Re: [Qemu-devel] [PATCH v5 12/18] qht: QEMU's fast, resizable and scalable Hash Table

[Sergey Fedorov <serge.fdrv@gmail.com>]

Wow, that's really great stuff!

On 14/05/16 06:34, Emilio G. Cota wrote:
> diff --git a/include/qemu/qht.h b/include/qemu/qht.h
> new file mode 100644
> index 0000000..c2ab8b8
> --- /dev/null
> +++ b/include/qemu/qht.h
> @@ -0,0 +1,66 @@
> +/*
> + * 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.
> + */
> +#ifndef QEMU_QHT_H
> +#define QEMU_QHT_H
> +
> +#include "qemu/osdep.h"
> +#include "qemu-common.h"

There's no need in qemu-common.h

> +#include "qemu/seqlock.h"
> +#include "qemu/qdist.h"
> +#include "qemu/rcu.h"

qemu/rcu.h is really required in qht.c, not here.

> +
> +struct qht {
> +    struct qht_map *map;
> +    unsigned int mode;
> +};
> +
> +struct qht_stats {
> +    size_t head_buckets;
> +    size_t used_head_buckets;
> +    size_t entries;
> +    struct qdist chain;
> +    struct qdist occupancy;
> +};
> +
> +typedef bool (*qht_lookup_func_t)(const void *obj, const void *userp);
> +typedef void (*qht_iter_func_t)(struct qht *ht, void *p, uint32_t h, void *up);

We could move ght_stats, qht_lookup_func_t and qht_iter_func_t closer to
the relevant functions declarations. Anyway, I think it's also fine to
keep them here. :)

Although the API is mostly intuitive some kernel-doc-style comments
wouldn’t hurt, I think. ;-)

> +
> +#define QHT_MODE_AUTO_RESIZE 0x1 /* auto-resize when heavily loaded */
> +
> +void qht_init(struct qht *ht, size_t n_elems, unsigned int mode);
> +
> +/* call only when there are no readers left */
> +void qht_destroy(struct qht *ht);
> +
> +/* call with an external lock held */
> +void qht_reset(struct qht *ht);
> +
> +/* call with an external lock held */
> +bool qht_reset_size(struct qht *ht, size_t n_elems);
> +
> +/* call with an external lock held */
> +bool qht_insert(struct qht *ht, void *p, uint32_t hash);
> +
> +/* call with an external lock held */
> +bool qht_remove(struct qht *ht, const void *p, uint32_t hash);
> +
> +/* call with an external lock held */
> +void qht_iter(struct qht *ht, qht_iter_func_t func, void *userp);
> +
> +/* call with an external lock held */
> +bool qht_resize(struct qht *ht, size_t n_elems);
> +
> +/* if @func is NULL, then pointer comparison is used */
> +void *qht_lookup(struct qht *ht, qht_lookup_func_t func, const void *userp,
> +                 uint32_t hash);
> +
> +/* pass @stats to qht_statistics_destroy() when done */
> +void qht_statistics_init(struct qht *ht, struct qht_stats *stats);
> +
> +void qht_statistics_destroy(struct qht_stats *stats);
> +
> +#endif /* QEMU_QHT_H */
(snip)
> diff --git a/util/qht.c b/util/qht.c
> new file mode 100644
> index 0000000..112f32d
> --- /dev/null
> +++ b/util/qht.c
> @@ -0,0 +1,703 @@
> +/*
> + * 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:
> + * - Writers and iterators must take an external lock.
> + * - NULL cannot be inserted as a pointer value.
> + * - Duplicate pointer values cannot be inserted.
> + *
> + * Features:
> + * - Optional auto-resizing: the hash table resizes up if the load surpasses
> + *   a certain threshold. Resizing is done concurrently with readers.
> + *
> + * 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 three advantages:

s/three/two/?

