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// Copyright 2019 The Chromium Authors
// 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