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qemu/util/qsp.c

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/*
* qsp.c - QEMU Synchronization Profiler
*
* Copyright (C) 2018, Emilio G. Cota <cota@braap.org>
*
* License: GNU GPL, version 2 or later.
* See the COPYING file in the top-level directory.
*
* QSP profiles the time spent in synchronization primitives, which can
* help diagnose performance problems, e.g. scalability issues when
* contention is high.
*
* The primitives currently supported are mutexes, recursive mutexes and
* condition variables. Note that not all related functions are intercepted;
* instead we profile only those functions that can have a performance impact,
* either due to blocking (e.g. cond_wait, mutex_lock) or cache line
* contention (e.g. mutex_lock, mutex_trylock).
*
* QSP's design focuses on speed and scalability. This is achieved
* by having threads do their profiling entirely on thread-local data.
* The appropriate thread-local data is found via a QHT, i.e. a concurrent hash
* table. To aggregate data in order to generate a report, we iterate over
* all entries in the hash table. Depending on the number of threads and
* synchronization objects this might be expensive, but note that it is
* very rarely called -- reports are generated only when requested by users.
*
* Reports are generated as a table where each row represents a call site. A
* call site is the triplet formed by the __file__ and __LINE__ of the caller
* as well as the address of the "object" (i.e. mutex, rec. mutex or condvar)
* being operated on. Optionally, call sites that operate on different objects
* of the same type can be coalesced, which can be particularly useful when
* profiling dynamically-allocated objects.
*
* Alternative designs considered:
*
* - Use an off-the-shelf profiler such as mutrace. This is not a viable option
* for us because QEMU has __malloc_hook set (by one of the libraries it
* uses); leaving this hook unset is required to avoid deadlock in mutrace.
*
* - Use a glib HT for each thread, protecting each HT with its own lock.
* This isn't simpler than the current design, and is 10% slower in the
* atomic_add-bench microbenchmark (-m option).
*
* - For reports, just use a binary tree as we aggregate data, instead of having
* an intermediate hash table. This would simplify the code only slightly, but
* would perform badly if there were many threads and objects to track.
*
* - Wrap operations on qsp entries with RCU read-side critical sections, so
* that qsp_reset() can delete entries. Unfortunately, the overhead of calling
* rcu_read_lock/unlock slows down atomic_add-bench -m by 24%. Having
* a snapshot that is updated on qsp_reset() avoids this overhead.
*
* Related Work:
* - Lennart Poettering's mutrace: http://0pointer.de/blog/projects/mutrace.html
* - Lozi, David, Thomas, Lawall and Muller. "Remote Core Locking: Migrating
* Critical-Section Execution to Improve the Performance of Multithreaded
* Applications", USENIX ATC'12.
*/
#include "qemu/osdep.h"
#include "qemu/thread.h"
#include "qemu/timer.h"
#include "qemu/qht.h"
#include "qemu/rcu.h"
#include "exec/tb-hash-xx.h"
enum QSPType {
QSP_MUTEX,
QSP_BQL_MUTEX,
QSP_REC_MUTEX,
QSP_CONDVAR,
};
struct QSPCallSite {
const void *obj;
const char *file; /* i.e. __FILE__; shortened later */
int line;
enum QSPType type;
};
typedef struct QSPCallSite QSPCallSite;
struct QSPEntry {
void *thread_ptr;
const QSPCallSite *callsite;
uint64_t n_acqs;
uint64_t ns;
unsigned int n_objs; /* count of coalesced objs; only used for reporting */
};
typedef struct QSPEntry QSPEntry;
struct QSPSnapshot {
struct rcu_head rcu;
struct qht ht;
};
typedef struct QSPSnapshot QSPSnapshot;
/* initial sizing for hash tables */
#define QSP_INITIAL_SIZE 64
/* If this file is moved, QSP_REL_PATH should be updated accordingly */
#define QSP_REL_PATH "util/qsp.c"
/* this file's full path. Used to present all call sites with relative paths */
static size_t qsp_qemu_path_len;
/* the address of qsp_thread gives us a unique 'thread ID' */
static __thread int qsp_thread;
/*
* Call sites are the same for all threads, so we track them in a separate hash
* table to save memory.
