scheduler.cpp

#include "pls/internal/scheduling/scheduler.h"

#include "context_switcher/context_switcher.h"

#include "pls/internal/build_flavour.h"
#include "pls/internal/base/futex_wrapper.h"
#include "pls/internal/base/error_handling.h"

#include <thread>

namespace pls::internal::scheduling {

scheduler::scheduler(unsigned int num_threads,
                     size_t computation_depth,
                     size_t stack_size,
                     bool reuse_thread) : scheduler(num_threads,
                                                    computation_depth,
                                                    stack_size,
                                                    reuse_thread,
                                                    base::mmap_stack_allocator{}) {}

scheduler::~scheduler() {
  terminate();
}
void scheduler::work_thread_main_loop() {
  auto &scheduler = thread_state::get().get_scheduler();
  while (true) {
    // Wait to be triggered
    scheduler.sync_barrier_.wait();

    // Check for shutdown
    if (scheduler.terminated_) {
      return;
    }

    scheduler.work_thread_work_section();

    // Sync back with main thread
    scheduler.sync_barrier_.wait();
  }
}

void scheduler::work_thread_work_section() {
  thread_state &my_state = thread_state::get();
  my_state.set_scheduler_active(true);
  unsigned const num_threads = my_state.get_scheduler().num_threads();

  if (my_state.get_thread_id() == 0) {
    // Main Thread, kick off by executing the user's main code block.
    main_thread_starter_function_->run();
  }

  unsigned int failed_steals = 0;
  while (!work_section_done_) {
#if PLS_PROFILING_ENABLED
    my_state.get_scheduler().profiler_.stealing_start(my_state.get_thread_id());
#endif
    PLS_ASSERT(check_task_chain(*my_state.get_active_task()), "Must start stealing with a clean task chain.");

    size_t target;
    do {
      target = my_state.get_rand() % num_threads;
    } while (target == my_state.get_thread_id());

    thread_state &target_state = my_state.get_scheduler().thread_state_for(target);
#if PLS_SLEEP_WORKERS_ON_EMPTY
    queue_empty_flag_retry_steal:
    // TODO: relax atomics for empty flag
    data_structures::stamped_integer target_queue_empty_flag = target_state.get_queue_empty_flag().load();
#endif

    base_task *stolen_task;
    base_task *chain_after_stolen_task;
    bool cas_success;
    do {
#if PLS_PROFILING_ENABLED
      my_state.get_scheduler().profiler_.stealing_cas_op(my_state.get_thread_id());
#endif
      std::tie(stolen_task, chain_after_stolen_task, cas_success) =
          target_state.get_task_manager().steal_task(my_state);
    } while (!cas_success); // re-try on cas-conflicts, as in classic ws

    if (stolen_task) {
      // Keep task chain consistent. We want to appear as if we are working an a branch upwards of the stolen task.
      stolen_task->next_ = chain_after_stolen_task;
      chain_after_stolen_task->prev_ = stolen_task;
      my_state.set_active_task(stolen_task);

      PLS_ASSERT(check_task_chain_forward(*my_state.get_active_task()),
                 "We are sole owner of this chain, it has to be valid!");

      // Execute the stolen task by jumping to it's continuation.
      PLS_ASSERT(stolen_task->continuation_.valid(),
                 "A task that we can steal must have a valid continuation for us to start working.");
      stolen_task->is_synchronized_ = false;
#if PLS_PROFILING_ENABLED
      my_state.get_scheduler().profiler_.stealing_end(my_state.get_thread_id(), true);
      my_state.get_scheduler().profiler_.task_start_running(my_state.get_thread_id(),
                                                            stolen_task->profiling_node_);
#endif
      context_switcher::switch_context(std::move(stolen_task->continuation_));
      // We will continue execution in this line when we finished the stolen work.
      failed_steals = 0;
    } else {
      // TODO: tune value for when we start yielding
      const unsigned YIELD_AFTER = 1;

