linux-kernel - [PATCH 4/9] rcu: Register rcu-lazy shrinker only for CONFIG_RCU

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Message-Id: <20230315194349.10798-4-joel@joelfernandes.org>
Date:   Wed, 15 Mar 2023 19:43:44 +0000
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>
Cc:     Zqiang <qiang1.zhang@...el.com>, rcu@...r.kernel.org
Subject: [PATCH 4/9] rcu: Register rcu-lazy shrinker only for CONFIG_RCU_LAZY=y kernels

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

The lazy_rcu_shrink_count() shrinker function is registered even in
kernels built with CONFIG_RCU_LAZY=n, in which case this function
uselessly consumes cycles learning that no CPU has any lazy callbacks
queued.

This commit therefore registers this shrinker function only in the kernels
built with CONFIG_RCU_LAZY=y, where it might actually do something useful.

Signed-off-by: Zqiang <qiang1.zhang@...el.com>
Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 kernel/rcu/tree_nocb.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 9e1c8caec5ce..f2280616f9d5 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1312,6 +1312,7 @@ int rcu_nocb_cpu_offload(int cpu)
 }
 EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
 
+#ifdef CONFIG_RCU_LAZY
 static unsigned long
 lazy_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
 {
@@ -1360,6 +1361,7 @@ static struct shrinker lazy_rcu_shrinker = {
 	.batch = 0,
 	.seeks = DEFAULT_SEEKS,
 };
+#endif // #ifdef CONFIG_RCU_LAZY
 
 void __init rcu_init_nohz(void)
 {
@@ -1391,8 +1393,10 @@ void __init rcu_init_nohz(void)
 	if (!rcu_state.nocb_is_setup)
 		return;
 
+#ifdef CONFIG_RCU_LAZY
 	if (register_shrinker(&lazy_rcu_shrinker, "rcu-lazy"))
 		pr_err("Failed to register lazy_rcu shrinker!\n");
+#endif // #ifdef CONFIG_RCU_LAZY
 
 	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
 		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
-- 
2.40.0.rc1.284.g88254d51c5-goog