[PATCH v2 16/35] preempt,rcu: warn on PREEMPT_RCU=n, preempt=full

[Ankur Arora]

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@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Link: https://lore.kernel.org/lkml/842f589e-5ea3-4c2b-9376-d718c14fabf5@paulmck-laptop/
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 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