/*
* qht.c - QEMU Hash Table, designed to scale for read-mostly workloads.
*
* Copyright (C) 2016, Emilio G. Cota <cota@braap.org>
*
* License: GNU GPL, version 2 or later.
* See the COPYING file in the top-level directory.
*
* Assumptions:
* - NULL cannot be inserted/removed as a pointer value.
* - Trying to insert an already-existing hash-pointer pair is OK. However,
* it is not OK to insert into the same hash table different hash-pointer
* pairs that have the same pointer value, but not the hashes.
* - Lookups are performed under an RCU read-critical section; removals
* must wait for a grace period to elapse before freeing removed objects.
*
* Features:
* - Reads (i.e. lookups and iterators) can be concurrent with other reads.
* Lookups that are concurrent with writes to the same bucket will retry
* via a seqlock; iterators acquire all bucket locks and therefore can be
* concurrent with lookups and are serialized wrt writers.
* - Writes (i.e. insertions/removals) can be concurrent with writes to
* different buckets; writes to the same bucket are serialized through a lock.
* - Optional auto-resizing: the hash table resizes up if the load surpasses
* a certain threshold. Resizing is done concurrently with readers; writes
* are serialized with the resize operation.
*
* The key structure is the bucket, which is cacheline-sized. Buckets
* contain a few hash values and pointers; the u32 hash values are stored in
* full so that resizing is fast. Having this structure instead of directly
* chaining items has two advantages:
* - Failed lookups fail fast, and touch a minimum number of cache lines.
* - Resizing the hash table with concurrent lookups is easy.
*
* There are two types of buckets:
* 1. "head" buckets are the ones allocated in the array of buckets in qht_map.
* 2. all "non-head" buckets (i.e. all others) are members of a chain that
* starts from a head bucket.
* Note that the seqlock and spinlock of a head bucket applies to all buckets
* chained to it; these two fields are unused in non-head buckets.
*
* On removals, we move the last valid item in the chain to the position of the
* just-removed entry. This makes lookups slightly faster, since the moment an
* invalid entry is found, the (failed) lookup is over.
*
* Resizing is done by taking all bucket spinlocks (so that no other writers can
* race with us) and then copying all entries into a new hash map. Then, the
* ht->map pointer is set, and the old map is freed once no RCU readers can see
* it anymore.
*
* Writers check for concurrent resizes by comparing ht->map before and after
* acquiring their bucket lock. If they don't match, a resize has occured
* while the bucket spinlock was being acquired.
*
* Related Work:
* - Idea of cacheline-sized buckets with full hashes taken from:
* David, Guerraoui & Trigonakis, "Asynchronized Concurrency:
* The Secret to Scaling Concurrent Search Data Structures", ASPLOS'15.
* - Why not RCU-based hash tables? They would allow us to get rid of the
* seqlock, but resizing would take forever since RCU read critical
* sections in QEMU take quite a long time.
* More info on relativistic hash tables:
* + Triplett, McKenney & Walpole, "Resizable, Scalable, Concurrent Hash
* Tables via Relativistic Programming", USENIX ATC'11.
* + Corbet, "Relativistic hash tables, part 1: Algorithms", @ lwn.net, 2014.
* https://lwn.net/Articles/612021/
*/
#include "qemu/osdep.h"
#include "qemu/qht.h"
#include "qemu/atomic.h"
#include "qemu/rcu.h"
//#define QHT_DEBUG
/*
* We want to avoid false sharing of cache lines. Most systems have 64-byte
* cache lines so we go with it for simplicity.
*
* Note that systems with smaller cache lines will be fine (the struct is
* almost 64-bytes); systems with larger cache lines might suffer from
* some false sharing.
*/
#define QHT_BUCKET_ALIGN 64
/* define these to keep sizeof(qht_bucket) within QHT_BUCKET_ALIGN */
#if HOST_LONG_BITS == 32
#define QHT_BUCKET_ENTRIES 6
#else /* 64-bit */
#define QHT_BUCKET_ENTRIES 4
#endif
/*
* Note: reading partially-updated pointers in @pointers could lead to
* segfaults. We thus access them with atomic_read/set; this guarantees
* that the compiler makes all those accesses atomic. We also need the
* volatile-like behavior in atomic_read, since otherwise the compiler
* might refetch the pointer.
