From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754373Ab2CWVRI (ORCPT ); Fri, 23 Mar 2012 17:17:08 -0400 Received: from e37.co.us.ibm.com ([32.97.110.158]:51581 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753046Ab2CWVRG (ORCPT ); Fri, 23 Mar 2012 17:17:06 -0400 Date: Fri, 23 Mar 2012 14:16:38 -0700 From: "Paul E. McKenney" To: Linus Torvalds Cc: Ingo Molnar , linux-kernel@vger.kernel.org, Peter Zijlstra , Thomas Gleixner , Andrew Morton Subject: Re: [GIT PULL] RCU changes for v3.4 Message-ID: <20120323211638.GA2450@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20120319152317.GA3932@gmail.com> <20120320033300.GH2393@linux.vnet.ibm.com> <20120323192143.GY2450@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12032321-7408-0000-0000-000003ADF8C2 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 23, 2012 at 12:39:59PM -0700, Linus Torvalds wrote: > On Fri, Mar 23, 2012 at 12:21 PM, Paul E. McKenney > wrote: > >> > >> Please? Every time I look at some profiles, that silly rcu_read_lock > >> is there in the profile. It's annoying. I'd rather see it in the > >> function that invokes it. > > > > Let me see what I can do... > > Thanks. To some degree, rcu_read_lock() is the more critical one, > because it is often in the much more critical path in the caller. In > particular, it's often at the beginning of a function, where a number > of arguments are "live", and calling it out-of-line also forces the > compiler to then save/restore those arguments (because they are > clobbered by the function call). > > rcu_read_unlock() is *usually* not as critical, and is obviously much > harder to inline anyway due to the whole complexity with needing to > check if an RCU sequence has ended. It often is at the end of the > function call in the caller, when the only thing like is often just > the single return value (if that). So it seldom looks nearly as bad in > any profiles, because it doesn't tend to have the same kind of bad > impact on the call site. Very good to hear! Especially since I am not seeing how to move ->rcu_read_unlock_special to a per-CPU variable given that rcu_boost() needs cross-task access to it. There is probably some obvious trick, but I will start with just __rcu_read_lock() for now. Thanx, Paul