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// Copyright 2018 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef BASE_SAMPLING_HEAP_PROFILER_POISSON_ALLOCATION_SAMPLER_H_
#define BASE_SAMPLING_HEAP_PROFILER_POISSON_ALLOCATION_SAMPLER_H_
#include <atomic>
#include <optional>
#include <vector>
#include "base/allocator/dispatcher/notification_data.h"
#include "base/allocator/dispatcher/reentry_guard.h"
#include "base/allocator/dispatcher/subsystem.h"
#include "base/base_export.h"
#include "base/compiler_specific.h"
#include "base/gtest_prod_util.h"
#include "base/memory/raw_ptr_exclusion.h"
#include "base/no_destructor.h"
#include "base/sampling_heap_profiler/lock_free_address_hash_set.h"
#include "base/synchronization/lock.h"
#include "base/thread_annotations.h"
namespace base {
class SamplingHeapProfilerTest;
// Stats about the allocation sampler.
struct BASE_EXPORT PoissonAllocationSamplerStats {
using AddressCacheBucketStats = LockFreeAddressHashSet::BucketStats;
PoissonAllocationSamplerStats(
size_t address_cache_hits,
size_t address_cache_misses,
size_t address_cache_max_size,
float address_cache_max_load_factor,
AddressCacheBucketStats address_cache_bucket_stats);
~PoissonAllocationSamplerStats();
PoissonAllocationSamplerStats(const PoissonAllocationSamplerStats&);
PoissonAllocationSamplerStats& operator=(
const PoissonAllocationSamplerStats&);
size_t address_cache_hits;
size_t address_cache_misses;
size_t address_cache_max_size;
float address_cache_max_load_factor;
AddressCacheBucketStats address_cache_bucket_stats;
};
// This singleton class implements Poisson sampling of the incoming allocations
// stream. It hooks onto base::allocator and base::PartitionAlloc.
// The only control parameter is sampling interval that controls average value
// of the sampling intervals. The actual intervals between samples are
// randomized using Poisson distribution to mitigate patterns in the allocation
// stream.
// Once accumulated allocation sizes fill up the current sample interval,
// a sample is generated and sent to the observers via |SampleAdded| call.
// When the corresponding memory that triggered the sample is freed observers
// get notified with |SampleRemoved| call.
//
class BASE_EXPORT PoissonAllocationSampler {
public:
class SamplesObserver {
public:
virtual ~SamplesObserver() = default;
virtual void SampleAdded(
void* address,
size_t size,
size_t total,
base::allocator::dispatcher::AllocationSubsystem type,
const char* context) = 0;
virtual void SampleRemoved(void* address) = 0;
};
// An instance of this class makes the sampler not report samples generated
// within the object scope for the current thread.
// It allows observers to allocate/deallocate memory while holding a lock
// without a chance to get into reentrancy problems.
class BASE_EXPORT ScopedMuteThreadSamples {
public:
ScopedMuteThreadSamples();
~ScopedMuteThreadSamples();
ScopedMuteThreadSamples(const ScopedMuteThreadSamples&) = delete;
ScopedMuteThreadSamples& operator=(const ScopedMuteThreadSamples&) = delete;
static bool IsMuted();
private:
bool was_muted_ = false;
};
// An instance of this class makes the sampler behave deterministically to
// ensure test results are repeatable. Does not support nesting.
class BASE_EXPORT ScopedSuppressRandomnessForTesting {
public:
ScopedSuppressRandomnessForTesting();
~ScopedSuppressRandomnessForTesting();
ScopedSuppressRandomnessForTesting(
const ScopedSuppressRandomnessForTesting&) = delete;
ScopedSuppressRandomnessForTesting& operator=(
const ScopedSuppressRandomnessForTesting&) = delete;
static bool IsSuppressed();
};
// An instance of this class makes the sampler only report samples with
// AllocatorType kManualForTesting, not those from hooked allocators. This
// allows unit tests to set test expectations based on only explicit calls to
// RecordAlloc and RecordFree.
