moved cooldown

Author: VSadov

src/libraries/System.Private.CoreLib/src/System/Threading/LowLevelLifoSemaphore.cs

@@ -202,6 +202,14 @@ private bool WaitAsWaiter(int timeoutMs, bool allowFastWake)
 
                 if (!allowFastWake)
                 {
+                    // The caller wants that the thread spends some time waiting as a matter of rate limiting
+                    // thus we will require a 4 usec cooldown before reintroducing the thread.
+                    // The sleep/wake transition typically takes care of the wait, but the blocker has fast
+                    // wake paths and the underlying OS API may have fast/trivial wake paths as well,
+                    // thus fast wakeups can happen and are hard to avoid completely.
+                    // So, if a fast wake happened when parking was desired, we hold up the thread a bit
+                    // before releasing.
+                    long cooldown = Stopwatch.Frequency * 4 / 1000000;
                     while (Stopwatch.GetTimestamp() - blockingStart < cooldown)
                     {
                         Thread.SpinWait(1);