scheduler.cpp

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

#include "context_switcher/context_switcher.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();
  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_) {
    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);
    auto[stolen_task, traded_task] = target_state.get_task_manager().steal_task(my_state);
    if (stolen_task) {
      // Keep task chain consistent. We want to appear as if we are working an a branch upwards of the stolen task.
      base_task *next_own_task = traded_task->next_;
      stolen_task->next_ = next_own_task;
      next_own_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;
      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 {
      failed_steals++;
      if (failed_steals >= num_threads) {
        std::this_thread::yield();
      }
    }
  }
}

void scheduler::sync() {
  thread_state &syncing_state = thread_state::get();

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

  if (active_task->is_synchronized_) {
    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.");
    return; // We cleanly synced to the last one finishing work on last_task
  }
}

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);
}

}