linux-kernel - [PATCH rcu 5/7] rcu: Add CONFIG_RCU_LAZY delays to call

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Message-Id: <20250116200239.3782374-5-paulmck@kernel.org>
Date: Thu, 16 Jan 2025 12:02:37 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org,
	kernel-team@...a.com,
	rostedt@...dmis.org,
	"Paul E. McKenney" <paulmck@...nel.org>
Subject: [PATCH rcu 5/7] rcu: Add CONFIG_RCU_LAZY delays to call_rcu() kernel-doc header

This commit adds a description of the energy-efficiency delays that
call_rcu() can impose, along with a pointer to call_rcu_hurry() for
latency-sensitive kernel code.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index befe35058c49a..229f427b8c821 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3193,6 +3193,13 @@ EXPORT_SYMBOL_GPL(call_rcu_hurry);
  *
  * Implementation of these memory-ordering guarantees is described here:
  * Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst.
+ *
+ * Specific to call_rcu() (as opposed to the other call_rcu*() functions),
+ * in kernels built with CONFIG_RCU_LAZY=y, call_rcu() might delay for many
+ * seconds before starting the grace period needed by the corresponding
+ * callback.  This delay can significantly improve energy-efficiency
+ * on low-utilization battery-powered devices.  To avoid this delay,
+ * in latency-sensitive kernel code, use call_rcu_hurry().
  */
 void call_rcu(struct rcu_head *head, rcu_callback_t func)
 {
-- 
2.40.1