[PATCH] rcu: Refactor rcu_{nmi,irq}_{enter,exit}()

[Byungchul Park]

When passing through irq or NMI contexts, the current code uses
->dynticks_nmi_nesting to detect if it's in the ourmost at the moment.

Here, the thing is that all the related functions, rcu_irq_enter(),
rcu_nmi_enter(), rcu_irq_exit() and rcu_nmi_exit() are carrying out
the check within each under the following call relationship so there
must be redundant conditional branches:

   rcu_irq_enter()
      /* A conditional branch with ->dynticks_nmi_nesting */
      rcu_nmi_enter()
         /* A conditional branch with ->dynticks_nmi_nesting */
      /* A conditional branch with ->dynticks_nmi_nesting */

   rcu_irq_exit()
      /* A conditional branch with ->dynticks_nmi_nesting */
      rcu_nmi_exit()
         /* A conditional branch with ->dynticks_nmi_nesting */
      /* A conditional branch with ->dynticks_nmi_nesting */

   rcu_nmi_enter()
      /* A conditional branch with ->dynticks_nmi_nesting */

   rcu_nmi_exit()
      /* A conditional branch with ->dynticks_nmi_nesting */

In order to make the code more readable and remove redundant
conditional branches, we need to refactor rcu_{nmi,irq}_{enter,exit}()
so they use a common function for each like:

   rcu_irq_enter() inlining rcu_nmi_enter_common(irq)
      /* A conditional branch with ->dynticks_nmi_nesting */

   rcu_irq_exit() inlining rcu_nmi_exit_common(irq)
      /* A conditional branch with ->dynticks_nmi_nesting */

   rcu_nmi_enter() inlining rcu_nmi_enter_common(nmi)
      /* A conditional branch with ->dynticks_nmi_nesting */

   rcu_nmi_exit() inlining rcu_nmi_exit_common(nmi)
      /* A conditional branch with ->dynticks_nmi_nesting */

Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
 kernel/rcu/tree.c | 59 ++++++++++++++++++++++++++++++++++++-------------------
 1 file changed, 39 insertions(+), 20 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index deb2508..cdae85f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -764,17 +764,17 @@ void rcu_user_enter(void)
 #endif /* CONFIG_NO_HZ_FULL */
 
 /**
- * rcu_nmi_exit - inform RCU of exit from NMI context
+ * rcu_nmi_exit_common - inform RCU of exit from NMI context
  *
  * If we are returning from the outermost NMI handler that interrupted an
  * RCU-idle period, update rdtp->dynticks and rdtp->dynticks_nmi_nesting
  * to let the RCU grace-period handling know that the CPU is back to
  * being RCU-idle.
  *
- * If you add or remove a call to rcu_nmi_exit(), be sure to test
+ * If you add or remove a call to rcu_nmi_exit_common(), be sure to test
  * with CONFIG_RCU_EQS_DEBUG=y.
  */
-void rcu_nmi_exit(void)
+static __always_inline void rcu_nmi_exit_common(bool irq)
 {
 	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
 
@@ -800,7 +800,22 @@ void rcu_nmi_exit(void)
 	/* This NMI interrupted an RCU-idle CPU, restore RCU-idleness. */
 	trace_rcu_dyntick(TPS("Startirq"), rdtp->dynticks_nmi_nesting, 0, rdtp->dynticks);
 	WRITE_ONCE(rdtp->dynticks_nmi_nesting, 0); /* Avoid store tearing. */
+
+	if (irq)
+		rcu_prepare_for_idle();
+
 	rcu_dynticks_eqs_enter();
+
+	if (irq)
+		rcu_dynticks_task_enter();
+}
+
+/**
+ * rcu_nmi_exit - inform RCU of exit from NMI context
+ */
+void rcu_nmi_exit(void)
+{
+	rcu_nmi_exit_common(false);
 }
 
 /**
@@ -824,14 +839,8 @@ void rcu_nmi_exit(void)
  */
 void rcu_irq_exit(void)
 {
-	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
-
 	lockdep_assert_irqs_disabled();
-	if (rdtp->dynticks_nmi_nesting == 1)
-		rcu_prepare_for_idle();
-	rcu_nmi_exit();
-	if (rdtp->dynticks_nmi_nesting == 0)
-		rcu_dynticks_task_enter();
+	rcu_nmi_exit_common(true);
 }
 
 /*
@@ -914,7 +923,7 @@ void rcu_user_exit(void)
 #endif /* CONFIG_NO_HZ_FULL */
 
 /**
- * rcu_nmi_enter - inform RCU of entry to NMI context
+ * rcu_nmi_enter_common - inform RCU of entry to NMI context
  *
  * If the CPU was idle from RCU's viewpoint, update rdtp->dynticks and
  * rdtp->dynticks_nmi_nesting to let the RCU grace-period handling know
@@ -922,10 +931,10 @@ void rcu_user_exit(void)
  * long as the nesting level does not overflow an int.  (You will probably
  * run out of stack space first.)
  *
- * If you add or remove a call to rcu_nmi_enter(), be sure to test
+ * If you add or remove a call to rcu_nmi_enter_common(), be sure to test
  * with CONFIG_RCU_EQS_DEBUG=y.
  */
-void rcu_nmi_enter(void)
+static __always_inline void rcu_nmi_enter_common(bool irq)
 {
 	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
 	long incby = 2;
@@ -942,7 +951,15 @@ void rcu_nmi_enter(void)
 	 * period (observation due to Andy Lutomirski).
 	 */
 	if (rcu_dynticks_curr_cpu_in_eqs()) {
+
+		if (irq)
+			rcu_dynticks_task_exit();
+
 		rcu_dynticks_eqs_exit();
+
+		if (irq)
+			rcu_cleanup_after_idle();
+
 		incby = 1;
 	}
 	trace_rcu_dyntick(incby == 1 ? TPS("Endirq") : TPS("++="),
@@ -954,6 +971,14 @@ void rcu_nmi_enter(void)
 }
 
 /**
+ * rcu_nmi_enter - inform RCU of entry to NMI context
+ */
+void rcu_nmi_enter(void)
+{
+	rcu_nmi_enter_common(false);
+}
+
+/**
  * rcu_irq_enter - inform RCU that current CPU is entering irq away from idle
  *
  * Enter an interrupt handler, which might possibly result in exiting
@@ -977,14 +1002,8 @@ void rcu_nmi_enter(void)
  */
 void rcu_irq_enter(void)
 {
-	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
-
 	lockdep_assert_irqs_disabled();
-	if (rdtp->dynticks_nmi_nesting == 0)
-		rcu_dynticks_task_exit();
-	rcu_nmi_enter();
-	if (rdtp->dynticks_nmi_nesting == 1)
-		rcu_cleanup_after_idle();
+	rcu_nmi_enter_common(true);
 }
 
 /*
-- 
1.9.1