> + * - 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 spinlocks (so that no readers-turned-writers
> + * can race with us) and then placing all elements into a new hash table. Last,
> + * the ht->map pointer is set, and the old map is freed once no RCU readers can
> + * see it anymore.
> + *
> + * 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/qht.h"
> +#include "qemu/atomic.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
> +
> +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);

Have you considered using separate structures for head buckets and
non-head buckets, e.g. "struct qht_head_bucket" and "struct
qht_added_bucket"? This would give us a little more entries per cache-line.

> +
> +/**
> + * 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: 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;

s/n/n_buckets/? (Actually, 'n_buckets' is already mentioned in a comment
below.)

> +    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(struct qht *ht, size_t n);
> +
> +static inline struct qht_map *qht_map__atomic_mb(const struct qht *ht)
> +{
> +    struct qht_map *map;
> +
> +    map = atomic_read(&ht->map);
> +    /* paired with smp_wmb() before setting ht->map */
> +    smp_rmb();
> +    return map;
> +}

Why don't just use atomic_rcu_read/set()? Looks like they were meant for
that exact purpose.

> +
> +/* helper for lockless bucket chain traversals */
> +static inline
> +struct qht_bucket *bucket_next__atomic_mb(const struct qht_bucket *b)
> +{
> +    struct qht_bucket *ret;
> +
> +    ret = atomic_read(&b->next);
> +    /*
> +     * This barrier guarantees that we will read a properly initialized b->next;
> +     * it is paired with an smp_wmb() before setting b->next.
> +     */
> +    smp_rmb();
> +    return ret;
> +}
> +
> +#ifdef QHT_DEBUG
> +static void qht_bucket_debug(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);
> +    assert(!corrupt);
> +}
> +
> +static void qht_map_debug(struct qht_map *map)
> +{
> +    int i;
> +
> +    for (i = 0; i < map->n; i++) {
> +        qht_bucket_debug(&map->buckets[i]);
> +    }
> +}
> +#else
> +static inline void qht_bucket_debug(struct qht_bucket *b)
> +{ }
> +
> +static inline void qht_map_debug(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 - 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; 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; i++) {
> +        struct qht_bucket *b = &map->buckets[i];
> +
> +        qemu_spin_unlock(&b->lock);
> +    }
> +}
> +
> +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; i++) {
> +        qht_chain_destroy(&map->buckets[i]);
> +    }
> +    qemu_vfree(map->buckets);
> +    g_free(map);
> +}
> +
> +static void qht_map_reclaim(struct rcu_head *rcu)
> +{
> +    struct qht_map *map = container_of(rcu, struct qht_map, rcu);
> +
> +    qht_map_destroy(map);
> +}
> +
> +static struct qht_map *qht_map_create(size_t n)
> +{
> +    struct qht_map *map;
> +    size_t i;
> +
> +    map = g_malloc(sizeof(*map));
> +    map->n = n;
> +
> +    map->n_added_buckets = 0;
> +    map->n_added_buckets_threshold = n / 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);
> +    for (i = 0; i < n; i++) {
> +        qht_head_init(&map->buckets[i]);
> +    }
> +    return map;
> +}
> +
> +static inline void qht_publish(struct qht *ht, struct qht_map *new)
> +{
> +    /* Readers should see a properly initialized map; pair with smp_rmb() */
> +    smp_wmb();
> +    atomic_set(&ht->map, new);
> +}
> +
> +void qht_init(struct qht *ht, size_t n_elems, unsigned int mode)
> +{
> +    struct qht_map *map;
> +    size_t n = qht_elems_to_buckets(n_elems);
> +
> +    ht->mode = mode;
> +    map = qht_map_create(n);
> +    qht_publish(ht, map);
> +}
> +
> +/* call only when there are no readers left */
> +void qht_destroy(struct qht *ht)
> +{
> +    qht_map_destroy(ht->map);
> +    memset(ht, 0, sizeof(*ht));
> +}
> +
> +static void qht_bucket_reset(struct qht_bucket *head)
> +{
> +    struct qht_bucket *b = head;
> +    int i;
> +
> +    qemu_spin_lock(&head->lock);
> +    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);
> +    qemu_spin_unlock(&head->lock);
> +}
> +
> +/* call with an external lock held */
> +void qht_reset(struct qht *ht)
> +{
> +    struct qht_map *map = ht->map;
> +    size_t i;
> +
> +    for (i = 0; i < map->n; i++) {
> +        qht_bucket_reset(&map->buckets[i]);
> +    }
> +    qht_map_debug(map);
> +}
> +
> +/* call with an external lock held */
> +bool qht_reset_size(struct qht *ht, size_t n_elems)
> +{
> +    struct qht_map *old = ht->map;
> +
> +    qht_reset(ht);
> +    if (old->n == qht_elems_to_buckets(n_elems)) {
> +        return false;
> +    }
> +    qht_init(ht, n_elems, ht->mode);
> +    call_rcu1(&old->rcu, qht_map_reclaim);
> +    return true;
> +}
> +
> +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) {
> +                void *p = atomic_read(&b->pointers[i]);

