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 BF6A4C43334 for ; Thu, 2 Jun 2022 07:29:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231752AbiFBH30 (ORCPT ); Thu, 2 Jun 2022 03:29:26 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46918 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231169AbiFBH3Y (ORCPT ); Thu, 2 Jun 2022 03:29:24 -0400 Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7F1BF235B2C for ; Thu, 2 Jun 2022 00:29:23 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.53]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4LDHgK6XHczjXLQ; Thu, 2 Jun 2022 15:28:29 +0800 (CST) Received: from [10.174.177.76] (10.174.177.76) by canpemm500002.china.huawei.com (7.192.104.244) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.24; Thu, 2 Jun 2022 15:29:20 +0800 Subject: Re: [PATCH 2/3] mm/swapfile: avoid confusing swap cache statistics To: David Hildenbrand CC: , , Hugh Dickins , Andrew Morton References: <20220527092626.31883-1-linmiaohe@huawei.com> <20220527092626.31883-3-linmiaohe@huawei.com> <20220530160409.c9b17085adb6112d8580f37d@linux-foundation.org> <33d6aec8-b4fc-aa37-27f4-f33984ea33d3@redhat.com> From: Miaohe Lin Message-ID: <1d550b5c-1409-cb9e-5de6-476d515c9a94@huawei.com> Date: Thu, 2 Jun 2022 15:29:20 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/6/1 15:53, David Hildenbrand wrote: > On 01.06.22 04:11, Miaohe Lin wrote: >> On 2022/5/31 20:58, David Hildenbrand wrote: >>> On 31.05.22 04:55, Miaohe Lin wrote: >>>> On 2022/5/31 7:04, Andrew Morton wrote: >>>>> On Fri, 27 May 2022 17:26:25 +0800 Miaohe Lin wrote: >>>>> >>>>>> At swapoff time, we're going to swap in the pages continuously. So calling >>>>>> lookup_swap_cache would confuse statistics. We should use find_get_page >>>>>> directly here. >>>>> >>>>> Why is the existing behaviour wrong? swapoff() has to swap stuff in to >>>>> be able to release the swap device. Why do you believe that this >>>>> swapin activity should not be accounted? >>>> >>>> IMHO, statistics, e.g. swap_cache_info.find_success, are used to show the effectiveness >>>> of the swap cache activity. So they should only reflect the memory accessing activity >>>> of the user. I think swapoff can't reflect the effectiveness of the swap cache activity >>>> because it just swaps in pages one by one. Or statistics should reflect all the activity >>>> of the user including swapoff? >>> >>> I'm wondering who cares and why? >> >> I thought it's used to show the effectiveness of the swapcache readahead algorithm. If nobody >> ever cares about it now, I'm fine to drop this patch. And could these statistics be removed >> since nobody cares about it? > > IIUC, they are printed (via show_swap_cache_info()), which is called via > show_free_areas() -- primarily used via show_mem(). show_mem() is > primarily used when OOM, when allocation fails and we warn, from the OOM > killer, on panic(). > > I am not sure how useful for (OOM ?) debugging the find_success vs. > find_total stats are at all. They are from ancient times. In > bb63be0a091c ("tmpfs: move swap_state stats update") we removed other > statistics that are "are relics of my 2.4.11 testing". Maybe > find_success and find_total can be similarly removed. Maybe add_total, del_total, find_success and find_total should be similarly removed altogether? It seems those can't provide useful info when OOM occurs? And we can thus avoid touching the swap_cache_info cacheline. > > data_race() indicates to me that these stats are somewhat best-effort > already. At least, this patch seems unneeded. Thanks! >