From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx180.postini.com [74.125.245.180]) by kanga.kvack.org (Postfix) with SMTP id 125AA6B0062 for ; Mon, 8 Apr 2013 13:29:07 -0400 (EDT) Received: from /spool/local by e8.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 8 Apr 2013 13:29:06 -0400 Received: from d01relay07.pok.ibm.com (d01relay07.pok.ibm.com [9.56.227.147]) by d01dlp03.pok.ibm.com (Postfix) with ESMTP id 04DDAC90073 for ; Mon, 8 Apr 2013 13:29:04 -0400 (EDT) Received: from d01av04.pok.ibm.com (d01av04.pok.ibm.com [9.56.224.64]) by d01relay07.pok.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id r38HStgW48758988 for ; Mon, 8 Apr 2013 13:28:58 -0400 Received: from d01av04.pok.ibm.com (loopback [127.0.0.1]) by d01av04.pok.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id r38HStCn022417 for ; Mon, 8 Apr 2013 13:28:55 -0400 Message-ID: <5162FE4D.7020308@linux.vnet.ibm.com> Date: Mon, 08 Apr 2013 10:28:45 -0700 From: Cody P Schafer MIME-Version: 1.0 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 Sender: owner-linux-mm@kvack.org List-ID: To: Gilad Ben-Yossef Cc: Andrew Morton , Mel Gorman , Linux MM , LKML 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org