Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935506Ab3DHWXu (ORCPT ); Mon, 8 Apr 2013 18:23:50 -0400 Received: from mail-qa0-f50.google.com ([209.85.216.50]:34569 "EHLO mail-qa0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763113Ab3DHWXt (ORCPT ); Mon, 8 Apr 2013 18:23:49 -0400 Message-ID: <51634375.2050205@gmail.com> Date: Mon, 08 Apr 2013 18:23:49 -0400 From: KOSAKI Motohiro User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:17.0) Gecko/20130307 Thunderbird/17.0.4 MIME-Version: 1.0 To: Cody P Schafer CC: Gilad Ben-Yossef , Andrew Morton , Mel Gorman , Linux MM , LKML , kosaki.motohiro@gmail.com Subject: Re: [PATCH 3/3] mm: when handling percpu_pagelist_fraction, use on_each_cpu() to set percpu pageset fields. References: <1365194030-28939-1-git-send-email-cody@linux.vnet.ibm.com> <1365194030-28939-4-git-send-email-cody@linux.vnet.ibm.com> <5162FE4D.7020308@linux.vnet.ibm.com> <51631F89.5090407@linux.vnet.ibm.com> In-Reply-To: <51631F89.5090407@linux.vnet.ibm.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2504 Lines: 61 (4/8/13 3:50 PM), Cody P Schafer wrote: > On 04/08/2013 10:28 AM, Cody P Schafer wrote: >> On 04/08/2013 05:20 AM, Gilad Ben-Yossef wrote: >>> On Fri, Apr 5, 2013 at 11:33 PM, Cody P Schafer >>> wrote: >>>> In free_hot_cold_page(), we rely on pcp->batch remaining stable. >>>> Updating it without being on the cpu owning the percpu pageset >>>> potentially destroys this stability. >>>> >>>> Change for_each_cpu() to on_each_cpu() to fix. >>> >>> Are you referring to this? - >> >> This was the case I noticed. >> >>> >>> 1329 if (pcp->count >= pcp->high) { >>> 1330 free_pcppages_bulk(zone, pcp->batch, pcp); >>> 1331 pcp->count -= pcp->batch; >>> 1332 } >>> >>> I'm probably missing the obvious but won't it be simpler to do this in >>> free_hot_cold_page() - >>> >>> 1329 if (pcp->count >= pcp->high) { >>> 1330 unsigned int batch = ACCESS_ONCE(pcp->batch); >>> 1331 free_pcppages_bulk(zone, batch, pcp); >>> 1332 pcp->count -= batch; >>> 1333 } >>> >> >> Potentially, yes. Note that this was simply the one case I noticed, >> rather than certainly the only case. >> >> I also wonder whether there could be unexpected interactions between >> ->high and ->batch not changing together atomically. For example, could >> adjusting this knob cause ->batch to rise enough that it is greater than >> the previous ->high? If the code above then runs with the previous >> ->high, ->count wouldn't be correct (checking this inside >> free_pcppages_bulk() might help on this one issue). >> >>> Now the batch value used is stable and you don't have to IPI every CPU >>> in the system just to change a config knob... >> >> Is this really considered an issue? I wouldn't have expected someone to >> adjust the config knob often enough (or even more than once) to cause >> problems. Of course as a "It'd be nice" thing, I completely agree. > > Would using schedule_on_each_cpu() instead of on_each_cpu() be an > improvement, in your opinion? No. As far as lightweight solusion work, we shouldn't introduce heavyweight code never. on_each_cpu() is really heavy weight especially when number of cpus are much than a thousand. -- 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/