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From: Ankur Arora <ankur.a.arora@oracle.com>
To: linux-kernel@vger.kernel.org
Cc: tglx@linutronix.de, peterz@infradead.org, torvalds@linux-foundation.org,
        paulmck@kernel.org, rostedt@goodmis.org, mark.rutland@arm.com,
        juri.lelli@redhat.com, joel@joelfernandes.org, raghavendra.kt@amd.com,
        sshegde@linux.ibm.com, boris.ostrovsky@oracle.com,
        konrad.wilk@oracle.com, Ankur Arora <ankur.a.arora@oracle.com>,
        Ingo Molnar <mingo@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>
Subject: [PATCH v2 16/35] preempt,rcu: warn on PREEMPT_RCU=n, preempt=full
Date: Mon, 27 May 2024 17:35:02 -0700
Message-Id: <20240528003521.979836-17-ankur.a.arora@oracle.com>
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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.

Warn if the user is switching to full preemption with PREEMPT_RCU=n.

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

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d7804e29182d..df8e333f2d8b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8943,6 +8943,10 @@ 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