fence_waiter_vk.cc

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// Copyright 2013 The Flutter 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 "impeller/renderer/backend/vulkan/fence_waiter_vk.h"
 
#include <algorithm>
#include <chrono>
#include <utility>
 
#include "flutter/fml/cpu_affinity.h"
#include "flutter/fml/thread.h"
#include "flutter/fml/trace_event.h"
#include "impeller/base/validation.h"
 
namespace impeller {
 
class WaitSetEntry {
 public:
  static std::shared_ptr<WaitSetEntry> Create(vk::UniqueFence p_fence,
                                              const fml::closure& p_callback) {
    return std::shared_ptr<WaitSetEntry>(
        new WaitSetEntry(std::move(p_fence), p_callback));
  }
 
  void UpdateSignalledStatus(const vk::Device& device) {
    if (is_signalled_) {
      return;
    }
    is_signalled_ = device.getFenceStatus(fence_.get()) == vk::Result::eSuccess;
  }
 
  const vk::Fence& GetFence() const { return fence_.get(); }
 
  bool IsSignalled() const { return is_signalled_; }
 
 private:
  vk::UniqueFence fence_;
  fml::ScopedCleanupClosure callback_;
  bool is_signalled_ = false;
 
  WaitSetEntry(vk::UniqueFence p_fence, const fml::closure& p_callback)
      : fence_(std::move(p_fence)),
        callback_(fml::ScopedCleanupClosure{p_callback}) {}
 
  FML_DISALLOW_COPY_ASSIGN_AND_MOVE(WaitSetEntry);
};
 
FenceWaiterVK::FenceWaiterVK(std::weak_ptr<DeviceHolder> device_holder)
    : device_holder_(std::move(device_holder)) {
  waiter_thread_ = std::make_unique<std::thread>([&]() { Main(); });
}
 
FenceWaiterVK::~FenceWaiterVK() {
  Terminate();
  waiter_thread_->join();
}
 
bool FenceWaiterVK::AddFence(vk::UniqueFence fence,
                             const fml::closure& callback) {
  TRACE_EVENT0("flutter", "FenceWaiterVK::AddFence");
  if (!fence || !callback) {
    return false;
  }
  {
    // Maintain the invariant that terminate_ is accessed only under the lock.
    std::scoped_lock lock(wait_set_mutex_);
    if (terminate_) {
      return false;
    }
    wait_set_.emplace_back(WaitSetEntry::Create(std::move(fence), callback));
  }
  wait_set_cv_.notify_one();
  return true;
}
 
static std::vector<vk::Fence> GetFencesForWaitSet(const WaitSet& set) {
  std::vector<vk::Fence> fences;
  for (const auto& entry : set) {
    if (!entry->IsSignalled()) {
      fences.emplace_back(entry->GetFence());
    }
  }
  return fences;
}
 
void FenceWaiterVK::Main() {
  fml::Thread::SetCurrentThreadName(
      fml::Thread::ThreadConfig{"io.flutter.impeller.fence_waiter"});
  // Since this thread mostly waits on fences, it doesn't need to be fast.
  fml::RequestAffinity(fml::CpuAffinity::kEfficiency);
 
  while (true) {
    // We'll read the terminate_ flag within the lock below.
    bool terminate = false;
 
    {
      std::unique_lock lock(wait_set_mutex_);
 
      // If there are no fences to wait on, wait on the condition variable.
      wait_set_cv_.wait(lock,
                        [&]() { return !wait_set_.empty() || terminate_; });
 
      // Still under the lock, check if the waiter has been terminated.
      terminate = terminate_;
    }
 
    if (terminate) {
      WaitUntilEmpty();
      break;
    }
 
    if (!Wait()) {
      break;
    }
  }
}
 
void FenceWaiterVK::WaitUntilEmpty() {
  // Note, there is no lock because once terminate_ is set to true, no other
  // fence can be added to the wait set. Just in case, here's a FML_DCHECK:
  FML_DCHECK(terminate_) << "Fence waiter must be terminated.";
  while (!wait_set_.empty() && Wait()) {
    // Intentionally empty.
  }
}
 
bool FenceWaiterVK::Wait() {
  // Snapshot the wait set and wait on the fences.
  WaitSet wait_set;
  {
    std::scoped_lock lock(wait_set_mutex_);
    wait_set = wait_set_;
  }
 
  using namespace std::literals::chrono_literals;
 
  // Check if the context had died in the meantime.
  auto device_holder = device_holder_.lock();
  if (!device_holder) {
    return false;
  }
 
  const auto& device = device_holder->GetDevice();
  // Wait for one or more fences to be signaled. Any additional fences added
  // to the waiter will be serviced in the next pass. If a fence that is going
  // to be signaled at an abnormally long deadline is the only one in the set,
  // a timeout will bail out the wait.
  auto fences = GetFencesForWaitSet(wait_set);
  if (fences.empty()) {
    return true;
  }
 
  auto result = device.waitForFences(
      /*fenceCount=*/fences.size(),
      /*pFences=*/fences.data(),
      /*waitAll=*/false,
      /*timeout=*/std::chrono::nanoseconds{100ms}.count());
  if (!(result == vk::Result::eSuccess || result == vk::Result::eTimeout)) {
    VALIDATION_LOG << "Fence waiter encountered an unexpected error. Tearing "
                      "down the waiter thread.";
    return false;
  }
 
  // One or more fences have been signaled. Find out which ones and update
  // their signaled statuses.
  {
    TRACE_EVENT0("impeller", "CheckFenceStatus");
    for (auto& entry : wait_set) {
      entry->UpdateSignalledStatus(device);
    }
    wait_set.clear();
  }
 
  // Quickly acquire the wait set lock and erase signaled entries. Make sure
  // the mutex is unlocked before calling the destructors of the erased
  // entries. These might touch allocators.
  WaitSet erased_entries;
  {
    static constexpr auto is_signalled = [](const auto& entry) {
      return entry->IsSignalled();
    };
    std::scoped_lock lock(wait_set_mutex_);
 
    // TODO(matanlurey): Iterate the list 1x by copying is_signaled into erased.
    std::copy_if(wait_set_.begin(), wait_set_.end(),
                 std::back_inserter(erased_entries), is_signalled);
    wait_set_.erase(
        std::remove_if(wait_set_.begin(), wait_set_.end(), is_signalled),
        wait_set_.end());
  }
 
  {
    TRACE_EVENT0("impeller", "ClearSignaledFences");
    // Erase the erased entries which will invoke callbacks.
    erased_entries.clear();  // Bit redundant because of scope but hey.
  }
 
  return true;
}
 
void FenceWaiterVK::Terminate() {
  {
    std::scoped_lock lock(wait_set_mutex_);
    terminate_ = true;
  }
  wait_set_cv_.notify_one();
}
 
}  // namespace impeller