[PATCH tip/core/rcu 07/15] rcu: Force wakeup of rcu_gp_kthread at grace-period end

[Paul E. McKenney]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>

The rcu_gp_kthread_wake() refuses to do a wakeup unless at least
one of the ->gp_flags bits are set, which normally will not be the
case when the last quiescent state is reported.  This results in
up to a 3-jiffy delay given default Kconfig settings.  This commit
therefore has rcu_report_qs_rsp() set RCU_GP_FLAG_FQS before invoking
rcu_gp_kthread_wake() in order to force a more immediate wakeup at
grace-period end, thus reducing grace-period latencies.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 kernel/rcu/tree.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 04bca6b1b9ca..a7615a0b684f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2137,6 +2137,7 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
 	__releases(rcu_get_root(rsp)->lock)
 {
 	WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
+	WRITE_ONCE(rsp->gp_flags, READ_ONCE(rsp->gp_flags) | RCU_GP_FLAG_FQS);
 	raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
 	rcu_gp_kthread_wake(rsp);
 }
-- 
1.8.1.5

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