rcu: Explain why rcu_all_qs() is a stub in preemptible TREE RCU

The cond_resched() function reports an RCU quiescent state only in
non-preemptible TREE RCU implementation.  This commit therefore adds a
comment explaining why cond_resched() does nothing in preemptible kernels.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2d9ff40f4661995e0668f485edd98859596883aa..6a03c3fac55cc448aaa3654a2ed3547d4a6e3fdf 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7781,6 +7781,17 @@ 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