rcu: Don't spawn rcub kthreads on root rcu_node structure

Now that offlining CPUs no longer moves leaf rcu_node structures'
->blkd_tasks lists to the root, there is no way for the root rcu_node
structure's ->blkd_task list to be nonempty, unless the root node is also
the sole leaf node.  This commit therefore refrains from creating an rcub
kthread for the root rcu_node structure unless it is also the sole leaf.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 2 insertions, 6 deletions

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index bcc6d9134d47..3e64bb197b1a 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1352,12 +1352,8 @@ static void __init rcu_spawn_boost_kthreads(void)
         for_each_possible_cpu(cpu)
                 per_cpu(rcu_cpu_has_work, cpu) = 0;
         BUG_ON(smpboot_register_percpu_thread(&rcu_cpu_thread_spec));
-        rnp = rcu_get_root(rcu_state_p);
-        (void)rcu_spawn_one_boost_kthread(rcu_state_p, rnp);
-        if (NUM_RCU_NODES > 1) {
-                rcu_for_each_leaf_node(rcu_state_p, rnp)
-                        (void)rcu_spawn_one_boost_kthread(rcu_state_p, rnp);
-        }
+        rcu_for_each_leaf_node(rcu_state_p, rnp)
+                (void)rcu_spawn_one_boost_kthread(rcu_state_p, rnp);
 }
 
 static void rcu_prepare_kthreads(int cpu)