*/
static struct qht qsp_callsite_ht;
static struct qht qsp_ht;
static QSPSnapshot *qsp_snapshot;
static bool qsp_initialized, qsp_initializing;
static const char * const qsp_typenames[] = {
[QSP_MUTEX] = "mutex",
[QSP_BQL_MUTEX] = "BQL mutex",
[QSP_REC_MUTEX] = "rec_mutex",
[QSP_CONDVAR] = "condvar",
};
QemuMutexLockFunc qemu_bql_mutex_lock_func = qemu_mutex_lock_impl;
QemuMutexLockFunc qemu_mutex_lock_func = qemu_mutex_lock_impl;
QemuMutexTrylockFunc qemu_mutex_trylock_func = qemu_mutex_trylock_impl;
QemuRecMutexLockFunc qemu_rec_mutex_lock_func = qemu_rec_mutex_lock_impl;
QemuRecMutexTrylockFunc qemu_rec_mutex_trylock_func =
qemu_rec_mutex_trylock_impl;
QemuCondWaitFunc qemu_cond_wait_func = qemu_cond_wait_impl;
/*
* It pays off to _not_ hash callsite->file; hashing a string is slow, and
* without it we still get a pretty unique hash.
*/
static inline
uint32_t do_qsp_callsite_hash(const QSPCallSite *callsite, uint64_t a)
{
uint64_t b = (uint64_t)(uintptr_t)callsite->obj;
uint32_t e = callsite->line;
uint32_t f = callsite->type;
return tb_hash_func7(a, b, e, f, 0);
}
static inline
uint32_t qsp_callsite_hash(const QSPCallSite *callsite)
{
return do_qsp_callsite_hash(callsite, 0);
}
static inline uint32_t do_qsp_entry_hash(const QSPEntry *entry, uint64_t a)
{
return do_qsp_callsite_hash(entry->callsite, a);
}
static uint32_t qsp_entry_hash(const QSPEntry *entry)
{
return do_qsp_entry_hash(entry, (uint64_t)(uintptr_t)entry->thread_ptr);
}
static uint32_t qsp_entry_no_thread_hash(const QSPEntry *entry)
{
return do_qsp_entry_hash(entry, 0);
}
/* without the objects we need to hash the file name to get a decent hash */
static uint32_t qsp_entry_no_thread_obj_hash(const QSPEntry *entry)
{
const QSPCallSite *callsite = entry->callsite;
uint64_t a = g_str_hash(callsite->file);
uint64_t b = callsite->line;
uint32_t e = callsite->type;
return tb_hash_func7(a, b, e, 0, 0);
}
static bool qsp_callsite_cmp(const void *ap, const void *bp)
{
const QSPCallSite *a = ap;
const QSPCallSite *b = bp;
return a == b ||
(a->obj == b->obj &&
a->line == b->line &&
a->type == b->type &&
(a->file == b->file || !strcmp(a->file, b->file)));
}
static bool qsp_callsite_no_obj_cmp(const void *ap, const void *bp)
{
const QSPCallSite *a = ap;
const QSPCallSite *b = bp;
return a == b ||
(a->line == b->line &&
a->type == b->type &&
(a->file == b->file || !strcmp(a->file, b->file)));
}
static bool qsp_entry_no_thread_cmp(const void *ap, const void *bp)
{
const QSPEntry *a = ap;
const QSPEntry *b = bp;
return qsp_callsite_cmp(a->callsite, b->callsite);
}
static bool qsp_entry_no_thread_obj_cmp(const void *ap, const void *bp)
{
const QSPEntry *a = ap;
const QSPEntry *b = bp;
return qsp_callsite_no_obj_cmp(a->callsite, b->callsite);
}
static bool qsp_entry_cmp(const void *ap, const void *bp)
{
const QSPEntry *a = ap;
const QSPEntry *b = bp;
return a->thread_ptr == b->thread_ptr &&
qsp_callsite_cmp(a->callsite, b->callsite);
}
/*
* Normally we'd call this from a constructor function, but we want it to work
* via libutil as well.
