123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215 |
- // Copyright 2019 The Chromium Authors. All rights reserved.
- // Use of this source code is governed by a BSD-style license that can be
- // found in the LICENSE file.
- #include "base/profiler/metadata_recorder.h"
- #include "base/metrics/histogram_macros.h"
- #include "third_party/abseil-cpp/absl/types/optional.h"
- namespace base {
- const size_t MetadataRecorder::MAX_METADATA_COUNT;
- MetadataRecorder::Item::Item(uint64_t name_hash,
- absl::optional<int64_t> key,
- absl::optional<PlatformThreadId> thread_id,
- int64_t value)
- : name_hash(name_hash), key(key), thread_id(thread_id), value(value) {}
- MetadataRecorder::Item::Item() : name_hash(0), value(0) {}
- MetadataRecorder::Item::Item(const Item& other) = default;
- MetadataRecorder::Item& MetadataRecorder::Item::Item::operator=(
- const Item& other) = default;
- MetadataRecorder::ItemInternal::ItemInternal() = default;
- MetadataRecorder::ItemInternal::~ItemInternal() = default;
- MetadataRecorder::MetadataRecorder() {
- // Ensure that we have necessary atomic support.
- DCHECK(items_[0].is_active.is_lock_free());
- DCHECK(items_[0].value.is_lock_free());
- }
- MetadataRecorder::~MetadataRecorder() = default;
- void MetadataRecorder::Set(uint64_t name_hash,
- absl::optional<int64_t> key,
- absl::optional<PlatformThreadId> thread_id,
- int64_t value) {
- AutoLock lock(write_lock_);
- // Acquiring the |write_lock_| ensures that:
- //
- // - We don't try to write into the same new slot at the same time as
- // another thread
- // - We see all writes by other threads (acquiring a mutex implies acquire
- // semantics)
- size_t item_slots_used = item_slots_used_.load(std::memory_order_relaxed);
- for (size_t i = 0; i < item_slots_used; ++i) {
- auto& item = items_[i];
- if (item.name_hash == name_hash && item.key == key &&
- item.thread_id == thread_id) {
- item.value.store(value, std::memory_order_relaxed);
- const bool was_active =
- item.is_active.exchange(true, std::memory_order_release);
- if (!was_active)
- inactive_item_count_--;
- return;
- }
- }
- item_slots_used = TryReclaimInactiveSlots(item_slots_used);
- if (item_slots_used == items_.size()) {
- // The metadata recorder is full, forcing us to drop this metadata. The
- // above UMA histogram counting occupied metadata slots should help us set a
- // max size that avoids this condition during normal Chrome use.
- return;
- }
- // Wait until the item is fully created before setting |is_active| to true and
- // incrementing |item_slots_used_|, which will signal to readers that the item
- // is ready.
- auto& item = items_[item_slots_used];
- item.name_hash = name_hash;
- item.key = key;
- item.thread_id = thread_id;
- item.value.store(value, std::memory_order_relaxed);
- item.is_active.store(true, std::memory_order_release);
- item_slots_used_.fetch_add(1, std::memory_order_release);
- }
- void MetadataRecorder::Remove(uint64_t name_hash,
- absl::optional<int64_t> key,
- absl::optional<PlatformThreadId> thread_id) {
- AutoLock lock(write_lock_);
- size_t item_slots_used = item_slots_used_.load(std::memory_order_relaxed);
- for (size_t i = 0; i < item_slots_used; ++i) {
- auto& item = items_[i];
- if (item.name_hash == name_hash && item.key == key &&
- item.thread_id == thread_id) {
- // A removed item will occupy its slot until that slot is reclaimed.
