linux-kernel - [PATCH percpu/for-3.11] percpu-refcount: use RCU-sched insted of normal RCU

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Date:	Sun, 16 Jun 2013 14:55:46 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	Michal Hocko <mhocko@...e.cz>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: [PATCH percpu/for-3.11] percpu-refcount: use RCU-sched insted of
 normal RCU

percpu-refcount was incorrectly using preempt_disable/enable() for RCU
critical sections against call_rcu().  6a24474da8 ("percpu-refcount:
consistently use plain (non-sched) RCU") fixed it by converting the
preepmtion operations with rcu_read_[un]lock() citing that there isn't
any advantage in using sched-RCU over using the usual one; however,
rcu_read_[un]lock() for the preemptible RCU implementation -
CONFIG_TREE_PREEMPT_RCU, chosen when CONFIG_PREEMPT - are slightly
more expensive than preempt_disable/enable().

In a contrived microbench which repeats the followings,

 - percpu_ref_get()
 - copy 32 bytes of data into percpu buffer
 - percpu_put_get()
 - copy 32 bytes of data into percpu buffer

rcu_read_[un]lock() used in percpu_ref_get/put() makes it go slower by
about 15% when compared to using sched-RCU.

As the RCU critical sections are extremely short, using sched-RCU
shouldn't have any latency implications.  Convert to RCU-sched.

Signed-off-by: Tejun Heo <tj@...nel.org>
Cc: Kent Overstreet <koverstreet@...gle.com>
Cc: Michal Hocko <mhocko@...e.cz>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Rusty Russell <rusty@...tcorp.com.au>
---
 include/linux/percpu-refcount.h |   12 ++++++------
 lib/percpu-refcount.c           |    2 +-
 2 files changed, 7 insertions(+), 7 deletions(-)

--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -105,7 +105,7 @@ static inline void percpu_ref_get(struct
 {
 	unsigned __percpu *pcpu_count;
 
-	rcu_read_lock();
+	rcu_read_lock_sched();
 
 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
 
@@ -114,7 +114,7 @@ static inline void percpu_ref_get(struct
 	else
 		atomic_inc(&ref->count);
 
-	rcu_read_unlock();
+	rcu_read_unlock_sched();
 }
 
 /**
@@ -134,7 +134,7 @@ static inline bool percpu_ref_tryget(str
 	unsigned __percpu *pcpu_count;
 	int ret = false;
 
-	rcu_read_lock();
+	rcu_read_lock_sched();
 
 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
 
@@ -143,7 +143,7 @@ static inline bool percpu_ref_tryget(str
 		ret = true;
 	}
 
-	rcu_read_unlock();
+	rcu_read_unlock_sched();
 
 	return ret;
 }
@@ -159,7 +159,7 @@ static inline void percpu_ref_put(struct
 {
 	unsigned __percpu *pcpu_count;
 
-	rcu_read_lock();
+	rcu_read_lock_sched();
 
 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
 
@@ -168,7 +168,7 @@ static inline void percpu_ref_put(struct
 	else if (unlikely(atomic_dec_and_test(&ref->count)))
 		ref->release(ref);
 
-	rcu_read_unlock();
+	rcu_read_unlock_sched();
 }
 
 #endif
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -154,5 +154,5 @@ void percpu_ref_kill_and_confirm(struct
 		(((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD);
 	ref->confirm_kill = confirm_kill;
 
-	call_rcu(&ref->rcu, percpu_ref_kill_rcu);
+	call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu);
 }
--
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