//
// The accumulated bytes on the thread that creates a
// ScopedMuteHookedSamplesForTesting will also be reset to 0, and restored
// when the object leaves scope. This gives tests a known state to start
// recording samples on one thread: a full interval must pass to record a
// sample. Other threads will still have a random number of accumulated bytes.
//
// Only one instance may exist at a time.
class BASE_EXPORT ScopedMuteHookedSamplesForTesting {
public:
ScopedMuteHookedSamplesForTesting();
~ScopedMuteHookedSamplesForTesting();
// Move-only.
ScopedMuteHookedSamplesForTesting(
const ScopedMuteHookedSamplesForTesting&) = delete;
ScopedMuteHookedSamplesForTesting& operator=(
const ScopedMuteHookedSamplesForTesting&) = delete;
ScopedMuteHookedSamplesForTesting(ScopedMuteHookedSamplesForTesting&&);
ScopedMuteHookedSamplesForTesting& operator=(
ScopedMuteHookedSamplesForTesting&&);
private:
intptr_t accumulated_bytes_snapshot_;
};
// Must be called early during the process initialization. It creates and
// reserves a TLS slot.
static void Init();
void AddSamplesObserver(SamplesObserver*);
// Note: After an observer is removed it is still possible to receive
// a notification to that observer. This is not a problem currently as
// the only client of this interface is the base::SamplingHeapProfiler,
// which is a singleton.
// If there's a need for this functionality in the future, one might
// want to put observers notification loop under a reader-writer lock.
void RemoveSamplesObserver(SamplesObserver*);
// Sets the mean number of bytes that will be allocated before taking a
// sample.
void SetSamplingInterval(size_t sampling_interval_bytes);
// Returns the current mean sampling interval, in bytes.
size_t SamplingInterval() const;
// Sets the max load factor before rebalancing the LockFreeAddressHashSet, or
// resets it to the default if `load_factor` is nulloptr.
void SetTargetHashSetLoadFactor(std::optional<float> load_factor);
// Returns statistics about the allocation sampler, and resets the running
// counts so that each call to this returns only stats about the period
// between calls.
PoissonAllocationSamplerStats GetAndResetStats();
ALWAYS_INLINE void OnAllocation(
const base::allocator::dispatcher::AllocationNotificationData&
allocation_data);
ALWAYS_INLINE void OnFree(
const base::allocator::dispatcher::FreeNotificationData& free_data);
static PoissonAllocationSampler* Get();
PoissonAllocationSampler(const PoissonAllocationSampler&) = delete;
PoissonAllocationSampler& operator=(const PoissonAllocationSampler&) = delete;
// Returns true if a ScopedMuteHookedSamplesForTesting exists. This can be
// read from any thread.
static bool AreHookedSamplesMuted() {
return profiling_state_.load(std::memory_order_relaxed) &
ProfilingStateFlag::kHookedSamplesMutedForTesting;
}
// Returns the number of allocated bytes that have been observed.
static intptr_t GetAccumulatedBytesForTesting();
private:
// Flags recording the state of the profiler. This does not use enum class so
// flags can be used in a bitmask.
enum ProfilingStateFlag {
// Set if profiling has ever been started in this session of Chrome. Once
// this is set, it is never reset. This is used to optimize the common case
// where profiling is never used.
kWasStarted = 1 << 0,
// Set if profiling is currently running. This flag is toggled on and off
// as sample observers are added and removed.
kIsRunning = 1 << 1,
// Set if a ScopedMuteHookedSamplesForTesting object exists.
kHookedSamplesMutedForTesting = 1 << 2,
};
using ProfilingStateFlagMask = int;
PoissonAllocationSampler();
~PoissonAllocationSampler() = delete;
static size_t GetNextSampleInterval(size_t base_interval);
// Return the set of sampled addresses. This is only valid to call after
// Init().
static LockFreeAddressHashSet& sampled_addresses_set();
// Atomically adds `flag` to `profiling_state_`. DCHECK's if it was already
// set. If `flag` is kIsRunning, also sets kWasStarted. Uses
// std::memory_order_relaxed semantics and therefore doesn't synchronize the
// state of any other memory with future readers. (See the comment in
// RecordFree() for why this is safe.)