* atomic_read's are of course not necessary when the bucket lock is held.
*
* If both ht->lock and b->lock are grabbed, ht->lock should always
* be grabbed first.
*/
struct qht_bucket {
QemuSpin lock;
QemuSeqLock sequence;
uint32_t hashes[QHT_BUCKET_ENTRIES];
void *pointers[QHT_BUCKET_ENTRIES];
struct qht_bucket *next;
} QEMU_ALIGNED(QHT_BUCKET_ALIGN);
QEMU_BUILD_BUG_ON(sizeof(struct qht_bucket) > QHT_BUCKET_ALIGN);
/**
* struct qht_map - structure to track an array of buckets
* @rcu: used by RCU. Keep it as the top field in the struct to help valgrind
* find the whole struct.
* @buckets: array of head buckets. It is constant once the map is created.
* @n_buckets: number of head buckets. It is constant once the map is created.
* @n_added_buckets: number of added (i.e. "non-head") buckets
* @n_added_buckets_threshold: threshold to trigger an upward resize once the
* number of added buckets surpasses it.
*
* Buckets are tracked in what we call a "map", i.e. this structure.
*/
struct qht_map {
struct rcu_head rcu;
struct qht_bucket *buckets;
size_t n_buckets;
size_t n_added_buckets;
size_t n_added_buckets_threshold;
};
/* trigger a resize when n_added_buckets > n_buckets / div */
#define QHT_NR_ADDED_BUCKETS_THRESHOLD_DIV 8
static void qht_do_resize_reset(struct qht *ht, struct qht_map *new,
bool reset);
static void qht_grow_maybe(struct qht *ht);
#ifdef QHT_DEBUG
#define qht_debug_assert(X) do { assert(X); } while (0)
static void qht_bucket_debug__locked(struct qht_bucket *b)
{
bool seen_empty = false;
bool corrupt = false;
int i;
do {
for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
if (b->pointers[i] == NULL) {
seen_empty = true;
continue;
}
if (seen_empty) {
fprintf(stderr, "%s: b: %p, pos: %i, hash: 0x%x, p: %p\n",
__func__, b, i, b->hashes[i], b->pointers[i]);
corrupt = true;
}
}
b = b->next;
} while (b);
qht_debug_assert(!corrupt);
}
static void qht_map_debug__all_locked(struct qht_map *map)
{
int i;
for (i = 0; i < map->n_buckets; i++) {
qht_bucket_debug__locked(&map->buckets[i]);
}
}
#else
#define qht_debug_assert(X) do { (void)(X); } while (0)
static inline void qht_bucket_debug__locked(struct qht_bucket *b)
{ }
static inline void qht_map_debug__all_locked(struct qht_map *map)
{ }
#endif /* QHT_DEBUG */
static inline size_t qht_elems_to_buckets(size_t n_elems)
{
return pow2ceil(n_elems / QHT_BUCKET_ENTRIES);
}
static inline void qht_head_init(struct qht_bucket *b)
{
memset(b, 0, sizeof(*b));
qemu_spin_init(&b->lock);
seqlock_init(&b->sequence);
}
static inline
struct qht_bucket *qht_map_to_bucket(struct qht_map *map, uint32_t hash)
{
return &map->buckets[hash & (map->n_buckets - 1)];
}
/* acquire all bucket locks from a map */
static void qht_map_lock_buckets(struct qht_map *map)
{
size_t i;
for (i = 0; i < map->n_buckets; i++) {
struct qht_bucket *b = &map->buckets[i];
qemu_spin_lock(&b->lock);
}
}
static void qht_map_unlock_buckets(struct qht_map *map)
{
size_t i;
for (i = 0; i < map->n_buckets; i++) {
struct qht_bucket *b = &map->buckets[i];
qemu_spin_unlock(&b->lock);
}
}
/*
* Call with at least a bucket lock held.
* @map should be the value read before acquiring the lock (or locks).
*/
static inline bool qht_map_is_stale__locked(struct qht *ht, struct qht_map *map)
{
return map != ht->map;
}
/*
* Grab all bucket locks, and set @pmap after making sure the map isn't stale.
