src/hotspot/share/gc/shared/workerThread.cpp

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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#include "gc/shared/gc_globals.hpp"
#include "gc/shared/workerThread.hpp"
#include "logging/log.hpp"
#include "memory/iterator.hpp"
#include "runtime/atomic.hpp"
#include "runtime/init.hpp"
#include "runtime/java.hpp"
#include "runtime/os.hpp"
#include "runtime/safepoint.hpp"

WorkerTaskDispatcher::WorkerTaskDispatcher() :
    _task(nullptr),
    _started(0),
    _not_finished(0),
    _start_semaphore(),
    _end_semaphore() {}

void WorkerTaskDispatcher::coordinator_distribute_task(WorkerTask* task, uint num_workers) {
  // No workers are allowed to read the state variables until they have been signaled.
  _task = task;
  _not_finished = num_workers;

  // Dispatch 'num_workers' number of tasks.
  _start_semaphore.signal(num_workers);

  // Wait for the last worker to signal the coordinator.
  _end_semaphore.wait();

  // No workers are allowed to read the state variables after the coordinator has been signaled.
  assert(_not_finished == 0, "%d not finished workers?", _not_finished);
  _task = nullptr;
  _started = 0;
}

void WorkerTaskDispatcher::worker_run_task() {
  // Wait for the coordinator to dispatch a task.
  _start_semaphore.wait();

  // Get and set worker id.
  const uint worker_id = Atomic::fetch_then_add(&_started, 1u);
  WorkerThread::set_worker_id(worker_id);

  // Run task.
  GCIdMark gc_id_mark(_task->gc_id());
  _task->work(worker_id);

  // Mark that the worker is done with the task.
  // The worker is not allowed to read the state variables after this line.
  const uint not_finished = Atomic::sub(&_not_finished, 1u);

  // The last worker signals to the coordinator that all work is completed.
  if (not_finished == 0) {
    _end_semaphore.signal();
  }
}

WorkerThreads::WorkerThreads(const char* name, uint max_workers) :
    _name(name),
    _workers(NEW_C_HEAP_ARRAY(WorkerThread*, max_workers, mtInternal)),
    _max_workers(max_workers),
    _created_workers(0),
    _active_workers(0),
    _dispatcher() {}

void WorkerThreads::initialize_workers() {
  const uint initial_active_workers = UseDynamicNumberOfGCThreads ? 1 : _max_workers;
  if (set_active_workers(initial_active_workers) != initial_active_workers) {
    vm_exit_during_initialization();
  }
}

WorkerThread* WorkerThreads::create_worker(uint name_suffix) {
  if (is_init_completed() && InjectGCWorkerCreationFailure) {
    return nullptr;
  }

  WorkerThread* const worker = new WorkerThread(_name, name_suffix, &_dispatcher);

  if (!os::create_thread(worker, os::gc_thread)) {
    delete worker;
    return nullptr;
  }

  on_create_worker(worker);

  os::start_thread(worker);

  return worker;
}

uint WorkerThreads::set_active_workers(uint num_workers) {
  assert(num_workers > 0 && num_workers <= _max_workers,
         "Invalid number of active workers %u (should be 1-%u)",
         num_workers, _max_workers);

  while (_created_workers < num_workers) {
    WorkerThread* const worker = create_worker(_created_workers);
    if (worker == nullptr) {
      log_error(gc, task)("Failed to create worker thread");
      break;
    }

    _workers[_created_workers] = worker;
    _created_workers++;
  }

  _active_workers = MIN2(_created_workers, num_workers);

  log_trace(gc, task)("%s: using %d out of %d workers", _name, _active_workers, _max_workers);

  return _active_workers;
}

void WorkerThreads::threads_do(ThreadClosure* tc) const {
  for (uint i = 0; i < _created_workers; i++) {
    tc->do_thread(_workers[i]);
  }
}

template <typename Function>
void WorkerThreads::threads_do_f(Function function) const {
  for (uint i = 0; i < _created_workers; i++) {
    function(_workers[i]);
  }
}

void WorkerThreads::set_indirect_states() {
#ifdef ASSERT
  const bool is_suspendible = Thread::current()->is_suspendible_thread();
  const bool is_safepointed = Thread::current()->is_VM_thread() && SafepointSynchronize::is_at_safepoint();

  threads_do_f([&](Thread* thread) {
    assert(!thread->is_indirectly_suspendible_thread(), "Unexpected");
    assert(!thread->is_indirectly_safepoint_thread(), "Unexpected");
    if (is_suspendible) {
      thread->set_indirectly_suspendible_thread();
    }
    if (is_safepointed) {
      thread->set_indirectly_safepoint_thread();
    }
  });
#endif
}

void WorkerThreads::clear_indirect_states() {
#ifdef ASSERT
  threads_do_f([&](Thread* thread) {
    thread->clear_indirectly_suspendible_thread();
    thread->clear_indirectly_safepoint_thread();
  });
#endif
}

void WorkerThreads::run_task(WorkerTask* task) {
  set_indirect_states();
  _dispatcher.coordinator_distribute_task(task, _active_workers);
  clear_indirect_states();
}

void WorkerThreads::run_task(WorkerTask* task, uint num_workers) {
  WithActiveWorkers with_active_workers(this, num_workers);
  run_task(task);
}

THREAD_LOCAL uint WorkerThread::_worker_id = UINT_MAX;

WorkerThread::WorkerThread(const char* name_prefix, uint name_suffix, WorkerTaskDispatcher* dispatcher) :
    _dispatcher(dispatcher) {
  set_name("%s#%u", name_prefix, name_suffix);
}

void WorkerThread::run() {
  os::set_priority(this, NearMaxPriority);

  while (true) {
    _dispatcher->worker_run_task();
  }
}