gecko-dev/dom/media/AsyncLogger.h

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9.1 KiB
C++

/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
/* Implementation of an asynchronous lock-free logging system. */
#ifndef mozilla_dom_AsyncLogger_h
#define mozilla_dom_AsyncLogger_h
#include <atomic>
#include <thread>
#include "mozilla/Logging.h"
#include "mozilla/Attributes.h"
#include "mozilla/MathAlgorithms.h"
#include "mozilla/Sprintf.h"
namespace mozilla {
namespace detail {
// This class implements a lock-free multiple producer single consumer queue of
// fixed size log messages, with the following characteristics:
// - Unbounded (uses a intrinsic linked list)
// - Allocates on Push. Push can be called on any thread.
// - Deallocates on Pop. Pop MUST always be called on the same thread for the
// life-time of the queue.
//
// In our scenario, the producer threads are real-time, they can't block. The
// consummer thread runs every now and then and empties the queue to a log
// file, on disk.
//
// Having fixed size messages and jemalloc is probably not the fastest, but
// allows having a simpler design, we count on the fact that jemalloc will get
// the memory from a thread-local source most of the time.
template <size_t MESSAGE_LENGTH>
class MPSCQueue {
public:
struct Message {
Message() { mNext.store(nullptr, std::memory_order_relaxed); }
Message(const Message& aMessage) = delete;
void operator=(const Message& aMessage) = delete;
char data[MESSAGE_LENGTH];
std::atomic<Message*> mNext;
};
// Creates a new MPSCQueue. Initially, the queue has a single sentinel node,
// pointed to by both mHead and mTail.
MPSCQueue()
// At construction, the initial message points to nullptr (it has no
// successor). It is a sentinel node, that does not contain meaningful
// data.
: mHead(new Message()), mTail(mHead.load(std::memory_order_relaxed)) {}
~MPSCQueue() {
Message dummy;
while (this->Pop(dummy.data)) {
}
Message* front = mHead.load(std::memory_order_relaxed);
delete front;
}
void Push(MPSCQueue<MESSAGE_LENGTH>::Message* aMessage) {
// The next two non-commented line are called A and B in this paragraph.
// Producer threads i, i-1, etc. are numbered in the order they reached
// A in time, thread i being the thread that has reached A first.
// Atomically, on line A the new `mHead` is set to be the node that was
// just allocated, with strong memory order. From now one, any thread
// that reaches A will see that the node just allocated is
// effectively the head of the list, and will make itself the new head
// of the list.
// In a bad case (when thread i executes A and then
// is not scheduled for a long time), it is possible that thread i-1 and
// subsequent threads create a seemingly disconnected set of nodes, but
// they all have the correct value for the next node to set as their
// mNext member on their respective stacks (in `prev`), and this is
// always correct. When the scheduler resumes, and line B is executed,
// the correct linkage is resumed.
// Before line B, since mNext for the node the was the last element of
// the queue still has an mNext of nullptr, Pop will not see the node
// added.
// For line A, it's critical to have strong ordering both ways (since
// it's going to possibly be read and write repeatidly by multiple
// threads)
// Line B can have weaker guarantees, it's only going to be written by a
// single thread, and we just need to ensure it's read properly by a
// single other one.
Message* prev = mHead.exchange(aMessage, std::memory_order_acq_rel);
prev->mNext.store(aMessage, std::memory_order_release);
}
// Allocates a new node, copy aInput to the new memory location, and pushes
// it to the end of the list.
void Push(const char aInput[MESSAGE_LENGTH]) {
// Create a new message, and copy the messages passed on argument to the
// new memory location. We are not touching the queue right now. The
// successor for this new node is set to be nullptr.
Message* msg = new Message();
strncpy(msg->data, aInput, MESSAGE_LENGTH);
Push(msg);
}
// Copy the content of the first message of the queue to aOutput, and
// frees the message. Returns true if there was a message, in which case
// `aOutput` contains a valid value. If the queue was empty, returns false,
// in which case `aOutput` is left untouched.
bool Pop(char aOutput[MESSAGE_LENGTH]) {
// Similarly, in this paragraph, the two following lines are called A
// and B, and threads are called thread i, i-1, etc. in order of
// execution of line A.
// On line A, the first element of the queue is acquired. It is simply a
// sentinel node.
