[PATCH 1/2] rcu: Explain why rcu_all_qs() is a stub in preemptible TREE RCU

[Frederic Weisbecker]

cond_resched() reports an RCU quiescent state only in non-preemptible
TREE RCU implementation. Provide an explanation for the different
behaviour in CONFIG_PREEMPT_RCU=y.

Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/sched/core.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index cf16f8fda9a6..db374cb38eb2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7780,6 +7780,19 @@ int __sched __cond_resched(void)
 		preempt_schedule_common();
 		return 1;
 	}
+	/*
+	 * A process spending a long time in the kernel space might
+	 * have too few opportunities to report quiescent states
+	 * when CONFIG_PREEMPT_RCU=n because then the tick can't know
+	 * if it's interrupting an RCU read side critical section. In the
+	 * absence of voluntary sleeps, the last resort resides in tracking
+	 * calls to cond_resched() which always imply quiescent states.
+	 *
+	 * On the other hand, preemptible RCU has a real RCU read side
+	 * tracking that allows the tick for reporting interrupted quiescent
+	 * states or, in the worst case, deferred quiescent states after
+	 * rcu_read_unlock().
+	 */
 #ifndef CONFIG_PREEMPT_RCU
 	rcu_all_qs();
 #endif
-- 
2.25.1