[RFC PATCH 48/86] rcu: handle quiescent states for PREEMPT_RCU=n

[Ankur Arora]

cond_resched() is used to provide urgent quiescent states for
read-side critical sections on PREEMPT_RCU=n configurations.
This was necessary because lacking preempt_count, there was no
way for the tick handler to know if we were executing in RCU
read-side critical section or not.

An always-on CONFIG_PREEMPT_COUNT, however, allows the tick to
reliably report quiescent states.

Accordingly, evaluate preempt_count() based quiescence in
rcu_flavor_sched_clock_irq().

Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h |  3 ++-
 kernel/sched/core.c      | 15 +--------------
 2 files changed, 3 insertions(+), 15 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index f87191e008ff..618f055f8028 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -963,7 +963,8 @@ 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() ||
+	    !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
 
 		/*
 		 * Get here if this CPU took its interrupt from user
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bf5df2b866df..15db5fb7acc7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8588,20 +8588,7 @@ int __sched _cond_resched(void)
 		preempt_schedule_common();
 		return 1;
 	}
-	/*
-	 * In preemptible kernels, ->rcu_read_lock_nesting tells the tick
-	 * whether the current CPU is in an RCU read-side critical section,
-	 * so the tick can report quiescent states even for CPUs looping
-	 * in kernel context.  In contrast, in non-preemptible kernels,
-	 * RCU readers leave no in-memory hints, which means that CPU-bound
-	 * processes executing in kernel context might never report an
-	 * RCU quiescent state.  Therefore, the following code causes
-	 * cond_resched() to report a quiescent state, but only when RCU
-	 * is in urgent need of one.
-	 */
-#ifndef CONFIG_PREEMPT_RCU
-	rcu_all_qs();
-#endif
+
 	return 0;
 }
 EXPORT_SYMBOL(_cond_resched);
-- 
2.31.1