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@kernel.org>
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 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 a50ed23bee77..15e9600d231d 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

On 2024-11-06 12:17:57 [-0800], Ankur Arora wrote:
> 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.

The problem is as you say CONFIG_PREEMPT_RCU=n + CONFIG_PREEMPTION=y.
You can't select any of those two directly but get here via
 PREEMPT_LAZY=y + PREEMPT_DYNAMIC=n.

Please spell it out to make it obvious. It is not a large group of
configurations, it is exactly this combo.

 With PREEMPT_LAZY=y + PREEMPT_DYNAMIC=n however we get PREEMPT_RCU=n
 which means no direct rcu_momentary_eqs() invocations and
 cond_resched() is an empty stub.

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

> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> Acked-by: Daniel Bristot de Oliveira <bristot@kernel.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>

Sebastian

Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes:

> On 2024-11-06 12:17:57 [-0800], Ankur Arora wrote:
>> 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.
>
> The problem is as you say CONFIG_PREEMPT_RCU=n + CONFIG_PREEMPTION=y.
> You can't select any of those two directly but get here via
>  PREEMPT_LAZY=y + PREEMPT_DYNAMIC=n.
>
> Please spell it out to make it obvious. It is not a large group of
> configurations, it is exactly this combo.

Makes sense.

Will do. Thanks.

Ankur

>  With PREEMPT_LAZY=y + PREEMPT_DYNAMIC=n however we get PREEMPT_RCU=n
>  which means no direct rcu_momentary_eqs() invocations and
>  cond_resched() is an empty stub.
>
>> Handle that by fallback to the explicit quiescent states via
>> rcu_momentary_eqs().
>
>> Cc: Paul E. McKenney <paulmck@kernel.org>
>> Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
>> Cc: Steven Rostedt <rostedt@goodmis.org>
>> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
>> Acked-by: Daniel Bristot de Oliveira <bristot@kernel.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>
> Sebastian


--
ankur