MINOR: tasks: restore the lower latency scheduling when niced tasks are present

In the past we used to reduce the number of tasks consulted at once when
some niced tasks were present in the run queue. This was dropped in 1.8
when the scheduler started to take batches. With the recent fixes it now
becomes possible to restore this behaviour which guarantees a better
latency between tasks when niced tasks are present. Thanks to this, with
the default number of 200 for tune.runqueue-depth, with a parasitic load
of 14000 requests per second, nice 0 gives 14000 rps, nice 1024 gives
12000 rps and nice -1024 gives 16000 rps. The amplitude widens if the
runqueue depth is lowered.

src/task.c

@@ -325,6 +325,9 @@ void process_runnable_tasks()
 	nb_tasks_cur = nb_tasks;
 	max_processed = global.tune.runqueue_depth;
 
+	if (likely(niced_tasks))
+		max_processed = (max_processed + 3) / 4;
+
 	/* Note: the grq lock is always held when grq is not null */
 
 	while (task_per_thread[tid].task_list_size < max_processed) {