Move task count bookeeping out of libstd

For libgreen, bookeeping should not be global but rather on a per-pool basis.
Inside libnative, it's known that there must be a global counter with a
mutex/cvar.

The benefit of taking this strategy is to remove this functionality from libstd
to allow fine-grained control of it through libnative/libgreen. Notably, helper
threads in libnative can manually decrement the global count so they don't count
towards the global count of threads. Also, the shutdown process of *all* sched
pools is now dependent on the number of tasks in the pool being 0 rather than
this only being a hardcoded solution for the initial sched pool in libgreen.

This involved adding a Local::try_take() method on the Local trait in order for
the channel wakeup to work inside of libgreen. The channel send was happening
from a SchedTask when there is no Task available in TLS, and now this is
possible to work (remote wakeups are always possible, just a little slower).

Author: alexcrichton

src/libgreen/lib.rs

@@ -25,18 +25,20 @@
 // NB this does *not* include globs, please keep it that way.
 #[feature(macro_rules)];
 
+// Allow check-stage0-green for now
+#[cfg(test, stage0)] extern mod green;
+
 use std::os;
 use std::rt::crate_map;
-use std::rt::local::Local;
 use std::rt::rtio;
-use std::rt::task::Task;
 use std::rt::thread::Thread;
 use std::rt;
 use std::sync::atomics::{SeqCst, AtomicUint, INIT_ATOMIC_UINT};
 use std::sync::deque;
 use std::task::TaskOpts;
 use std::util;
 use std::vec;
+use std::sync::arc::UnsafeArc;
 
 use sched::{Shutdown, Scheduler, SchedHandle, TaskFromFriend, NewNeighbor};
 use sleeper_list::SleeperList;
@@ -118,14 +120,6 @@ pub fn run(main: proc()) -> int {
         os::set_exit_status(rt::DEFAULT_ERROR_CODE);
     }
 
-    // Once the main task has exited and we've set our exit code, wait for all
-    // spawned sub-tasks to finish running. This is done to allow all schedulers
-    // to remain active while there are still tasks possibly running.
-    unsafe {
-        let mut task = Local::borrow(None::<Task>);
-        task.get().wait_for_other_tasks();
-    }
-
     // Now that we're sure all tasks are dead, shut down the pool of schedulers,
     // waiting for them all to return.
     pool.shutdown();
@@ -164,6 +158,17 @@ pub struct SchedPool {
     priv deque_pool: deque::BufferPool<~task::GreenTask>,
     priv sleepers: SleeperList,
     priv factory: fn() -> ~rtio::EventLoop,
+    priv task_state: TaskState,
+    priv tasks_done: Port<()>,
+}
+
+/// This is an internal state shared among a pool of schedulers. This is used to
+/// keep track of how many tasks are currently running in the pool and then
+/// sending on a channel once the entire pool has been drained of all tasks.
+#[deriving(Clone)]
+struct TaskState {
+    cnt: UnsafeArc<AtomicUint>,
+    done: SharedChan<()>,
 }
 
 impl SchedPool {
@@ -182,6 +187,7 @@ impl SchedPool {
         assert!(nscheds > 0);
 
         // The pool of schedulers that will be returned from this function
+        let (p, state) = TaskState::new();
         let mut pool = SchedPool {
             threads: ~[],
             handles: ~[],
@@ -192,6 +198,8 @@ impl SchedPool {
             deque_pool: deque::BufferPool::new(),
             next_friend: 0,
             factory: factory,
+            task_state: state,
+            tasks_done: p,
         };
 
         // Create a work queue for each scheduler, ntimes. Create an extra
@@ -210,21 +218,30 @@ impl SchedPool {
                                             (pool.factory)(),
                                             worker,
                                             pool.stealers.clone(),
-                                            pool.sleepers.clone());
+                                            pool.sleepers.clone(),
+                                            pool.task_state.clone());
             pool.handles.push(sched.make_handle());
             let sched = sched;
-            pool.threads.push(do Thread::start {
-                sched.bootstrap();
-            });
+            pool.threads.push(do Thread::start { sched.bootstrap(); });
         }
 
         return pool;
     }
 
+    /// Creates a new task configured to run inside of this pool of schedulers.
+    /// This is useful to create a task which can then be sent to a specific
+    /// scheduler created by `spawn_sched` (and possibly pin it to that
+    /// scheduler).
     pub fn task(&mut self, opts: TaskOpts, f: proc()) -> ~GreenTask {
         GreenTask::configure(&mut self.stack_pool, opts, f)
     }
 
+    /// Spawns a new task into this pool of schedulers, using the specified
+    /// options to configure the new task which is spawned.
+    ///
+    /// New tasks are spawned in a round-robin fashion to the schedulers in this
+    /// pool, but tasks can certainly migrate among schedulers once they're in
+    /// the pool.
     pub fn spawn(&mut self, opts: TaskOpts, f: proc()) {
         let task = self.task(opts, f);
 
