With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent
states for read-side critical sections via rcu_all_qs().
One reason why this was needed, was lacking preempt-count, the tick
handler has no way of knowing whether it is executing in a read-side
critical section or not.

With PREEMPT_LAZY=y, there can be configurations with PREEMPT_COUNT=y,
PREEMPT_RCU=n, where cond_resched() is a stub that does not provide
quiescent states via rcu_all_qs().

So, use the availability of preempt_count() to report quiescent states
in rcu_flavor_sched_clock_irq().

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 kernel/rcu/tree_plugin.h | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 1c7cbd145d5e..da324d66034b 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -974,13 +974,16 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
  */
 static void rcu_flavor_sched_clock_irq(int user)
 {
-	if (user || rcu_is_cpu_rrupt_from_idle()) {
+	if (user || rcu_is_cpu_rrupt_from_idle() ||
+	     (IS_ENABLED(CONFIG_PREEMPT_COUNT) &&
+	      !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)))) {
 
 		/*
 		 * Get here if this CPU took its interrupt from user
-		 * mode or from the idle loop, and if this is not a
-		 * nested interrupt.  In this case, the CPU is in
-		 * a quiescent state, so note it.
+		 * mode, from the idle loop without this being a nested
+		 * interrupt, or while not holding a preempt count (but
+		 * with PREEMPT_COUNT=y. In this case, the CPU is in a
+		 * quiescent state, so note it.
 		 *
 		 * No memory barrier is required here because rcu_qs()
 		 * references only CPU-local variables that other CPUs
-- 
2.43.5

On 2024-11-06 12:17:56 [-0800], Ankur Arora wrote:
> With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent
> states for read-side critical sections via rcu_all_qs().
> One reason why this was needed, was lacking preempt-count, the tick
> handler has no way of knowing whether it is executing in a read-side
> critical section or not.
> 
> With PREEMPT_LAZY=y, there can be configurations with PREEMPT_COUNT=y,
> PREEMPT_RCU=n, where cond_resched() is a stub that does not provide
> quiescent states via rcu_all_qs().

With PREEMPT_LAZY=y && PREEMPT_DYNAMIC=n we get PREEMPT_COUNT=y and
PREEMPT_RCU=n. In this configuration cond_resched() is an empty stub and
does not provide quiescent states via rcu_all_qs(). PREEMPT_RCU=y
provides this information via rcu_read_unlock() and its nesting counter.

> So, use the availability of preempt_count() to report quiescent states
> in rcu_flavor_sched_clock_irq().

Okay. You might also want to update the cond_resched() comment,
	s@In preemptible kernels, ->rcu_read_lock_nesting@
	  In PREEMPT_RCU kernels, ->rcu_read_lock_nesting@

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>

Sebastian

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

> On 2024-11-06 12:17:56 [-0800], Ankur Arora wrote:
>> With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent
>> states for read-side critical sections via rcu_all_qs().
>> One reason why this was needed, was lacking preempt-count, the tick
>> handler has no way of knowing whether it is executing in a read-side
>> critical section or not.
>>
>> With PREEMPT_LAZY=y, there can be configurations with PREEMPT_COUNT=y,
>> PREEMPT_RCU=n, where cond_resched() is a stub that does not provide
>> quiescent states via rcu_all_qs().
>
> With PREEMPT_LAZY=y && PREEMPT_DYNAMIC=n we get PREEMPT_COUNT=y and
> PREEMPT_RCU=n. In this configuration cond_resched() is an empty stub and
> does not provide quiescent states via rcu_all_qs(). PREEMPT_RCU=y
> provides this information via rcu_read_unlock() and its nesting counter.
>
>> So, use the availability of preempt_count() to report quiescent states
>> in rcu_flavor_sched_clock_irq().
>
> Okay. You might also want to update the cond_resched() comment,
> 	s@In preemptible kernels, ->rcu_read_lock_nesting@
> 	  In PREEMPT_RCU kernels, ->rcu_read_lock_nesting@

Good point. Will add.

> Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Thanks!

Ankur

>> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>
> Sebastian


--
ankur