linux-kernel - [PATCH 5/5] rcu/nocb: Initialize nocb kthreads only for boot CPU prior SMP initialization

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Message-Id: <20220216154208.696069-6-frederic@kernel.org>
Date:   Wed, 16 Feb 2022 16:42:08 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Uladzislau Rezki <uladzislau.rezki@...y.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: [PATCH 5/5] rcu/nocb: Initialize nocb kthreads only for boot CPU prior SMP initialization

rcu_spawn_gp_kthread() is called as an early initcall, which means that
SMP initialization hasn't happened yet and the boot CPU is the only one
online. Therefore only the NOCB kthreads related to the boot CPU need
to be created at this stage.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc: Uladzislau Rezki <uladzislau.rezki@...y.com>
Cc: Joel Fernandes <joel@...lfernandes.org>
Cc: Boqun Feng <boqun.feng@...il.com>
---
 kernel/rcu/tree.c      |  6 +++++-
 kernel/rcu/tree.h      |  1 -
 kernel/rcu/tree_nocb.h | 20 --------------------
 3 files changed, 5 insertions(+), 22 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 4a98417c2e28..3e9a8ec222e4 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4526,9 +4526,13 @@ 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();
 	/* This is a pre-SMP initcall, we expect a single CPU */
 	WARN_ON(num_online_cpus() > 1);
+	/*
+	 * Those kthreads couldn't be created on rcu_init() -> rcutree_prepare_cpu()
+	 * due to rcu_scheduler_fully_active.
+	 */
+	rcu_spawn_cpu_nocb_kthread(smp_processor_id());
 	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 48c772fc353f..e3296ecde277 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -443,7 +443,6 @@ static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level);
 static bool do_nocb_deferred_wakeup(struct rcu_data *rdp);
 static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
 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);
 static void rcu_nocb_lock(struct rcu_data *rdp);
 static void rcu_nocb_unlock(struct rcu_data *rdp);
diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 494e65445773..9d6d4786bc70 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1266,22 +1266,6 @@ 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_state.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);
@@ -1538,10 +1522,6 @@ 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)
 {
 }
-- 
2.25.1