Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755511Ab3FPVzy (ORCPT ); Sun, 16 Jun 2013 17:55:54 -0400 Received: from mail-yh0-f54.google.com ([209.85.213.54]:58583 "EHLO mail-yh0-f54.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751835Ab3FPVzw (ORCPT ); Sun, 16 Jun 2013 17:55:52 -0400 Date: Sun, 16 Jun 2013 14:55:46 -0700 From: Tejun Heo To: Kent Overstreet Cc: Michal Hocko , "Paul E. McKenney" , Rusty Russell , linux-kernel@vger.kernel.org Subject: [PATCH percpu/for-3.11] percpu-refcount: use RCU-sched insted of normal RCU Message-ID: <20130616215546.GB28587@htj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3044 Lines: 102 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 Cc: Kent Overstreet Cc: Michal Hocko Cc: "Paul E. McKenney" Cc: Rusty Russell --- 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); } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/