*/
static void qsp_do_init(void)
{
/* make sure this file's path in the tree is up to date with QSP_REL_PATH */
g_assert(strstr(__FILE__, QSP_REL_PATH));
qsp_qemu_path_len = strlen(__FILE__) - strlen(QSP_REL_PATH);
qht_init(&qsp_ht, qsp_entry_cmp, QSP_INITIAL_SIZE,
QHT_MODE_AUTO_RESIZE | QHT_MODE_RAW_MUTEXES);
qht_init(&qsp_callsite_ht, qsp_callsite_cmp, QSP_INITIAL_SIZE,
QHT_MODE_AUTO_RESIZE | QHT_MODE_RAW_MUTEXES);
}
static __attribute__((noinline)) void qsp_init__slowpath(void)
{
if (atomic_cmpxchg(&qsp_initializing, false, true) == false) {
qsp_do_init();
atomic_set(&qsp_initialized, true);
} else {
while (!atomic_read(&qsp_initialized)) {
cpu_relax();
}
}
}
/* qsp_init() must be called from _all_ exported functions */
static inline void qsp_init(void)
{
if (likely(atomic_read(&qsp_initialized))) {
return;
}
qsp_init__slowpath();
}
static QSPCallSite *qsp_callsite_find(const QSPCallSite *orig)
{
QSPCallSite *callsite;
uint32_t hash;
hash = qsp_callsite_hash(orig);
callsite = qht_lookup(&qsp_callsite_ht, orig, hash);
if (callsite == NULL) {
void *existing = NULL;
callsite = g_new(QSPCallSite, 1);
memcpy(callsite, orig, sizeof(*callsite));
qht_insert(&qsp_callsite_ht, callsite, hash, &existing);
if (unlikely(existing)) {
g_free(callsite);
callsite = existing;
}
}
return callsite;
}
static QSPEntry *
qsp_entry_create(struct qht *ht, const QSPEntry *entry, uint32_t hash)
{
QSPEntry *e;
void *existing = NULL;
e = g_new0(QSPEntry, 1);
e->thread_ptr = entry->thread_ptr;
e->callsite = qsp_callsite_find(entry->callsite);
qht_insert(ht, e, hash, &existing);
if (unlikely(existing)) {
g_free(e);
e = existing;
}
return e;
}
static QSPEntry *
qsp_entry_find(struct qht *ht, const QSPEntry *entry, uint32_t hash)
{
QSPEntry *e;
e = qht_lookup(ht, entry, hash);
if (e == NULL) {
e = qsp_entry_create(ht, entry, hash);
}
return e;
}
/*
* Note: Entries are never removed, so callers do not have to be in an RCU
* read-side critical section.
*/
static QSPEntry *qsp_entry_get(const void *obj, const char *file, int line,
enum QSPType type)
{
QSPCallSite callsite = {
.obj = obj,
.file = file,
.line = line,
.type = type,
};
QSPEntry orig;
uint32_t hash;
qsp_init();
orig.thread_ptr = &qsp_thread;
orig.callsite = &callsite;
hash = qsp_entry_hash(&orig);
return qsp_entry_find(&qsp_ht, &orig, hash);
}
/*
* @e is in the global hash table; it is only written to by the current thread,
* so we write to it atomically (as in "write once") to prevent torn reads.
*/
static inline void do_qsp_entry_record(QSPEntry *e, int64_t delta, bool acq)
{
atomic_set_u64(&e->ns, e->ns + delta);
if (acq) {
atomic_set_u64(&e->n_acqs, e->n_acqs + 1);
}
}
static inline void qsp_entry_record(QSPEntry *e, int64_t delta)
{
do_qsp_entry_record(e, delta, true);
}
#define QSP_GEN_VOID(type_, qsp_t_, func_, impl_) \
static void func_(type_ *obj, const char *file, int line) \
{ \
QSPEntry *e; \
int64_t t0, t1; \
\
t0 = get_clock(); \
impl_(obj, file, line); \
t1 = get_clock(); \
\
e = qsp_entry_get(obj, file, line, qsp_t_); \
qsp_entry_record(e, t1 - t0); \
}
#define QSP_GEN_RET1(type_, qsp_t_, func_, impl_) \
static int func_(type_ *obj, const char *file, int line) \
{ \
QSPEntry *e; \
int64_t t0, t1; \
int err; \
\
t0 = get_clock(); \
err = impl_(obj, file, line); \
t1 = get_clock(); \
