rcu: Improve rcu_note_voluntary_context_switch() reporting

We expect a quiescent state of TASKS_RCU when cond_resched_tasks_rcu_qs()
is called, no matter whether it actually be scheduled or not. However,
it currently doesn't report the quiescent state when the task enters
into __schedule() as it's called with preempt = true. So make it report
the quiescent state unconditionally when cond_resched_tasks_rcu_qs() is
called.

And in TINY_RCU, even though the quiescent state of rcu_bh also should
be reported when the tick interrupt comes from user, it doesn't. So make
it reported.

Lastly in TREE_RCU, rcu_note_voluntary_context_switch() should be
reported when the tick interrupt comes from not only user but also idle,
as an extended quiescent state.
Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Simplify rcutiny portion given no RCU-tasks for !PREEMPT. ]

include/linux/rcupdate.h

@@ -194,8 +194,8 @@ static inline void exit_tasks_rcu_finish(void) { }
  */
 #define cond_resched_tasks_rcu_qs() \
 do { \
-	if (!cond_resched()) \
 	rcu_note_voluntary_context_switch_lite(current); \
+	cond_resched(); \
 } while (0)
 
 /*

kernel/rcu/tiny.c

@@ -122,10 +122,8 @@ void rcu_check_callbacks(int user)
 {
 	if (user)
 		rcu_sched_qs();
-	else if (!in_softirq())
+	if (user || !in_softirq())
 		rcu_bh_qs();
-	if (user)
-		rcu_note_voluntary_context_switch(current);
 }
 
 /*

kernel/rcu/tree.c

@@ -2645,6 +2645,7 @@ void rcu_check_callbacks(int user)
 
 		rcu_sched_qs();
 		rcu_bh_qs();
+		rcu_note_voluntary_context_switch(current);
 
 	} else if (!in_softirq()) {
 
@@ -2660,8 +2661,7 @@ void rcu_check_callbacks(int user)
 	rcu_preempt_check_callbacks();
 	if (rcu_pending())
 		invoke_rcu_core();
-	if (user)
-		rcu_note_voluntary_context_switch(current);
+
 	trace_rcu_utilization(TPS("End scheduler-tick"));
 }