we cut down
	 * the amount of promotions.
	 */
	switch (stats_assess(&mq->hotspot_stats)) {
	case Q_POOR:
		threshold_level /= 4u;
		break;

	case Q_FAIR:
		threshold_level /= 2u;
		break;

	case Q_WELL:
		break;
	}

	mq->read_promote_level = NR_HOTSPOT_LEVELS - threshold_level;
	mq->write_promote_level = (NR_HOTSPOT_LEVELS - threshold_level) + 2u;
}

/*
 * If the hotspot queue is performing badly, then we try and move entries
 * around more quickly.
 */
static void update_level_jump(struct smq_policy *mq)
{
	switch (stats_assess(&mq->hotspot_stats)) {
	case Q_POOR:
		mq->hotspot_level_jump = 4u;
		break;

	case Q_FAIR:
		mq->hotspot_level_jump = 2u;
		break;

	case Q_WELL:
		mq->hotspot_level_jump = 1u;
		break;
	}
}

static void end_hotspot_period(struct smq_policy *mq)
{
	clear_bitset(mq->hotspot_hit_bits, mq->nr_hotspot_blocks);
	update_promote_levels(mq);

	if (time_after(jiffies, mq->next_hotspot_period)) {
		update_level_jump(mq);
		q_redistribute(&mq->hotspot);
		stats_reset(&mq->hotspot_stats);
		mq->next_hotspot_period = jiffies + HOTSPOT_UPDATE_PERIOD;
	}
}

static void end_cache_period(struct smq_policy *mq)
{
	if (time_after(jiffies, mq->next_cache_period)) {
		clear_bitset(mq->cache_hit_bits, from_cblock(mq->cache_size));

		q_redistribute(&mq->dirty);
		q_redistribute(&mq->clean);
		stats_reset(&mq->cache_stats);

		mq->next_cache_period = jiffies + CACHE_UPDATE_PERIOD;
	}
}

static int demote_cblock(struct smq_policy *mq,
			 struct policy_locker *locker,
			 dm_oblock_t *oblock)
{
	struct entry *demoted = q_peek(&mq->clean, mq->clean.nr_levels, false);
	if (!demoted)
		/*
		 * We could get a block from mq->dirty, but that
		 * would add extra latency to the triggering bio as it
		 * waits for the writeback.  Better to not promote this
		 * time and hope there's a clean block next time this block
		 * is hit.
		 */
		return -ENOSPC;

	if (locker->fn(locker, demoted->oblock))
		/*
		 * We couldn't lock this block.
		 */
		return -EBUSY;

	del(mq, demoted);
	*oblock = demoted->oblock;
	free_entry(&mq->cache_alloc, demoted);

	return 0;
}

enum promote_result {
	PROMOTE_NOT,
	PROMOTE_TEMPORARY,
	PROMOTE_PERMANENT
};

/*
 * Converts a boolean into a promote result.
 */
static enum promote_result maybe_promote(bool promote)
{
	return promote ? PROMOTE_PERMANENT : PROMOTE_NOT;
}

static enum promote_result should_promote(struct smq_policy *mq, struct entry *hs_e, struct bio *bio,
					  bool fast_promote)
{
	if (bio_data_dir(bio) == WRITE) {
		if (!allocator_empty(&mq->cache_alloc) && fast_promote)
			return PROMOTE_TEMPORARY;

		else
			return maybe_promote(hs_e->level >= mq->write_promote_level);
	} else
		return maybe_promote(hs_e->level >= mq->read_promote_level);
}

static void insert_in_cache(struct smq_policy *mq, dm_oblock_t oblock,
			    struct policy_locker *locker,
			    struct policy_result *result, enum promote_result pr)
{
	int r;
	struct entry *e;

	if (allocator_empty(&mq->cache_alloc)) {
		result->op = POLICY_REPLACE;
		r = demote_cblock(mq, locker, &result->old_oblock);
		if (r) {
			result->op = POLICY_MISS;
			return;
		}

	} else
		result->op = POLICY_NEW;

	e = alloc_entry(&mq->cache_alloc);
	BUG_ON(!e);
	e->oblock = oblock;

	if (pr == PROMOTE_TEMPORARY)
		push(mq, e);
	else
		push_new(mq, e);

	result->cblock = infer_cblock(mq, e);
}

static dm_oblock_t to_hblock(struct smq_policy *mq, dm_oblock_t b)
{
	sector_t r = from_oblock(b);
	(void) sector_div(r, mq->cache_blocks_per_hotspot_block);
	return to_oblock(r);
}

static struct entry *update_hotspot_queue(struct smq_policy *mq, dm_oblock_t b, struct bio *bio)
{
	unsigned hi;
	dm_oblock_t hb = to_hblock(mq, b);
	struct entry *e = h_lookup(&mq->hotspot_table, hb);

	if (e) {
		stats_level_accessed(&mq->hotspot_stats, e->level);

		hi = get_index(&mq->hotspot_alloc, e);
		q_requeue(&mq->hotspot, e,
			  test_and_set_bit(hi, mq->hotspot_hit_bits) ?
			  0u : mq->hotspot_level_jump);

