linux-kernel - [PATCH] rcu: Avoid unneeded function call in rcu_read

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Date:   Thu, 26 Aug 2021 22:21:22 -0400
From:   Waiman Long <longman@...hat.com>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        Waiman Long <longman@...hat.com>
Subject: [PATCH] rcu: Avoid unneeded function call in rcu_read_unlock()

Since commit aa40c138cc8f3 ("rcu: Report QS for outermost
PREEMPT=n rcu_read_unlock() for strict GPs"). A real function call
rcu_read_unlock_strict() is added to the inlined rcu_read_unlock().
The rcu_read_unlock_strict() call is only needed if the performance
sagging CONFIG_RCU_STRICT_GRACE_PERIOD option is set. This config
option isn't set for most production kernels while the function call
overhead remains.

To provide a slight performance improvement, the
CONFIG_RCU_STRICT_GRACE_PERIOD config check is moved from
rcu_read_unlock_strict() to __rcu_read_unlock() so that the function
call can be compiled out in most cases.

Besides, the GPL exported rcu_read_unlock_strict() also impact the
the compilation of non-GPL kernel modules as rcu_read_unlock() is a
frequently used kernel API.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 include/linux/rcupdate.h | 3 ++-
 kernel/rcu/tree_plugin.h | 3 +--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index d9680b798b21..945594770d57 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -71,7 +71,8 @@ static inline void __rcu_read_lock(void)
 static inline void __rcu_read_unlock(void)
 {
 	preempt_enable();
-	rcu_read_unlock_strict();
+	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
+		rcu_read_unlock_strict();
 }

 static inline int rcu_preempt_depth(void)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index de1dc3bb7f70..7fa518bef15d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -844,8 +844,7 @@ void rcu_read_unlock_strict(void)
 {
 	struct rcu_data *rdp;

-	if (!IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) ||
-	   irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
+	if (irqs_disabled() || preempt_count() || !rcu_state.gp_kthread)
 		return;
 	rdp = this_cpu_ptr(&rcu_data);
 	rcu_report_qs_rdp(rdp);
-- 
2.18.1