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Diffstat (limited to 'util/qsp.c')
| -rw-r--r-- | util/qsp.c | 633 |
1 files changed, 633 insertions, 0 deletions
diff --git a/util/qsp.c b/util/qsp.c new file mode 100644 index 0000000000..c5fce4b442 --- /dev/null +++ b/util/qsp.c @@ -0,0 +1,633 @@ +/* + * 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. Focusing on call sites instead of just on objects might + * seem puzzling. However, it is a sensible choice since otherwise dealing with + * dynamically-allocated objects becomes difficult (e.g. what to do when an + * object is destroyed, or reused?). Furthermore, the call site info is of most + * importance, since it is callers, and not objects, what cause wait time. + * + * 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. + * + * 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 "exec/tb-hash-xx.h" + +enum QSPType { + QSP_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; +#ifndef CONFIG_ATOMIC64 + /* + * If we cannot update the counts atomically, then use a seqlock. + * We don't need an associated lock because the updates are thread-local. + */ + QemuSeqLock sequence; +#endif +}; +typedef struct QSPEntry QSPEntry; + +/* 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 bool qsp_initialized, qsp_initializing; + +static const char * const qsp_typenames[] = { + [QSP_MUTEX] = "mutex", + [QSP_REC_MUTEX] = "rec_mutex", + [QSP_CONDVAR] = "condvar", +}; + +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); +} + +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_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_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); +} + +/* + * @from is in the global hash table; read it atomically if the host + * supports it, otherwise use the seqlock. + */ +static void qsp_entry_aggregate(QSPEntry *to, const QSPEntry *from) +{ +#ifdef CONFIG_ATOMIC64 + to->ns += atomic_read__nocheck(&from->ns); + to->n_acqs += atomic_read__nocheck(&from->n_acqs); +#else + unsigned int version; + uint64_t ns, n_acqs; + + do { + version = seqlock_read_begin(&from->sequence); + ns = atomic_read__nocheck(&from->ns); + n_acqs = atomic_read__nocheck(&from->n_acqs); + } while (seqlock_read_retry(&from->sequence, version)); + + to->ns += ns; + to->n_acqs += n_acqs; +#endif +} + +/* + * @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. + * If the host doesn't support u64 atomics, use the seqlock. + */ +static inline void do_qsp_entry_record(QSPEntry *e, int64_t delta, bool acq) +{ +#ifndef CONFIG_ATOMIC64 + seqlock_write_begin(&e->sequence); +#endif + atomic_set__nocheck(&e->ns, e->ns + delta); + if (acq) { + atomic_set__nocheck(&e->n_acqs, e->n_acqs + 1); + } +#ifndef CONFIG_ATOMIC64 + seqlock_write_end(&e->sequence); +#endif +} + +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_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_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_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; + const QSPCallSite *ca; + const QSPCallSite *cb; + + if (a->ns > b->ns) { + return -1; + } else if (a->ns < b->ns) { + return 1; + } + 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(struct qht *ht, void *p, uint32_t h, void *userp) +{ + QSPEntry *e = p; + GTree *tree = userp; + + g_tree_insert(tree, e, NULL); +} + +static void qsp_aggregate(struct qht *global_ht, 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); + qsp_entry_aggregate(agg, e); +} + +static void qsp_mktree(GTree *tree) +{ + struct qht ht; + + /* 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); + + /* sort the hash table elements by using a tree */ + qht_iter(&ht, qsp_sort, tree); + + /* free the hash table, but keep the elements (those are in the tree now) */ + qht_destroy(&ht); +} + +/* 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; +}; +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->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]; + + pr(f, "%-9s %14p %s%*s %13.5f %12" PRIu64 " %12.2f\n", e->typename, + e->obj, e->callsite_at, callsite_len - (int)strlen(e->callsite_at), + "", e->time_s, e->n_acqs, e->ns_avg * 1e-3); + } + + 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) +{ + GTree *tree = g_tree_new_full(qsp_tree_cmp, NULL, 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); + g_tree_foreach(tree, qsp_tree_report, &rep); + g_tree_destroy(tree); + + pr_report(&rep, f, cpu_fprintf); + report_destroy(&rep); +} |