Re: [Qemu-devel] [PATCH 08/10] qht: QEMU's fast, resizable and scalable Hash Table

[Alex Bennée]

Emilio G. Cota <address@hidden> writes:

> This is a hash table with optional auto-resizing and MRU promotion for
> reads and writes. Its implementation goal is to stay fast while
> scaling for read-mostly workloads.
>
> A hash table with these features will be necessary for the scalability
> of the ongoing MTTCG work; before those changes arrive we can already
> benefit from the single-threaded speedup that qht also provides.
>
> Signed-off-by: Emilio G. Cota <address@hidden>
> ---
>  include/qemu/qht.h | 150 ++++++++++++++++++
>  util/Makefile.objs |   2 +-
>  util/qht.c         | 458 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 609 insertions(+), 1 deletion(-)
>  create mode 100644 include/qemu/qht.h
>  create mode 100644 util/qht.c
>
> diff --git a/include/qemu/qht.h b/include/qemu/qht.h
> new file mode 100644
> index 0000000..63244e4
> --- /dev/null
> +++ b/include/qemu/qht.h
> @@ -0,0 +1,150 @@
> +/*
> + * Copyright (C) 2016, Emilio G. Cota <address@hidden>
> + *
> + * License: GNU GPL, version 2 or later.
> + *   See the COPYING file in the top-level directory.
> + */
> +#ifndef QHT_H
> +#define QHT_H
> +
> +#include <stdbool.h>
> +#include <assert.h>
> +#include <stdint.h>
> +#include <stdio.h>
> +
> +#include "qemu/osdep.h"
> +#include "qemu-common.h"
> +#include "qemu/spinlock.h"
> +#include "qemu/seqlock.h"
> +#include "qemu/rcu.h"
> +
> +#if HOST_LONG_BITS == 32
> +#define QHT_BUCKET_ENTRIES 6
> +#else /* 64-bit */
> +#define QHT_BUCKET_ENTRIES 4
> +#endif

Are these purely a function of the total size of qht_bucket bellow and keeping
it nicely cacheline aligned?

> +
> +struct qht_bucket {
> +    QemuSpinLock lock;
> +    QemuSeqLock sequence;
> +    uint32_t hashes[QHT_BUCKET_ENTRIES];
> +    void *pointers[QHT_BUCKET_ENTRIES];
> +    struct qht_bucket *next;
> +} QEMU_CACHELINE_ALIGNED;

For the TLB code we have compile time checks QEMU_BUILD_BUG_ON() to warn
the developers if we go over a boundary. Might be worth adding a similar
here.

> +struct qht_map {
> +    struct qht_bucket *buckets;
> +    uint64_t n;
> +    uint64_t n_items;
> +    uint64_t n_items_threshold;
> +    struct rcu_head rcu;
> +};
> +
> +struct qht {
> +    struct qht_map *map;
> +    unsigned int mode;
> +};
> +
> +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);
> +
> +#define QHT_MODE_MRU_LOOKUP  0x1 /* move looked-up items to head */
> +#define QHT_MODE_MRU_INSERT  0x2 /* insert new elements at the head */
> +#define QHT_MODE_AUTO_RESIZE 0x4 /* auto-resize when heavily loaded */
> +
> +void qht_init(struct qht *ht, uint64_t n_elems, unsigned int mode);
> +
> +/* call only when there are no readers left */
> +void qht_destroy(struct qht *ht);
> +

What's the rationale for making the caller responsible for locking here?
Are we going to be protected by tb_lock in the MTTCG case rather than a
finer grained locking inside the qht?

