[patch 08/15] sched, hotplug: Move sync_rcu to be with set_cpu_active(false)

[Thomas Gleixner]

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

The sync_rcu stuff is specificically for clearing bits in the active
mask, such that everybody will observe the bit cleared and will not
consider the cleared CPU for load-balancing etc.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 kernel/cpu.c        |   15 ---------------
 kernel/sched/core.c |   14 ++++++++++++++
 2 files changed, 14 insertions(+), 15 deletions(-)

--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -691,21 +691,6 @@ static int takedown_cpu(unsigned int cpu
 	struct cpuhp_cpu_state *st = per_cpu_ptr(&cpuhp_state, cpu);
 	int err;
 
-	/*
-	 * By now we've cleared cpu_active_mask, wait for all preempt-disabled
-	 * and RCU users of this state to go away such that all new such users
-	 * will observe it.
-	 *
-	 * For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu() might
-	 * not imply sync_sched(), so wait for both.
-	 *
-	 * Do sync before park smpboot threads to take care the rcu boost case.
-	 */
-	if (IS_ENABLED(CONFIG_PREEMPT))
-		synchronize_rcu_mult(call_rcu, call_rcu_sched);
-	else
-		synchronize_rcu();
-
 	/* Park the hotplug thread */
 	kthread_park(per_cpu_ptr(&cpuhp_state, cpu)->thread);
 
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7345,6 +7345,20 @@ int sched_cpu_deactivate(unsigned int cp
 	int ret;
 
 	set_cpu_active(cpu, false);
+	/*
+	 * We've cleared cpu_active_mask, wait for all preempt-disabled and RCU
+	 * users of this state to go away such that all new such users will
+	 * observe it.
+	 *
+	 * For CONFIG_PREEMPT we have preemptible RCU and its sync_rcu() might
+	 * not imply sync_sched(), so wait for both.
+	 *
+	 * Do sync before park smpboot threads to take care the rcu boost case.
+	 */
+	if (IS_ENABLED(CONFIG_PREEMPT))
+		synchronize_rcu_mult(call_rcu, call_rcu_sched);
+	else
+		synchronize_rcu();
 
 	if (!sched_smp_initialized)
 		return 0;