runtime: cleanup and add config options (#1807)

* runtime: cleanup and add config options

This patch finishes the cleanup as part of the transition to Tokio 0.2.
A number of changes were made to take advantage of having all Tokio
types in a single crate. Also, fixes using Tokio types from
`spawn_blocking`.

* Many threads, one resource driver

Previously, in the threaded scheduler, a resource driver (mio::Poll /
timer combo) was created per thread. This was more or less fine, except
it required balancing across the available drivers. When using a
resource driver from **outside** of the thread pool, balancing is
tricky. The change was original done to avoid having a dedicated driver
thread.

Now, instead of creating many resource drivers, a single resource driver
is used. Each scheduler thread will attempt to "lock" the resource
driver before parking on it. If the resource driver is already locked,
the thread uses a condition variable to park. Contention should remain
low as, under load, the scheduler avoids using the drivers.

* Add configuration options to enable I/O / time

New configuration options are added to `runtime::Builder` to allow
enabling I/O and time drivers on a runtime instance basis. This is
useful when wanting to create lightweight runtime instances to execute
compute only tasks.

* Bug fixes

The condition variable parker is updated to the same algorithm used in
`std`. This is motivated by some potential deadlock cases discovered by
`loom`.

The basic scheduler is fixed to fairly schedule tasks. `push_front` was
accidentally used instead of `push_back`.

I/O, time, and spawning now work from within `spawn_blocking` closures.

* Misc cleanup

The threaded scheduler is no longer generic over `P :Park`. Instead, it
is hard coded to a specific parker. Tests, including loom tests, are
updated to use `Runtime` directly. This provides greater coverage.

The `blocking` module is moved back into `runtime` as all usage is
within `runtime` itself.

1 files changed, 4 insertions, 6 deletions

diff --git a/tokio/src/sync/mod.rs b/tokio/src/sync/mod.rs
index 3ee563c3..2954eb85 100644
--- a/tokio/src/sync/mod.rs
+++ b/tokio/src/sync/mod.rs
@@ -1,3 +1,5 @@
+#![cfg_attr(loom, allow(dead_code, unreachable_pub, unused_imports))]
+
 //! Future-aware synchronization
 //!
 //! This module is enabled with the **`sync`** feature flag.
@@ -33,22 +35,18 @@ cfg_sync! {
 }
 
 cfg_not_sync! {
-    cfg_atomic_waker! {
+    cfg_resource_drivers! {
         mod task;
         pub(crate) use task::AtomicWaker;
     }
 
-    cfg_rt_threaded! {
+    cfg_rt_core! {
         pub(crate) mod oneshot;
     }
 
     cfg_signal! {
         pub(crate) mod mpsc;
         pub(crate) mod semaphore;
-
-        cfg_not_rt_threaded! {
-            pub(crate) mod oneshot;
-        }
     }
 }