linux-kernel - [PATCH 4/6] rcu/nocb: Create nocb kthreads on all CPUs as long as the "rcu

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Date:   Wed, 17 Nov 2021 16:56:35 +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>,
        Uladzislau Rezki <urezki@...il.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: [PATCH 4/6] rcu/nocb: Create nocb kthreads on all CPUs as long as the "rcu_nocb=" is passed

In order to be able to (de-)offload any CPU using cpuset in the future,
create a NOCB kthread for all possible CPUs. For now this is done only
as long as the "rcu_nocb=" kernel parameter is passed to avoid
the unnecessary overhead for most users.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Uladzislau Rezki <urezki@...il.com>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: Joel Fernandes <joel@...lfernandes.org>
---
 kernel/rcu/tree_nocb.h | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 9fe4be10fde7..1871f15b8472 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1221,11 +1221,8 @@ static void rcu_spawn_one_nocb_kthread(int cpu)
 	struct rcu_data *rdp_gp;
 	struct task_struct *t;
 
-	/*
-	 * If this isn't a no-CBs CPU or if it already has an rcuo kthread,
-	 * then nothing to do.
-	 */
-	if (!rcu_is_nocb_cpu(cpu) || rdp->nocb_cb_kthread)
+	/* If it already has an rcuo kthread, then nothing to do. */
+	if (rdp->nocb_cb_kthread)
 		return;
 
 	/* If we didn't spawn the GP kthread first, reorganize! */
@@ -1253,7 +1250,7 @@ static void rcu_spawn_one_nocb_kthread(int cpu)
  */
 static void rcu_spawn_cpu_nocb_kthread(int cpu)
 {
-	if (rcu_scheduler_fully_active)
+	if (rcu_scheduler_fully_active && rcu_nocb_is_setup)
 		rcu_spawn_one_nocb_kthread(cpu);
 }
 
@@ -1268,7 +1265,7 @@ static void __init rcu_spawn_nocb_kthreads(void)
 	int cpu;
 
 	if (rcu_nocb_is_setup) {
-		for_each_online_cpu(cpu)
+		for_each_possible_cpu(cpu)
 			rcu_spawn_cpu_nocb_kthread(cpu);
 	}
 }
@@ -1303,7 +1300,7 @@ static void __init rcu_organize_nocb_kthreads(void)
 	 * Should the corresponding CPU come online in the future, then
 	 * we will spawn the needed set of rcu_nocb_kthread() kthreads.
 	 */
-	for_each_cpu(cpu, rcu_nocb_mask) {
+	for_each_possible_cpu(cpu) {
 		rdp = per_cpu_ptr(&rcu_data, cpu);
 		if (rdp->cpu >= nl) {
 			/* New GP kthread, set up for CBs & next GP. */
@@ -1327,7 +1324,8 @@ static void __init rcu_organize_nocb_kthreads(void)
 				pr_cont(" %d", cpu);
 		}
 		rdp->nocb_gp_rdp = rdp_gp;
-		list_add_tail(&rdp->nocb_entry_rdp, &rdp_gp->nocb_head_rdp);
+		if (cpumask_test_cpu(cpu, rcu_nocb_mask))
+			list_add_tail(&rdp->nocb_entry_rdp, &rdp_gp->nocb_head_rdp);
 	}
 	if (gotnocbs && dump_tree)
 		pr_cont("%s\n", gotnocbscbs ? "" : " (self only)");
-- 
2.25.1