Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935844Ab3DHTuq (ORCPT ); Mon, 8 Apr 2013 15:50:46 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:48398 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S935727Ab3DHTup (ORCPT ); Mon, 8 Apr 2013 15:50:45 -0400 Message-ID: <51631F89.5090407@linux.vnet.ibm.com> Date: Mon, 08 Apr 2013 12:50:33 -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> <5162FE4D.7020308@linux.vnet.ibm.com> In-Reply-To: <5162FE4D.7020308@linux.vnet.ibm.com> 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: 13040819-7182-0000-0000-0000062485B7 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2226 Lines: 55 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? -- 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/