[PATCH] rcu: Only check tasks blocked on leaf rcu_nodes for PREEMPT_RCU

[Zqiang]

In PREEMPT_RCU kernel, for multi-node rcu tree, if the RCU read
critical section is preempted, the current task are only queued
leaf rcu_node blkd list, for single-node rcu tree, the root node
is also leaf node.

This commit add rcu_is_leaf_node() to filter out checks for non-leaf
rcu_node.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index b2219577fbe2..a9df11ec65af 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -693,6 +693,8 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 
 	RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!\n");
 	raw_lockdep_assert_held_rcu_node(rnp);
+	if (!rcu_is_leaf_node(rnp))
+		goto end;
 	if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
 		dump_blkd_tasks(rnp, 10);
 	if (rcu_preempt_has_tasks(rnp) &&
@@ -703,6 +705,7 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 		trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
 						rnp->gp_seq, t->pid);
 	}
+end:
 	WARN_ON_ONCE(rnp->qsmask);
 }
 
@@ -1178,7 +1181,8 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
  */
 static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
 {
-	rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
+	if (rcu_is_leaf_node(rnp))
+		rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
 }
 
 /*
-- 
2.25.1