rt: cap fifo scheduler slot to avoid starvation (#2349)

The work-stealing scheduler includes an optimization where each worker
includes a single slot to store the **last** scheduled task. Tasks in
scheduler's LIFO slot are executed next. This speeds up and reduces
latency with message passing patterns.

Previously, this optimization was susceptible to starving other tasks in
certain cases. If two tasks ping-ping between each other without ever
yielding, the worker would never execute other tasks.

An early PR (#2160) introduced a form of pre-emption. Each task is
allocated a per-poll operation budget. Tokio resources will return ready
until the budget is depleted, at which point, Tokio resources will
always return `Pending`.

This patch leverages the operation budget to limit the LIFO scheduler
optimization. When executing tasks from the LIFO slot, the budget is
**not** reset. Once the budget goes to zero, the task in the LIFO slot
is pushed to the back of the queue.

1 files changed, 7 insertions, 0 deletions

diff --git a/tokio/src/coop.rs b/tokio/src/coop.rs
index 19302559..78905e3c 100644
--- a/tokio/src/coop.rs
+++ b/tokio/src/coop.rs
@@ -98,6 +98,13 @@ where
     })
 }
 
+cfg_rt_threaded! {
+    #[inline(always)]
+    pub(crate) fn has_budget_remaining() -> bool {
+        HITS.with(|hits| hits.get() > 0)
+    }
+}
+
 cfg_blocking_impl! {
     /// Forcibly remove the budgeting constraints early.
     pub(crate) fn stop() {