rcu: Create rcuo kthreads only for onlined CPUs

RCU currently uses for_each_possible_cpu() to spawn rcuo kthreads,
which can result in more rcuo kthreads than one would expect, for
example, derRichard reported 64 CPUs worth of rcuo kthreads on an
8-CPU image.  This commit therefore creates rcuo kthreads only for
those CPUs that actually come online.

This was reported by derRichard on the OFTC IRC network.

Reported-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Tested-by: Paul Gortmaker <paul.gortmaker@windriver.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9be47f43903b..b49c8433f834 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3442,6 +3442,7 @@ static int rcu_cpu_notify(struct notifier_block *self,
         case CPU_UP_PREPARE_FROZEN:
                 rcu_prepare_cpu(cpu);
                 rcu_prepare_kthreads(cpu);
+                rcu_spawn_all_nocb_kthreads(cpu);
                 break;
         case CPU_ONLINE:
         case CPU_DOWN_FAILED:
@@ -3506,8 +3507,8 @@ static int __init rcu_spawn_gp_kthread(void)
                 raw_spin_lock_irqsave(&rnp->lock, flags);
                 rsp->gp_kthread = t;
                 raw_spin_unlock_irqrestore(&rnp->lock, flags);
-                rcu_spawn_nocb_kthreads(rsp);
         }
+        rcu_spawn_nocb_kthreads();
         rcu_spawn_boost_kthreads();
         return 0;
 }