	} else {
		stats_miss(&mq->hotspot_stats);

		e = alloc_entry(&mq->hotspot_alloc);
		if (!e) {
			e = q_pop(&mq->hotspot);
			if (e) {
				h_remove(&mq->hotspot_table, e);
				hi = get_index(&mq->hotspot_alloc, e);
				clear_bit(hi, mq->hotspot_hit_bits);
			}

		}

		if (e) {
			e->oblock = hb;
			q_push(&mq->hotspot, e);
			h_insert(&mq->hotspot_table, e);
		}
	}

	return e;
}

/*
 * Looks the oblock up in the hash table, then decides whether to put in
 * pre_cache, or cache etc.
 */
static int map(struct smq_policy *mq, struct bio *bio, dm_oblock_t oblock,
	       bool can_migrate, bool fast_promote,
	       struct policy_locker *locker, struct policy_result *result)
{
	struct entry *e, *hs_e;
	enum promote_result pr;

	hs_e = update_hotspot_queue(mq, oblock, bio);

	e = h_lookup(&mq->table, oblock);
	if (e) {
		stats_level_accessed(&mq->cache_stats, e->level);

		requeue(mq, e);
		result->op = POLICY_HIT;
		result->cblock = infer_cblock(mq, e);

	} else {
		stats_miss(&mq->cache_stats);

		pr = should_promote(mq, hs_e, bio, fast_promote);
		if (pr == PROMOTE_NOT)
			result->op = POLICY_MISS;

		else {
			if (!can_migrate) {
				result->op = POLICY_MISS;
				return -EWOULDBLOCK;
			}

			insert_in_cache(mq, oblock, locker, result, pr);
		}
	}

	return 0;
}

/*----------------------------------------------------------------*/

/*
 * Public interface, via the policy struct.  See dm-cache-policy.h for a
 * description of these.
 */

static struct smq_policy *to_smq_policy(struct dm_cache_policy *p)
{
	return container_of(p, struct smq_policy, policy);
}

static void smq_destroy(struct dm_cache_policy *p)
{
	struct smq_policy *mq = to_smq_policy(p);

	h_exit(&mq->hotspot_table);
	h_exit(&mq->table);
	free_bitset(mq->hotspot_hit_bits);
	free_bitset(mq->cache_hit_bits);
	space_exit(&mq->es);
	kfree(mq);
}

static void copy_tick(struct smq_policy *mq)
{
	unsigned long flags, tick;

	spin_lock_irqsave(&mq->tick_lock, flags);
	tick = mq->tick_protected;
	if (tick != mq->tick) {
		update_sentinels(mq);
		end_hotspot_period(mq);
		end_cache_period(mq);
		mq->tick = tick;
	}
	spin_unlock_irqrestore(&mq->tick_lock, flags);
}

static bool maybe_lock(struct smq_policy *mq, bool can_block)
{
	if (can_block) {
		mutex_lock(&mq->lock);
		return true;
	} else
		return mutex_trylock(&mq->lock);
}

static int smq_map(struct dm_cache_policy *p, dm_oblock_t oblock,
		   bool can_block, bool can_migrate, bool fast_promote,
		   struct bio *bio, struct policy_locker *locker,
		   struct policy_result *result)
{
	int r;
	struct smq_policy *mq = to_smq_policy(p);

	result->op = POLICY_MISS;

	if (!maybe_lock(mq, can_block))
		return -EWOULDBLOCK;

	copy_tick(mq);
	r = map(mq, bio, oblock, can_migrate, fast_promote, locker, result);
	mutex_unlock(&mq->lock);

	return r;
}

static int smq_lookup(struct dm_cache_policy *p, dm_oblock_t oblock, dm_cblock_t *cblock)
{
	int r;
	struct smq_policy *mq = to_smq_policy(p);
	struct entry *e;

	if (!mutex_trylock(&mq->lock))
		return -EWOULDBLOCK;

	e = h_lookup(&mq->table, oblock);
	if (e) {
		*cblock = infer_cblock(mq, e);
		r = 0;
	} else
		r = -ENOENT;

