Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934970Ab3DHR3S (ORCPT ); Mon, 8 Apr 2013 13:29:18 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:54403 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934770Ab3DHR3R (ORCPT ); Mon, 8 Apr 2013 13:29:17 -0400 Message-ID: <5162FE4D.7020308@linux.vnet.ibm.com> Date: Mon, 08 Apr 2013 10:28:45 -0700 From: Cody P Schafer User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130308 Thunderbird/17.0.4 MIME-Version: 1.0 To: Gilad Ben-Yossef CC: Andrew Morton , Mel Gorman , Linux MM , LKML 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> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13040817-5806-0000-0000-000020A43450 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2030 Lines: 50 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. -- 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/