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

From: Frederic Weisbecker
Date: Fri Feb 11 2022 - 09:58:05 EST


On Fri, Feb 04, 2022 at 03:07:59PM -0800, Paul E. McKenney wrote:
> From: David Woodhouse <dwmw@xxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

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 */

/*