	mutex_unlock(&mq->lock);

	return r;
}

static void __smq_set_clear_dirty(struct smq_policy *mq, dm_oblock_t oblock, bool set)
{
	struct entry *e;

	e = h_lookup(&mq->table, oblock);
	BUG_ON(!e);

	del(mq, e);
	e->dirty = set;
	push(mq, e);
}

static void smq_set_dirty(struct dm_cache_policy *p, dm_oblock_t oblock)
{
	struct smq_policy *mq = to_smq_policy(p);

	mutex_lock(&mq->lock);
	__smq_set_clear_dirty(mq, oblock, true);
	mutex_unlock(&mq->lock);
}

static void smq_clear_dirty(struct dm_cache_policy *p, dm_oblock_t oblock)
{
	struct smq_policy *mq = to_smq_policy(p);

	mutex_lock(&mq->lock);
	__smq_set_clear_dirty(mq, oblock, false);
	mutex_unlock(&mq->lock);
}

static int smq_load_mapping(struct dm_cache_policy *p,
			    dm_oblock_t oblock, dm_cblock_t cblock,
			    uint32_t hint, bool hint_valid)
{
	struct smq_policy *mq = to_smq_policy(p);
	struct entry *e;

	e = alloc_particular_entry(&mq->cache_alloc, from_cblock(cblock));
	e->oblock = oblock;
	e->dirty = false;	/* this gets corrected in a minute */
	e->level = hint_valid ? min(hint, NR_CACHE_LEVELS - 1) : 1;
	push(mq, e);

	return 0;
}

static int smq_save_hints(struct smq_policy *mq, struct queue *q,
			  policy_walk_fn fn, void *context)
{
	int r;
	unsigned level;
	struct entry *e;

	for (level = 0; level < q->nr_levels; level++)
		for (e = l_head(q->es, q->qs + level); e; e = l_next(q->es, e)) {
			if (!e->sentinel) {
				r = fn(context, infer_cblock(mq, e),
				       e->oblock, e->level);
				if (r)
					return r;
			}
		}

	return 0;
}

static int smq_walk_mappings(struct dm_cache_policy *p, policy_walk_fn fn,
			     void *context)
{
	struct smq_policy *mq = to_smq_policy(p);
	int r = 0;

	mutex_lock(&mq->lock);

	r = smq_save_hints(mq, &mq->clean, fn, context);
	if (!r)
		r = smq_save_hints(mq, &mq->dirty, fn, context);

	mutex_unlock(&mq->lock);

	return r;
}

static void __remove_mapping(struct smq_policy *mq, dm_oblock_t oblock)
{
	struct entry *e;

	e = h_lookup(&mq->table, oblock);
	BUG_ON(!e);

	del(mq, e);
	free_entry(&mq->cache_alloc, e);
}

static void smq_remove_mapping(struct dm_cache_policy *p, dm_oblock_t oblock)
{
	struct smq_policy *mq = to_smq_policy(p);

	mutex_lock(&mq->lock);
	__remove_mapping(mq, oblock);
	mutex_unlock(&mq->lock);
}

static int __remove_cblock(struct smq_policy *mq, dm_cblock_t cblock)
{
	struct entry *e = get_entry(&mq->cache_alloc, from_cblock(cblock));

	if (!e || !e->allocated)
		return -ENODATA;

	del(mq, e);
	free_entry(&mq->cache_alloc, e);

	return 0;
}

static int smq_remove_cblock(struct dm_cache_policy *p, dm_cblock_t cblock)
{
	int r;
	struct smq_policy *mq = to_smq_policy(p);

	mutex_lock(&mq->lock);
	r = __remove_cblock(mq, cblock);
	mutex_unlock(&mq->lock);

	return r;
}


#define CLEAN_TARGET_CRITICAL 5u /* percent */

static bool clean_target_met(struct smq_policy *mq, bool critical)
{
	if (critical) {
		/*
		 * Cache entries may not be populated.  So we're cannot rely on the
		 * size of the clean queue.
		 */
		unsigned nr_clean = from_cblock(mq->cache_size) - q_size(&mq->dirty);
		unsigned target = from_cblock(mq->cache_size) * CLEAN_TARGET_CRITICAL / 100u;

		return nr_clean >= target;
	} else
		return !q_size(&mq->dirty);
}

static int __smq_writeback_work(struct smq_policy *mq, dm_oblock_t *oblock,
				dm_cblock_t *cblock, bool critical_only)
{
	struct entry *e = NULL;
	bool target_met = clean_target_met(mq, critical_only);

	if (critical_only)
		/*
		 * Always try and keep the bottom level clean.
		 */
		e = pop_old(mq, &mq->dirty, target_met ? 1u : mq->dirty.nr_levels);

	else
		e = pop_old(mq, &mq->dirty, mq->dirty.nr_levels);

	if (!e)
		return -ENODATA;

