From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 5D03AC433DF for ; Wed, 15 Jul 2020 11:20:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2FDCE2063A for ; Wed, 15 Jul 2020 11:20:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729592AbgGOLUS (ORCPT ); Wed, 15 Jul 2020 07:20:18 -0400 Received: from [195.135.220.15] ([195.135.220.15]:37118 "EHLO mx2.suse.de" rhost-flags-FAIL-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1725855AbgGOLUR (ORCPT ); Wed, 15 Jul 2020 07:20:17 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id C81F3B0BD; Wed, 15 Jul 2020 11:20:19 +0000 (UTC) Subject: Re: [PATCH] mm: vmstat: fix /proc/sys/vm/stat_refresh generating false warnings To: Roman Gushchin , Andrew Morton Cc: Johannes Weiner , Michal Hocko , linux-mm@kvack.org, kernel-team@fb.com, linux-kernel@vger.kernel.org, Hugh Dickins References: <20200714173747.3315771-1-guro@fb.com> From: Vlastimil Babka Message-ID: Date: Wed, 15 Jul 2020 13:20:17 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200714173747.3315771-1-guro@fb.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/14/20 7:37 PM, Roman Gushchin wrote: > I've noticed a number of warnings like "vmstat_refresh: nr_free_cma > -5" or "vmstat_refresh: nr_zone_write_pending -11" on our production > hosts. The numbers of these warnings were relatively low and stable, > so it didn't look like we are systematically leaking the counters. > The corresponding vmstat counters also looked sane. > > These warnings are generated by the vmstat_refresh() function, which > assumes that atomic zone and numa counters can't go below zero. > However, on a SMP machine it's not quite right: due to per-cpu > caching it can in theory be as low as -(zone threshold) * NR_CPUs. > > For instance, let's say all cma pages are in use and NR_FREE_CMA_PAGES > reached 0. Then we've reclaimed a small number of cma pages on each > CPU except CPU0, so that most percpu NR_FREE_CMA_PAGES counters are > slightly positive (the atomic counter is still 0). Then somebody on > CPU0 consumes all these pages. The number of pages can easily exceed > the threshold and a negative value will be committed to the atomic > counter. > > To fix the problem and avoid generating false warnings, let's just > relax the condition and warn only if the value is less than minus > the maximum theoretically possible drift value, which is 125 * > number of online CPUs. It will still allow to catch systematic leaks, > but will not generate bogus warnings. > > Signed-off-by: Roman Gushchin > Cc: Hugh Dickins > Signed-off-by: Roman Gushchin Acked-by: Vlastimil Babka