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// Copyright 2019 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#ifndef ABSL_STRINGS_CORDZ_INFO_H_
#define ABSL_STRINGS_CORDZ_INFO_H_
#include <atomic>
#include <cstdint>
#include <functional>
#include "absl/base/config.h"
#include "absl/base/thread_annotations.h"
#include "absl/strings/internal/cord_internal.h"
#include "absl/strings/internal/cordz_handle.h"
#include "absl/strings/internal/cordz_statistics.h"
#include "absl/strings/internal/cordz_update_tracker.h"
#include "absl/synchronization/mutex.h"
#include "absl/types/span.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace cord_internal {
// CordzInfo tracks a profiled Cord. Each of these objects can be in two places.
// If a Cord is alive, the CordzInfo will be in the global_cordz_infos map, and
// can also be retrieved via the linked list starting with
// global_cordz_infos_head and continued via the cordz_info_next() method. When
// a Cord has reached the end of its lifespan, the CordzInfo object will be
// migrated out of the global_cordz_infos list and the global_cordz_infos_map,
// and will either be deleted or appended to the global_delete_queue. If it is
// placed on the global_delete_queue, the CordzInfo object will be cleaned in
// the destructor of a CordzSampleToken object.
class CordzInfo : public CordzHandle {
public:
using MethodIdentifier = CordzUpdateTracker::MethodIdentifier;
// All profiled Cords should be accompanied by a call to TrackCord.
// TrackCord creates a CordzInfo instance which tracks important metrics of
// the sampled cord. CordzInfo instances are placed in a global list which is
// used to discover and snapshot all actively tracked cords.
// Callers are responsible for calling UntrackCord() before the tracked Cord
// instance is deleted, or to stop tracking the sampled Cord.
// Callers are also responsible for guarding changes to `rep` through the
// Lock() and Unlock() calls, and calling SetCordRep() if the root of the
// sampled cord changes before the old root has been unreffed and/or deleted.
// `method` identifies the Cord method which initiated the cord to be sampled.
static CordzInfo* TrackCord(
CordRep* rep, MethodIdentifier method = MethodIdentifier::kUnknown);
// Stops tracking changes for a sampled cord, and deletes the provided info.
// This function must be called before the sampled cord instance is deleted,
// and before the root cordrep of the sampled cord is unreffed.
// This function may extend the lifetime of the cordrep in cases where the
// CordInfo instance is being held by a concurrent collection thread.
static void UntrackCord(CordzInfo* cordz_info);
// Identical to TrackCord(), except that this function fills the
// `parent_stack` and `parent_method` properties of the returned CordzInfo
// instance from the provided `src` instance if `src` is not null.
// This function should be used for sampling 'copy constructed' cords.
static CordzInfo* TrackCord(
CordRep* rep, const CordzInfo* src,
MethodIdentifier method = MethodIdentifier::kUnknown);
CordzInfo() = delete;
CordzInfo(const CordzInfo&) = delete;
CordzInfo& operator=(const CordzInfo&) = delete;
// Retrieves the oldest existing CordzInfo.
static CordzInfo* Head(const CordzSnapshot& snapshot);
// Retrieves the next oldest existing CordzInfo older than 'this' instance.
CordzInfo* Next(const CordzSnapshot& snapshot) const;
// Locks this instance for the update identified by `method`.
// Increases the count for `method` in `update_tracker`.
void Lock(MethodIdentifier method) ABSL_EXCLUSIVE_LOCK_FUNCTION(mutex_);
// Unlocks this instance. If the contained `rep` has been set to null
// indicating the Cord has been cleared or is otherwise no longer sampled,
// then this method will delete this CordzInfo instance.
void Unlock() ABSL_UNLOCK_FUNCTION(mutex_);
// Updates the `rep' property of this instance. This methods is invoked by
// Cord logic each time the root node of a sampled Cord changes, and before
// the old root reference count is deleted. This guarantees that collection
// code can always safely take a reference on the tracked cord.
// Requires a lock to be held through the `Lock()` method.
// TODO(b/117940323): annotate with ABSL_EXCLUSIVE_LOCKS_REQUIRED once all
// Cord code is in a state where this can be proven true by the compiler.
void SetCordRep(CordRep* rep);
// Returns the current value of `rep_` for testing purposes only.
CordRep* GetCordRepForTesting() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
return rep_;
}
// Returns the stack trace for where the cord was first sampled. Cords are
// potentially sampled when they promote from an inlined cord to a tree or
// ring representation, which is not necessarily the location where the cord
// was first created. Some cords are created as inlined cords, and only as
// data is added do they become a non-inlined cord. However, typically the
// location represents reasonably well where the cord is 'created'.
absl::Span<void* const> GetStack() const;
// Returns the stack trace for a sampled cord's 'parent stack trace'. This
// value may be set if the cord is sampled (promoted) after being created
// from, or being assigned the value of an existing (sampled) cord.
absl::Span<void* const> GetParentStack() const;
// Retrieves the CordzStatistics associated with this Cord. The statistics
// are only updated when a Cord goes through a mutation, such as an Append
// or RemovePrefix.
CordzStatistics GetCordzStatistics() const;
// Records size metric for this CordzInfo instance.
void RecordMetrics(int64_t size) {
size_.store(size, std::memory_order_relaxed);
}
private:
static constexpr int kMaxStackDepth = 64;
explicit CordzInfo(CordRep* rep, const CordzInfo* src,
MethodIdentifier method);
~CordzInfo() override;
// Returns the parent method from `src`, which is either `parent_method_` or
// `method_` depending on `parent_method_` being kUnknown.
// Returns kUnknown if `src` is null.
static MethodIdentifier GetParentMethod(const CordzInfo* src);
// Fills the provided stack from `src`, copying either `parent_stack_` or
// `stack_` depending on `parent_stack_` being empty, returning the size of
// the parent stack.
// Returns 0 if `src` is null.
static int FillParentStack(const CordzInfo* src, void** stack);
void Track();
void Untrack();
// 'Unsafe' head/next/prev accessors not requiring the lock being held.
// These are used exclusively for iterations (Head / Next) where we enforce
// a token being held, so reading an 'old' / deleted pointer is fine.
static CordzInfo* ci_head_unsafe() ABSL_NO_THREAD_SAFETY_ANALYSIS {
return ci_head_.load(std::memory_order_acquire);
}
CordzInfo* ci_next_unsafe() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
return ci_next_.load(std::memory_order_acquire);
}
CordzInfo* ci_prev_unsafe() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
return ci_prev_.load(std::memory_order_acquire);
}
static absl::Mutex ci_mutex_;
static std::atomic<CordzInfo*> ci_head_ ABSL_GUARDED_BY(ci_mutex_);
std::atomic<CordzInfo*> ci_prev_ ABSL_GUARDED_BY(ci_mutex_){nullptr};
std::atomic<CordzInfo*> ci_next_ ABSL_GUARDED_BY(ci_mutex_){nullptr};
mutable absl::Mutex mutex_;
CordRep* rep_ ABSL_GUARDED_BY(mutex_);
void* stack_[kMaxStackDepth];
void* parent_stack_[kMaxStackDepth];
const int stack_depth_;
const int parent_stack_depth_;
const MethodIdentifier method_;
const MethodIdentifier parent_method_;
CordzUpdateTracker update_tracker_;
const absl::Time create_time_;
// Last recorded size for the cord.
std::atomic<int64_t> size_{0};
};
} // namespace cord_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_STRINGS_CORDZ_INFO_H_
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