static void SetProfilingStateFlag(ProfilingStateFlag flag);
// Atomically removes `flag` from `profiling_state_`. DCHECK's if it was not
// already set. Uses std::memory_order_relaxed semantics and therefore doesn't
// synchronize the state of any other memory with future readers. (See the
// comment in RecordFree() for why this is safe.)
static void ResetProfilingStateFlag(ProfilingStateFlag flag);
void DoRecordAllocation(const ProfilingStateFlagMask state,
void* address,
size_t size,
base::allocator::dispatcher::AllocationSubsystem type,
const char* context);
void DoRecordFree(void* address);
void BalanceAddressesHashSet() EXCLUSIVE_LOCKS_REQUIRED(mutex_);
Lock mutex_;
// The |observers_| list is guarded by |mutex_|, however a copy of it
// is made before invoking the observers (to avoid performing expensive
// operations under the lock) as such the SamplesObservers themselves need
// to be thread-safe and support being invoked racily after
// RemoveSamplesObserver().
//
// This class handles allocation, so it must never use raw_ptr<T>. In
// particular, raw_ptr<T> with `enable_backup_ref_ptr_instance_tracer`
// developer option allocates memory, which would cause reentrancy issues:
// allocating memory while allocating memory.
// More details in https://crbug.com/340815319
RAW_PTR_EXCLUSION std::vector<SamplesObserver*> observers_ GUARDED_BY(mutex_);
// Fast, thread-safe access to the current profiling state.
static std::atomic<ProfilingStateFlagMask> profiling_state_;
// Running counts for PoissonAllocationSamplerStats. These are all atomic or
// mutex-guarded because they're updated from multiple threads. The atomics
// can always be accessed using std::memory_order_relaxed since each value is
// separately recorded in UMA and no other memory accesses depend on it. Some
// values are correlated (eg. `address_cache_hits_` and
// `address_cache_misses_`), and this might see a write to one but not the
// other, but this shouldn't cause enough errors in the aggregated UMA metrics
// to be worth adding overhead to avoid it.
std::atomic<size_t> address_cache_hits_;
std::atomic<size_t> address_cache_misses_;
size_t address_cache_max_size_ GUARDED_BY(mutex_) = 0;
// The max load factor that's observed in sampled_addresses_set().
float address_cache_max_load_factor_ GUARDED_BY(mutex_) = 0;
// The load factor that will trigger rebalancing in sampled_addresses_set().
// By definition `address_cache_max_load_factor_` will never exceed this.
float address_cache_target_load_factor_ GUARDED_BY(mutex_) = 1.0;
friend class NoDestructor<PoissonAllocationSampler>;
friend class PoissonAllocationSamplerStateTest;
friend class SamplingHeapProfilerTest;
FRIEND_TEST_ALL_PREFIXES(PoissonAllocationSamplerTest, MuteHooksWithoutInit);
FRIEND_TEST_ALL_PREFIXES(PoissonAllocationSamplerLoadFactorTest,
BalanceSampledAddressesSet);
FRIEND_TEST_ALL_PREFIXES(SamplingHeapProfilerTest, HookedAllocatorMuted);
};
ALWAYS_INLINE void PoissonAllocationSampler::OnAllocation(
const base::allocator::dispatcher::AllocationNotificationData&
allocation_data) {
// The allocation hooks may be installed before the sampler is started. Check
// if its ever been started first to avoid extra work on the fast path,
// because it's the most common case.
const ProfilingStateFlagMask state =
profiling_state_.load(std::memory_order_relaxed);
if (!(state & ProfilingStateFlag::kWasStarted)) [[likely]] {
return;
}
const auto type = allocation_data.allocation_subsystem();
// When sampling is muted for testing, only handle manual calls to
// RecordAlloc. (This doesn't need to be checked in RecordFree because muted
// allocations won't be added to sampled_addresses_set(), so RecordFree
// already skips them.)
if ((state & ProfilingStateFlag::kHookedSamplesMutedForTesting) &&
type !=
base::allocator::dispatcher::AllocationSubsystem::kManualForTesting)
[[unlikely]] {
return;
}
// Note: ReentryGuard prevents from recursions introduced by malloc and
// initialization of thread local storage which happen in the allocation path
// only (please see docs of ReentryGuard for full details).
