rcu: Force wakeup of rcu_gp_kthread at grace-period end

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@linux.vnet.ibm.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 213f644d6fb1..7d369b829598 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2139,6 +2139,7 @@ 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);
 }