[RFC] rcu: Warn that rcu ktheads cannot be spawned

[Byungchul Park <byungchul.park@xxxxxxx>]

Hello rcu folks,

I thought it'd better to announce it if those spawnings fail because of
!rcu_scheduler_fully_active.

Of course, with the current code, it never happens though.

Thoughts?

Thanks,
Byungchul

---8<---
>From 58a33a85c70f82c406319b4752af95cf6ceb73a3 Mon Sep 17 00:00:00 2001
From: Byungchul Park <byungchul.park@xxxxxxx>
Date: Mon, 24 Jun 2019 17:08:26 +0900
Subject: [RFC] rcu: Warn that rcu ktheads cannot be spawned

In case that rcu ktheads cannot be spawned due to
!rcu_scheduler_fully_active, it'd be better to anounce it.

While at it, because the return value of rcu_spawn_one_boost_kthread()
is not used any longer, changed the return type from int to void.

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
 kernel/rcu/tree_plugin.h | 31 +++++++++++++++++++------------
 1 file changed, 19 insertions(+), 12 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 1102765..7d74193 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1131,7 +1131,7 @@ static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
  * already exist.  We only create this kthread for preemptible RCU.
  * Returns zero if all is well, a negated errno otherwise.
  */
-static int rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
+static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
 {
 	int rnp_index = rnp - rcu_get_root();
 	unsigned long flags;
@@ -1139,25 +1139,24 @@ static int rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
 	struct task_struct *t;
 
 	if (!IS_ENABLED(CONFIG_PREEMPT_RCU))
-		return 0;
+		return;
 
-	if (!rcu_scheduler_fully_active || rcu_rnp_online_cpus(rnp) == 0)
-		return 0;
+	if (rcu_rnp_online_cpus(rnp) == 0)
+		return;
 
 	rcu_state.boost = 1;
 	if (rnp->boost_kthread_task != NULL)
-		return 0;
+		return;
 	t = kthread_create(rcu_boost_kthread, (void *)rnp,
 			   "rcub/%d", rnp_index);
 	if (IS_ERR(t))
-		return PTR_ERR(t);
+		return;
 	raw_spin_lock_irqsave_rcu_node(rnp, flags);
 	rnp->boost_kthread_task = t;
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	sp.sched_priority = kthread_prio;
 	sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
 	wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
-	return 0;
 }
 
 static void rcu_cpu_kthread_setup(unsigned int cpu)
@@ -1264,8 +1263,12 @@ static void __init rcu_spawn_boost_kthreads(void)
 		per_cpu(rcu_data.rcu_cpu_has_work, cpu) = 0;
 	if (WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec), "%s: Could not start rcub kthread, OOM is now expected behavior\n", __func__))
 		return;
+
+	if (WARN_ON(!rcu_scheduler_fully_active))
+		return;
+
 	rcu_for_each_leaf_node(rnp)
-		(void)rcu_spawn_one_boost_kthread(rnp);
+		rcu_spawn_one_boost_kthread(rnp);
 }
 
 static void rcu_prepare_kthreads(int cpu)
@@ -1273,9 +1276,11 @@ static void rcu_prepare_kthreads(int cpu)
 	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
 	struct rcu_node *rnp = rdp->mynode;
 
+	if (WARN_ON(!rcu_scheduler_fully_active))
+		return;
+
 	/* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
-	if (rcu_scheduler_fully_active)
-		(void)rcu_spawn_one_boost_kthread(rnp);
+	rcu_spawn_one_boost_kthread(rnp);
 }
 
 #else /* #ifdef CONFIG_RCU_BOOST */
@@ -2198,8 +2203,10 @@ static void rcu_spawn_one_nocb_kthread(int cpu)
  */
 static void rcu_spawn_cpu_nocb_kthread(int cpu)
 {
-	if (rcu_scheduler_fully_active)
-		rcu_spawn_one_nocb_kthread(cpu);
+	if (WARN_ON(!rcu_scheduler_fully_active))
+		return;
+
+	rcu_spawn_one_nocb_kthread(cpu);
 }
 
 /*
-- 
1.9.1