linux-kernel - [PATCH rcu 7/9] rcu: limit PREEMPT

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Message-Id: <20250116201327.3782963-7-paulmck@kernel.org>
Date: Thu, 16 Jan 2025 12:13:25 -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,
	Ankur Arora <ankur.a.arora@...cle.com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <frederic@...nel.org>
Subject: [PATCH rcu 7/9] rcu: limit PREEMPT_RCU configurations

From: Ankur Arora <ankur.a.arora@...cle.com>

PREEMPT_LAZY can be enabled stand-alone or alongside PREEMPT_DYNAMIC
which allows for dynamic switching of preemption models.

The choice of PREEMPT_RCU or not, however, is fixed at compile time.

Given that PREEMPT_RCU makes some trade-offs to optimize for latency
as opposed to throughput, configurations with limited preemption
might prefer the stronger forward-progress guarantees of PREEMPT_RCU=n.

Accordingly, explicitly limit PREEMPT_RCU=y to the latency oriented
preemption models: PREEMPT, PREEMPT_RT, and the runtime configurable
model PREEMPT_DYNAMIC.

This means the throughput oriented models, PREEMPT_NONE,
PREEMPT_VOLUNTARY, and PREEMPT_LAZY will run with PREEMPT_RCU=n.

Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 582d461c9ab7d..80242957fa417 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -18,7 +18,7 @@ config TREE_RCU

 config PREEMPT_RCU
 	bool
-	default y if PREEMPTION
+	default y if (PREEMPT || PREEMPT_RT || PREEMPT_DYNAMIC)
 	select TREE_RCU
 	help
 	  This option selects the RCU implementation that is
-- 
2.40.1