Split build system into library and driver

1 files changed, 709 insertions, 0 deletions

diff --git a/libbuild2/scheduler.hxx b/libbuild2/scheduler.hxx
new file mode 100644
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--- /dev/null
+++ b/libbuild2/scheduler.hxx
@@ -0,0 +1,709 @@
+// file      : libbuild2/scheduler.hxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef LIBBUILD2_SCHEDULER_HXX
+#define LIBBUILD2_SCHEDULER_HXX
+
+#include <list>
+#include <mutex>
+#include <tuple>
+#include <atomic>
+#include <type_traits>        // aligned_storage, etc
+#include <condition_variable>
+
+#include <libbuild2/types.hxx>
+#include <libbuild2/utility.hxx>
+
+#include <libbuild2/export.hxx>
+
+namespace build2
+{
+  // Scheduler of tasks and threads. Works best for "substantial" tasks (e.g.,
+  // running a process), where in comparison thread synchronization overhead
+  // is negligible.
+  //
+  // A thread (called "master") may need to perform several tasks which can be
+  // done in parallel (e.g., update all the prerequisites or run all the
+  // tests). To acomplish this, the master, via a call to async(), can ask the
+  // scheduler to run a task in another thread (called "helper"). If a helper
+  // is available, then the task is executed asynchronously by such a helper.
+  // Otherwise, the task is (normally) executed synchronously as part of the
+  // wait() call below. However, in certain cases (serial execution or full
+  // queue), the task may be executed synchronously as part of the async()
+  // call itself. Once the master thread has scheduled all the tasks, it calls
+  // wait() to await for their completion.
+  //
+  // The scheduler makes sure that only a certain number of threads (for
+  // example, the number of available hardware threads) are "active" at any
+  // given time. When a master thread calls wait(), it is "suspended" until
+  // all its asynchronous tasks are completed (at which point it becomes
+  // "ready"). A suspension of a master results in either another ready master
+  // being "resumed" or another helper thread becoming available.
+  //
+  // On completion of a task a helper thread returns to the scheduler which
+  // can again lead either to a ready master being resumed (in which case the
+  // helper is suspended) or the helper becoming available to perform another
+  // task.
+  //
+  // Note that suspended threads are not reused as helpers. Rather, a new
+  // helper thread is always created if none is available. This is done to
+  // allow a ready master to continue as soon as possible. If it were reused
+  // as a helper, then it could be blocked on a nested wait() further down the
+  // stack. All this means that the number of threads created by the scheduler
+  // will normally exceed the maximum active allowed.
+  //
+  class LIBBUILD2_SYMEXPORT scheduler
+  {
+  public:
+    using atomic_count = std::atomic<size_t>;
+
+    // F should return void and not throw any exceptions. The way the result
+    // of a task is communicated back to the master thread is ad hoc, usually
+    // via "out" arguments. Such result(s) can only be retrieved by the master
+    // once its task count reaches the start count.
+    //
+    // The argument passing semantics is the same as for std::thread. In
+    // particular, lvalue-references are passed as copies (use ref()/cref()
+    // for the by-reference semantics), except the case where the task is
+    // executed synchronously and as part of the async() call itself (this
+    // subtlety can become important when passing shared locks; you would
+    // only want it to be copied if the task is queued).
+    //
+    // Return true if the task was queued and false if it was executed
+    // synchronously.
+    //
+    // If the scheduler is shutdown, throw system_error(ECANCELED).
+    //
+    template <typename F, typename... A>
+    bool
+    async (size_t start_count, atomic_count& task_count, F&&, A&&...);
+
+    template <typename F, typename... A>
+    bool
+    async (atomic_count& task_count, F&& f, A&&... a)
+    {
+      return async (0, task_count, forward<F> (f), forward<A> (a)...);
+    }
+
+    // Wait until the task count reaches the start count or less. If the
+    // scheduler is shutdown while waiting, throw system_error(ECANCELED).
