Ensure yield-equivalent with an already-expired timeout.

PiperOrigin-RevId: 264920977

pkg/sentry/kernel/task_block.go

@@ -15,6 +15,7 @@
 package kernel
 
 import (
+	"runtime"
 	"time"
 
 	ktime "gvisor.dev/gvisor/pkg/sentry/kernel/time"
@@ -121,6 +122,17 @@ func (t *Task) block(C <-chan struct{}, timerChan <-chan struct{}) error {
 	// Deactive our address space, we don't need it.
 	interrupt := t.SleepStart()
 
+	// If the request is not completed, but the timer has already expired,
+	// then ensure that we run through a scheduler cycle. This is because
+	// we may see applications relying on timer slack to yield the thread.
+	// For example, they may attempt to sleep for some number of nanoseconds,
+	// and expect that this will actually yield the CPU and sleep for at
+	// least microseconds, e.g.:
+	// https://github.com/LMAX-Exchange/disruptor/commit/6ca210f2bcd23f703c479804d583718e16f43c07
+	if len(timerChan) > 0 {
+		runtime.Gosched()
+	}
+
 	select {
 	case <-C:
 		t.SleepFinish(true)