Merge remote-tracking branch 'rcu/rcu/next'

# Conflicts:
#	kernel/time/tick-internal.h

1 files changed, 11 insertions, 0 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ceae25ea8a0e..8a80361a4831 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7857,6 +7857,17 @@ 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