Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755629Ab3FPXNK (ORCPT ); Sun, 16 Jun 2013 19:13:10 -0400 Received: from mail-ye0-f182.google.com ([209.85.213.182]:42670 "EHLO mail-ye0-f182.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755408Ab3FPXNI (ORCPT ); Sun, 16 Jun 2013 19:13:08 -0400 Date: Sun, 16 Jun 2013 16:13:03 -0700 From: Tejun Heo To: Kent Overstreet Cc: Michal Hocko , "Paul E. McKenney" , Rusty Russell , linux-kernel@vger.kernel.org Subject: Re: [PATCH percpu/for-3.11] percpu-refcount: use RCU-sched insted of normal RCU Message-ID: <20130616231303.GE28587@htj.dyndns.org> References: <20130616215546.GB28587@htj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130616215546.GB28587@htj.dyndns.org> 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: 1592 Lines: 38 On Sun, Jun 16, 2013 at 02:55:46PM -0700, Tejun Heo wrote: > 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 Applied to percpu/for-3.11 with acks added. Thanks! -- tejun -- 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/