From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751536Ab3KTG6u (ORCPT ); Wed, 20 Nov 2013 01:58:50 -0500 Received: from e28smtp01.in.ibm.com ([122.248.162.1]:32822 "EHLO e28smtp01.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750858Ab3KTG6t (ORCPT ); Wed, 20 Nov 2013 01:58:49 -0500 Date: Wed, 20 Nov 2013 14:58:40 +0800 From: Wei Yang To: Tejun Heo Cc: Wei Yang , cl@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] percpu: stop the loop when a cpu belongs to a new group Message-ID: <20131120065840.GA10839@weiyang.vnet.ibm.com> Reply-To: Wei Yang References: <1382345893-6644-1-git-send-email-weiyang@linux.vnet.ibm.com> <20131027123008.GJ14934@mtj.dyndns.org> <20131028030055.GC15642@weiyang.vnet.ibm.com> <20131028113120.GB11541@mtj.dyndns.org> <20131028151746.GA7548@weiyang.vnet.ibm.com> <20131120030056.GA15273@weiyang.vnet.ibm.com> <20131120055121.GA13754@mtj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131120055121.GA13754@mtj.dyndns.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13112006-4790-0000-0000-00000B6DFD93 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Nov 20, 2013 at 12:51:21AM -0500, Tejun Heo wrote: >Hello, > >On Wed, Nov 20, 2013 at 11:00:56AM +0800, Wei Yang wrote: >> What do you think about this one? >> >> > >> >From bd70498b9df47b25ff20054e24bb510c5430c0c3 Mon Sep 17 00:00:00 2001 >> >From: Wei Yang >> >Date: Thu, 10 Oct 2013 09:42:14 +0800 >> >Subject: [PATCH] percpu: optimize group assignment when cpu_distance_fn is >> > NULL >> > >> >When cpu_distance_fn is NULL, all CPUs belongs to group 0. The original logic >> >will continue to go through each CPU and its predecessor. cpu_distance_fn is >> >always NULL when pcpu_build_alloc_info() is called from pcpu_page_first_chunk(). >> > >> >By applying this patch, the time complexity will drop to O(n) form O(n^2) in >> >case cpu_distance_fn is NULL. > >The test was put in the inner loop because the nesting was already too >deep and cpu_distance_fn is unlikely to be NULL on machines where the >number of CPUs is high enough to matter. If that O(n^2) loop is gonna >be a problem, it's gonna be a problem on large NUMA machines and we'll >have to do something about it for cases where cpu_distance_fn exists >anyway. Tejun, Yep, hope this will not bring some problem on a large NUMA machie when cpu_distance_fn is not NULL. > >The patch is just extremely marginal. Ah well... why not? I'll apply >it once -rc1 drops. > >Thanks. > >-- >tejun -- Richard Yang Help you, Help me