ruy/blocking_counter.h

/* Copyright 2019 Google LLC. All Rights Reserved.

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

    http://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 RUY_RUY_BLOCKING_COUNTER_H_
#define RUY_RUY_BLOCKING_COUNTER_H_

#include <atomic>
#include <condition_variable>  // NOLINT(build/c++11) // IWYU pragma: keep
#include <mutex>               // NOLINT(build/c++11) // IWYU pragma: keep

#include "ruy/time.h"

namespace ruy {

// A BlockingCounter lets one thread to wait for N events to occur.
// This is how the master thread waits for all the worker threads
// to have finished working.
// The waiting is done using a naive spinlock waiting for the atomic
// count_ to hit the value 0. This is acceptable because in our usage
// pattern, BlockingCounter is used only to synchronize threads after
// short-lived tasks (performing parts of the same GEMM). It is not used
// for synchronizing longer waits (resuming work on the next GEMM).
class BlockingCounter {
 public:
  BlockingCounter() : count_(0) {}

  // Sets/resets the counter; initial_count is the number of
  // decrementing events that the Wait() call will be waiting for.
  void Reset(int initial_count);

  // Decrements the counter; if the counter hits zero, signals
  // the threads that were waiting for that, and returns true.
  // Otherwise (if the decremented count is still nonzero),
  // returns false.
  bool DecrementCount();

  // Waits for the N other threads (N having been set by Reset())
  // to hit the BlockingCounter.
  //
  // Will first spin-wait for `spin_duration` before reverting to passive wait.
  void Wait(const Duration spin_duration);

 private:
  std::atomic<int> count_;

  // The condition variable and mutex allowing to passively wait for count_
  // to reach the value zero, in the case of longer waits.
  std::condition_variable count_cond_;
  std::mutex count_mutex_;
};

}  // namespace ruy

#endif  // RUY_RUY_BLOCKING_COUNTER_H_