rt: initial implementation of new threaded runtime (#5823)

This patch includes an initial implementation of a new multi-threaded
runtime. The new runtime aims to increase the scheduler throughput by
speeding up how it dispatches work to peer worker threads. This
implementation improves most benchmarks by about ~10% when the number of
threads is below 16. As threads increase, mutex contention deteriorates
performance.

Because the new scheduler is not yet ready to replace the old one, the
patch introduces it as an unstable runtime flavor with a warning that it
isn't production ready. Work to improve the scalability of the runtime
will most likely require more intrusive changes across Tokio, so I am
opting to merge with master to avoid larger conflicts.

.github/workflows/loom.yml

@@ -8,7 +8,9 @@ on:
 name: Loom
 
 env:
-  RUSTFLAGS: -Dwarnings
+  RUSTFLAGS: -Dwarnings --cfg loom --cfg tokio_unstable -C debug_assertions
+  LOOM_MAX_PREEMPTIONS: 2
+  LOOM_MAX_BRANCHES: 10000
   RUST_BACKTRACE: 1
   # Change to specific Rust release to pin
   rust_stable: stable
@@ -17,26 +19,66 @@ permissions:
   contents: read
 
 jobs:
-  loom:
-    name: loom
+  loom-sync:
+    name: loom tokio::sync
     # base_ref is null when it's not a pull request
-    if: github.repository_owner == 'tokio-rs' && (contains(github.event.pull_request.labels.*.name, 'R-loom') || (github.base_ref == null))
+    if: github.repository_owner == 'tokio-rs' && (contains(github.event.pull_request.labels.*.name, 'R-loom-sync') || (github.base_ref == null))
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install Rust ${{ env.rust_stable }}
+        uses: dtolnay/rust-toolchain@master
+        with:
+            toolchain: ${{ env.rust_stable }}
+      - uses: Swatinem/rust-cache@v2
+      - name: run tests
+        run: cargo test --lib --release --features full -- --nocapture sync::tests
+        working-directory: tokio
+
+  loom-time-driver:
+    name: loom time driver
+    # base_ref is null when it's not a pull request
+    if: github.repository_owner == 'tokio-rs' && (contains(github.event.pull_request.labels.*.name, 'R-loom-time-driver') || (github.base_ref == null))
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install Rust ${{ env.rust_stable }}
+        uses: dtolnay/rust-toolchain@master
+        with:
+            toolchain: ${{ env.rust_stable }}
+      - uses: Swatinem/rust-cache@v2
+      - name: run tests
+        run: cargo test --lib --release --features full -- --nocapture runtime::time::tests
+        working-directory: tokio
+
+  loom-current-thread:
+    name: loom current-thread scheduler
+    # base_ref is null when it's not a pull request
+    if: github.repository_owner == 'tokio-rs' && (contains(github.event.pull_request.labels.*.name, 'R-loom-current-thread') || (github.base_ref == null))
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install Rust ${{ env.rust_stable }}
+        uses: dtolnay/rust-toolchain@master
+        with:
+            toolchain: ${{ env.rust_stable }}
+      - uses: Swatinem/rust-cache@v2
+      - name: run tests
+        run: cargo test --lib --release --features full -- --nocapture loom_current_thread
+        working-directory: tokio
+
+  loom-multi-thread:
+    name: loom multi-thread scheduler
+    # base_ref is null when it's not a pull request
+    if: github.repository_owner == 'tokio-rs' && (contains(github.event.pull_request.labels.*.name, 'R-loom-multi-thread') || (github.base_ref == null))
     runs-on: ubuntu-latest
     strategy:
       matrix:
         include:
-          - scope: --skip loom_pool
-            max_preemptions: 2
-          - scope: loom_pool::group_a
-            max_preemptions: 2
-          - scope: loom_pool::group_b
-            max_preemptions: 2
-          - scope: loom_pool::group_c
-            max_preemptions: 2
-          - scope: loom_pool::group_d
-            max_preemptions: 2
-          - scope: time::driver
-            max_preemptions: 2
+          - scope: loom_multi_thread::group_a
+          - scope: loom_multi_thread::group_b
+          - scope: loom_multi_thread::group_c
+          - scope: loom_multi_thread::group_d
     steps:
       - uses: actions/checkout@v3
       - name: Install Rust ${{ env.rust_stable }}
@@ -45,10 +87,34 @@ jobs:
             toolchain: ${{ env.rust_stable }}
       - uses: Swatinem/rust-cache@v2
       - name: loom ${{ matrix.scope }}
-        run: cargo test --lib --release --features full -- --nocapture $SCOPE
+        run: cargo test --lib --release --features full -- $SCOPE
         working-directory: tokio
         env:
-          RUSTFLAGS: --cfg loom --cfg tokio_unstable -Dwarnings -C debug-assertions
-          LOOM_MAX_PREEMPTIONS: ${{ matrix.max_preemptions }}
-          LOOM_MAX_BRANCHES: 10000
           SCOPE: ${{ matrix.scope }}
+
+  loom-multi-thread-alt:
+    name: loom ALT multi-thread scheduler
+    # base_ref is null when it's not a pull request
+    if: github.repository_owner == 'tokio-rs' && (contains(github.event.pull_request.labels.*.name, 'R-loom-multi-thread-alt') || (github.base_ref == null))
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        include:
+          - scope: loom_multi_thread_alt::group_a
+          - scope: loom_multi_thread_alt::group_b
+          - scope: loom_multi_thread_alt::group_c
+          - scope: loom_multi_thread_alt::group_d
+    steps:
+      - uses: actions/checkout@v3
+      - name: Install Rust ${{ env.rust_stable }}
+        uses: dtolnay/rust-toolchain@master
+        with:
+            toolchain: ${{ env.rust_stable }}
+      - uses: Swatinem/rust-cache@v2
+      - name: loom ${{ matrix.scope }}
+        run: cargo test --lib --release --features full -- $SCOPE
+        working-directory: tokio
+        env:
+          SCOPE: ${{ matrix.scope }}
+          # TODO: remove this before stabilizing
+          LOOM_MAX_PREEMPTIONS: 1