From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx189.postini.com [74.125.245.189]) by kanga.kvack.org (Postfix) with SMTP id EF4736B004D for ; Mon, 8 Apr 2013 15:50:50 -0400 (EDT) Received: from /spool/local by e7.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 8 Apr 2013 15:50:49 -0400 Received: from d01relay02.pok.ibm.com (d01relay02.pok.ibm.com [9.56.227.234]) by d01dlp02.pok.ibm.com (Postfix) with ESMTP id B0F9C6E804F for ; Mon, 8 Apr 2013 15:50:38 -0400 (EDT) Received: from d01av05.pok.ibm.com (d01av05.pok.ibm.com [9.56.224.195]) by d01relay02.pok.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id r38Jof16270692 for ; Mon, 8 Apr 2013 15:50:41 -0400 Received: from d01av05.pok.ibm.com (loopback [127.0.0.1]) by d01av05.pok.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id r38Joejf022729 for ; Mon, 8 Apr 2013 15:50:41 -0400 Message-ID: <51631F89.5090407@linux.vnet.ibm.com> Date: Mon, 08 Apr 2013 12:50:33 -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> <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 Sender: owner-linux-mm@kvack.org List-ID: To: Gilad Ben-Yossef Cc: Andrew Morton , Mel Gorman , Linux MM , LKML 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, 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