linux-kernel - [PATCH rcu 6/7] rcu: Clarify RCU_LAZY and RCU_LAZY_DEFAULT

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Message-Id: <20250218054547.7364-7-boqun.feng@gmail.com>
Date: Mon, 17 Feb 2025 21:45:46 -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 6/7] rcu: Clarify RCU_LAZY and RCU_LAZY_DEFAULT_OFF help text

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

This commit wordsmiths the RCU_LAZY and RCU_LAZY_DEFAULT_OFF Kconfig
options' help text.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Boqun Feng <boqun.feng@...il.com>
---
 kernel/rcu/Kconfig | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index b9b6bc55185d..2bb22dac3b5a 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -323,21 +323,27 @@ config RCU_LAZY
 	depends on RCU_NOCB_CPU
 	default n
 	help
-	  To save power, batch RCU callbacks and flush after delay, memory
-	  pressure, or callback list growing too big.
+	  To save power, batch RCU callbacks and delay starting the
+	  corresponding grace  period for multiple seconds.  The grace
+	  period will be started after this delay, in case of memory
+	  pressure, or if the corresponding CPU's callback list grows
+	  too large.
 
-	  Requires rcu_nocbs=all to be set.
+	  These delays happen only on rcu_nocbs CPUs, that is, CPUs
+	  whose callbacks have been offloaded.
 
-	  Use rcutree.enable_rcu_lazy=0 to turn it off at boot time.
+	  Use the rcutree.enable_rcu_lazy=0 kernel-boot parameter to
+	  globally disable these delays.
 
 config RCU_LAZY_DEFAULT_OFF
 	bool "Turn RCU lazy invocation off by default"
 	depends on RCU_LAZY
 	default n
 	help
-	  Allows building the kernel with CONFIG_RCU_LAZY=y yet keep it default
-	  off. Boot time param rcutree.enable_rcu_lazy=1 can be used to switch
-	  it back on.
+	  Build the kernel with CONFIG_RCU_LAZY=y, but cause the kernel
+	  to boot with these energy-efficiency delays disabled.  Use the
+	  rcutree.enable_rcu_lazy=0 kernel-boot parameter to override
+	  the this option at boot time, thus re-enabling these delays.
 
 config RCU_DOUBLE_CHECK_CB_TIME
 	bool "RCU callback-batch backup time check"
-- 
2.39.5 (Apple Git-154)