> +/* call with an external lock held */
> +void qht_reset(struct qht *ht);
> +
> +/* call with an external lock held */
> +void qht_reset_size(struct qht *ht, uint64_t n_elems);
> +
> +/* call with an external lock held */
> +void 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 */
> +void qht_grow(struct qht *ht);
> +
> +void qht_bucket_mru(struct qht_bucket *b, struct qht_bucket *orig,
> +                    const void *p, int pos);
> +
> +static inline
> +struct qht_bucket *qht_map_to_bucket(struct qht_map *map, uint32_t hash)
> +{
> +    return &map->buckets[hash & (map->n - 1)];
> +}
> +
> +static inline
> +void *__qht_lookup(struct qht_bucket *b, struct qht_bucket **far_bucket,
> +                    int *pos, qht_lookup_func_t func, const void *userp,
> +                    uint32_t hash)

Aside the inline is there any reason to keep such an implementation
detail in the header. I certainly couldn't measure a difference after I
moved them into qht.c.

> +{
> +    unsigned int count = 0;
> +    int i;
> +
> +    do {
> +        for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> +            if (b->hashes[i] == hash) {
> +                void *p = atomic_read(&b->pointers[i]);
> +
> +                /*
> +                 * p could be NULL if we're racing with a writer. We could 
> use
> +                 * barriers here but for performance we only issue the ones
> +                 * in the seqlock.
> +                 */
> +                if (likely(p && func(p, userp))) {

A comment about saving the position for MRU might be worthwhile here.

> +                    if (unlikely(count)) {
> +                        *far_bucket = b;
> +                        *pos = i;
> +                    }
> +                    return p;
> +                }
> +            }
> +        }
> +        count++;
> +        b = 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();
> +    } while (b);
> +    return NULL;
> +}
> +
> +static inline void *qht_lookup(struct qht *ht, qht_lookup_func_t func,
> +                                const void *userp, uint32_t hash)
> +{
> +    struct qht_bucket *far_bucket = NULL;
> +    struct qht_bucket *b;
> +    struct qht_map *map;
> +    uint32_t version;
> +    int pos = 0;
> +    void *ret;
> +
> +    map = atomic_read(&ht->map);
> +    /* paired with smp_wmb() before setting ht->map */
> +    smp_rmb();
> +    b = qht_map_to_bucket(map, hash);
> +
> +    do {
> +        version = seqlock_read_begin(&b->sequence);
> +        ret = __qht_lookup(b, &far_bucket, &pos, func, userp, hash);
> +    } while (seqlock_read_retry(&b->sequence, version));

OK I was slightly confused by this until I got further along. Maybe some
comments in the qht_bucket structure to point out the seqlock is
important when a bucket is at the head of the list.

> +    if ((ht->mode & QHT_MODE_MRU_LOOKUP) && unlikely(far_bucket)) {
> +        qht_bucket_mru(b, far_bucket, ret, pos);
> +    }
> +    return ret;
> +}
> +
> +#endif /* QHT_H */
> diff --git a/util/Makefile.objs b/util/Makefile.objs
> index a8a777e..ae893dc 100644
> --- a/util/Makefile.objs
> +++ b/util/Makefile.objs
> @@ -1,4 +1,4 @@
> -util-obj-y = osdep.o cutils.o unicode.o qemu-timer-common.o
> +util-obj-y = osdep.o cutils.o unicode.o qemu-timer-common.o qht.o
>  util-obj-$(CONFIG_POSIX) += compatfd.o
>  util-obj-$(CONFIG_POSIX) += event_notifier-posix.o
>  util-obj-$(CONFIG_POSIX) += mmap-alloc.o
> diff --git a/util/qht.c b/util/qht.c
> new file mode 100644
> index 0000000..590cd8a
> --- /dev/null
> +++ b/util/qht.c
> @@ -0,0 +1,458 @@
> +/*
> + * qht.c - QEMU Hash Table, designed to scale for read-mostly workloads.
> + *
> + * Copyright (C) 2016, Emilio G. Cota <address@hidden>
> + *
> + * 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.
> + * - A hash value of 0 is invalid.
> + * - NULL cannot be inserted as a pointer value.
> + *
> + * Features:
> + * - Optional auto-resizing: the hash table resizes up if the load surpasses
> + *   a certain threshold. Resizing is done concurrently with readers.
> + * - Optional bucket MRU promotion policy.
> + *
> + * 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:
> + * - Failed lookups fail fast, and touch a minimum number of cache lines.
> + * - Resizing the hash table with concurrent lookups is easy.
> + * - We can have a few Most-Recently-Used (MRU) hash-pointer pairs in the 
> same
> + *   head bucket. This helps scalability, since MRU promotions (i.e. writes 
> to
> + *   the bucket) become less common.
> + *
> + * For concurrent lookups we use a per-bucket seqlock; per-bucket spinlocks
> + * allow readers (lookups) to upgrade to writers and thus implement an MRU
> + * promotion policy; these MRU-induced writes do not touch the cache lines of
> + * other head buckets.
> + *
> + * Note that 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.
> + *
> + * 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/