@@ -262,7 +279,8 @@ impl SchedPool {
                                         (self.factory)(),
                                         worker,
                                         self.stealers.clone(),
-                                        self.sleepers.clone());
+                                        self.sleepers.clone(),
+                                        self.task_state.clone());
         let ret = sched.make_handle();
         self.handles.push(sched.make_handle());
         let sched = sched;
@@ -271,9 +289,28 @@ impl SchedPool {
         return ret;
     }
 
+    /// Consumes the pool of schedulers, waiting for all tasks to exit and all
+    /// schedulers to shut down.
+    ///
+    /// This function is required to be called in order to drop a pool of
+    /// schedulers, it is considered an error to drop a pool without calling
+    /// this method.
+    ///
+    /// This only waits for all tasks in *this pool* of schedulers to exit, any
+    /// native tasks or extern pools will not be waited on
     pub fn shutdown(mut self) {
         self.stealers = ~[];
 
+        // Wait for everyone to exit. We may have reached a 0-task count
+        // multiple times in the past, meaning there could be several buffered
+        // messages on the `tasks_done` port. We're guaranteed that after *some*
+        // message the current task count will be 0, so we just receive in a
+        // loop until everything is totally dead.
+        while self.task_state.active() {
+            self.tasks_done.recv();
+        }
+
+        // Now that everyone's gone, tell everything to shut down.
         for mut handle in util::replace(&mut self.handles, ~[]).move_iter() {
             handle.send(Shutdown);
         }
@@ -283,6 +320,31 @@ impl SchedPool {
     }
 }
 
+impl TaskState {
+    fn new() -> (Port<()>, TaskState) {
+        let (p, c) = SharedChan::new();
+        (p, TaskState {
+            cnt: UnsafeArc::new(AtomicUint::new(0)),
+            done: c,
+        })
+    }
+
+    fn increment(&mut self) {
+        unsafe { (*self.cnt.get()).fetch_add(1, SeqCst); }
+    }
+
+    fn active(&self) -> bool {
+        unsafe { (*self.cnt.get()).load(SeqCst) != 0 }
+    }
+
+    fn decrement(&mut self) {
+        let prev = unsafe { (*self.cnt.get()).fetch_sub(1, SeqCst) };
+        if prev == 1 {
+            self.done.send(());
+        }
+    }
+}
+
 impl Drop for SchedPool {
     fn drop(&mut self) {
         if self.threads.len() > 0 {

src/libgreen/sched.rs

@@ -19,6 +19,7 @@ use std::unstable::mutex::Mutex;
 use std::unstable::raw;
 use mpsc = std::sync::mpsc_queue;
 
+use TaskState;
 use context::Context;
 use coroutine::Coroutine;
 use sleeper_list::SleeperList;
@@ -85,6 +86,9 @@ pub struct Scheduler {
     /// A flag to tell the scheduler loop it needs to do some stealing
     /// in order to introduce randomness as part of a yield
     steal_for_yield: bool,
+    /// Bookeeping for the number of tasks which are currently running around
+    /// inside this pool of schedulers
+    task_state: TaskState,
 
     // n.b. currently destructors of an object are run in top-to-bottom in order
     //      of field declaration. Due to its nature, the pausable idle callback
@@ -120,11 +124,12 @@ impl Scheduler {
                event_loop: ~EventLoop,
                work_queue: deque::Worker<~GreenTask>,
                work_queues: ~[deque::Stealer<~GreenTask>],
-               sleeper_list: SleeperList)
+               sleeper_list: SleeperList,
+               state: TaskState)
         -> Scheduler {
 
         Scheduler::new_special(pool_id, event_loop, work_queue, work_queues,
-                               sleeper_list, true, None)
+                               sleeper_list, true, None, state)
 
     }
 
@@ -134,7 +139,8 @@ impl Scheduler {
                        work_queues: ~[deque::Stealer<~GreenTask>],
                        sleeper_list: SleeperList,
                        run_anything: bool,
-                       friend: Option<SchedHandle>)
+                       friend: Option<SchedHandle>,
+                       state: TaskState)
         -> Scheduler {
 
         let (consumer, producer) = mpsc::queue(());
@@ -156,7 +162,8 @@ impl Scheduler {
             rng: new_sched_rng(),
             idle_callback: None,
             yield_check_count: 0,
-            steal_for_yield: false
+            steal_for_yield: false,
+            task_state: state,
         };
 
         sched.yield_check_count = reset_yield_check(&mut sched.rng);
@@ -756,6 +763,7 @@ impl Scheduler {
         let _cur = self.change_task_context(cur, stask, |sched, mut dead_task| {
             let coroutine = dead_task.coroutine.take_unwrap();
             coroutine.recycle(&mut sched.stack_pool);
+            sched.task_state.decrement();
         });
         fail!("should never return!");
     }
@@ -955,11 +963,10 @@ mod test {
     use std::rt::task::Task;
     use std::rt::local::Local;
 