Why do we need this atomic_read() and other (looking a bit inconsistent)
atomic operations on 'b->pointers' and 'b->hash'? if we always have to
access them protected properly by a seqlock together with a spinlock?

> +
> +                if (likely(p) && likely(func(p, userp))) {
> +                    return p;
> +                }
> +            }
> +        }
> +        b = bucket_next__atomic_mb(b);
> +    } 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)
> +{
> +    uint32_t version;
> +    void *ret;
> +
> +    do {
> +        version = seqlock_read_begin(&b->sequence);

seqlock_read_begin() returns "unsigned".

> +        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;
> +    uint32_t version;
> +    void *ret;
> +
> +    map = qht_map__atomic_mb(ht);
> +    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;
> +
> +    for (;;) {
> +        if (b == NULL) {
> +            b = qemu_memalign(QHT_BUCKET_ALIGN, sizeof(*b));
> +            memset(b, 0, sizeof(*b));
> +            new = b;
> +            atomic_inc(&map->n_added_buckets);
> +            if (unlikely(qht_map_needs_resize(map)) && needs_resize) {
> +                *needs_resize = true;
> +            }
> +        }
> +        for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> +            if (b->pointers[i]) {
> +                if (unlikely(b->pointers[i] == p)) {
> +                    return false;
> +                }
> +                continue;
> +            }
> +            /* found an empty key: acquire the seqlock and write */
> +            seqlock_write_begin(&head->sequence);
> +            if (new) {
> +                /*
> +                 * This barrier is paired with smp_rmb() after reading
> +                 * b->next when not holding b->lock.
> +                 */
> +                smp_wmb();
> +                atomic_set(&prev->next, b);
> +            }
> +            atomic_set(&b->hashes[i], hash);
> +            atomic_set(&b->pointers[i], p);
> +            seqlock_write_end(&head->sequence);
> +            return true;
> +        }
> +        prev = b;
> +        b = b->next;
> +    }
> +}
> +
> +/* call with an external lock held */
> +bool qht_insert(struct qht *ht, void *p, uint32_t hash)
> +{
> +    struct qht_map *map = ht->map;
> +    struct qht_bucket *b = qht_map_to_bucket(map, hash);
> +    bool needs_resize = false;
> +    bool ret;
> +
> +    /* NULL pointers are not supported */
> +    assert(p);

Maybe wrap such assertions in a macro, something like tcg_debug_assert()?

