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 1E70EC433F5 for ; Wed, 23 Mar 2022 01:55:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240830AbiCWB4l (ORCPT ); Tue, 22 Mar 2022 21:56:41 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33476 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240157AbiCWB4k (ORCPT ); Tue, 22 Mar 2022 21:56:40 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id 839A456202 for ; Tue, 22 Mar 2022 18:55:11 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1648000510; 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=GQneE/rU6uYhLlhfDWprcrRBY9/cQlczmjL+8igVejo=; b=gT7R/uCegvH/p5aLMmbZgYko1Tu3Ewg1dz2OOxuXoltL+IgqlR4x89ml9aDa2HRNgYlGx8 1o/JOcrGsPkbQjKHCM4JonHzRVQP6O4DyYtkXQrFObCcOd3QraudDwmugwV6pj7crmPC+L uNFM8CF6+XlUt/szX6aRfysj85uAxjU= 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-588-37AbWpS6Mxu4ESKL-r-VHw-1; Tue, 22 Mar 2022 21:55:07 -0400 X-MC-Unique: 37AbWpS6Mxu4ESKL-r-VHw-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 B87BE3C11A0B; Wed, 23 Mar 2022 01:55:06 +0000 (UTC) Received: from [10.22.16.140] (unknown [10.22.16.140]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6AF3043E90C; Wed, 23 Mar 2022 01:55:06 +0000 (UTC) Message-ID: <2263666d-5eef-b1fe-d5e3-b166a3185263@redhat.com> Date: Tue, 22 Mar 2022 21:55:05 -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> From: Waiman Long In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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 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. 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))                 return; Cheers, Longman