+    use {TaskState, PoolConfig, SchedPool};
     use basic;
     use sched::{TaskFromFriend, PinnedTask};
     use task::{GreenTask, HomeSched};
-    use PoolConfig;
-    use SchedPool;
 
     fn pool() -> SchedPool {
         SchedPool::new(PoolConfig {
@@ -1078,14 +1085,16 @@ mod test {
             let (normal_worker, normal_stealer) = pool.deque();
             let (special_worker, special_stealer) = pool.deque();
             let queues = ~[normal_stealer, special_stealer];
+            let (_p, state) = TaskState::new();
 
             // Our normal scheduler
             let mut normal_sched = ~Scheduler::new(
                 1,
                 basic::event_loop(),
                 normal_worker,
                 queues.clone(),
-                sleepers.clone());
+                sleepers.clone(),
+                state.clone());
 
             let normal_handle = normal_sched.make_handle();
             let friend_handle = normal_sched.make_handle();
@@ -1098,7 +1107,8 @@ mod test {
                 queues.clone(),
                 sleepers.clone(),
                 false,
-                Some(friend_handle));
+                Some(friend_handle),
+                state);
 
             let special_handle = special_sched.make_handle();
 

src/libgreen/task.rs

@@ -33,11 +33,32 @@ use stack::StackPool;
 /// The necessary fields needed to keep track of a green task (as opposed to a
 /// 1:1 task).
 pub struct GreenTask {
+    /// Coroutine that this task is running on, otherwise known as the register
+    /// context and the stack that this task owns. This field is optional to
+    /// relinquish ownership back to a scheduler to recycle stacks at a later
+    /// date.
     coroutine: Option<Coroutine>,
+
+    /// Optional handle back into the home sched pool of this task. This field
+    /// is lazily initialized.
     handle: Option<SchedHandle>,
+
+    /// Slot for maintaining ownership of a scheduler. If a task is running,
+    /// this value will be Some(sched) where the task is running on "sched".
     sched: Option<~Scheduler>,
+
+    /// Temporary ownership slot of a std::rt::task::Task object. This is used
+    /// to squirrel that libstd task away while we're performing green task
+    /// operations.
     task: Option<~Task>,
+
+    /// Dictates whether this is a sched task or a normal green task
     task_type: TaskType,
+
+    /// Home pool that this task was spawned into. This field is lazily
+    /// initialized until when the task is initially scheduled, and is used to
+    /// make sure that tasks are always woken up in the correct pool of
+    /// schedulers.
     pool_id: uint,
 
     // See the comments in the scheduler about why this is necessary
@@ -147,10 +168,15 @@ impl GreenTask {
             // cleanup job after we have re-acquired ownership of the green
             // task.
             let mut task: ~GreenTask = unsafe { GreenTask::from_uint(ops) };
-            task.sched.get_mut_ref().run_cleanup_job();
+            task.pool_id = {
+                let sched = task.sched.get_mut_ref();
+                sched.run_cleanup_job();
+                sched.task_state.increment();
+                sched.pool_id
+            };
 
             // Convert our green task to a libstd task and then execute the code
-            // requeted. This is the "try/catch" block for this green task and
+            // requested. This is the "try/catch" block for this green task and
             // is the wrapper for *all* code run in the task.
             let mut start = Some(start);
             let task = task.swap().run(|| start.take_unwrap()());
@@ -350,6 +376,14 @@ impl Runtime for GreenTask {
         self.put_task(to_wake);
         assert!(self.sched.is_none());
 
+        // Optimistically look for a local task, but if one's not available to
+        // inspect (in order to see if it's in the same sched pool as we are),
+        // then just use our remote wakeup routine and carry on!
+        let mut running_task: ~Task = match Local::try_take() {
+            Some(task) => task,
+            None => return self.reawaken_remotely()
+        };
+
         // Waking up a green thread is a bit of a tricky situation. We have no
         // guarantee about where the current task is running. The options we
         // have for where this current task is running are:
@@ -368,7 +402,6 @@ impl Runtime for GreenTask {
         //
         // In case 2 and 3, we need to remotely reawaken ourself in order to be
         // transplanted back to the correct scheduler pool.
-        let mut running_task: ~Task = Local::take();
         match running_task.maybe_take_runtime::<GreenTask>() {
             Some(mut running_green_task) => {
                 running_green_task.put_task(running_task);