Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 44D67C433EF for ; Thu, 6 Jan 2022 11:00:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238244AbiAFLA5 (ORCPT ); Thu, 6 Jan 2022 06:00:57 -0500 Received: from smtp-out1.suse.de ([195.135.220.28]:51152 "EHLO smtp-out1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S238102AbiAFLAy (ORCPT ); Thu, 6 Jan 2022 06:00:54 -0500 Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 0F04321106; Thu, 6 Jan 2022 11:00:53 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1641466853; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=IEvBu7dHeD6Gxq79SWvMNCBIMerIX/L+M1nvgZS7GIA=; b=c9tjKvCDKtEOwp/kJiBfMqCa2Hx8pjG8zls8M2T3zZUkvtbMScBxARnqXHD8qHriSx68g6 Fzb9hhw1w49D5ywmlk5xCq7JwW/Hi54rHTHaoCaBtESyOKlx6dj/yhu++1VWpbefA8QVzz 2z+FuFvRmU7Jv+/GSo8XqEphvhVWcng= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id B46BA13C3E; Thu, 6 Jan 2022 11:00:52 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id cdVkK+TL1mHWEAAAMHmgww (envelope-from ); Thu, 06 Jan 2022 11:00:52 +0000 Date: Thu, 6 Jan 2022 12:00:51 +0100 From: Michal =?iso-8859-1?Q?Koutn=FD?= To: Muchun Song Cc: willy@infradead.org, akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com, shakeelb@google.com, guro@fb.com, shy828301@gmail.com, alexs@kernel.org, richard.weiyang@gmail.com, david@fromorbit.com, trond.myklebust@hammerspace.com, anna.schumaker@netapp.com, jaegeuk@kernel.org, chao@kernel.org, kari.argillander@gmail.com, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-nfs@vger.kernel.org, zhengqi.arch@bytedance.com, duanxiongchun@bytedance.com, fam.zheng@bytedance.com, smuchun@gmail.com Subject: Re: [PATCH v5 10/16] mm: list_lru: allocate list_lru_one only when needed Message-ID: <20220106110051.GA470@blackbody.suse.cz> References: <20211220085649.8196-1-songmuchun@bytedance.com> <20211220085649.8196-11-songmuchun@bytedance.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211220085649.8196-11-songmuchun@bytedance.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 20, 2021 at 04:56:43PM +0800, Muchun Song wrote: (Thanks for pointing me here.) > -void memcg_drain_all_list_lrus(int src_idx, struct mem_cgroup *dst_memcg) > +void memcg_drain_all_list_lrus(struct mem_cgroup *src, struct mem_cgroup *dst) > { > + struct cgroup_subsys_state *css; > struct list_lru *lru; > + int src_idx = src->kmemcg_id; > + > + /* > + * Change kmemcg_id of this cgroup and all its descendants to the > + * parent's id, and then move all entries from this cgroup's list_lrus > + * to ones of the parent. > + * > + * After we have finished, all list_lrus corresponding to this cgroup > + * are guaranteed to remain empty. So we can safely free this cgroup's > + * list lrus in memcg_list_lru_free(). > + * > + * Changing ->kmemcg_id to the parent can prevent memcg_list_lru_alloc() > + * from allocating list lrus for this cgroup after memcg_list_lru_free() > + * call. > + */ > + rcu_read_lock(); > + css_for_each_descendant_pre(css, &src->css) { > + struct mem_cgroup *memcg; > + > + memcg = mem_cgroup_from_css(css); > + memcg->kmemcg_id = dst->kmemcg_id; > + } > + rcu_read_unlock(); Do you envision using this function anywhere else beside offlining? If not, you shouldn't need traversing whole subtree because normally parents are offlined only after children (see cgroup_subsys_state.online_cnt). > > mutex_lock(&list_lrus_mutex); > list_for_each_entry(lru, &memcg_list_lrus, list) > - memcg_drain_list_lru(lru, src_idx, dst_memcg); > + memcg_drain_list_lru(lru, src_idx, dst); > mutex_unlock(&list_lrus_mutex); If you do, then here you only drain list_lru of the subtree root but not the descendants anymore. So I do get that mem_cgroup.kmemcg_id refernces the "effective" kmemcg_id after offlining, so that proper list_lrus are used afterwards. I wonder -- is this necessary when objcgs are reparented too? IOW would anyone query the offlined child's kmemcg_id? (Maybe it's worth explaining better in the commit message, I think even current approach is OK (better safe than sorry).) Thanks, Michal