[PATCH v3 1/2] rcu-tasks: Stop RCU Tasks scanning no-idle tasks which record on dyntick-idle entry

[Zqiang]

When the RCU Tasks scanning on-rq tasks, there is no need to scanning
no-idle tasks(have invoked rcu_user_enter()) which record on dyntick-idle
entry, at this time, these tasks are not remain within an RCU Tasks
read-side critical section.

This commit skip scanning tasks which record on dyntick-idle entry in
rcu_tasks_pertask().

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 kernel/rcu/tasks.h | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 195c97e85bcb..a28337f280e4 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -809,10 +809,16 @@ static void rcu_tasks_pregp_step(struct list_head *hop)
 	synchronize_rcu();
 }
 
+static bool task_is_on_dyntick_idle(struct task_struct *t)
+{
+	return IS_ENABLED(CONFIG_NO_HZ_FULL) && t->rcu_tasks_idle_cpu >= 0;
+}
+
 /* Per-task initial processing. */
 static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
 {
-	if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(t)) {
+	if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(t) &&
+				!task_is_on_dyntick_idle(t)) {
 		get_task_struct(t);
 		t->rcu_tasks_nvcsw = READ_ONCE(t->nvcsw);
 		WRITE_ONCE(t->rcu_tasks_holdout, true);
@@ -842,8 +848,7 @@ static void check_holdout_task(struct task_struct *t,
 	if (!READ_ONCE(t->rcu_tasks_holdout) ||
 	    t->rcu_tasks_nvcsw != READ_ONCE(t->nvcsw) ||
 	    !READ_ONCE(t->on_rq) ||
-	    (IS_ENABLED(CONFIG_NO_HZ_FULL) &&
-	     !is_idle_task(t) && t->rcu_tasks_idle_cpu >= 0)) {
+	    task_is_on_dyntick_idle(t)) {
 		WRITE_ONCE(t->rcu_tasks_holdout, false);
 		list_del_init(&t->rcu_tasks_holdout_list);
 		put_task_struct(t);
-- 
2.25.1