Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1765844AbZDBWtr (ORCPT ); Thu, 2 Apr 2009 18:49:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755363AbZDBWth (ORCPT ); Thu, 2 Apr 2009 18:49:37 -0400 Received: from 124x34x33x190.ap124.ftth.ucom.ne.jp ([124.34.33.190]:60304 "EHLO master.linux-sh.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759575AbZDBWtg (ORCPT ); Thu, 2 Apr 2009 18:49:36 -0400 Date: Fri, 3 Apr 2009 07:44:56 +0900 From: Paul Mundt To: Ingo Molnar Cc: "Paul E. McKenney" , Andrew Morton , linux-kernel@vger.kernel.org, niv@us.ibm.com, dvhltc@us.ibm.com, dhowells@redhat.com, kernel@wantstofly.org, matthew@wil.cx Subject: Re: [PATCH] v3 RCU: the bloatwatch edition Message-ID: <20090402224455.GA13777@linux-sh.org> Mail-Followup-To: Paul Mundt , Ingo Molnar , "Paul E. McKenney" , Andrew Morton , linux-kernel@vger.kernel.org, niv@us.ibm.com, dvhltc@us.ibm.com, dhowells@redhat.com, kernel@wantstofly.org, matthew@wil.cx References: <20090203183426.GA14409@linux.vnet.ibm.com> <20090329203118.GA14005@linux.vnet.ibm.com> <20090402223605.GB19007@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090402223605.GB19007@elte.hu> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1875 Lines: 37 On Fri, Apr 03, 2009 at 12:36:05AM +0200, Ingo Molnar wrote: > * Paul E. McKenney wrote: > > This patch is a version of RCU designed for (!SMP && EMBEDDED) > > provided as a proof of concept of a small-footprint RCU > > implementation. In particular, the implementation of > > synchronize_rcu() is extremely lightweight and high performance. > > It passes rcutorture testing in each of the four relevant > > configurations (combinations of NO_HZ and PREEMPT) on x86. This > > saves about 900 bytes compared to Classic RCU, and a couple > > kilobytes compared to Hierarchical RCU: > > Andrew, what do you think? > > A worry is yet another RCU variant - we already have 3. > > A trick we could use would be to put it into Documentation/rcu/, > linked in via some clever Makefile magic and only usable if a > ultra-embedded developer does a build with something like > CONFIG_RCU_TINY=y. That way there's no real maintenance and testing > overhead. > > It _does_ have documentation value beyond the ~900 bytes: it's the > simplest and smallest possible still-working UP RCU implementation > so it would be easy to teach RCU concepts via that, gradually. > A similar argument could have been used for tiny-shmem when it was first integrated. As this is hiding behind CONFIG_EMBEDDED, most users are not going to run in to it, so the confusion of 1 more RCU variant is not likely to be a problem for those that aren't actively seeking it out. So, personally I think it is a good idea, and I have no reservations about default enabling it for a number of more constrained SH platforms. -- 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/