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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id C9D69C433EF for ; Mon, 28 Mar 2022 00:57:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S237172AbiC1A7C (ORCPT ); Sun, 27 Mar 2022 20:59:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39738 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233105AbiC1A7B (ORCPT ); Sun, 27 Mar 2022 20:59:01 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id A82201E3DB for ; Sun, 27 Mar 2022 17:57:21 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1648429040; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=hCoHUvo0otD1KBtAxsU+h6m7YgOeGv2vtjDIUeHjlH0=; b=LFxjf/hVw2Ij3jM0ox7HIFI8z5eSvSh0ritmTINu+/ayEckrjSnmdZtM4vNOT0oy5dxhMq dh2//3agnANL0Lr2OWkFCUbWQxOPGeg7ZBGbwVyyaBptdxHX9PQuLtoqJPxzdFiaUzHEtd D3g2N3jz4ytVMI0xDzjD+ydoeyipA+A= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-427-l-72ovXrOy6M1r66HAMhYw-1; Sun, 27 Mar 2022 20:57:17 -0400 X-MC-Unique: l-72ovXrOy6M1r66HAMhYw-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 833413811A24; Mon, 28 Mar 2022 00:57:16 +0000 (UTC) Received: from [10.22.16.95] (unknown [10.22.16.95]) by smtp.corp.redhat.com (Postfix) with ESMTP id 442F0401E01; Mon, 28 Mar 2022 00:57:16 +0000 (UTC) Message-ID: Date: Sun, 27 Mar 2022 20:57:15 -0400 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [PATCH-mm v3] mm/list_lru: Optimize memcg_reparent_list_lru_node() Content-Language: en-US To: Muchun Song Cc: Andrew Morton , Linux Memory Management List , LKML , Roman Gushchin References: <20220309144000.1470138-1-longman@redhat.com> <2263666d-5eef-b1fe-d5e3-b166a3185263@redhat.com> From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.85 on 10.11.54.9 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/22/22 22:12, Muchun Song wrote: > On Wed, Mar 23, 2022 at 9:55 AM Waiman Long wrote: >> On 3/22/22 21:06, Muchun Song wrote: >>> On Wed, Mar 9, 2022 at 10:40 PM Waiman Long wrote: >>>> Since commit 2c80cd57c743 ("mm/list_lru.c: fix list_lru_count_node() >>>> to be race free"), we are tracking the total number of lru >>>> entries in a list_lru_node in its nr_items field. In the case of >>>> memcg_reparent_list_lru_node(), there is nothing to be done if nr_items >>>> is 0. We don't even need to take the nlru->lock as no new lru entry >>>> could be added by a racing list_lru_add() to the draining src_idx memcg >>>> at this point. >>> Hi Waiman, >>> >>> Sorry for the late reply. Quick question: what if there is an inflight >>> list_lru_add()? How about the following race? >>> >>> CPU0: CPU1: >>> list_lru_add() >>> spin_lock(&nlru->lock) >>> l = list_lru_from_kmem(memcg) >>> memcg_reparent_objcgs(memcg) >>> memcg_reparent_list_lrus(memcg) >>> memcg_reparent_list_lru() >>> memcg_reparent_list_lru_node() >>> if (!READ_ONCE(nlru->nr_items)) >>> // Miss reparenting >>> return >>> // Assume 0->1 >>> l->nr_items++ >>> // Assume 0->1 >>> nlru->nr_items++ >>> >>> IIUC, we use nlru->lock to serialise this scenario. >> I guess this race is theoretically possible but very unlikely since it >> means a very long pause between list_lru_from_kmem() and the increment >> of nr_items. > It is more possible in a VM. > >> How about the following changes to make sure that this race can't happen? >> >> diff --git a/mm/list_lru.c b/mm/list_lru.c >> index c669d87001a6..c31a0a8ad4e7 100644 >> --- a/mm/list_lru.c >> +++ b/mm/list_lru.c >> @@ -395,9 +395,10 @@ static void memcg_reparent_list_lru_node(struct >> list_lru *lru, int nid, >> struct list_lru_one *src, *dst; >> >> /* >> - * If there is no lru entry in this nlru, we can skip it >> immediately. >> + * If there is no lru entry in this nlru and the nlru->lock is free, >> + * we can skip it immediately. >> */ >> - if (!READ_ONCE(nlru->nr_items)) >> + if (!READ_ONCE(nlru->nr_items) && !spin_is_locked(&nlru->lock)) > I think we also should insert a smp_rmb() between those two loads. Thinking about this some more, I believe that adding spin_is_locked() check will be enough for x86. However, that will likely not be enough for arches with a more relaxed memory semantics. So the safest way to avoid this possible race is to move the check to within the lock critical section, though that comes with a slightly higher overhead for the 0 nr_items case. I will send out a patch to correct that. Thanks for bring this possible race to my attention. Cheers, Longman