      failed_steals++;
      if (failed_steals >= YIELD_AFTER) {
        std::this_thread::yield();
#if PLS_PROFILING_ENABLED
        my_state.get_scheduler().profiler_.stealing_end(my_state.get_thread_id(), false);
#endif
#if PLS_SLEEP_WORKERS_ON_EMPTY
        switch (target_queue_empty_flag.value) {
          case EMPTY_QUEUE_STATE::QUEUE_NON_EMPTY: {
            // We found the queue empty, but the flag says it should still be full.
            // We want to declare it empty, bet we need to re-check the queue in a sub-step to avoid races.
            data_structures::stamped_integer
                maybe_empty_flag{target_queue_empty_flag.stamp + 1, EMPTY_QUEUE_STATE::QUEUE_MAYBE_EMPTY};
            if (target_state.get_queue_empty_flag().compare_exchange_strong(target_queue_empty_flag,
                                                                            maybe_empty_flag)) {
              goto queue_empty_flag_retry_steal;
            }
            break;
          }
          case EMPTY_QUEUE_STATE::QUEUE_MAYBE_EMPTY: {
            // We found the queue empty and it was already marked as maybe empty.
            // We can safely mark it empty and increment the central counter.
            data_structures::stamped_integer
                empty_flag{target_queue_empty_flag.stamp + 1, EMPTY_QUEUE_STATE::QUEUE_EMPTY};
            if (target_state.get_queue_empty_flag().compare_exchange_strong(target_queue_empty_flag, empty_flag)) {
              // We marked it empty, now its our duty to modify the central counter
              my_state.get_scheduler().empty_queue_increase_counter();
            }
            break;
          }
          case EMPTY_QUEUE_STATE::QUEUE_EMPTY: {
            // The queue was already marked empty, just do nothing
            break;
          }
        }
        // Disregarding if we found the thread empty, we should check if we can put ourself to sleep
        my_state.get_scheduler().empty_queue_try_sleep_worker();
#endif
      }
    }
  }
  my_state.set_scheduler_active(false);
}

void scheduler::sync_internal() {
  if (thread_state::is_scheduler_active()) {
    thread_state &syncing_state = thread_state::get();

    base_task *active_task = syncing_state.get_active_task();
    base_task *spawned_task = active_task->next_;

#if PLS_PROFILING_ENABLED
    syncing_state.get_scheduler().profiler_.task_finish_stack_measure(syncing_state.get_thread_id(),
                                                                      active_task->stack_memory_,
                                                                      active_task->stack_size_,
                                                                      active_task->profiling_node_);
    syncing_state.get_scheduler().profiler_.task_stop_running(syncing_state.get_thread_id(),
                                                              active_task->profiling_node_);
    auto *next_dag_node =
        syncing_state.get_scheduler().profiler_.task_sync(syncing_state.get_thread_id(), active_task->profiling_node_);
    active_task->profiling_node_ = next_dag_node;
#endif

    if (active_task->is_synchronized_) {
#if PLS_PROFILING_ENABLED
      thread_state::get().get_scheduler().profiler_.task_start_running(thread_state::get().get_thread_id(),
                                                                       thread_state::get().get_active_task()->profiling_node_);
#endif
      return; // We are already the sole owner of last_task
    } else {
      auto continuation =
          spawned_task->run_as_task([active_task, spawned_task, &syncing_state](context_switcher::continuation cont) {
            active_task->continuation_ = std::move(cont);
            syncing_state.set_active_task(spawned_task);
            return slow_return(syncing_state);
          });

      PLS_ASSERT(!continuation.valid(),
                 "We only return to a sync point, never jump to it directly."
                 "This must therefore never return an unfinished fiber/continuation.");
#if PLS_PROFILING_ENABLED
      thread_state::get().get_scheduler().profiler_.task_start_running(thread_state::get().get_thread_id(),
                                                                       thread_state::get().get_active_task()->profiling_node_);
#endif
      return; // We cleanly synced to the last one finishing work on last_task
    }
  } else {
    // Scheduler not active
    return;
  }
}

context_switcher::continuation scheduler::slow_return(thread_state &calling_state) {
  base_task *this_task = calling_state.get_active_task();
  PLS_ASSERT(this_task->depth_ > 0,
             "Must never try to return from a task at level 0 (no last task), as we must have a target to return to.");
  base_task *last_task = this_task->prev_;

