// 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 key, absl::optional 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 key, absl::optional 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 key, absl::optional 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