+    // Return the value of task count. Note that this is a synchronizaiton
+    // point (i.e., the task count is checked with memory_order_acquire).
+    //
+    // Note that it is valid to wait on another thread's task count (that is,
+    // without making any async() calls in this thread). However, if the start
+    // count differs from the one passed to async(), then whomever sets the
+    // start count to this alternative value must also call resume() below
+    // in order to signal waiting threads.
+    //
+    // Note also that in this case (waiting on someone else's start count),
+    // the async() call could execute the tasks synchronously without ever
+    // incrementing the task count. Thus if waiting on another thread's start
+    // count starts before/during async() calls, then it must be "gated" with
+    // an alternative (lower) start count.
+    //
+    // Finally, if waiting on someone else's start count, it may be unsafe
+    // (from the deadlock's point of view) to continue working through our own
+    // queue (i.e., we may block waiting on a task that has been queued before
+    // us which in turn may end up waiting on "us").
+    //
+    enum work_queue
+    {
+      work_none, // Don't work own queue.
+      work_one,  // Work own queue rechecking the task count after every task.
+      work_all   // Work own queue before rechecking the task count.
+    };
+
+    size_t
+    wait (size_t start_count,
+          const atomic_count& task_count,
+          work_queue = work_all);
+
+    size_t
+    wait (const atomic_count& task_count, work_queue wq = work_all)
+    {
+      return wait (0, task_count, wq);
+    }
+
+    // Resume threads waiting on this task count.
+    //
+    void
+    resume (const atomic_count& task_count);
+
+    // An active thread that is about to wait for potentially significant time
+    // on something other than task_count (e.g., mutex, condition variable)
+    // should deactivate itself with the scheduler and then reactivate once
+    // done waiting.
+    //
+    void
+    deactivate ();
+
+    void
+    activate (bool collision = false);
+
+    // Sleep for the specified duration, deactivating the thread before going
+    // to sleep and re-activating it after waking up (which means this
+    // function may sleep potentially significantly longer than requested).
+    //
+    void
+    sleep (const duration&);
+
+    // Startup and shutdown.
+    //
+  public:
+    // Unless already shut down, call shutdown() but ignore errors.
+    //
+    ~scheduler ();
+
+    // Create a shut down scheduler.
+    //
+    scheduler () = default;
+
+    // Create a started up scheduler.
+    //
+    // The initial active argument is the number of threads to assume are
+    // already active (e.g., the calling thread). It must not be 0 (since
+    // someone has to schedule the first task).
+    //
+    // If the maximum threads or task queue depth arguments are unspecified,
+    // then appropriate defaults are used.
+    //
+    explicit
+    scheduler (size_t max_active,
+               size_t init_active = 1,
+               size_t max_threads = 0,
+               size_t queue_depth = 0,
+               optional<size_t> max_stack = nullopt)
+    {
+      startup (max_active, init_active, max_threads, queue_depth, max_stack);
+    }
+
+    // Start the scheduler.
+    //
+    void
+    startup (size_t max_active,
+             size_t init_active = 1,
+             size_t max_threads = 0,
+             size_t queue_depth = 0,
+             optional<size_t> max_stack = nullopt);
+
+    // Return true if the scheduler was started up.
+    //
+    // Note: can only be called from threads that have observed creation,
+    // startup, or shutdown.
+    //
+    bool
+    started () const {return !shutdown_;}
+
+    // Tune a started up scheduler.
+    //
+    // Currently one cannot increase the number of max_active. Pass 0 to
+    // restore the initial value.
+    //
+    // Note that tuning can only be done while the scheduler is inactive, that
+    // is, no threads are executing a task or are suspended. For example, in a
+    // setup with a single initial active thread that would be after a return
+    // from the top-level wait() call.
+    //
+    void
+    tune (size_t max_active);
+
+    // Return true if the scheduler is configured to run tasks serially.