*
* Pairs with qht_map_unlock_buckets(), hence the pass-by-reference.
*
* Note: callers cannot have ht->lock held.
*/
static inline
void qht_map_lock_buckets__no_stale(struct qht *ht, struct qht_map **pmap)
{
struct qht_map *map;
map = atomic_rcu_read(&ht->map);
qht_map_lock_buckets(map);
if (likely(!qht_map_is_stale__locked(ht, map))) {
*pmap = map;
return;
}
qht_map_unlock_buckets(map);
/* we raced with a resize; acquire ht->lock to see the updated ht->map */
qemu_mutex_lock(&ht->lock);
map = ht->map;
qht_map_lock_buckets(map);
qemu_mutex_unlock(&ht->lock);
*pmap = map;
return;
}
/*
* Get a head bucket and lock it, making sure its parent map is not stale.
* @pmap is filled with a pointer to the bucket's parent map.
*
* Unlock with qemu_spin_unlock(&b->lock).
*
* Note: callers cannot have ht->lock held.
*/
static inline
struct qht_bucket *qht_bucket_lock__no_stale(struct qht *ht, uint32_t hash,
struct qht_map **pmap)
{
struct qht_bucket *b;
struct qht_map *map;
map = atomic_rcu_read(&ht->map);
b = qht_map_to_bucket(map, hash);
qemu_spin_lock(&b->lock);
if (likely(!qht_map_is_stale__locked(ht, map))) {
*pmap = map;
return b;
}
qemu_spin_unlock(&b->lock);
/* we raced with a resize; acquire ht->lock to see the updated ht->map */
qemu_mutex_lock(&ht->lock);
map = ht->map;
b = qht_map_to_bucket(map, hash);
qemu_spin_lock(&b->lock);
qemu_mutex_unlock(&ht->lock);
*pmap = map;
return b;
}
static inline bool qht_map_needs_resize(struct qht_map *map)
{
return atomic_read(&map->n_added_buckets) > map->n_added_buckets_threshold;
}
static inline void qht_chain_destroy(struct qht_bucket *head)
{
struct qht_bucket *curr = head->next;
struct qht_bucket *prev;
while (curr) {
prev = curr;
curr = curr->next;
qemu_vfree(prev);
}
}
/* pass only an orphan map */
static void qht_map_destroy(struct qht_map *map)
{
size_t i;
for (i = 0; i < map->n_buckets; i++) {
qht_chain_destroy(&map->buckets[i]);
}
qemu_vfree(map->buckets);
g_free(map);
}
static struct qht_map *qht_map_create(size_t n_buckets)
{
struct qht_map *map;
size_t i;
map = g_malloc(sizeof(*map));
map->n_buckets = n_buckets;
map->n_added_buckets = 0;
map->n_added_buckets_threshold = n_buckets /
QHT_NR_ADDED_BUCKETS_THRESHOLD_DIV;
/* let tiny hash tables to at least add one non-head bucket */
if (unlikely(map->n_added_buckets_threshold == 0)) {
map->n_added_buckets_threshold = 1;
}
map->buckets = qemu_memalign(QHT_BUCKET_ALIGN,
sizeof(*map->buckets) * n_buckets);
for (i = 0; i < n_buckets; i++) {
qht_head_init(&map->buckets[i]);
}
return map;
}
void qht_init(struct qht *ht, size_t n_elems, unsigned int mode)
{
struct qht_map *map;
size_t n_buckets = qht_elems_to_buckets(n_elems);
ht->mode = mode;
qemu_mutex_init(&ht->lock);
map = qht_map_create(n_buckets);
atomic_rcu_set(&ht->map, map);
}
/* call only when there are no readers/writers left */
void qht_destroy(struct qht *ht)
{
qht_map_destroy(ht->map);
memset(ht, 0, sizeof(*ht));
}
static void qht_bucket_reset__locked(struct qht_bucket *head)
{
struct qht_bucket *b = head;
int i;
seqlock_write_begin(&head->sequence);
do {
for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
if (b->pointers[i] == NULL) {
goto done;
}
atomic_set(&b->hashes[i], 0);
atomic_set(&b->pointers[i], NULL);
}
b = b->next;
} while (b);
done:
seqlock_write_end(&head->sequence);
}
/* call with all bucket locks held */
static void qht_map_reset__all_locked(struct qht_map *map)
{
size_t i;
for (i = 0; i < map->n_buckets; i++) {
