linux-kernel - [PATCH 14/30] preempt,rcu: warn on PREEMPT

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Date: Mon, 12 Feb 2024 21:55:38 -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 14/30] preempt,rcu: warn on PREEMPT_RCU=n, preempt=full

The combination of PREEMPT_RCU=n and (PREEMPT_AUTO=y, preempt=full)
works at cross purposes: the RCU read side critical sections disable
preemption, while preempt=full schedules eagerly to minimize
latency.

For now, warn if the user is switching to full preemption with
PREEMPT_RCU=n.

Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>
Suggested-by: Paul E. McKenney <paulmck@...nel.org>
Link: https://lore.kernel.org/lkml/842f589e-5ea3-4c2b-9376-d718c14fabf5@paulmck-laptop/
Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
---
 kernel/sched/core.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e5d631197f35..425e4f03e0af 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8922,6 +8922,10 @@ static void __sched_dynamic_update(int mode)
 		break;
 
 	case preempt_dynamic_full:
+		if (!IS_ENABLED(CONFIG_PREEMPT_RCU))
+			pr_warn("%s: preempt=full is not recommended with CONFIG_PREEMPT_RCU=n",
+				PREEMPT_MODE);
+
 		preempt_dynamic_mode = preempt_dynamic_undefined;
 		break;
 	}
-- 
2.31.1