+    //
+    // Note: can only be called from threads that have observed startup.
+    //
+    bool
+    serial () const {return max_active_ == 1;}
+
+    // Wait for all the helper threads to terminate. Throw system_error on
+    // failure. Note that the initially active threads are not waited for.
+    // Return scheduling statistics.
+    //
+    struct stat
+    {
+      size_t thread_max_active     = 0; // max # of active threads allowed.
+      size_t thread_max_total      = 0; // max # of total threads allowed.
+      size_t thread_helpers        = 0; // # of helper threads created.
+      size_t thread_max_waiting    = 0; // max # of waiters at any time.
+
+      size_t task_queue_depth      = 0; // # of entries in a queue (capacity).
+      size_t task_queue_full       = 0; // # of times task queue was full.
+      size_t task_queue_remain     = 0; // # of tasks remaining in queue.
+
+      size_t wait_queue_slots      = 0; // # of wait slots (buckets).
+      size_t wait_queue_collisions = 0; // # of times slot had been occupied.
+    };
+
+    stat
+    shutdown ();
+
+    // Progress monitoring.
+    //
+    // Setting and clearing of the monitor is not thread-safe. That is, it
+    // should be set before any tasks are queued and cleared after all of
+    // them have completed.
+    //
+    // The counter must go in one direction, either increasing or decreasing,
+    // and should contain the initial value during the call. Zero threshold
+    // value is reserved.
+    //
+    struct monitor_guard
+    {
+      explicit
+      monitor_guard (scheduler* s = nullptr): s_ (s) {}
+      monitor_guard (monitor_guard&& x): s_ (x.s_) {x.s_ = nullptr;}
+      monitor_guard& operator= (monitor_guard&& x)
+      {
+        if (&x != this)
+        {
+          s_ = x.s_;
+          x.s_ = nullptr;
+        }
+        return *this;
+      }
+
+      ~monitor_guard ()
+      {
+        if (s_ != nullptr)
+        {
+          lock l (s_->wait_idle ()); // See monitor() for details.
+          s_->monitor_count_ = nullptr;
+          s_->monitor_func_  = nullptr;
+        }
+      }
+
+      explicit operator bool () const {return s_ != nullptr;}
+
+    private:
+      scheduler* s_;
+    };
+
+    monitor_guard
+    monitor (atomic_count&, size_t threshold, function<size_t (size_t)>);
+
+    // If initially active thread(s) (besides the one that calls startup())
+    // exist before the call to startup(), then they must call join() before
+    // executing any tasks. The two common cases where you don't have to call
+    // join are a single active thread that calls startup()/shutdown() or
+    // active thread(s) that are created after startup().
+    //
+    void
+    join ()
+    {
+      assert (queue () == nullptr);
+
+      // Lock the mutex to make sure the values set in startup() are visible
+      // in this thread.
+      //
+      lock l (mutex_);
+    }
+
+    // If initially active thread(s) participate in multiple schedulers and/or
+    // sessions (intervals between startup() and shutdown()), then they must
+    // call leave() before joining another scheduler/session. Note that this
+    // applies to the active thread that calls shutdown(). Note that a thread
+    // can only participate in one scheduler at a time.
+    //
+    void
+    leave ()
+    {
+      queue (nullptr);
+    }
+
+    // Return the number of hardware threads or 0 if unable to determine.
+    //
+    static size_t
+    hardware_concurrency ()
+    {
+      return std::thread::hardware_concurrency ();
+    }
+
+    // Return a prime number that can be used as a lock shard size that's
+    // appropriate for the scheduler's concurrency. Use power of two values
+    // for mul for higher-contention shards and for div for lower-contention
+    // ones. Always return 1 for serial execution.
+    //
+    // Note: can only be called from threads that have observed startup.
+    //
+    size_t
+    shard_size (size_t mul = 1, size_t div = 1) const;
+
+    // Assuming all the task have been executed, busy-wait for all the threads
+    // to become idle. Return the lock over the scheduler mutex. Normally you
+    // don't need to call this function directly.