qht_bucket_reset__locked(&map->buckets[i]);
}
qht_map_debug__all_locked(map);
}
void qht_reset(struct qht *ht)
{
struct qht_map *map;
qht_map_lock_buckets__no_stale(ht, &map);
qht_map_reset__all_locked(map);
qht_map_unlock_buckets(map);
}
static inline void qht_do_resize(struct qht *ht, struct qht_map *new)
{
qht_do_resize_reset(ht, new, false);
}
static inline void qht_do_resize_and_reset(struct qht *ht, struct qht_map *new)
{
qht_do_resize_reset(ht, new, true);
}
bool qht_reset_size(struct qht *ht, size_t n_elems)
{
struct qht_map *new = NULL;
struct qht_map *map;
size_t n_buckets;
n_buckets = qht_elems_to_buckets(n_elems);
qemu_mutex_lock(&ht->lock);
map = ht->map;
if (n_buckets != map->n_buckets) {
new = qht_map_create(n_buckets);
}
qht_do_resize_and_reset(ht, new);
qemu_mutex_unlock(&ht->lock);
return !!new;
}
static inline
void *qht_do_lookup(struct qht_bucket *head, qht_lookup_func_t func,
const void *userp, uint32_t hash)
{
struct qht_bucket *b = head;
int i;
do {
for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
if (atomic_read(&b->hashes[i]) == hash) {
/* The pointer is dereferenced before seqlock_read_retry,
* so (unlike qht_insert__locked) we need to use
* atomic_rcu_read here.
*/
void *p = atomic_rcu_read(&b->pointers[i]);
if (likely(p) && likely(func(p, userp))) {
return p;
}
}
}
b = atomic_rcu_read(&b->next);
} while (b);
return NULL;
}
static __attribute__((noinline))
void *qht_lookup__slowpath(struct qht_bucket *b, qht_lookup_func_t func,
const void *userp, uint32_t hash)
{
unsigned int version;
void *ret;
do {
version = seqlock_read_begin(&b->sequence);
ret = qht_do_lookup(b, func, userp, hash);
} while (seqlock_read_retry(&b->sequence, version));
return ret;
}
void *qht_lookup(struct qht *ht, qht_lookup_func_t func, const void *userp,
uint32_t hash)
{
struct qht_bucket *b;
struct qht_map *map;
unsigned int version;
void *ret;
map = atomic_rcu_read(&ht->map);
b = qht_map_to_bucket(map, hash);
version = seqlock_read_begin(&b->sequence);
ret = qht_do_lookup(b, func, userp, hash);
if (likely(!seqlock_read_retry(&b->sequence, version))) {
return ret;
}
/*
* Removing the do/while from the fastpath gives a 4% perf. increase when
* running a 100%-lookup microbenchmark.
*/
return qht_lookup__slowpath(b, func, userp, hash);
}
/* call with head->lock held */
static bool qht_insert__locked(struct qht *ht, struct qht_map *map,
struct qht_bucket *head, void *p, uint32_t hash,
bool *needs_resize)
{
struct qht_bucket *b = head;
struct qht_bucket *prev = NULL;
struct qht_bucket *new = NULL;
int i;
do {
for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
if (b->pointers[i]) {
if (unlikely(b->pointers[i] == p)) {
return false;
}
} else {
goto found;
}
}
prev = b;
b = b->next;
} while (b);
b = qemu_memalign(QHT_BUCKET_ALIGN, sizeof(*b));
memset(b, 0, sizeof(*b));
new = b;
i = 0;
atomic_inc(&map->n_added_buckets);
if (unlikely(qht_map_needs_resize(map)) && needs_resize) {
*needs_resize = true;
}
found:
/* found an empty key: acquire the seqlock and write */
seqlock_write_begin(&head->sequence);
if (new) {
atomic_rcu_set(&prev->next, b);
}
/* smp_wmb() implicit in seqlock_write_begin. */
atomic_set(&b->hashes[i], hash);
atomic_set(&b->pointers[i], p);
seqlock_write_end(&head->sequence);
return true;
}
static __attribute__((noinline)) void qht_grow_maybe(struct qht *ht)
{
struct qht_map *map;
/*
* If the lock is taken it probably means there's an ongoing resize,
* so bail out.