Very helpful links, thanks ;-)

> + */
> +#include "qemu/qht.h"
> +#include "qemu/atomic.h"
> +
> +static inline uint64_t qht_elems_to_buckets(uint64_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_spinlock_init(&b->lock);
> +    seqlock_init(&b->sequence);
> +}
> +
> +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)
> +{
> +    uint64_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(uint64_t n)
> +{
> +    struct qht_map *map;
> +    uint64_t i;
> +
> +    assert(n <= (1ULL << 32)); /* we're using a 32-bit hash func */
> +    map = g_malloc(sizeof(*map));
> +    map->n = n;
> +    map->n_items = 0;
> +    map->n_items_threshold = n * QHT_BUCKET_ENTRIES / 2;
> +    map->buckets = qemu_memalign(QEMU_CACHELINE, 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, uint64_t n_elems, unsigned int mode)
> +{
> +    struct qht_map *map;
> +    uint64_t n = qht_elems_to_buckets(n_elems);
> +
> +    map = qht_map_create(n);
> +    ht->mode = mode;
> +    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 *b)

I think Paolo has already mentioned __ names.

> +{
> +    int i;
> +
> +    do {
> +        for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> +            atomic_set(&b->hashes[i], 0);
> +            atomic_set(&b->pointers[i], NULL);
> +        }
> +        b = b->next;
> +    } while (b);
> +}
> +
> +static void qht_bucket_reset(struct qht_bucket *b)
> +{
> +    qemu_spinlock_lock(&b->lock);
> +    seqlock_write_begin(&b->sequence);
> +    __qht_bucket_reset(b);

Personally I'd just import the __qht_bucket_reset into here to make it
clear the seqlock protects us from using values halfway from being
reset. Otherwise a quick comment to repeat the head text that effect on
the above function.

> +    seqlock_write_end(&b->sequence);
> +    qemu_spinlock_unlock(&b->lock);
> +}
> +
> +/* call with an external lock held */
> +void qht_reset(struct qht *ht)
> +{
> +    struct qht_map *map = ht->map;
> +    uint64_t i;
> +
> +    for (i = 0; i < map->n; i++) {
> +        qht_bucket_reset(&map->buckets[i]);
> +    }
> +}
> +
> +/* call with an external lock held */
> +void qht_reset_size(struct qht *ht, uint64_t n_elems)
> +{
> +    struct qht_map *old = ht->map;
> +
> +    qht_reset(ht);
> +    if (old->n == qht_elems_to_buckets(n_elems)) {
> +        return;
> +    }
> +    qht_init(ht, n_elems, ht->mode);
> +    call_rcu1(&old->rcu, qht_map_reclaim);
> +}
> +
> +/* Can only be called when at least orig is the 3rd link i.e. 
> head->2nd->orig */
> +static void
> +__qht_move_bucket_to_front(struct qht_bucket *head, struct qht_bucket *orig)
> +{
> +    struct qht_bucket *b = head->next;
> +
> +    for (;;) {
> +        if (b->next == orig) {
> +            atomic_set(&b->next, orig->next);
> +            atomic_set(&orig->next, head->next);
> +            atomic_set(&head->next, orig);
> +            return;
> +        }
> +        b = b->next;
> +    }
> +}
> +
> +static inline void __qht_bucket_mru_head(struct qht_bucket *b, int pos)
> +{
> +    uint32_t orig_hash = b->hashes[pos];
> +    void *orig_p = b->pointers[pos];
> +    int i;
> +
> +    for (i = 0; i < pos; i++) {
> +        atomic_set(&b->hashes[i + 1], b->hashes[i]);
> +        atomic_set(&b->pointers[i + 1], b->pointers[i]);
> +    }
> +    atomic_set(&b->hashes[0], orig_hash);
> +    atomic_set(&b->pointers[0], orig_p);
> +}
> +
> +
> +/* call with head->lock held */
> +static inline void
> +__qht_bucket_mru(struct qht_bucket *head, struct qht_bucket *orig, int pos)
> +{
> +    uint32_t *dest_hash;
> +    void **dest_p;
> +    void *p;
> +    uint32_t hash;
> +    int i;
> +
> +    if (head == orig) {
> +        return __qht_bucket_mru_head(head, pos);
> +    }

