[PATCH rcu 4/5] rcu: Initialize boost kthread only for boot node prior SMP initialization

["Paul E. McKenney" <paulmck@kernel.org>]

From: Frederic Weisbecker <frederic@kernel.org>

The rcu_spawn_gp_kthread() function is called as an early initcall,
which means that SMP initialization hasn't happened yet and only the
boot CPU is online.  Therefore, create only the boost kthread for the
leaf node of the boot CPU.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Uladzislau Rezki <uladzislau.rezki@sony.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree.c        |  5 ++++-
 kernel/rcu/tree.h        |  1 -
 kernel/rcu/tree_plugin.h | 16 ----------------
 3 files changed, 4 insertions(+), 18 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e6a9e5744e45..70b33c55d39a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4480,6 +4480,7 @@ static int __init rcu_spawn_gp_kthread(void)
 	struct rcu_node *rnp;
 	struct sched_param sp;
 	struct task_struct *t;
+	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
 	rcu_scheduler_fully_active = 1;
 	t = kthread_create(rcu_gp_kthread, NULL, "%s", rcu_state.name);
@@ -4498,7 +4499,9 @@ static int __init rcu_spawn_gp_kthread(void)
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	wake_up_process(t);
 	rcu_spawn_nocb_kthreads();
-	rcu_spawn_boost_kthreads();
+	/* This is a pre-SMP initcall, we expect a single CPU */
+	WARN_ON(num_online_cpus() > 1);
+	rcu_spawn_one_boost_kthread(rdp->mynode);
 	rcu_spawn_core_kthreads();
 	return 0;
 }
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index f6a3d54585c9..b2a0f2613ab9 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -422,7 +422,6 @@ static void rcu_preempt_boost_start_gp(struct rcu_node *rnp);
 static bool rcu_is_callbacks_kthread(void);
 static void rcu_cpu_kthread_setup(unsigned int cpu);
 static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp);
-static void __init rcu_spawn_boost_kthreads(void);
 static bool rcu_preempt_has_tasks(struct rcu_node *rnp);
 static bool rcu_preempt_need_deferred_qs(struct task_struct *t);
 static void rcu_preempt_deferred_qs(struct task_struct *t);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 8360d86db1c0..b139635f33bd 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1226,18 +1226,6 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 	free_cpumask_var(cm);
 }
 
-/*
- * Spawn boost kthreads -- called as soon as the scheduler is running.
- */
-static void __init rcu_spawn_boost_kthreads(void)
-{
-	struct rcu_node *rnp;
-
-	rcu_for_each_leaf_node(rnp)
-		if (rcu_rnp_online_cpus(rnp))
-			rcu_spawn_one_boost_kthread(rnp);
-}
-
 #else /* #ifdef CONFIG_RCU_BOOST */
 
 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
@@ -1263,10 +1251,6 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 {
 }
 
-static void __init rcu_spawn_boost_kthreads(void)
-{
-}
-
 #endif /* #else #ifdef CONFIG_RCU_BOOST */
 
 /*
-- 
2.31.1.189.g2e36527f23