[PATCH] rcu: Only boost rcu reader tasks with lower priority than boost kthreads

[Zqiang <qiang1.zhang@intel.com>]

When RCU_BOOST is enabled, the boost kthreads will boosting readers
who are blocking a given grace period, if the current reader tasks
have a higher priority than boost kthreads(the boost kthreads priority
not always 1, if the kthread_prio is set), boosting is useless, skip
current task and select next task to boosting, reduce the time for a
given grace period.

Signed-off-by: Zqiang <qiang1.zhang@intel.com>
---
 kernel/rcu/tree_plugin.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index c3d212bc5338..d35b6da66bbd 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -12,6 +12,7 @@
  */
 
 #include "../locking/rtmutex_common.h"
+#include <linux/sched/deadline.h>
 
 static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
 {
@@ -1065,13 +1066,20 @@ static int rcu_boost(struct rcu_node *rnp)
 	 * section.
 	 */
 	t = container_of(tb, struct task_struct, rcu_node_entry);
+	if (!rnp->exp_tasks && (dl_task(t) || t->prio <= current->prio)) {
+		tb = rcu_next_node_entry(t, rnp);
+		WRITE_ONCE(rnp->boost_tasks, tb);
+		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+		goto end;
+	}
+
 	rt_mutex_init_proxy_locked(&rnp->boost_mtx.rtmutex, t);
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	/* Lock only for side effect: boosts task t's priority. */
 	rt_mutex_lock(&rnp->boost_mtx);
 	rt_mutex_unlock(&rnp->boost_mtx);  /* Then keep lockdep happy. */
 	rnp->n_boosts++;
-
+end:
 	return READ_ONCE(rnp->exp_tasks) != NULL ||
 	       READ_ONCE(rnp->boost_tasks) != NULL;
 }
-- 
2.25.1