[PATCH 6/7] osnoise: handle quiescent states for PREEMPT_RCU=n, PREEMPTION=y

[Ankur Arora]

To reduce RCU noise for nohz_full configurations, osnoise depends
on cond_resched() providing quiescent states for PREEMPT_RCU=n
configurations. And, for PREEMPT_RCU=y configurations does this
by directly calling rcu_momentary_eqs().

With PREEMPT_LAZY=y, however, we can have configurations with
(PREEMPTION=y, PREEMPT_RCU=n), which means neither of the above
can help.

Handle that by fallback to the explicit quiescent states via
rcu_momentary_eqs().

Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Acked-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 kernel/trace/trace_osnoise.c | 22 ++++++++++++----------
 1 file changed, 12 insertions(+), 10 deletions(-)

diff --git a/kernel/trace/trace_osnoise.c b/kernel/trace/trace_osnoise.c
index 1439064f65d6..4f4ebf3f15f0 100644
--- a/kernel/trace/trace_osnoise.c
+++ b/kernel/trace/trace_osnoise.c
@@ -1538,18 +1538,20 @@ static int run_osnoise(void)
 		/*
 		 * In some cases, notably when running on a nohz_full CPU with
 		 * a stopped tick PREEMPT_RCU has no way to account for QSs.
-		 * This will eventually cause unwarranted noise as PREEMPT_RCU
-		 * will force preemption as the means of ending the current
-		 * grace period. We avoid this problem by calling
-		 * rcu_momentary_eqs(), which performs a zero duration
-		 * EQS allowing PREEMPT_RCU to end the current grace period.
-		 * This call shouldn't be wrapped inside an RCU critical
-		 * section.
+		 * This will eventually cause unwarranted noise as RCU forces
+		 * preemption as the means of ending the current grace period.
+		 * We avoid this by calling rcu_momentary_eqs(), which performs
+		 * a zero duration EQS allowing RCU to end the current grace
+		 * period. This call shouldn't be wrapped inside an RCU
+		 * critical section.
 		 *
-		 * Note that in non PREEMPT_RCU kernels QSs are handled through
-		 * cond_resched()
+		 * For non-PREEMPT_RCU kernels with cond_resched() (non-
+		 * PREEMPT_LAZY configurations), QSs are handled through
+		 * cond_resched(). For PREEMPT_LAZY kernels, we fallback to
+		 * the zero duration QS via rcu_momentary_eqs().
 		 */
-		if (IS_ENABLED(CONFIG_PREEMPT_RCU)) {
+		if (IS_ENABLED(CONFIG_PREEMPT_RCU) ||
+		    (!IS_ENABLED(CONFIG_PREEMPT_RCU) && IS_ENABLED(CONFIG_PREEMPTION))) {
 			if (!disable_irq)
 				local_irq_disable();
 
-- 
2.43.5