// On line B, we acquire the node that has the data we want. If B is
// null, then only the sentinel node was present in the queue, we can
// safely return false.
// mTail can be loaded with relaxed ordering, since it's not written nor
// read by any other thread (this queue is single consumer).
// mNext can be written to by one of the producer, so it's necessary to
// ensure those writes are seen, hence the stricter ordering.
Message* tail = mTail.load(std::memory_order_relaxed);
Message* next = tail->mNext.load(std::memory_order_acquire);
if (next == nullptr) {
return false;
}
strncpy(aOutput, next->data, MESSAGE_LENGTH);
// Simply shift the queue one node further, so that the sentinel node is
// now pointing to the correct most ancient node. It contains stale data,
// but this data will never be read again.
// It's only necessary to ensure the previous load on this thread is not
// reordered past this line, so release ordering is sufficient here.
mTail.store(next, std::memory_order_release);
// This thread is now the only thing that points to `tail`, it can be
// safely deleted.
delete tail;
return true;
}
private:
// An atomic pointer to the most recent message in the queue.
std::atomic<Message*> mHead;
// An atomic pointer to a sentinel node, that points to the oldest message
// in the queue.
std::atomic<Message*> mTail;
MPSCQueue(const MPSCQueue&) = delete;
void operator=(const MPSCQueue&) = delete;
public:
// The goal here is to make it easy on the allocator. We pack a pointer in the
// message struct, and we still want to do power of two allocations to
// minimize allocator slop. The allocation size are going to be constant, so
// the allocation is probably going to hit the thread local cache in jemalloc,
// making it cheap and, more importantly, lock-free enough.
static const size_t MESSAGE_PADDING = sizeof(Message::mNext);
private:
static_assert(IsPowerOfTwo(MESSAGE_LENGTH + MESSAGE_PADDING),
"MPSCQueue internal allocations must have a size that is a"
"power of two ");
};
} // end namespace detail
// This class implements a lock-free asynchronous logger, that outputs to
// MOZ_LOG.
// Any thread can use this logger without external synchronization and without
// being blocked. This log is suitable for use in real-time audio threads.
// Log formatting is best done externally, this class implements the output
// mechanism only.
// This class uses a thread internally, and must be started and stopped
// manually.
// If logging is disabled, all the calls are no-op.
class AsyncLogger {
public:
static const uint32_t MAX_MESSAGE_LENGTH =
512 - detail::MPSCQueue<sizeof(void*)>::MESSAGE_PADDING;
// aLogModuleName is the name of the MOZ_LOG module.
explicit AsyncLogger(const char* aLogModuleName)
: mThread(nullptr), mLogModule(aLogModuleName), mRunning(false) {}
~AsyncLogger() {
if (Enabled()) {
Stop();
}
}
void Start() {
MOZ_ASSERT(!mRunning, "Double calls to AsyncLogger::Start");
if (Enabled()) {
mRunning = true;
Run();
}
}
void Stop() {
if (Enabled()) {
if (mRunning) {
mRunning = false;
mThread->join();
}
} else {
MOZ_ASSERT(!mRunning && !mThread);
}
}
void Log(const char* format, ...) MOZ_FORMAT_PRINTF(2, 3) {
if (Enabled()) {
auto* msg = new detail::MPSCQueue<MAX_MESSAGE_LENGTH>::Message();
va_list args;
va_start(args, format);
VsprintfLiteral(msg->data, format, args);
va_end(args);
mMessageQueue.Push(msg);
}
}
bool Enabled() {
return MOZ_LOG_TEST(mLogModule, mozilla::LogLevel::Verbose);
}
private:
void Run() {
MOZ_ASSERT(Enabled());
mThread.reset(new std::thread([this]() {
while (mRunning) {
char message[MAX_MESSAGE_LENGTH];
while (mMessageQueue.Pop(message) && mRunning) {
MOZ_LOG(mLogModule, mozilla::LogLevel::Verbose, ("%s", message));
}
Sleep();
}
}));
}
void Sleep() { std::this_thread::sleep_for(std::chrono::milliseconds(10)); }
std::unique_ptr<std::thread> mThread;
mozilla::LazyLogModule mLogModule;
detail::MPSCQueue<MAX_MESSAGE_LENGTH> mMessageQueue;
std::atomic<bool> mRunning;
};
} // end namespace mozilla
#endif // mozilla_dom_AsyncLogger_h