rcu: Register rcu-lazy shrinker only for CONFIG_RCU_LAZY=y kernels

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@intel.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 9e1c8caec5ceb983a0422954c0383a191643b3c2..f2280616f9d51bdac29f94fc2381696af790ec7a 100644 (file)
--- 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");