[PATCH v2 17/35] rcu: handle quiescent states for PREEMPT_RCU=n, PREEMPT_COUNT=y

Ankur Arora
2024-05-28T00:36:38Z

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 necessary: 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_AUTO=y, there can be configurations with (PREEMPT_COUNT=y,
PREEMPT_RCU=n). This means that cond_resched() is a stub which does
not provide for 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 36a8b5dbf5b5..741476c841a1 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -963,13 +963,16 @@ 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.
+ * 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
--
2.31.1