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=-6.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,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 55352C388F2 for ; Thu, 22 Oct 2020 07:05:04 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 66E1821534 for ; Thu, 22 Oct 2020 07:05:03 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 66E1821534 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=suse.de Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 9089A6B005D; Thu, 22 Oct 2020 03:05:02 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 8B8F36B0062; Thu, 22 Oct 2020 03:05:02 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 75A9C6B0068; Thu, 22 Oct 2020 03:05:02 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0111.hostedemail.com [216.40.44.111]) by kanga.kvack.org (Postfix) with ESMTP id 4821C6B005D for ; Thu, 22 Oct 2020 03:05:02 -0400 (EDT) Received: from smtpin27.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id E350D8995516 for ; Thu, 22 Oct 2020 07:05:01 +0000 (UTC) X-FDA: 77398674402.27.key64_07042ab2724f Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin27.hostedemail.com (Postfix) with ESMTP id BADAF3D663 for ; Thu, 22 Oct 2020 07:05:01 +0000 (UTC) X-HE-Tag: key64_07042ab2724f X-Filterd-Recvd-Size: 5241 Received: from mx2.suse.de (mx2.suse.de [195.135.220.15]) by imf44.hostedemail.com (Postfix) with ESMTP for ; Thu, 22 Oct 2020 07:05:01 +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 D9F03AD4A; Thu, 22 Oct 2020 07:04:59 +0000 (UTC) References: <87lfg2ob83.fsf@suse.de> <20201019095812.25710-1-rpalethorpe@suse.com> <87mu0hwik7.fsf@suse.de> <20201020172402.GD46039@blackbook> User-agent: mu4e 1.4.13; emacs 27.1 From: Richard Palethorpe To: Michal =?utf-8?Q?Koutn=C3=BD?= Cc: Shakeel Butt , Linux MM , LKML , LTP List , Roman Gushchin , Johannes Weiner , Andrew Morton , Christoph Lameter , Michal Hocko , Tejun Heo , Vlastimil Babka Subject: Re: [PATCH v3] mm: memcg/slab: Stop reparented obj_cgroups from charging root Reply-To: rpalethorpe@suse.de In-reply-to: <20201020172402.GD46039@blackbook> Date: Thu, 22 Oct 2020 08:04:58 +0100 Message-ID: <87sga6vizp.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable 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: Hello, Michal Koutn=C3=BD writes: > Hi. > > On Tue, Oct 20, 2020 at 06:52:08AM +0100, Richard Palethorpe wrote: >> I don't think that is relevant as we get the memcg from objcg->memcg >> which is set during reparenting. I suppose however, we can determine if >> the objcg was reparented by inspecting memcg->objcg. > +1 > >> If we just check use_hierarchy then objects directly charged to the >> memcg where use_hierarchy=3D0 will not be uncharged. However, maybe it is >> better to check if it was reparented and if use_hierarchy=3D0. > I think (I had to make a table) the yielded condition would be: > > if ((memcg->use_hierarchy && reparented) || (!mem_cgroup_is_root(memcg) &= & !reparented)) This looks correct, but I don't think we need to check for reparenting after all. We have the following unique scenarious: use_hierarchy=3D1, memcg=3D?, reparented=3D?: Because use_hierarchy=3D1 any descendants will have charged the current memcg, including root, so we can simply uncharge regardless of the memcg or objcg. use_hierarchy=3D0, memcg!=3Droot, reparented=3D?: When use_hierarchy=3D0, objcgs are *only* reparented to root, so if we are not root the objcg must belong to us. use_hierarchy=3D0, memcg=3Droot, reparented=3D?: We must have been reparented because root is not initialised with an objcg, but use_hierarchy=3D0 so we can not uncharge. So I believe that the following is sufficient. if (memcg->use_hierarchy || !mem_cgroup_is_root(memcg)) > __memcg_kmem_uncharge(memcg, nr_pages); > > (I admit it's not very readable.) > > > Michal For the record, I did create the following patch which checks for reparenting, but it appears to be unecessary. ---- diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 6877c765b8d0..0285f760f003 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -257,6 +257,14 @@ struct cgroup_subsys_state *vmpressure_to_css(struct v= mpressure *vmpr) #ifdef CONFIG_MEMCG_KMEM extern spinlock_t css_set_lock; +/* Assumes objcg originated from a descendant of memcg or is memcg's */ +static bool obj_cgroup_did_charge(const struct obj_cgroup *objcg, + const struct mem_cgroup *memcg) +{ + return memcg->use_hierarchy || + rcu_access_pointer(memcg->objcg) =3D=3D objcg; +} + static void obj_cgroup_release(struct percpu_ref *ref) { struct obj_cgroup *objcg =3D container_of(ref, struct obj_cgroup, r= efcnt); @@ -291,7 +299,7 @@ static void obj_cgroup_release(struct percpu_ref *ref) spin_lock_irqsave(&css_set_lock, flags); memcg =3D obj_cgroup_memcg(objcg); - if (nr_pages) + if (nr_pages && obj_cgroup_did_charge(objcg, memcg)) __memcg_kmem_uncharge(memcg, nr_pages); list_del(&objcg->list); mem_cgroup_put(memcg); @@ -3100,6 +3108,7 @@ static bool consume_obj_stock(struct obj_cgroup *objc= g, unsigned int nr_bytes) static void drain_obj_stock(struct memcg_stock_pcp *stock) { struct obj_cgroup *old =3D stock->cached_objcg; + struct mem_cgroup *memcg; if (!old) return; @@ -3110,7 +3119,9 @@ static void drain_obj_stock(struct memcg_stock_pcp *s= tock) if (nr_pages) { rcu_read_lock(); - __memcg_kmem_uncharge(obj_cgroup_memcg(old), nr_pag= es); + memcg =3D obj_cgroup_memcg(old); + if (obj_cgroup_did_charge(old, memcg)) + __memcg_kmem_uncharge(memcg, nr_pages); rcu_read_unlock(); } --=20 Thank you, Richard.