+    //
+    using lock = std::unique_lock<std::mutex>;
+
+    lock
+    wait_idle ();
+
+  private:
+    void
+    activate_helper (lock&);
+
+    void
+    create_helper (lock&);
+
+    // We restrict ourselves to a single pointer as an argument in hope of
+    // a small object optimization. Return NULL.
+    //
+    // Note that the return type is void* to make the function usable with
+    // pthreads (see scheduler.cxx for details).
+    //
+    static void*
+    helper (void*);
+
+    size_t
+    suspend (size_t start_count, const atomic_count& task_count);
+
+    // Task encapsulation.
+    //
+    template <typename F, typename... A>
+    struct task_type
+    {
+      using func_type = std::decay_t<F>;
+      using args_type = std::tuple<std::decay_t<A>...>;
+
+      atomic_count* task_count;
+      size_t start_count;
+      func_type func;
+      args_type args;
+
+      template <size_t... i>
+      void
+      thunk (std::index_sequence<i...>)
+      {
+        move (func) (std::get<i> (move (args))...);
+      }
+    };
+
+    template <typename F, typename... A>
+    static void
+    task_thunk (scheduler&, lock&, void*);
+
+    template <typename T>
+    static std::decay_t<T>
+    decay_copy (T&& x) {return forward<T> (x);}
+
+  private:
+    // Monitor.
+    //
+    atomic_count*              monitor_count_ = nullptr;  // NULL if not used.
+    atomic_count               monitor_tshold_;           // 0 means locked.
+    size_t                     monitor_init_;             // Initial count.
+    function<size_t (size_t)>  monitor_func_;
+
+    std::mutex mutex_;
+    bool shutdown_ = true;  // Shutdown flag.
+
+    optional<size_t> max_stack_;
+
+    // The constraints that we must maintain:
+    //
+    //                  active <= max_active
+    // (init_active + helpers) <= max_threads (soft; see activate_helper())
+    //
+    // Note that the first three are immutable between startup() and
+    // shutdown() so can be accessed without a lock (but see join()).
+    //
+    size_t init_active_ = 0; // Initially active threads.
+    size_t max_active_  = 0; // Maximum number of active threads.
+    size_t max_threads_ = 0; // Maximum number of total threads.
+
+    size_t helpers_     = 0; // Number of helper threads created so far.
+
+    // Every thread that we manage must be accounted for in one of these
+    // counters. And their sum should equal (init_active + helpers).
+    //
+    size_t active_   = 0;  // Active master threads executing a task.
+    size_t idle_     = 0;  // Idle helper threads waiting for a task.
+    size_t waiting_  = 0;  // Suspended master threads waiting for their tasks.
+    size_t ready_    = 0;  // Ready master thread waiting to become active.
+    size_t starting_ = 0;  // Helper threads starting up.
+
+    // Original values (as specified during startup) that can be altered via
+    // tuning.
+    //
+    size_t orig_max_active_ = 0;
+
+    std::condition_variable idle_condv_;  // Idle helpers queue.
+    std::condition_variable ready_condv_; // Ready masters queue.
+
+    // Statistics counters.
+    //
+    size_t stat_max_waiters_;
+    size_t stat_wait_collisions_;
+
+    // Progress counter.
+    //
+    // We increment it for each active->waiting->ready->active transition
+    // and it is used for deadlock detection (see deactivate()).
+    //
+    size_t progress_;
+
+    // Wait queue.
+    //
+    // A wait slot blocks a bunch of threads. When they are (all) unblocked,
+    // they re-examine their respective conditions and either carry on or
+    // block again.
+    //
+    // The wait queue is a shard of slots. A thread picks a slot based on the
+    // address of its task count variable. How many slots do we need? This
+    // depends on the number of waiters that we can have which cannot be
+    // greater than the total number of threads.