- const bool was_active =
- item.is_active.exchange(false, std::memory_order_relaxed);
- if (was_active)
- inactive_item_count_++;
- return;
- }
- }
- }
- MetadataRecorder::MetadataProvider::MetadataProvider(
- MetadataRecorder* metadata_recorder,
- PlatformThreadId thread_id)
- : metadata_recorder_(metadata_recorder),
- thread_id_(thread_id),
- auto_lock_(metadata_recorder->read_lock_) {}
- MetadataRecorder::MetadataProvider::~MetadataProvider() = default;
- size_t MetadataRecorder::MetadataProvider::GetItems(
- ItemArray* const items) const {
- return metadata_recorder_->GetItems(items, thread_id_);
- }
- size_t MetadataRecorder::GetItems(ItemArray* const items,
- PlatformThreadId thread_id) const {
- // If a writer adds a new item after this load, it will be ignored. We do
- // this instead of calling item_slots_used_.load() explicitly in the for loop
- // bounds checking, which would be expensive.
- //
- // Also note that items are snapshotted sequentially and that items can be
- // modified mid-snapshot by non-suspended threads. This means that there's a
- // small chance that some items, especially those that occur later in the
- // array, may have values slightly "in the future" from when the sample was
- // actually collected. It also means that the array as returned may have never
- // existed in its entirety, although each name/value pair represents a
- // consistent item that existed very shortly after the thread was supended.
- size_t item_slots_used = item_slots_used_.load(std::memory_order_acquire);
- size_t write_index = 0;
- for (size_t read_index = 0; read_index < item_slots_used; ++read_index) {
- const auto& item = items_[read_index];
- // Because we wait until |is_active| is set to consider an item active and
- // that field is always set last, we ignore half-created items.
- if (item.is_active.load(std::memory_order_acquire) &&
- (!item.thread_id.has_value() || item.thread_id == thread_id)) {
- (*items)[write_index++] =
- Item{item.name_hash, item.key, item.thread_id,
- item.value.load(std::memory_order_relaxed)};
- }
- }
- return write_index;
- }
- size_t MetadataRecorder::TryReclaimInactiveSlots(size_t item_slots_used) {
- const size_t remaining_slots = MAX_METADATA_COUNT - item_slots_used;
- if (inactive_item_count_ == 0 || inactive_item_count_ < remaining_slots) {
- // This reclaiming threshold has a few nice properties:
- //
- // - It avoids reclaiming when no items have been removed
- // - It makes doing so more likely as free slots become more scarce
- // - It makes doing so less likely when the benefits are lower
- return item_slots_used;
- }
- if (read_lock_.Try()) {
- // The lock isn't already held by a reader or another thread reclaiming
- // slots.
- item_slots_used = ReclaimInactiveSlots(item_slots_used);
- read_lock_.Release();
- }
- return item_slots_used;
- }
- size_t MetadataRecorder::ReclaimInactiveSlots(size_t item_slots_used) {
- // From here until the end of the reclamation, we can safely use
- // memory_order_relaxed for all reads and writes. We don't need
- // memory_order_acquire because acquiring the write mutex gives acquire
- // semantics and no other threads can write after we hold that mutex. We don't
- // need memory_order_release because no readers can read until we release the
- // read mutex, which itself has release semantics.
- size_t first_inactive_item_idx = 0;
- size_t last_active_item_idx = item_slots_used - 1;
- while (first_inactive_item_idx < last_active_item_idx) {
- ItemInternal& inactive_item = items_[first_inactive_item_idx];
- ItemInternal& active_item = items_[last_active_item_idx];
- if (inactive_item.is_active.load(std::memory_order_relaxed)) {
- // Keep seeking forward to an inactive item.
- ++first_inactive_item_idx;
- continue;
- }
- if (!active_item.is_active.load(std::memory_order_relaxed)) {
- // Keep seeking backward to an active item. Skipping over this item
- // indicates that we're freeing the slot at this index.
- --last_active_item_idx;
- item_slots_used--;
- continue;
- }
- inactive_item.name_hash = active_item.name_hash;
- inactive_item.value.store(active_item.value.load(std::memory_order_relaxed),
- std::memory_order_relaxed);
- inactive_item.is_active.store(true, std::memory_order_relaxed);
- ++first_inactive_item_idx;
- --last_active_item_idx;
- item_slots_used--;
- }
- item_slots_used_.store(item_slots_used, std::memory_order_relaxed);
- return item_slots_used;
- }
- } // namespace base
|