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

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Message-Id: <20250218054547.7364-6-boqun.feng@gmail.com>
Date: Mon, 17 Feb 2025 21:45:45 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: rcu@...r.kernel.org
Cc: "Paul E. McKenney" <paulmck@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Uladzislau Rezki <urezki@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	Jonathan Corbet <corbet@....net>,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH rcu 5/7] rcu: Add CONFIG_RCU_LAZY delays to call_rcu() kernel-doc header

From: "Paul E. McKenney" <paulmck@...nel.org>

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>
Signed-off-by: Boqun Feng <boqun.feng@...il.com>
---
 kernel/rcu/tree.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2cd193ed854c..780e4b30febb 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3172,6 +3172,13 @@ 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)
 {
-- 
2.39.5 (Apple Git-154)