Add yielding to scheduler loop.

We yield after num_thread failed steals in a row. This parameter can be tuned for better performance, but we stick to a sensible default just to prevent massive spinning.

lib/pls/src/internal/scheduling/scheduler.cpp

@@ -68,6 +68,7 @@ void scheduler::work_thread_work_section() {
     main_thread_starter_function_->run();
   }
 
+  unsigned int failed_steals = 0;
   while (!work_section_done_) {
     PLS_ASSERT(my_task_manager.check_task_chain(), "Must start stealing with a clean task chain.");
 
@@ -111,6 +112,12 @@ void scheduler::work_thread_work_section() {
       stolen_task->is_synchronized_ = false;
       context_switcher::switch_context(std::move(stolen_task->continuation_));
       // We will continue execution in this line when we finished the stolen work.
+      failed_steals = 0;
+    } else {
+      failed_steals++;
+      if (failed_steals >= num_threads) {
+        base::this_thread::yield();
+      }
     }
   }
 }