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Subject: [PATCH 14/30] preempt,rcu: warn on PREEMPT_RCU=n, preempt=full
Date: Mon, 12 Feb 2024 21:55:38 -0800
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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.

For now, 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 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