*/
if (qemu_mutex_trylock(&ht->lock)) {
return;
}
map = ht->map;
/* another thread might have just performed the resize we were after */
if (qht_map_needs_resize(map)) {
struct qht_map *new = qht_map_create(map->n_buckets * 2);
qht_do_resize(ht, new);
}
qemu_mutex_unlock(&ht->lock);
}
bool qht_insert(struct qht *ht, void *p, uint32_t hash)
{
struct qht_bucket *b;
struct qht_map *map;
bool needs_resize = false;
bool ret;
/* NULL pointers are not supported */
qht_debug_assert(p);
b = qht_bucket_lock__no_stale(ht, hash, &map);
ret = qht_insert__locked(ht, map, b, p, hash, &needs_resize);
qht_bucket_debug__locked(b);
qemu_spin_unlock(&b->lock);
if (unlikely(needs_resize) && ht->mode & QHT_MODE_AUTO_RESIZE) {
qht_grow_maybe(ht);
}
return ret;
}
static inline bool qht_entry_is_last(struct qht_bucket *b, int pos)
{
if (pos == QHT_BUCKET_ENTRIES - 1) {
if (b->next == NULL) {
return true;
}
return b->next->pointers[0] == NULL;
}
return b->pointers[pos + 1] == NULL;
}
static void
qht_entry_move(struct qht_bucket *to, int i, struct qht_bucket *from, int j)
{
qht_debug_assert(!(to == from && i == j));
qht_debug_assert(to->pointers[i]);
qht_debug_assert(from->pointers[j]);
atomic_set(&to->hashes[i], from->hashes[j]);
atomic_set(&to->pointers[i], from->pointers[j]);
atomic_set(&from->hashes[j], 0);
atomic_set(&from->pointers[j], NULL);
}
/*
* Find the last valid entry in @head, and swap it with @orig[pos], which has
* just been invalidated.
*/
static inline void qht_bucket_remove_entry(struct qht_bucket *orig, int pos)
{
struct qht_bucket *b = orig;
struct qht_bucket *prev = NULL;
int i;
if (qht_entry_is_last(orig, pos)) {
orig->hashes[pos] = 0;
atomic_set(&orig->pointers[pos], NULL);
return;
}
do {
for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
if (b->pointers[i]) {
continue;
}
if (i > 0) {
return qht_entry_move(orig, pos, b, i - 1);
}
qht_debug_assert(prev);
return qht_entry_move(orig, pos, prev, QHT_BUCKET_ENTRIES - 1);
}
prev = b;
b = b->next;
} while (b);
/* no free entries other than orig[pos], so swap it with the last one */
qht_entry_move(orig, pos, prev, QHT_BUCKET_ENTRIES - 1);
}
/* call with b->lock held */
static inline
bool qht_remove__locked(struct qht_map *map, struct qht_bucket *head,
const void *p, uint32_t hash)
{
struct qht_bucket *b = head;
int i;
do {
for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
void *q = b->pointers[i];
if (unlikely(q == NULL)) {
return false;
}
if (q == p) {
qht_debug_assert(b->hashes[i] == hash);
seqlock_write_begin(&head->sequence);
qht_bucket_remove_entry(b, i);
seqlock_write_end(&head->sequence);
return true;
}
}
b = b->next;
} while (b);
return false;
}
bool qht_remove(struct qht *ht, const void *p, uint32_t hash)
{
struct qht_bucket *b;
struct qht_map *map;
bool ret;
/* NULL pointers are not supported */
qht_debug_assert(p);
b = qht_bucket_lock__no_stale(ht, hash, &map);
ret = qht_remove__locked(map, b, p, hash);
qht_bucket_debug__locked(b);
qemu_spin_unlock(&b->lock);
return ret;
}
static inline void qht_bucket_iter(struct qht *ht, struct qht_bucket *b,
qht_iter_func_t func, void *userp)
{
int i;
do {
for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
