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 97879C433E1 for ; Wed, 15 Jul 2020 15:43:45 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 6317D20663 for ; Wed, 15 Jul 2020 15:43:45 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6317D20663 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.cz Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 0DA376B000C; Wed, 15 Jul 2020 11:43:45 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 063626B000D; Wed, 15 Jul 2020 11:43:45 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E94046B000E; Wed, 15 Jul 2020 11:43:44 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0080.hostedemail.com [216.40.44.80]) by kanga.kvack.org (Postfix) with ESMTP id D538B6B000C for ; Wed, 15 Jul 2020 11:43:44 -0400 (EDT) Received: from smtpin03.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 9427F824999B for ; Wed, 15 Jul 2020 15:43:44 +0000 (UTC) X-FDA: 77040730368.03.scale76_0b032f126efa Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin03.hostedemail.com (Postfix) with ESMTP id EF1C028A246 for ; Wed, 15 Jul 2020 15:43:43 +0000 (UTC) X-HE-Tag: scale76_0b032f126efa X-Filterd-Recvd-Size: 2795 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf08.hostedemail.com (Postfix) with ESMTP for ; Wed, 15 Jul 2020 15:43:43 +0000 (UTC) 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 X-Rspamd-Queue-Id: EF1C028A246 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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