[PATCH tip/core/rcu 09/16] cpu: Wait for RCU grace periods concurrently

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

In kernels built with CONFIG_PREEMPT, _cpu_down() waits for RCU and
RCU-sched grace periods back-to-back, incurring quite a bit more latency
than required.  This commit therefore uses the new synchronize_rcu_mult()
to allow waiting for both grace periods concurrently.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/cpu.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 9c9c9fab16cc..d63b062b6267 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -380,14 +380,14 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
 	 * will observe it.
 	 *
 	 * For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu() might
-	 * not imply sync_sched(), so explicitly call both.
+	 * not imply sync_sched(), so wait for both.
 	 *
 	 * Do sync before park smpboot threads to take care the rcu boost case.
 	 */
-#ifdef CONFIG_PREEMPT
-	synchronize_sched();
-#endif
-	synchronize_rcu();
+	if (IS_ENABLED(CONFIG_PREEMPT))
+		synchronize_rcu_mult(call_rcu, call_rcu_sched);
+	else
+		synchronize_rcu();
 
 	smpboot_park_threads(cpu);
 
-- 
1.8.1.5