if (b->pointers[i] == NULL) {
return;
}
func(ht, b->pointers[i], b->hashes[i], userp);
}
b = b->next;
} while (b);
}
/* call with all of the map's locks held */
static inline void qht_map_iter__all_locked(struct qht *ht, struct qht_map *map,
qht_iter_func_t func, void *userp)
{
size_t i;
for (i = 0; i < map->n_buckets; i++) {
qht_bucket_iter(ht, &map->buckets[i], func, userp);
}
}
void qht_iter(struct qht *ht, qht_iter_func_t func, void *userp)
{
struct qht_map *map;
map = atomic_rcu_read(&ht->map);
qht_map_lock_buckets(map);
/* Note: ht here is merely for carrying ht->mode; ht->map won't be read */
qht_map_iter__all_locked(ht, map, func, userp);
qht_map_unlock_buckets(map);
}
static void qht_map_copy(struct qht *ht, void *p, uint32_t hash, void *userp)
{
struct qht_map *new = userp;
struct qht_bucket *b = qht_map_to_bucket(new, hash);
/* no need to acquire b->lock because no thread has seen this map yet */
qht_insert__locked(ht, new, b, p, hash, NULL);
}
/*
* Atomically perform a resize and/or reset.
* Call with ht->lock held.
*/
static void qht_do_resize_reset(struct qht *ht, struct qht_map *new, bool reset)
{
struct qht_map *old;
old = ht->map;
qht_map_lock_buckets(old);
if (reset) {
qht_map_reset__all_locked(old);
}
if (new == NULL) {
qht_map_unlock_buckets(old);
return;
}
g_assert(new->n_buckets != old->n_buckets);
qht_map_iter__all_locked(ht, old, qht_map_copy, new);
qht_map_debug__all_locked(new);
atomic_rcu_set(&ht->map, new);
qht_map_unlock_buckets(old);
call_rcu(old, qht_map_destroy, rcu);
}
bool qht_resize(struct qht *ht, size_t n_elems)
{
size_t n_buckets = qht_elems_to_buckets(n_elems);
size_t ret = false;
qemu_mutex_lock(&ht->lock);
if (n_buckets != ht->map->n_buckets) {
struct qht_map *new;
new = qht_map_create(n_buckets);
qht_do_resize(ht, new);
ret = true;
}
qemu_mutex_unlock(&ht->lock);
return ret;
}
/* pass @stats to qht_statistics_destroy() when done */
void qht_statistics_init(struct qht *ht, struct qht_stats *stats)
{
struct qht_map *map;
int i;
map = atomic_rcu_read(&ht->map);
stats->used_head_buckets = 0;
stats->entries = 0;
qdist_init(&stats->chain);
qdist_init(&stats->occupancy);
/* bail out if the qht has not yet been initialized */
if (unlikely(map == NULL)) {
stats->head_buckets = 0;
return;
}
stats->head_buckets = map->n_buckets;
for (i = 0; i < map->n_buckets; i++) {
struct qht_bucket *head = &map->buckets[i];
struct qht_bucket *b;
unsigned int version;
size_t buckets;
size_t entries;
int j;
do {
version = seqlock_read_begin(&head->sequence);
buckets = 0;
entries = 0;
b = head;
do {
for (j = 0; j < QHT_BUCKET_ENTRIES; j++) {
if (atomic_read(&b->pointers[j]) == NULL) {
break;
}
entries++;
}
buckets++;
b = atomic_rcu_read(&b->next);
} while (b);
} while (seqlock_read_retry(&head->sequence, version));
if (entries) {
qdist_inc(&stats->chain, buckets);
qdist_inc(&stats->occupancy,
(double)entries / QHT_BUCKET_ENTRIES / buckets);
stats->used_head_buckets++;
stats->entries += entries;
} else {
qdist_inc(&stats->occupancy, 0);
}
}
}
void qht_statistics_destroy(struct qht_stats *stats)
{
qdist_destroy(&stats->occupancy);
qdist_destroy(&stats->chain);
}