\
e = qsp_entry_get(obj, file, line, qsp_t_); \
do_qsp_entry_record(e, t1 - t0, !err); \
return err; \
}
QSP_GEN_VOID(QemuMutex, QSP_BQL_MUTEX, qsp_bql_mutex_lock, qemu_mutex_lock_impl)
QSP_GEN_VOID(QemuMutex, QSP_MUTEX, qsp_mutex_lock, qemu_mutex_lock_impl)
QSP_GEN_RET1(QemuMutex, QSP_MUTEX, qsp_mutex_trylock, qemu_mutex_trylock_impl)
QSP_GEN_VOID(QemuRecMutex, QSP_REC_MUTEX, qsp_rec_mutex_lock,
qemu_rec_mutex_lock_impl)
QSP_GEN_RET1(QemuRecMutex, QSP_REC_MUTEX, qsp_rec_mutex_trylock,
qemu_rec_mutex_trylock_impl)
#undef QSP_GEN_RET1
#undef QSP_GEN_VOID
static void
qsp_cond_wait(QemuCond *cond, QemuMutex *mutex, const char *file, int line)
{
QSPEntry *e;
int64_t t0, t1;
t0 = get_clock();
qemu_cond_wait_impl(cond, mutex, file, line);
t1 = get_clock();
e = qsp_entry_get(cond, file, line, QSP_CONDVAR);
qsp_entry_record(e, t1 - t0);
}
bool qsp_is_enabled(void)
{
return atomic_read(&qemu_mutex_lock_func) == qsp_mutex_lock;
}
void qsp_enable(void)
{
atomic_set(&qemu_mutex_lock_func, qsp_mutex_lock);
atomic_set(&qemu_mutex_trylock_func, qsp_mutex_trylock);
atomic_set(&qemu_bql_mutex_lock_func, qsp_bql_mutex_lock);
atomic_set(&qemu_rec_mutex_lock_func, qsp_rec_mutex_lock);
atomic_set(&qemu_rec_mutex_trylock_func, qsp_rec_mutex_trylock);
atomic_set(&qemu_cond_wait_func, qsp_cond_wait);
}
void qsp_disable(void)
{
atomic_set(&qemu_mutex_lock_func, qemu_mutex_lock_impl);
atomic_set(&qemu_mutex_trylock_func, qemu_mutex_trylock_impl);
atomic_set(&qemu_bql_mutex_lock_func, qemu_mutex_lock_impl);
atomic_set(&qemu_rec_mutex_lock_func, qemu_rec_mutex_lock_impl);
atomic_set(&qemu_rec_mutex_trylock_func, qemu_rec_mutex_trylock_impl);
atomic_set(&qemu_cond_wait_func, qemu_cond_wait_impl);
}
static gint qsp_tree_cmp(gconstpointer ap, gconstpointer bp, gpointer up)
{
const QSPEntry *a = ap;
const QSPEntry *b = bp;
enum QSPSortBy sort_by = *(enum QSPSortBy *)up;
const QSPCallSite *ca;
const QSPCallSite *cb;
switch (sort_by) {
case QSP_SORT_BY_TOTAL_WAIT_TIME:
if (a->ns > b->ns) {
return -1;
} else if (a->ns < b->ns) {
return 1;
}
break;
case QSP_SORT_BY_AVG_WAIT_TIME:
{
double avg_a = a->n_acqs ? a->ns / a->n_acqs : 0;
double avg_b = b->n_acqs ? b->ns / b->n_acqs : 0;
if (avg_a > avg_b) {
return -1;
} else if (avg_a < avg_b) {
return 1;
}
break;
}
default:
g_assert_not_reached();
}
ca = a->callsite;
cb = b->callsite;
/* Break the tie with the object's address */
if (ca->obj < cb->obj) {
return -1;
} else if (ca->obj > cb->obj) {
return 1;
} else {
int cmp;
/* same obj. Break the tie with the callsite's file */
cmp = strcmp(ca->file, cb->file);
if (cmp) {
return cmp;
}
/* same callsite file. Break the tie with the callsite's line */
g_assert(ca->line != cb->line);
if (ca->line < cb->line) {
return -1;
} else if (ca->line > cb->line) {
return 1;
} else {
/* break the tie with the callsite's type */
return cb->type - ca->type;
}
}
}
static void qsp_sort(void *p, uint32_t h, void *userp)
{
QSPEntry *e = p;
GTree *tree = userp;
g_tree_insert(tree, e, NULL);
}
static void qsp_aggregate(void *p, uint32_t h, void *up)
{
struct qht *ht = up;
const QSPEntry *e = p;
QSPEntry *agg;
uint32_t hash;
hash = qsp_entry_no_thread_hash(e);
agg = qsp_entry_find(ht, e, hash);
/*
* The entry is in the global hash table; read from it atomically (as in
* "read once").
