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