linux-kernel - [PATCH rcu 04/10] rcu: Create per-cpu rcuc kthreads only when rcutree.use

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Date:   Fri,  4 Feb 2022 15:24:00 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, Zqiang <qiang1.zhang@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 04/10] rcu: Create per-cpu rcuc kthreads only when rcutree.use_softirq=0

From: Zqiang <qiang1.zhang@...el.com>

The per-CPU "rcuc" kthreads are used only by kernels booted with
rcutree.use_softirq=0, but they are nevertheless unconditionally created
by kernels built with CONFIG_RCU_BOOST=y.  This results in "rcuc"
kthreads being created that are never actually used.  This commit
therefore refrains from creating these kthreads unless the kernel
is actually booted with rcutree.use_softirq=0.

Acked-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Signed-off-by: Zqiang <qiang1.zhang@...el.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a4c25a6283b0b..4e5e37e5ee3c9 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2894,7 +2894,7 @@ static int __init rcu_spawn_core_kthreads(void)
 
 	for_each_possible_cpu(cpu)
 		per_cpu(rcu_data.rcu_cpu_has_work, cpu) = 0;
-	if (!IS_ENABLED(CONFIG_RCU_BOOST) && use_softirq)
+	if (use_softirq)
 		return 0;
 	WARN_ONCE(smpboot_register_percpu_thread(&rcu_cpu_thread_spec),
 		  "%s: Could not start rcuc kthread, OOM is now expected behavior\n", __func__);
-- 
2.31.1.189.g2e36527f23