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=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable 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 F0647C43603 for ; Tue, 17 Dec 2019 14:14:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D22C421739 for ; Tue, 17 Dec 2019 14:14:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728806AbfLQOOP (ORCPT ); Tue, 17 Dec 2019 09:14:15 -0500 Received: from relay.sw.ru ([185.231.240.75]:46658 "EHLO relay.sw.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728695AbfLQOOO (ORCPT ); Tue, 17 Dec 2019 09:14:14 -0500 Received: from dhcp-172-16-24-104.sw.ru ([172.16.24.104]) by relay.sw.ru with esmtp (Exim 4.92.3) (envelope-from ) id 1ihDbP-0005Tm-9c; Tue, 17 Dec 2019 17:13:31 +0300 Subject: Re: [PATCH v2] mm/hugetlb: Defer freeing of huge pages if in non-task context To: Waiman Long , Michal Hocko Cc: Mike Kravetz , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Matthew Wilcox , Davidlohr Bueso , Andi Kleen , "Aneesh Kumar K.V" References: <20191217012508.31495-1-longman@redhat.com> <20191217093143.GC31063@dhcp22.suse.cz> <87c2ff49-999e-3196-791f-36e3d42ad79c@virtuozzo.com> <0b8a59a0-517f-1387-ad00-cb47fb5fc50c@redhat.com> From: Kirill Tkhai Message-ID: <99caa26d-e14d-ed38-f56a-e6aee203251a@virtuozzo.com> Date: Tue, 17 Dec 2019 17:13:31 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <0b8a59a0-517f-1387-ad00-cb47fb5fc50c@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 17.12.2019 17:00, Waiman Long wrote: > On 12/17/19 5:50 AM, Kirill Tkhai wrote: >> On 17.12.2019 12:31, Michal Hocko wrote: >>> On Mon 16-12-19 20:25:08, Waiman Long wrote: >>> [...] >>>> Both the hugetbl_lock and the subpool lock can be acquired in >>>> free_huge_page(). One way to solve the problem is to make both locks >>>> irq-safe. >>> Please document why we do not take this, quite natural path and instead >>> we have to come up with an elaborate way instead. I believe the primary >>> motivation is that some operations under those locks are quite >>> expensive. Please add that to the changelog and ideally to the code as >>> well. We probably want to fix those anyway and then this would be a >>> temporary workaround. >>> >>>> Another alternative is to defer the freeing to a workqueue job. >>>> >>>> This patch implements the deferred freeing by adding a >>>> free_hpage_workfn() work function to do the actual freeing. The >>>> free_huge_page() call in a non-task context saves the page to be freed >>>> in the hpage_freelist linked list in a lockless manner. >>> Do we need to over complicate this (presumably) rare event by a lockless >>> algorithm? Why cannot we use a dedicated spin lock for for the linked >>> list manipulation? This should be really a trivial code without an >>> additional burden of all the lockless subtleties. >> Why not llist_add()/llist_del_all() ? >> > The llist_add() and llist_del_all() are just simple helpers. Because > this lockless case involve synchronization of two variables, the llist > helpers do not directly apply here. So the rests cannot be used. It will > look awkward it is partially converted to use the helpers. If we convert > to use a lock as suggested by Michal, using the helpers will be an > overkill as xchg() will not be needed. I don't understand you. What are two variables? Why can't you simply do the below? diff --git a/mm/hugetlb.c b/mm/hugetlb.c index ac65bb5e38ac..e8ec753f3d92 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -1136,7 +1136,7 @@ static inline void ClearPageHugeTemporary(struct page *page) page[2].mapping = NULL; } -void free_huge_page(struct page *page) +static void __free_huge_page(struct page *page) { /* * Can't pass hstate in here because it is called from the @@ -1199,6 +1199,35 @@ void free_huge_page(struct page *page) spin_unlock(&hugetlb_lock); } +static struct llist_head hpage_freelist = LLIST_HEAD_INIT; + +static void free_hpage_workfn(struct work_struct *work) +{ + struct llist_node *node; + struct page *page; + + node = llist_del_all(&hpage_freelist); + + while (node) { + page = container_of(node, struct page, mapping); + node = node->next; + __free_huge_page(page); + } +} + +static DECLARE_WORK(free_hpage_work, free_hpage_workfn); + +void free_huge_page(struct page *page) +{ + if (!in_task()) { + if (llist_add((struct llist_node *)&page->mapping, &hpage_freelist)) + schedule_work(&free_hpage_work); + return; + } + + __free_huge_page(page); +} + static void prep_new_huge_page(struct hstate *h, struct page *page, int nid) { INIT_LIST_HEAD(&page->lru);