[PATCH rcu 5/7] rcu: Add CONFIG_RCU_LAZY delays to call_rcu() kernel-doc header

[Boqun Feng <boqun.feng@gmail.com>]

From: "Paul E. McKenney" <paulmck@kernel.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@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.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)