	*oblock = e->oblock;
	*cblock = infer_cblock(mq, e);
	e->dirty = false;
	push_new(mq, e);

	return 0;
}

static int smq_writeback_work(struct dm_cache_policy *p, dm_oblock_t *oblock,
			      dm_cblock_t *cblock, bool critical_only)
{
	int r;
	struct smq_policy *mq = to_smq_policy(p);

	mutex_lock(&mq->lock);
	r = __smq_writeback_work(mq, oblock, cblock, critical_only);
	mutex_unlock(&mq->lock);

	return r;
}

static void __force_mapping(struct smq_policy *mq,
			    dm_oblock_t current_oblock, dm_oblock_t new_oblock)
{
	struct entry *e = h_lookup(&mq->table, current_oblock);

	if (e) {
		del(mq, e);
		e->oblock = new_oblock;
		e->dirty = true;
		push(mq, e);
	}
}

static void smq_force_mapping(struct dm_cache_policy *p,
			      dm_oblock_t current_oblock, dm_oblock_t new_oblock)
{
	struct smq_policy *mq = to_smq_policy(p);

	mutex_lock(&mq->lock);
	__force_mapping(mq, current_oblock, new_oblock);
	mutex_unlock(&mq->lock);
}

static dm_cblock_t smq_residency(struct dm_cache_policy *p)
{
	dm_cblock_t r;
	struct smq_policy *mq = to_smq_policy(p);

	mutex_lock(&mq->lock);
	r = to_cblock(mq->cache_alloc.nr_allocated);
	mutex_unlock(&mq->lock);

	return r;
}

static void smq_tick(struct dm_cache_policy *p)
{
	struct smq_policy *mq = to_smq_policy(p);
	unsigned long flags;

	spin_lock_irqsave(&mq->tick_lock, flags);
	mq->tick_protected++;
	spin_unlock_irqrestore(&mq->tick_lock, flags);
}

/* Init the policy plugin interface function pointers. */
static void init_policy_functions(struct smq_policy *mq)
{
	mq->policy.destroy = smq_destroy;
	mq->policy.map = smq_map;
	mq->policy.lookup = smq_lookup;
	mq->policy.set_dirty = smq_set_dirty;
	mq->policy.clear_dirty = smq_clear_dirty;
	mq->policy.load_mapping = smq_load_mapping;
	mq->policy.walk_mappings = smq_walk_mappings;
	mq->policy.remove_mapping = smq_remove_mapping;
	mq->policy.remove_cblock = smq_remove_cblock;
	mq->policy.writeback_work = smq_writeback_work;
	mq->policy.force_mapping = smq_force_mapping;
	mq->policy.residency = smq_residency;
	mq->policy.tick = smq_tick;
}

static bool too_many_hotspot_blocks(sector_t origin_size,
				    sector_t hotspot_block_size,
				    unsigned nr_hotspot_blocks)
{
	return (hotspot_block_size * nr_hotspot_blocks) > origin_size;
}

static void calc_hotspot_params(sector_t origin_size,
				sector_t cache_block_size,
				unsigned nr_cache_blocks,
				sector_t *hotspot_block_size,
				unsigned *nr_hotspot_blocks)
{
	*hotspot_block_size = cache_block_size * 16u;
	*nr_hotspot_blocks = max(nr_cache_blocks / 4u, 1024u);

	while ((*hotspot_block_size > cache_block_size) &&
	       too_many_hotspot_blocks(origin_size, *hotspot_block_size, *nr_hotspot_blocks))
		*hotspot_block_size /= 2u;
}

static struct dm_cache_policy *smq_create(dm_cblock_t cache_size,
					  sector_t origin_size,
					  sector_t cache_block_size)
{
	unsigned i;
	unsigned nr_sentinels_per_queue = 2u * NR_CACHE_LEVELS;
	unsigned total_sentinels = 2u * nr_sentinels_per_queue;
	struct smq_policy *mq = kzalloc(sizeof(*mq), GFP_KERNEL);

	if (!mq)
		return NULL;

	init_policy_functions(mq);
	mq->cache_size = cache_size;
	mq->cache_block_size = cache_block_size;

	calc_hotspot_params(origin_size, cache_block_size, from_cblock(cache_size),
			    &mq->hotspot_block_size, &mq->nr_hotspot_blocks);

