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

Paul E. McKenney
2018-06-23T17:48:01Z

On Fri, Jun 22, 2018 at 03:23:51PM +0900, Byungchul Park wrote:
> On Fri, Jun 22, 2018 at 03:12:06PM +0900, Byungchul Park wrote:
> > 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 */
> ^
> Precisely, ->dynticks
> > /* 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 */
> ^
> Precisely, ->dynticks
> >
> > 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 */
> ^
> Precisely, ->dynticks
> >
> > 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 */
> ^
> Precisely, ->dynticks
> >
> > rcu_nmi_exit() inlining rcu_nmi_exit_common(nmi)
> > /* A conditional branch with ->dynticks_nmi_nesting */

I queued this for testing and further review, updating the commit log
as follows. Please let me know if I messed up something.

Thanx, Paul

------------------------------------------------------------------------

commit 5e5ea52645b197fb7ae2f59f7927079b91e91aa0
Author: Byungchul Park <byungchul.park@lge.com>
Date: Fri Jun 22 15:12:06 2018 +0900

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

When entering or exiting irq or NMI handlers, the current code uses
->dynticks_nmi_nesting to detect if it is in the outermost handler,
that is, the one interrupting or returning to an RCU-idle context (the
idle loop or nohz_full usermode execution). When entering the outermost
handler via an interrupt (as opposed to NMI), it is necessary to invoke
rcu_dynticks_task_exit() just before the CPU is marked non-idle from an
RCU perspective and to invoke rcu_cleanup_after_idle() just after the
CPU is marked non-idle. Similarly, when exiting the outermost handler
via an interrupt, it is necessary to invoke rcu_prepare_for_idle() just
before marking the CPU idle and to invoke rcu_dynticks_task_enter()
just after marking the CPU idle.

The decision to execute these four functions is currently taken in
rcu_irq_enter() and rcu_irq_exit() as follows:

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

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

This works, but the conditional branches in rcu_irq_enter() and
rcu_irq_exit() are redundant with those in rcu_nmi_enter() and
rcu_nmi_exit(), respectively. Redundant branches are not something
we want in the to/from-idle fastpaths, so this commit refactors
rcu_{nmi,irq}_{enter,exit}() so they use a common inlined function passed
a constant argument as follows:

rcu_irq_enter() inlining rcu_nmi_enter_common(irq=true)
/* A conditional branch with ->dynticks */

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

rcu_nmi_enter() inlining rcu_nmi_enter_common(irq=false)
/* A conditional branch with ->dynticks */

rcu_nmi_exit() inlining rcu_nmi_exit_common(irq=false)
/* A conditional branch with ->dynticks_nmi_nesting */

The combination of the constant function argument and the inlining allows
the compiler to discard the conditionals that previously controlled
execution of rcu_dynticks_task_exit(), rcu_cleanup_after_idle(),
rcu_prepare_for_idle(), and rcu_dynticks_task_enter(). This reduces both
the to-idle and from-idle path lengths by two conditional branches each,
and improves readability as well.

Suggested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Byungchul Park <byungchul.park@lge.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8788ddbc0d13..4ed74f10c852 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -773,17 +773,17 @@ 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);

@@ -809,7 +809,22 @@ 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);
}

/**
@@ -833,14 +848,8 @@ 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);
}

/*
@@ -923,7 +932,7 @@ 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
@@ -931,10 +940,10 @@ 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;
@@ -951,7 +960,15 @@ 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("++="),
@@ -962,6 +979,14 @@ void rcu_nmi_enter(void)
barrier();
}

+/**
+ * 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
*
@@ -986,14 +1011,8 @@ 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);
}

/*