OK this is calling out for a comment because I'm having trouble
following the logic. I think we are pushing the last entry in the head
bucket out the bucket the MRU entry is coming from while shuffling the
rest up for the new entry.

> +    for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> +        if (i == QHT_BUCKET_ENTRIES - 1) {
> +            dest_hash = &orig->hashes[pos];
> +            dest_p = &o rig->pointers[pos];
> +        } else {
> +            dest_hash = &head->hashes[i + 1];
> +            dest_p = &head->pointers[i + 1];
> +        }
> +        hash = *dest_hash;
> +        p = *dest_p;
> +
> +        atomic_set(dest_hash, head->hashes[i]);
> +        atomic_set(dest_p, head->pointers[i]);
> +
> +        atomic_set(&head->hashes[i], hash);
> +        atomic_set(&head->pointers[i], p);
> +    }

So the bucket that has just swapped the MRU entry for an evicted entry
gets placed after head. Is there a chance we end up just bouncing the
head->next bucket each hit?

> +    if (head->next != orig) {
> +        __qht_move_bucket_to_front(head, orig);
> +    }
> +}
> +
> +void qht_bucket_mru(struct qht_bucket *head, struct qht_bucket *orig,
> +                    const void *p, int pos)
> +{
> +    qemu_spinlock_lock(&head->lock);
> +    if (unlikely(orig->pointers[pos] != p)) {
> +        /* while we acquired the lock, the bucket was updated, so bail out */
> +        goto out;
> +    }
> +    seqlock_write_begin(&head->sequence);
> +    __qht_bucket_mru(head, orig, pos);
> +    seqlock_write_end(&head->sequence);
> + out:
> +    qemu_spinlock_unlock(&head->lock);
> +}
> +
> +/* call with b->lock held */
> +static void __qht_insert(struct qht *ht, struct qht_map *map,
> +                         struct qht_bucket *b, void *p, uint32_t hash)
> +{
> +    struct qht_bucket *head = b;
> +    struct qht_bucket *prev = NULL;
> +    struct qht_bucket *new = NULL;
> +    unsigned int count = 0;
> +    int i;
> +
> +    for (;;) {
> +        if (b == NULL) {
> +            b = qemu_memalign(QEMU_CACHELINE, sizeof(*b));
> +            memset(b, 0, sizeof(*b));
> +            new = b;
> +        }
> +        for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> +            if (b->hashes[i]) {
> +                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);
> +            if ((ht->mode & QHT_MODE_MRU_INSERT) && count) {
> +                __qht_bucket_mru(head, b, i);
> +            }
> +            seqlock_write_end(&head->sequence);
> +            map->n_items++;
> +            return;
> +        }
> +        prev = b;
> +        b = b->next;
> +        count++;
> +    }
> +}
> +
> +/* call with an external lock held */
> +void 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);
> +
> +    qemu_spinlock_lock(&b->lock);
> +    __qht_insert(ht, map, b, p, hash);
> +    qemu_spinlock_unlock(&b->lock);
> +
> +    if ((ht->mode & QHT_MODE_AUTO_RESIZE) &&
> +        unlikely(map->n_items > map->n_items_threshold)) {
> +        qht_grow(ht);
> +    }
> +}
> +
> +/* call with b->lock held */
> +static inline bool __qht_remove(struct qht_map *map, struct qht_bucket *b,
> +                                const void *p, uint32_t hash)
