Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752088Ab3DJGWg (ORCPT ); Wed, 10 Apr 2013 02:22:36 -0400 Received: from mail-la0-f51.google.com ([209.85.215.51]:47785 "EHLO mail-la0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751287Ab3DJGWf (ORCPT ); Wed, 10 Apr 2013 02:22:35 -0400 MIME-Version: 1.0 X-Originating-IP: [212.179.42.66] In-Reply-To: References: <1365550099-6795-1-git-send-email-cody@linux.vnet.ibm.com> <1365550099-6795-4-git-send-email-cody@linux.vnet.ibm.com> Date: Wed, 10 Apr 2013 09:22:34 +0300 Message-ID: Subject: Re: [PATCH v2 03/10] mm/page_alloc: insert memory barriers to allow async update of pcp batch and high From: Gilad Ben-Yossef To: Cody P Schafer Cc: Andrew Morton , Simon Jeons , KOSAKI Motohiro , Mel Gorman , Linux MM , LKML Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3708 Lines: 123 On Wed, Apr 10, 2013 at 9:19 AM, Gilad Ben-Yossef wrote: > On Wed, Apr 10, 2013 at 2:28 AM, Cody P Schafer wrote: >> In pageset_set_batch() and setup_pagelist_highmark(), ensure that batch >> is always set to a safe value (1) prior to updating high, and ensure >> that high is fully updated before setting the real value of batch. >> >> Suggested by Gilad Ben-Yossef in this thread: >> >> https://lkml.org/lkml/2013/4/9/23 >> >> Also reproduces his proposed comment. >> >> Signed-off-by: Cody P Schafer >> --- >> mm/page_alloc.c | 19 +++++++++++++++++++ >> 1 file changed, 19 insertions(+) >> >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index d259599..a07bd4c 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -4007,11 +4007,26 @@ static int __meminit zone_batchsize(struct zone *zone) >> #endif >> } >> >> +static void pageset_update_prep(struct per_cpu_pages *pcp) >> +{ >> + /* >> + * We're about to mess with PCP in an non atomic fashion. Put an >> + * intermediate safe value of batch and make sure it is visible before >> + * any other change >> + */ >> + pcp->batch = 1; >> + smp_wmb(); >> +} >> + >> /* a companion to setup_pagelist_highmark() */ >> static void pageset_set_batch(struct per_cpu_pageset *p, unsigned long batch) >> { >> struct per_cpu_pages *pcp = &p->pcp; >> + pageset_update_prep(pcp); >> + >> pcp->high = 6 * batch; >> + smp_wmb(); >> + >> pcp->batch = max(1UL, 1 * batch); >> } >> >> @@ -4039,7 +4054,11 @@ static void setup_pagelist_highmark(struct per_cpu_pageset *p, >> struct per_cpu_pages *pcp; >> >> pcp = &p->pcp; >> + pageset_update_prep(pcp); >> + >> pcp->high = high; >> + smp_wmb(); >> + >> pcp->batch = max(1UL, high/4); >> if ((high/4) > (PAGE_SHIFT * 8)) >> pcp->batch = PAGE_SHIFT * 8; >> -- >> 1.8.2 >> > > That is very good. > However, now we've created a "protocol" for updating ->high and ->batch: > > 1. Call pageset_update_prep(pcp) > 2. Update ->high > 3. smp_wmb() > 4. Update ->batch > > But that protocol is not documented anywhere and someone that reads > the code two > years from now will not be aware of it or why it is done like that. > > How about if we create: > > /* > * pcp->high and pcp->batch values are related and dependent on one another: > * ->batch must never be higher then ->high. > * The following function updates them in a safe manner without a > costly atomic transaction. > */ > static void pageset_update(struct per_cpu_pages *pcp, unsigned int > high, unsigned int batch) > { > /* start with a fail safe value for batch */ > pcp->batch = 1; > smp_wmb(); > > /* Update high, then batch, in order */ > pcp->high = high; > smp_wmb(); > pcp->batch = batch; > } > > And use that at the update sites? then the protocol becomes explicit. Oh, and other then that it looks good to me, so assuming you do that - Reviewed-By: Gilad Ben-Yossef Many thanks, Gilad -- Gilad Ben-Yossef Chief Coffee Drinker gilad@benyossef.com Israel Cell: +972-52-8260388 US Cell: +1-973-8260388 http://benyossef.com "If you take a class in large-scale robotics, can you end up in a situation where the homework eats your dog?" -- Jean-Baptiste Queru -- 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/