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=-3.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=no 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 BD81EC433E7 for ; Fri, 16 Oct 2020 15:05:25 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6D89920829 for ; Fri, 16 Oct 2020 15:05:25 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2406832AbgJPPFY convert rfc822-to-8bit (ORCPT ); Fri, 16 Oct 2020 11:05:24 -0400 Received: from mx2.suse.de ([195.135.220.15]:55660 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2406455AbgJPPFY (ORCPT ); Fri, 16 Oct 2020 11:05:24 -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 594B3B1AB; Fri, 16 Oct 2020 15:05:22 +0000 (UTC) References: <20201014190749.24607-1-rpalethorpe@suse.com> <20201016094702.GA95052@blackbook> <87sgaesba0.fsf@suse.de> User-agent: mu4e 1.4.13; emacs 27.1 From: Richard Palethorpe To: Michal =?utf-8?Q?Koutn=C3=BD?= Cc: Roman Gushchin , ltp@lists.linux.it, Johannes Weiner , Andrew Morton , "Shakeel Butt" , Christoph Lameter , Michal Hocko , Tejun Heo , Vlastimil Babka , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH] mm: memcg/slab: Stop reparented obj_cgroups from charging root Reply-To: rpalethorpe@suse.de In-reply-to: <87sgaesba0.fsf@suse.de> Date: Fri, 16 Oct 2020 16:05:21 +0100 Message-ID: <87pn5irz2m.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Richard Palethorpe writes: > Hello Michal, > > Michal Koutný writes: > >> Hello. >> >> On Wed, Oct 14, 2020 at 08:07:49PM +0100, Richard Palethorpe wrote: >>> SLAB objects which outlive their memcg are moved to their parent >>> memcg where they may be uncharged. However if they are moved to the >>> root memcg, uncharging will result in negative page counter values as >>> root has no page counters. >> Why do you think those are reparented objects? If those are originally >> charged in a non-root cgroup, then the charge value should be propagated up the >> hierarchy, including root memcg, so if they're later uncharged in root >> after reparenting, it should still break even. (Or did I miss some stock >> imbalance?) > > I traced it and can see they are reparented objects and that the root > groups counters are zero (or negative if I run madvise06 multiple times) > before a drain takes place. I'm guessing this is because the root group > has 'use_hierachy' set to false so that the childs page_counter parents > are set to NULL. However I will check, because I'm not sure about > either. Yes, it appears that use_hierarchy=0 which is probably because the test mounts cgroup v1, creates a child group within that and does not set use_hierarchy on the root. On v2 root always has use_hierarchy enabled. > >> >> (But the patch seems justifiable to me as objects (not)charged directly to >> root memcg may be incorrectly uncharged.) >> >> Thanks, >> Michal I'm don't know if that could happen without reparenting. I suppose if use_hierarchy=1 then actually this patch will result in root being overcharged, so perhaps it should also check for use_hierarchy? -- Thank you, Richard.