> +{
> +    int i;
> +
> +    do {
> +        for (i = 0; i < QHT_BUCKET_ENTRIES; i++) {
> +            if (b->hashes[i] == hash && b->pointers[i] == p) {
> +                atomic_set(&b->hashes[i], 0);
> +                atomic_set(&b->pointers[i], NULL);
> +                map->n_items--;
> +                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_spinlock_lock(&b->lock);
> +    seqlock_write_begin(&b->sequence);
> +    ret = __qht_remove(map, b, p, hash);
> +    seqlock_write_end(&b->sequence);
> +    qemu_spinlock_unlock(&b->lock);
> +    return ret;
> +}
> +
> +/*
> + * acquire all spinlocks from a map, so that writers cannot race with
> + * readers-turned-writers.
> + */
> +static void qht_lock(struct qht *ht)
> +{
> +    struct qht_map *map = ht->map;
> +    uint64_t i;
> +
> +    /* if readers cannot upgrade, do nothing */
> +    if (!(ht->mode & QHT_MODE_MRU_LOOKUP)) {
> +        return;
> +    }
> +    for (i = 0; i < map->n; i++) {
> +        struct qht_bucket *b = &map->buckets[i];
> +
> +        qemu_spinlock_lock(&b->lock);
> +    }
> +}
> +
> +static void qht_unlock(struct qht *ht)
> +{
> +    struct qht_map *map = ht->map;
> +    uint64_t i;
> +
> +    if (!(ht->mode & QHT_MODE_MRU_LOOKUP)) {
> +        return;
> +    }
> +    for (i = 0; i < map->n; i++) {
> +        struct qht_bucket *b = &map->buckets[i];
> +
> +        qemu_spinlock_unlock(&b->lock);
> +    }
> +}
> +
> +/* external lock + all of the map's locks held (if !MRU_LOOKUP) */
> +static inline void __qht_map_iter(struct qht *ht, struct qht_map *map,
> +                                  qht_iter_func_t func, void *userp)
> +{
> +    uint64_t i;
> +
> +    for (i = 0; i < map->n; i++) {
> +        struct qht_bucket *b = &map->buckets[i];
> +        int j;
> +
> +        do {
> +            for (j = 0; j < QHT_BUCKET_ENTRIES; j++) {
> +                if (b->hashes[j]) {
> +                    func(ht, b->pointers[j], b->hashes[j], userp);
> +                }
> +            }
> +            b = b->next;
> +        } while (b);
> +    }
> +}
> +
> +/* call with an external lock held */
> +void qht_iter(struct qht *ht, qht_iter_func_t func, void *userp)
> +{
> +    qht_lock(ht);
> +    __qht_map_iter(ht, ht->map, func, userp);
> +    qht_unlock(ht);
> +}
> +
> +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(ht, new, b, p, hash);
> +}
> +
> +/* call with an external lock held */
> +void qht_grow(struct qht *ht)
> +{
> +    struct qht_map *old = ht->map;
> +    struct qht_map *new;
> +    uint64_t n = old->n * 2;
> +
> +    if (unlikely(n > (1ULL << 32))) {
> +        return;
> +    }
> +    new = qht_map_create(n);
> +    qht_iter(ht, qht_map_copy, new);
> +
> +    qht_publish(ht, new);
> +    call_rcu1(&old->rcu, qht_map_reclaim);
> +}

This looks good and I can see this being a useful utility function
across QEMU. My main problem was following exactly what happens when
entries are moved around for the MRU case. As users don't have to have a
deep knowledge of the implementation details this isn't a major issue
although a few more expansive comments or ASCII diagrams may help if
you are feeling up to it ;-)

--
Alex Bennée