On Fri, Feb 11, 2022 at 03:57:57PM +0100, Frederic Weisbecker wrote:
> 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 */
>
> /*
nocb kthread creation is similar but it depends on the gp kthread.
So we can't rely on rcu_init() -> rcu_prepare_cpu() and we must keep
the early_initcall() -> rcu_spawn_gp_kthread().
That would become (untested again):
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index da8ac2b6f8cc..9284625a9a50 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4525,7 +4525,7 @@ static int __init rcu_spawn_gp_kthread(void)
smp_store_release(&rcu_state.gp_kthread, t); /* ^^^ */
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
wake_up_process(t);
- rcu_spawn_nocb_kthreads();
+ rcu_spawn_cpu_nocb_kthread(smp_processor_id());
rcu_spawn_core_kthreads();
return 0;
}
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 636d0546a4e9..711f6eb7f7e1 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1277,22 +1277,6 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
WRITE_ONCE(rdp->nocb_gp_kthread, rdp_gp->nocb_gp_kthread);
}
-/*
- * Once the scheduler is running, spawn rcuo kthreads for all online
- * no-CBs CPUs. This assumes that the early_initcall()s happen before
- * non-boot CPUs come online -- if this changes, we will need to add
- * some mutual exclusion.
- */
-static void __init rcu_spawn_nocb_kthreads(void)
-{
- int cpu;
-
- if (rcu_nocb_is_setup) {
- for_each_online_cpu(cpu)
- rcu_spawn_cpu_nocb_kthread(cpu);
- }
-}
-
/* How many CB CPU IDs per GP kthread? Default of -1 for sqrt(nr_cpu_ids). */
static int rcu_nocb_gp_stride = -1;
module_param(rcu_nocb_gp_stride, int, 0444);
@@ -1549,10 +1533,6 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
{
}
-static void __init rcu_spawn_nocb_kthreads(void)
-{
-}
-
static void show_rcu_nocb_state(struct rcu_data *rdp)
{
}