+    //
+    // The pointer to the task count is used to identify the already waiting
+    // group of threads for collision statistics.
+    //
+    struct wait_slot
+    {
+      std::mutex mutex;
+      std::condition_variable condv;
+      size_t waiters = 0;
+      const atomic_count* task_count;
+      bool shutdown = true;
+    };
+
+    size_t wait_queue_size_; // Proportional to max_threads.
+    unique_ptr<wait_slot[]> wait_queue_;
+
+    // Task queue.
+    //
+    // Each queue has its own mutex plus we have an atomic total count of the
+    // queued tasks. Note that it should only be modified while holding one
+    // of the queue locks.
+    //
+    atomic_count queued_task_count_;
+
+    // For now we only support trivially-destructible tasks.
+    //
+    struct task_data
+    {
+      std::aligned_storage<sizeof (void*) * 8>::type data;
+      void (*thunk) (scheduler&, lock&, void*);
+    };
+
+    // We have two requirements: Firstly, we want to keep the master thread
+    // (the one that called wait()) busy working though its own queue for as
+    // long as possible before (if at all) it "reincarnates" as a helper. The
+    // main reason for this is the limited number of helpers we can create.
+    //
+    // Secondly, we don't want to block wait() longer than necessary since the
+    // master thread can do some work with the result. Plus, overall, we want
+    // to "unwind" task hierarchies as soon as possible since they hold up
+    // resources such as thread's stack. All this means that the master thread
+    // can only work through tasks that it has queued at this "level" of the
+    // async()/wait() calls since we know that wait() cannot return until
+    // they are done.
+    //
+    // To satisfy the first requirement, the master and helper threads get the
+    // tasks from different ends of the queue: master from the back while
+    // helpers from the front. And the master always adds new tasks to the
+    // back.
+    //
+    // To satisfy the second requirement, the master thread stores the index
+    // of the first task it has queued at this "level" and makes sure it
+    // doesn't try to deque any task beyond that.
+    //
+    size_t task_queue_depth_; // Multiple of max_active.
+
+    struct task_queue
+    {
+      std::mutex mutex;
+      bool shutdown = false;
+
+      size_t stat_full = 0; // Number of times push() returned NULL.
+
+      // Our task queue is circular with head being the index of the first
+      // element and tail -- of the last. Since this makes the empty and one
+      // element cases indistinguishable, we also keep the size.
+      //
+      // The mark is an index somewhere between (figuratively speaking) head
+      // and tail, if enabled. If the mark is hit, then it is disabled until
+      // the queue becomes empty or it is reset by a push.
+      //
+      size_t head = 0;
+      size_t mark = 0;
+      size_t tail = 0;
+      size_t size = 0;
+
+      unique_ptr<task_data[]> data;
+
+      task_queue (size_t depth): data (new task_data[depth]) {}
+    };
+
+    // Task queue API. Expects the queue mutex to be locked.
+    //
+
+    // Push a new task to the queue returning a pointer to the task data to be
+    // filled or NULL if the queue is full.
+    //
+    task_data*
+    push (task_queue& tq)
+    {
+      size_t& s (tq.size);
+      size_t& t (tq.tail);
+      size_t& m (tq.mark);
+
+      if (s != task_queue_depth_)
+      {
+        //                                         normal  wrap empty
+        //                                         |       |    |
+        t = s != 0 ? (t != task_queue_depth_ - 1 ? t + 1 : 0) : t;
+        s++;
+
+        if (m == task_queue_depth_) // Enable the mark if first push.
+          m = t;
+
+        queued_task_count_.fetch_add (1, std::memory_order_release);
+        return &tq.data[t];
+      }
+
+      return nullptr;
+    }
+
+    bool
+    empty_front (task_queue& tq) const {return tq.size == 0;}
+
+    void
+    pop_front (task_queue& tq, lock& ql)
+    {
+      size_t& s (tq.size);
+      size_t& h (tq.head);
+      size_t& m (tq.mark);
+
+      bool a (h == m); // Adjust mark?
