Re: [PATCH rcu 3/9] rcu: Add mutex for rcu boost kthread spawning and affinity setting

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Feb 04, 2022 at 03:07:59PM -0800, Paul E. McKenney wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> As we handle parallel CPU bringup, we will need to take care to avoid
> spawning multiple boost threads, or race conditions when setting their
> affinity. Spotted by Paul McKenney.
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>

Speaking of, we have:

rcu_init()
   for_each_online_cpu(cpu) // should be boot CPU only at this stage ?
       rcutree_prepare_cpu(cpu)
           rcu_spawn_one_boost_kthread(cpu)


early_initcall()
    rcu_spawn_gp_kthread()
        rcu_spawn_boost_kthreads()
	    rcu_for_each_leaf_node(rnp)
	        rcu_rnp_online_cpus(rnp) // as above, only boot CPU at this stage.
                    rcu_spawn_one_boost_kthread(cpu)

cpu_up()
    rcutree_prepare_cpu(cpu)
        rcu_spawn_one_boost_kthread(cpu)


My guess is that we could remove rcu_spawn_boost_kthreads() and simplify
rcu_init(). Something like this (untested yet):

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 86eec6a0f1a1..da8ac2b6f8cc 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4526,7 +4526,6 @@ 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();
 	rcu_spawn_core_kthreads();
 	return 0;
 }
@@ -4813,7 +4812,7 @@ static void __init kfree_rcu_batch_init(void)
 
 void __init rcu_init(void)
 {
-	int cpu;
+	int cpu = smp_processor_id();
 
 	rcu_early_boot_tests();
 
@@ -4833,11 +4832,10 @@ void __init rcu_init(void)
 	 * or the scheduler are operational.
 	 */
 	pm_notifier(rcu_pm_notify, 0);
-	for_each_online_cpu(cpu) {
-		rcutree_prepare_cpu(cpu);
-		rcu_cpu_starting(cpu);
-		rcutree_online_cpu(cpu);
-	}
+
+	rcutree_prepare_cpu(cpu);
+	rcu_cpu_starting(cpu);
+	rcutree_online_cpu(cpu);
 
 	/* Create workqueue for Tree SRCU and for expedited GPs. */
 	rcu_gp_wq = alloc_workqueue("rcu_gp", WQ_MEM_RECLAIM, 0);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 6082dd23408f..90925a589774 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 */
 
 /*