From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932398Ab0JTHfQ (ORCPT ); Wed, 20 Oct 2010 03:35:16 -0400 Received: from one.firstfloor.org ([213.235.205.2]:44844 "EHLO one.firstfloor.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932320Ab0JTHfN (ORCPT ); Wed, 20 Oct 2010 03:35:13 -0400 Date: Wed, 20 Oct 2010 09:35:12 +0200 From: Andi Kleen To: Eric Dumazet Cc: Shaohua Li , lkml , Ingo Molnar , "hpa@zytor.com" , Andi Kleen , "Chen, Tim C" Subject: Re: [PATCH 1/2]percpu: introduce read mostly percpu API Message-ID: <20101020073511.GC20124@basil.fritz.box> References: <1287544022.4571.7.camel@sli10-conroe.sh.intel.com> <1287551880.2700.79.camel@edumazet-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1287551880.2700.79.camel@edumazet-laptop> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Oct 20, 2010 at 07:18:00AM +0200, Eric Dumazet wrote: > Le mercredi 20 octobre 2010 à 11:07 +0800, Shaohua Li a écrit : > > Add a new readmostly percpu section and api, next patch will use it. > > > > Signed-off-by: Shaohua Li > > --- > > > Could you precisely describe why grouping together read mostly percpu > variables is a win ? Especially when you add in your next patch a single > variable ? Not Shaohua, but I can explain it: There is some per cpu data which is read by other CPUs. In this case it's a win to use the separate section because you avoid false sharing and the cache line can be kept in shared mode on all CPUs. The next patch has an example of such data: data which is read by another CPU to send something to the target CPU. I think the concept is useful and makes a lot of sense. The alternative would be __read_mostly NR_CPUS arrays, but we all know that is a bad idea because it wastes too much memory on CONFIG_MAX_SMP setups. -Andi