qemu

qsp.c (24568B)

---

 /*
  * 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/qemu-print.h"
 #include "qemu/thread.h"
 #include "qemu/timer.h"
 #include "qemu/qht.h"
 #include "qemu/rcu.h"
 #include "qemu/xxhash.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;
     aligned_uint64_t n_acqs;
     aligned_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;
 QemuCondTimedWaitFunc qemu_cond_timedwait_func = qemu_cond_timedwait_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 ab)
 {
     uint64_t cd = (uint64_t)(uintptr_t)callsite->obj;
     uint32_t e = callsite->line;
     uint32_t f = callsite->type;
 
     return qemu_xxhash6(ab, cd, e, f);
 }
 
 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 ab = g_str_hash(callsite->file);
     uint64_t cd = callsite->line;
     uint32_t e = callsite->type;
 
     return qemu_xxhash5(ab, cd, e);
 }
 
 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 (qatomic_cmpxchg(&qsp_initializing, false, true) == false) {
         qsp_do_init();
         qatomic_set(&qsp_initialized, true);
     } else {
         while (!qatomic_read(&qsp_initialized)) {
             cpu_relax();
         }
     }
 }
 
 /* qsp_init() must be called from _all_ exported functions */
 static inline void qsp_init(void)
 {
     if (likely(qatomic_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)
 {
     qatomic_set_u64(&e->ns, e->ns + delta);
     if (acq) {
         qatomic_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);
 }
 
 static bool
 qsp_cond_timedwait(QemuCond *cond, QemuMutex *mutex, int ms,
                    const char *file, int line)
 {
     QSPEntry *e;
     int64_t t0, t1;
     bool ret;
 
     t0 = get_clock();
     ret = qemu_cond_timedwait_impl(cond, mutex, ms, file, line);
     t1 = get_clock();
 
     e = qsp_entry_get(cond, file, line, QSP_CONDVAR);
     qsp_entry_record(e, t1 - t0);
     return ret;
 }
 
 bool qsp_is_enabled(void)
 {
     return qatomic_read(&qemu_mutex_lock_func) == qsp_mutex_lock;
 }
 
 void qsp_enable(void)
 {
     qatomic_set(&qemu_mutex_lock_func, qsp_mutex_lock);
     qatomic_set(&qemu_mutex_trylock_func, qsp_mutex_trylock);
     qatomic_set(&qemu_bql_mutex_lock_func, qsp_bql_mutex_lock);
     qatomic_set(&qemu_rec_mutex_lock_func, qsp_rec_mutex_lock);
     qatomic_set(&qemu_rec_mutex_trylock_func, qsp_rec_mutex_trylock);
     qatomic_set(&qemu_cond_wait_func, qsp_cond_wait);
     qatomic_set(&qemu_cond_timedwait_func, qsp_cond_timedwait);
 }
 
 void qsp_disable(void)
 {
     qatomic_set(&qemu_mutex_lock_func, qemu_mutex_lock_impl);
     qatomic_set(&qemu_mutex_trylock_func, qemu_mutex_trylock_impl);
     qatomic_set(&qemu_bql_mutex_lock_func, qemu_mutex_lock_impl);
     qatomic_set(&qemu_rec_mutex_lock_func, qemu_rec_mutex_lock_impl);
     qatomic_set(&qemu_rec_mutex_trylock_func, qemu_rec_mutex_trylock_impl);
     qatomic_set(&qemu_cond_wait_func, qemu_cond_wait_impl);
     qatomic_set(&qemu_cond_timedwait_func, qemu_cond_timedwait_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 += qatomic_read_u64(&e->ns);
     agg->n_acqs += qatomic_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)
 {
     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.
      */
     WITH_RCU_READ_LOCK_GUARD() {
         QSPSnapshot *snap = qatomic_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);
         }
     }
 
     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)
 {
     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");
 
     qemu_printf("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';
     qemu_printf("%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);
         qemu_printf("%s", s->str);
         g_string_free(s, TRUE);
     }
 
     qemu_printf("%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(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);
     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 = qatomic_xchg(&qsp_snapshot, new);
     if (old) {
         call_rcu(old, qsp_snapshot_destroy, rcu);
     }
 }