	mq->cache_blocks_per_hotspot_block = div64_u64(mq->hotspot_block_size, mq->cache_block_size);
	mq->hotspot_level_jump = 1u;
	if (space_init(&mq->es, total_sentinels + mq->nr_hotspot_blocks + from_cblock(cache_size))) {
		DMERR("couldn't initialize entry space");
		goto bad_pool_init;
	}

	init_allocator(&mq->writeback_sentinel_alloc, &mq->es, 0, nr_sentinels_per_queue);
        for (i = 0; i < nr_sentinels_per_queue; i++)
		get_entry(&mq->writeback_sentinel_alloc, i)->sentinel = true;

	init_allocator(&mq->demote_sentinel_alloc, &mq->es, nr_sentinels_per_queue, total_sentinels);
        for (i = 0; i < nr_sentinels_per_queue; i++)
		get_entry(&mq->demote_sentinel_alloc, i)->sentinel = true;

	init_allocator(&mq->hotspot_alloc, &mq->es, total_sentinels,
		       total_sentinels + mq->nr_hotspot_blocks);

	init_allocator(&mq->cache_alloc, &mq->es,
		       total_sentinels + mq->nr_hotspot_blocks,
		       total_sentinels + mq->nr_hotspot_blocks + from_cblock(cache_size));

	mq->hotspot_hit_bits = alloc_bitset(mq->nr_hotspot_blocks);
	if (!mq->hotspot_hit_bits) {
		DMERR("couldn't allocate hotspot hit bitset");
		goto bad_hotspot_hit_bits;
	}
	clear_bitset(mq->hotspot_hit_bits, mq->nr_hotspot_blocks);

	if (from_cblock(cache_size)) {
		mq->cache_hit_bits = alloc_bitset(from_cblock(cache_size));
		if (!mq->cache_hit_bits && mq->cache_hit_bits) {
			DMERR("couldn't allocate cache hit bitset");
			goto bad_cache_hit_bits;
		}
		clear_bitset(mq->cache_hit_bits, from_cblock(mq->cache_size));
	} else
		mq->cache_hit_bits = NULL;

	mq->tick_protected = 0;
	mq->tick = 0;
	mutex_init(&mq->lock);
	spin_lock_init(&mq->tick_lock);

	q_init(&mq->hotspot, &mq->es, NR_HOTSPOT_LEVELS);
	mq->hotspot.nr_top_levels = 8;
	mq->hotspot.nr_in_top_levels = min(mq->nr_hotspot_blocks / NR_HOTSPOT_LEVELS,
					   from_cblock(mq->cache_size) / mq->cache_blocks_per_hotspot_block);

	q_init(&mq->clean, &mq->es, NR_CACHE_LEVELS);
	q_init(&mq->dirty, &mq->es, NR_CACHE_LEVELS);

	stats_init(&mq->hotspot_stats, NR_HOTSPOT_LEVELS);
	stats_init(&mq->cache_stats, NR_CACHE_LEVELS);

	if (h_init(&mq->table, &mq->es, from_cblock(cache_size)))
		goto bad_alloc_table;

	if (h_init(&mq->hotspot_table, &mq->es, mq->nr_hotspot_blocks))
		goto bad_alloc_hotspot_table;

	sentinels_init(mq);
	mq->write_promote_level = mq->read_promote_level = NR_HOTSPOT_LEVELS;

	mq->next_hotspot_period = jiffies;
	mq->next_cache_period = jiffies;

	return &mq->policy;

bad_alloc_hotspot_table:
	h_exit(&mq->table);
bad_alloc_table:
	free_bitset(mq->cache_hit_bits);
bad_cache_hit_bits:
	free_bitset(mq->hotspot_hit_bits);
bad_hotspot_hit_bits:
	space_exit(&mq->es);
bad_pool_init:
	kfree(mq);

	return NULL;
}

/*----------------------------------------------------------------*/

static struct dm_cache_policy_type smq_policy_type = {
	.name = "smq",
	.version = {1, 0, 0},
	.hint_size = 4,
	.owner = THIS_MODULE,
	.create = smq_create
};

static int __init smq_init(void)
{
	int r;

	r = dm_cache_policy_register(&smq_policy_type);
	if (r) {
		DMERR("register failed %d", r);
		return -ENOMEM;
	}

	return 0;
}

static void __exit smq_exit(void)
{
	dm_cache_policy_unregister(&smq_policy_type);
}

module_init(smq_init);
module_exit(smq_exit);

MODULE_AUTHOR("Joe Thornber <dm-devel@redhat.com>");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("smq cache policy");