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

Byungchul Park
2018-06-22T06:12:15Z

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@linux.vnet.ibm.com>
Signed-off-by: Byungchul Park <byungchul.park@lge.com>
---
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