[PATCH tip/core/rcu 06/27] rcu: Mark task as .need_qs less aggressively

[Paul E. McKenney]

If any scheduling-clock interrupt interrupts an RCU-preempt read-side
critical section, the interrupted task's ->rcu_read_unlock_special.b.need_qs
field is set.  This causes the outermost rcu_read_unlock() to incur the
extra overhead of calling into rcu_read_unlock_special().  This commit
reduces that overhead by setting ->rcu_read_unlock_special.b.need_qs only
if the grace period has been in effect for more than one second.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index dbfe90191e19..0239cf8a4be6 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -730,6 +730,7 @@ rcu_preempt_check_blocked_tasks(struct rcu_state *rsp, struct rcu_node *rnp)
  */
 static void rcu_preempt_check_callbacks(void)
 {
+	struct rcu_state *rsp = &rcu_preempt_state;
 	struct task_struct *t = current;
 
 	if (t->rcu_read_lock_nesting == 0) {
@@ -738,7 +739,9 @@ static void rcu_preempt_check_callbacks(void)
 	}
 	if (t->rcu_read_lock_nesting > 0 &&
 	    __this_cpu_read(rcu_data_p->core_needs_qs) &&
-	    __this_cpu_read(rcu_data_p->cpu_no_qs.b.norm))
+	    __this_cpu_read(rcu_data_p->cpu_no_qs.b.norm) &&
+	    !t->rcu_read_unlock_special.b.need_qs &&
+	    time_after(jiffies, rsp->gp_start + HZ))
 		t->rcu_read_unlock_special.b.need_qs = true;
 }
 
-- 
2.17.1