linux-kernel - [PATCH 12/30] rcu: limit PREEMPT_RCU to full preemption under PREEMPT

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Date: Mon, 12 Feb 2024 21:55:36 -0800
From: Ankur Arora <ankur.a.arora@...cle.com>
To: linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, peterz@...radead.org, torvalds@...ux-foundation.org,
        paulmck@...nel.org, akpm@...ux-foundation.org, luto@...nel.org,
        bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
        willy@...radead.org, mgorman@...e.de, jpoimboe@...nel.org,
        mark.rutland@....com, jgross@...e.com, andrew.cooper3@...rix.com,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...LAB.COM, richard@....at, mjguzik@...il.com,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        Ankur Arora <ankur.a.arora@...cle.com>
Subject: [PATCH 12/30] rcu: limit PREEMPT_RCU to full preemption under PREEMPT_AUTO

Under PREEMPT_AUTO, CONFIG_PREEMPTION is enabled, and much like
PREEMPT_DYNAMIC, PREEMPT_AUTO also allows for dynamic switching
of preemption models.

The RCU model, however, is fixed at compile time.

Now, RCU typically selects PREEMPT_RCU if CONFIG_PREEMPTION is enabled.
Given the trade-offs between PREEMPT_RCU=y and PREEMPT_RCU=n, some
configurations might prefer the stronger forward-progress guarantees
of PREEMPT_RCU=n.

Accordingly, select PREEMPT_RCU=y only if the user selects
CONFIG_PREEMPT at compile time.

Suggested-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
 kernel/rcu/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index bdd7eadb33d8..d0ecc8ef17a7 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_DYNAMIC || PREEMPT_RT)
 	select TREE_RCU
 	help
 	  This option selects the RCU implementation that is
-- 
2.31.1