Default value of the config structure's fields must be off.

src/sched_policies/detect_combined_workers.c

@@ -171,7 +171,7 @@ void _starpu_sched_find_worker_combinations(struct starpu_machine_topology_s *to
 {
 	struct starpu_machine_config_s *config = _starpu_get_machine_config();
 
-	if ((config->user_conf && config->user_conf->single_combined_worker) || starpu_get_env_number("STARPU_SINGLE_COMBINED_WORKER") > 0)
+	if (config->user_conf && config->user_conf->single_combined_worker > 0 || starpu_get_env_number("STARPU_SINGLE_COMBINED_WORKER") > 0)
 		combine_all_cpu_workers(topology);
 	else {
 #ifdef STARPU_HAVE_HWLOC