*/
agg->ns += atomic_read_u64(&e->ns);
agg->n_acqs += atomic_read_u64(&e->n_acqs);
}
static void qsp_iter_diff(void *p, uint32_t hash, void *htp)
{
struct qht *ht = htp;
QSPEntry *old = p;
QSPEntry *new;
new = qht_lookup(ht, old, hash);
/* entries are never deleted, so we must have this one */
g_assert(new != NULL);
/* our reading of the stats happened after the snapshot was taken */
g_assert(new->n_acqs >= old->n_acqs);
g_assert(new->ns >= old->ns);
new->n_acqs -= old->n_acqs;
new->ns -= old->ns;
/* No point in reporting an empty entry */
if (new->n_acqs == 0 && new->ns == 0) {
bool removed = qht_remove(ht, new, hash);
g_assert(removed);
g_free(new);
}
}
static void qsp_diff(struct qht *orig, struct qht *new)
{
qht_iter(orig, qsp_iter_diff, new);
}
static void qsp_iter_callsite_coalesce(void *p, uint32_t h, void *htp)
{
struct qht *ht = htp;
QSPEntry *old = p;
QSPEntry *e;
uint32_t hash;
hash = qsp_entry_no_thread_obj_hash(old);
e = qht_lookup(ht, old, hash);
if (e == NULL) {
e = qsp_entry_create(ht, old, hash);
e->n_objs = 1;
} else if (e->callsite->obj != old->callsite->obj) {
e->n_objs++;
}
e->ns += old->ns;
e->n_acqs += old->n_acqs;
}
static void qsp_ht_delete(void *p, uint32_t h, void *htp)
{
g_free(p);
}
static void qsp_mktree(GTree *tree, bool callsite_coalesce)
{
QSPSnapshot *snap;
struct qht ht, coalesce_ht;
struct qht *htp;
/*
* First, see if there's a prior snapshot, so that we read the global hash
* table _after_ the snapshot has been created, which guarantees that
* the entries we'll read will be a superset of the snapshot's entries.
*
* We must remain in an RCU read-side critical section until we're done
* with the snapshot.
*/
rcu_read_lock();
snap = atomic_rcu_read(&qsp_snapshot);
/* Aggregate all results from the global hash table into a local one */
qht_init(&ht, qsp_entry_no_thread_cmp, QSP_INITIAL_SIZE,
QHT_MODE_AUTO_RESIZE | QHT_MODE_RAW_MUTEXES);
qht_iter(&qsp_ht, qsp_aggregate, &ht);
/* compute the difference wrt the snapshot, if any */
if (snap) {
qsp_diff(&snap->ht, &ht);
}
/* done with the snapshot; RCU can reclaim it */
rcu_read_unlock();
htp = &ht;
if (callsite_coalesce) {
qht_init(&coalesce_ht, qsp_entry_no_thread_obj_cmp, QSP_INITIAL_SIZE,
QHT_MODE_AUTO_RESIZE | QHT_MODE_RAW_MUTEXES);
qht_iter(&ht, qsp_iter_callsite_coalesce, &coalesce_ht);
/* free the previous hash table, and point htp to coalesce_ht */
qht_iter(&ht, qsp_ht_delete, NULL);
qht_destroy(&ht);
htp = &coalesce_ht;
}
/* sort the hash table elements by using a tree */
qht_iter(htp, qsp_sort, tree);
/* free the hash table, but keep the elements (those are in the tree now) */
qht_destroy(htp);
}
/* free string with g_free */
static char *qsp_at(const QSPCallSite *callsite)
{
GString *s = g_string_new(NULL);
const char *shortened;
/* remove the absolute path to qemu */
if (unlikely(strlen(callsite->file) < qsp_qemu_path_len)) {
shortened = callsite->file;
} else {
shortened = callsite->file + qsp_qemu_path_len;
}
g_string_append_printf(s, "%s:%u", shortened, callsite->line);
return g_string_free(s, FALSE);
}
struct QSPReportEntry {
const void *obj;
char *callsite_at;
const char *typename;
double time_s;
double ns_avg;
uint64_t n_acqs;
unsigned int n_objs;
};
typedef struct QSPReportEntry QSPReportEntry;
struct QSPReport {
QSPReportEntry *entries;