> +
> +    qemu_spin_lock(&b->lock);
> +    ret = qht_insert__locked(ht, map, b, p, hash, &needs_resize);
> +    qht_bucket_debug(b);
> +    qemu_spin_unlock(&b->lock);
> +
> +    if (unlikely(needs_resize) && ht->mode & QHT_MODE_AUTO_RESIZE) {
> +        qht_do_resize(ht, map->n * 2);
> +    }
> +    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)
> +{
> +    assert(!(to == from && i == j));
> +    assert(to->pointers[i] == NULL);
> +    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_fill_hole(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)) {
> +        return;
> +    }
> +    do {
> +        for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {

We could iterate in the opposite direction: from the last entry in a
qht_bucket to the first. It would allow us to fast-forward to the next
qht_bucket in a chain in case of non-NULL last entry and speed-up the
search.

> +            if (b->pointers[i] || (b == orig && i == pos)) {
> +                continue;
> +            }
> +            if (i > 0) {
> +                return qht_entry_move(orig, pos, b, i - 1);
> +            }
> +            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) {
> +                assert(b->hashes[i] == hash);
> +                seqlock_write_begin(&head->sequence);
> +                atomic_set(&b->hashes[i], 0);
> +                atomic_set(&b->pointers[i], NULL);

Could we better avoid zeroing these here? (At least when debugging is
disabled.)

> +                qht_bucket_fill_hole(b, i);
> +                seqlock_write_end(&head->sequence);
> +                return true;
> +            }
> +        }
> +        b = b->next;
> +    } while (b);
> +    return false;
> +}
> +
> +/* call with an external lock held */
> +bool qht_remove(struct qht *ht, const void *p, uint32_t hash)
> +{
> +    struct qht_map *map = ht->map;
> +    struct qht_bucket *b = qht_map_to_bucket(map, hash);
> +    bool ret;
> +
> +    qemu_spin_lock(&b->lock);
> +    ret = qht_remove__locked(map, b, p, hash);
> +    qht_bucket_debug(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);
> +}
> +
> +/* external lock + all of the map's locks held */
> +static inline void qht_map_iter__locked(struct qht *ht, struct qht_map *map,
> +                                        qht_iter_func_t func, void *userp)
> +{
> +    size_t i;
> +
> +    for (i = 0; i < map->n; i++) {
> +        qht_bucket_iter(ht, &map->buckets[i], func, userp);
> +    }
> +}
> +
> +/* call with an external lock held */
> +void qht_iter(struct qht *ht, qht_iter_func_t func, void *userp)
> +{
> +    qht_map_lock_buckets(ht->map);
> +    qht_map_iter__locked(ht, ht->map, func, userp);
> +    qht_map_unlock_buckets(ht->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);
> +}
> +
> +/* call with an external lock held */
> +static void qht_do_resize(struct qht *ht, size_t n)
> +{
> +    struct qht_map *old = ht->map;
> +    struct qht_map *new;
> +
> +    g_assert_cmpuint(n, !=, old->n);

Maybe use g_assert*() consistently? (I mead using either assert() or
g_assert*() variants.)

Kind regards,
Sergey

> +    new = qht_map_create(n);
> +    qht_iter(ht, qht_map_copy, new);
> +    qht_map_debug(new);
> +
> +    qht_publish(ht, new);
> +    call_rcu1(&old->rcu, qht_map_reclaim);
> +}
> +
> +/* call with an external lock held */
> +bool qht_resize(struct qht *ht, size_t n_elems)
> +{
> +    size_t n = qht_elems_to_buckets(n_elems);
> +
> +    if (n == ht->map->n) {
> +        return false;
> +    }
> +    qht_do_resize(ht, n);
> +    return true;
> +}
> +
> +/* 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 = qht_map__atomic_mb(ht);
> +
> +    stats->head_buckets = map->n;
> +    stats->used_head_buckets = 0;
> +    stats->entries = 0;
> +    qdist_init(&stats->chain);
> +    qdist_init(&stats->occupancy);
> +
> +    for (i = 0; i < map->n; i++) {
> +        struct qht_bucket *head = &map->buckets[i];
> +        struct qht_bucket *b;
> +        uint32_t 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 = bucket_next__atomic_mb(b);
> +            } 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);
> +}