allocator::dispatcher::ReentryGuard reentry_guard;
if (!reentry_guard) [[unlikely]] {
return;
}
DoRecordAllocation(state, allocation_data.address(), allocation_data.size(),
type, allocation_data.type_name());
}
ALWAYS_INLINE void PoissonAllocationSampler::OnFree(
const base::allocator::dispatcher::FreeNotificationData& free_data) {
// The allocation hooks may be installed before the sampler is started. Check
// if its ever been started first to avoid extra work on the fast path,
// because it's the most common case. Note that DoRecordFree still needs to be
// called if the sampler was started but is now stopped, to track allocations
// that were recorded while the sampler was still running.
//
// Relaxed ordering is safe here because there's only one case where
// RecordAlloc and RecordFree MUST see the same value of `profiling_state_`.
// Assume thread A updates `profiling_state_` from 0 to kWasStarted |
// kIsRunning, thread B calls RecordAlloc, and thread C calls RecordFree.
// (Something else could update `profiling_state_` to remove kIsRunning before
// RecordAlloc or RecordFree.)
//
// 1. If RecordAlloc(p) sees !kWasStarted or !kIsRunning it will return
// immediately, so p won't be in sampled_address_set(). So no matter what
// RecordFree(p) sees it will also return immediately.
//
// 2. If RecordFree() is called with a pointer that was never passed to
// RecordAlloc(), again it will return immediately no matter what it sees.
//
// 3. If RecordAlloc(p) sees kIsRunning it will put p in
// sampled_address_set(). In this case RecordFree(p) MUST see !kWasStarted
// or it will return without removing p:
//
// 3a. If the program got p as the return value from malloc() and passed it
// to free(), then RecordFree() happens-after RecordAlloc() and
// therefore will see the same value of `profiling_state_` as
// RecordAlloc() for all memory orders. (Proof: using the definitions
// of sequenced-after, happens-after and inter-thread happens-after
// from https://en.cppreference.com/w/cpp/atomic/memory_order, malloc()
// calls RecordAlloc() so its return is sequenced-after RecordAlloc();
// free() inter-thread happens-after malloc's return because it
// consumes the result; RecordFree() is sequenced-after its caller,
// free(); therefore RecordFree() interthread happens-after
// RecordAlloc().)
// 3b. If the program is freeing a random pointer which coincidentally was
// also returned from malloc(), such that free(p) does not happen-after
// malloc(), then there is already an unavoidable race condition. If
// the profiler sees malloc() before free(p), then it will add p to
// sampled_addresses_set() and then remove it; otherwise it will do
// nothing in RecordFree() and add p to sampled_addresses_set() in
// RecordAlloc(), recording a potential leak. Reading
// `profiling_state_` with relaxed ordering adds another possibility:
// if the profiler sees malloc() with kWasStarted and then free without
// kWasStarted, it will add p to sampled_addresses_set() in
// RecordAlloc() and then do nothing in RecordFree(). This has the same
// outcome as the existing race.
const ProfilingStateFlagMask state =
profiling_state_.load(std::memory_order_relaxed);
if (!(state & ProfilingStateFlag::kWasStarted)) [[likely]] {
return;
}
void* const address = free_data.address();
if (address == nullptr) [[unlikely]] {
return;
}
if (!sampled_addresses_set().Contains(address)) [[likely]] {
address_cache_misses_.fetch_add(1, std::memory_order_relaxed);
return;
}
address_cache_hits_.fetch_add(1, std::memory_order_relaxed);
if (ScopedMuteThreadSamples::IsMuted()) [[unlikely]] {
return;
}
// Note: ReentryGuard prevents from recursions introduced by malloc and
// initialization of thread local storage which happen in the allocation path
// only (please see docs of ReentryGuard for full details). Therefore, the
// DoNotifyFree doesn't need to be guarded.
DoRecordFree(address);
}
} // namespace base
#endif // BASE_SAMPLING_HEAP_PROFILER_POISSON_ALLOCATION_SAMPLER_H_
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