+      task_data& td (tq.data[h]);
+
+      //                                         normal  wrap empty
+      //                                         |       |    |
+      h = s != 1 ? (h != task_queue_depth_ - 1 ? h + 1 : 0) : h;
+
+      if (--s == 0 || a)
+        m = h; // Reset or adjust the mark.
+
+      execute (ql, td);
+    }
+
+    bool
+    empty_back (task_queue& tq) const
+    {
+      return tq.size == 0 || tq.mark == task_queue_depth_;
+    }
+
+    void
+    pop_back (task_queue& tq, lock& ql)
+    {
+      size_t& s (tq.size);
+      size_t& t (tq.tail);
+      size_t& m (tq.mark);
+
+      bool a (t == m); // Adjust mark?
+
+      task_data& td (tq.data[t]);
+
+      // Save the old queue mark and disable it in case the task we are about
+      // to run adds sub-tasks. The first push(), if any, will reset it.
+      //
+      size_t om (m);
+      m = task_queue_depth_;
+
+      //                     normal  wrap                     empty
+      //                     |       |                        |
+      t = s != 1 ? (t != 0 ? t - 1 : task_queue_depth_ - 1) : t;
+      --s;
+
+      execute (ql, td);
+
+      // Restore the old mark (which we might have to adjust).
+      //
+      if (s == 0)
+        m = t;                 // Reset the mark.
+      else if (a)
+        m = task_queue_depth_; // Disable the mark.
+      else
+        // What happens if head goes past the old mark? In this case we will
+        // get into the empty queue state before we end up making any (wrong)
+        // decisions based on this value. Unfortunately there is no way to
+        // detect this (and do some sanity asserts) since things can wrap
+        // around.
+        //
+        // To put it another way, the understanding here is that after the
+        // task returns we will either have an empty queue or there will still
+        // be tasks between the old mark and the current tail, something along
+        // these lines:
+        //
+        // OOOOOXXXXOOO
+        //   |  |  |
+        //   m  h  t
+        //
+        m = om;
+    }
+
+    void
+    execute (lock& ql, task_data& td)
+    {
+      queued_task_count_.fetch_sub (1, std::memory_order_release);
+
+      // The thunk moves the task data to its stack, releases the lock,
+      // and continues to execute the task.
+      //
+      td.thunk (*this, ql, &td.data);
+
+      // See if we need to call the monitor (see also the serial version
+      // in async()).
+      //
+      if (monitor_count_ != nullptr)
+      {
+        // Note that we don't care if we don't see the updated values right
+        // away.
+        //
+        if (size_t t = monitor_tshold_.load (memory_order_relaxed))
+        {
+          // "Lock" the monitor by setting threshold to 0.
+          //
+          if (monitor_tshold_.compare_exchange_strong (
+                t,
+                0,
+                memory_order_release,
+                memory_order_relaxed))
+          {
+            // Now we are the only ones messing with this.
+            //
+            size_t v (monitor_count_->load (memory_order_relaxed));
+
+            if (v != monitor_init_)
+            {
+              // See which direction we are going.
+              //
+              if (v > monitor_init_ ? (v >= t) : (v <= t))
+                t = monitor_func_ (v);
+            }
+
+            monitor_tshold_.store (t, memory_order_release);
+          }
+        }
+      }
+
+      ql.lock ();
+    }
+
+    // Each thread has its own queue which are stored in this list.
+    //
+    std::list<task_queue> task_queues_;
+
+    task_queue&
+    create_queue ();
+
+    static task_queue*
+    queue () noexcept;
+
+    static void
+    queue (task_queue*) noexcept;
+  };
+}
+
+#include <libbuild2/scheduler.txx>
+
+#endif // LIBBUILD2_SCHEDULER_HXX