scheduling_group_demo: add explicit yields since future::get() no lon…

…ger does

With 7e2483a, future::get() is no longer an implicit yield. This causes
the threads spawned in scheduling_group_demo to never yield and the demo
no longer works.

Adding explicit yields makes it work again.
Message-Id: <[email protected]>

demos/scheduling_group_demo.cc

@@ -80,6 +80,7 @@ run_compute_intensive_tasks(seastar::scheduling_group sg, done_func done, unsign
                     return task(counter);
                 });
             }).get();
+            thread::maybe_yield();
         }
     });
 }
@@ -92,6 +93,7 @@ run_compute_intensive_tasks_in_threads(seastar::scheduling_group sg, done_func d
         return seastar::async(attr, [done, &counter, task] {
             while (!done()) {
                 task(counter).get();
+                thread::maybe_yield();
             }
         });
     });
@@ -113,6 +115,7 @@ run_with_duty_cycle(float utilization, std::chrono::steady_clock::duration perio
         while (!done()) {
             while (!combined_done()) {
                 task(std::cref(combined_done)).get();
+                thread::maybe_yield();
             }
             cv.wait([&] {
                 return done() || duty_toggle;