[PATCH v2] rcu: Make defer_qs_iw as hard irq-work under RT kernel with strict grace period configuration

[Zqiang]

When the CONFIG_RCU_STRICT_GRACE_PERIOD is enabled, on non-PREEMPT_RT
kernel, the init_irq_work() make the defer_qs_iw irq-work execute in
interrupt context. however, on PREEMPT_RT kernel, the init_irq_work()
make defer_qs_iq irq-work execute in rt-fifo irq_work kthreads. when
system booting, there are a lot of defer_qs_iw irq-work to be processed
in rt-fifo irq_work kthreads, it occupies boot CPU for long time and
cause other kthread cannot get the boot CPU, the boot process occurs
hang. use IRQ_WORK_INIT_HARD() to initialize defer_qs_iw irq-work, can
ensure the defer_qs_iw irq-work execute in interrupt context, under
PREEMPT_RT kernel with the CONFIG_RCU_STRICT_GRACE_PERIOD is enabled.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 v1->v2:
 Use IRQ_WORK_INIT_HARD initialize defer_qs_iw only under RT kernel
 with CONFIG_RCU_STRICT_GRACE_PERIOD is enabled. 

 kernel/rcu/tree_plugin.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 3037c2536e1f..e6196a23793c 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -661,7 +661,13 @@ static void rcu_read_unlock_special(struct task_struct *t)
 			    expboost && !rdp->defer_qs_iw_pending && cpu_online(rdp->cpu)) {
 				// Get scheduler to re-evaluate and call hooks.
 				// If !IRQ_WORK, FQS scan will eventually IPI.
-				init_irq_work(&rdp->defer_qs_iw, rcu_preempt_deferred_qs_handler);
+				if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) &&
+							IS_ENABLED(CONFIG_PREEMPT_RT))
+					rdp->defer_qs_iw = IRQ_WORK_INIT_HARD(
+								rcu_preempt_deferred_qs_handler);
+				else
+					init_irq_work(&rdp->defer_qs_iw,
+							rcu_preempt_deferred_qs_handler);
 				rdp->defer_qs_iw_pending = true;
 				irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
 			}
-- 
2.25.1