size_t n_entries;
size_t max_n_entries;
};
typedef struct QSPReport QSPReport;
static gboolean qsp_tree_report(gpointer key, gpointer value, gpointer udata)
{
const QSPEntry *e = key;
QSPReport *report = udata;
QSPReportEntry *entry;
if (report->n_entries == report->max_n_entries) {
return TRUE;
}
entry = &report->entries[report->n_entries];
report->n_entries++;
entry->obj = e->callsite->obj;
entry->n_objs = e->n_objs;
entry->callsite_at = qsp_at(e->callsite);
entry->typename = qsp_typenames[e->callsite->type];
entry->time_s = e->ns * 1e-9;
entry->n_acqs = e->n_acqs;
entry->ns_avg = e->n_acqs ? e->ns / e->n_acqs : 0;
return FALSE;
}
static void
pr_report(const QSPReport *rep, FILE *f, fprintf_function pr)
{
char *dashes;
size_t max_len = 0;
int callsite_len = 0;
int callsite_rspace;
int n_dashes;
size_t i;
/* find out the maximum length of all 'callsite' fields */
for (i = 0; i < rep->n_entries; i++) {
const QSPReportEntry *e = &rep->entries[i];
size_t len = strlen(e->callsite_at);
if (len > max_len) {
max_len = len;
}
}
callsite_len = MAX(max_len, strlen("Call site"));
/* white space to leave to the right of "Call site" */
callsite_rspace = callsite_len - strlen("Call site");
pr(f, "Type Object Call site%*s Wait Time (s) "
" Count Average (us)\n", callsite_rspace, "");
/* build a horizontal rule with dashes */
n_dashes = 79 + callsite_rspace;
dashes = g_malloc(n_dashes + 1);
memset(dashes, '-', n_dashes);
dashes[n_dashes] = '\0';
pr(f, "%s\n", dashes);
for (i = 0; i < rep->n_entries; i++) {
const QSPReportEntry *e = &rep->entries[i];
GString *s = g_string_new(NULL);
g_string_append_printf(s, "%-9s ", e->typename);
if (e->n_objs > 1) {
g_string_append_printf(s, "[%12u]", e->n_objs);
} else {
g_string_append_printf(s, "%14p", e->obj);
}
g_string_append_printf(s, " %s%*s %13.5f %12" PRIu64 " %12.2f\n",
e->callsite_at,
callsite_len - (int)strlen(e->callsite_at), "",
e->time_s, e->n_acqs, e->ns_avg * 1e-3);
pr(f, "%s", s->str);
g_string_free(s, TRUE);
}
pr(f, "%s\n", dashes);
g_free(dashes);
}
static void report_destroy(QSPReport *rep)
{
size_t i;
for (i = 0; i < rep->n_entries; i++) {
QSPReportEntry *e = &rep->entries[i];
g_free(e->callsite_at);
}
g_free(rep->entries);
}
void qsp_report(FILE *f, fprintf_function cpu_fprintf, size_t max,
enum QSPSortBy sort_by, bool callsite_coalesce)
{
GTree *tree = g_tree_new_full(qsp_tree_cmp, &sort_by, g_free, NULL);
QSPReport rep;
qsp_init();
rep.entries = g_new0(QSPReportEntry, max);
rep.n_entries = 0;
rep.max_n_entries = max;
qsp_mktree(tree, callsite_coalesce);
g_tree_foreach(tree, qsp_tree_report, &rep);
g_tree_destroy(tree);
pr_report(&rep, f, cpu_fprintf);
report_destroy(&rep);
}
static void qsp_snapshot_destroy(QSPSnapshot *snap)
{
qht_iter(&snap->ht, qsp_ht_delete, NULL);
qht_destroy(&snap->ht);
g_free(snap);
}
void qsp_reset(void)
{
QSPSnapshot *new = g_new(QSPSnapshot, 1);
QSPSnapshot *old;
qsp_init();
qht_init(&new->ht, qsp_entry_cmp, QSP_INITIAL_SIZE,
QHT_MODE_AUTO_RESIZE | QHT_MODE_RAW_MUTEXES);
/* take a snapshot of the current state */
qht_iter(&qsp_ht, qsp_aggregate, &new->ht);
/* replace the previous snapshot, if any */
old = atomic_xchg(&qsp_snapshot, new);
if (old) {
call_rcu(old, qsp_snapshot_destroy, rcu);
}
}
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