  // Slow return means we try to finish the child 'this_task' of 'last_task' and we
  // do not know if we are the last child to finish.
  // If we are not the last one, we get a spare task chain for our resources and can return to the main scheduling loop.
  base_task *pop_result = calling_state.get_task_manager().pop_clean_task_chain(last_task);

  if (pop_result != nullptr) {
    base_task *clean_chain = pop_result;

    // We got a clean chain to fill up our resources.
    PLS_ASSERT(last_task->depth_ == clean_chain->depth_,
               "Resources must only reside in the correct depth!");
    PLS_ASSERT(last_task != clean_chain,
               "We want to swap out the last task and its chain to use a clean one, thus they must differ.");
    PLS_ASSERT(check_task_chain_backward(*clean_chain),
               "Can only acquire clean chains for clean returns!");

    // Acquire it/merge it with our task chain.
    this_task->prev_ = clean_chain;
    clean_chain->next_ = this_task;

    base_task *active_task = clean_chain;
    while (active_task->depth_ > 0) {
      active_task = active_task->prev_;
    }
    calling_state.set_active_task(active_task);

    // Jump back to the continuation in main scheduling loop.
    context_switcher::continuation result_cont = std::move(thread_state::get().main_continuation());
    PLS_ASSERT(result_cont.valid(), "Must return a valid continuation.");
    return result_cont;
  } else {
    // Make sure that we are owner of this full continuation/task chain.
    last_task->next_ = this_task;

    // We are the last one working on this task. Thus the sync must be finished, continue working.
    calling_state.set_active_task(last_task);
    last_task->is_synchronized_ = true;

    // Jump to parent task and continue working on it.
    context_switcher::continuation result_cont = std::move(last_task->continuation_);
    PLS_ASSERT(result_cont.valid(), "Must return a valid continuation.");
    return result_cont;
  }
}

base_task &scheduler::task_chain_at(unsigned int depth, thread_state &calling_state) {
  // TODO: possible optimize with cache array at steal events
  base_task *result = calling_state.get_active_task();
  while (result->depth_ > depth) {
    result = result->prev_;
  }
  while (result->depth_ < depth) {
    result = result->next_;
  }

  return *result;
}

void scheduler::terminate() {
  if (terminated_) {
    return;
  }

  terminated_ = true;
  sync_barrier_.wait();

  for (unsigned int i = 0; i < num_threads_; i++) {
    if (reuse_thread_ && i == 0) {
      continue;
    }
    worker_threads_[i].join();
  }
}

bool scheduler::check_task_chain_forward(base_task &start_task) {
  base_task *current = &start_task;
  while (current->next_) {
    if (current->next_->prev_ != current) {
      return false;
    }
    current = current->next_;
  }
  return true;
}

bool scheduler::check_task_chain_backward(base_task &start_task) {
  base_task *current = &start_task;
  while (current->prev_) {
    if (current->prev_->next_ != current) {
      return false;
    }
    current = current->prev_;
  }
  return true;
}

bool scheduler::check_task_chain(base_task &start_task) {
  return check_task_chain_backward(start_task) && check_task_chain_forward(start_task);
}

#if PLS_SLEEP_WORKERS_ON_EMPTY
// TODO: relax memory orderings

void scheduler::empty_queue_try_sleep_worker() {
  int32_t counter_value = empty_queue_counter_.load();
  if (counter_value == num_threads()) {
#if PLS_PROFILING_ENABLED
    get_profiler().sleep_start(thread_state::get().get_thread_id());
#endif
    threads_sleeping_++;
    base::futex_wait((int32_t *) &empty_queue_counter_, num_threads());
    threads_sleeping_--;
    base::futex_wakeup((int32_t *) &empty_queue_counter_, 1);
#if PLS_PROFILING_ENABLED
    get_profiler().sleep_stop(thread_state::get().get_thread_id());
#endif
  }
}

void scheduler::empty_queue_increase_counter() {
  empty_queue_counter_.fetch_add(1);
}

void scheduler::empty_queue_decrease_counter_and_wake() {
  empty_queue_counter_.fetch_sub(1);
  if (threads_sleeping_.load() > 0) {
    base::futex_wakeup((int32_t *) &empty_queue_counter_, 1);
  }
}
#endif

}