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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 9CEE7C433EF for ; Mon, 6 Jun 2022 03:15:26 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 0D3C48D0003; Sun, 5 Jun 2022 23:15:26 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 082178D0002; Sun, 5 Jun 2022 23:15:26 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E62FE8D0003; Sun, 5 Jun 2022 23:15:25 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0011.hostedemail.com [216.40.44.11]) by kanga.kvack.org (Postfix) with ESMTP id D1A398D0002 for ; Sun, 5 Jun 2022 23:15:25 -0400 (EDT) Received: from smtpin08.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay10.hostedemail.com (Postfix) with ESMTP id A6E992BB for ; Mon, 6 Jun 2022 03:15:25 +0000 (UTC) X-FDA: 79546345410.08.682140A Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by imf28.hostedemail.com (Postfix) with ESMTP id 94BA3C0037 for ; Mon, 6 Jun 2022 03:14:40 +0000 (UTC) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.56]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4LGdpn0WD2z1K9G1; Mon, 6 Jun 2022 11:13:05 +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; Mon, 6 Jun 2022 11:14:50 +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> <1d550b5c-1409-cb9e-5de6-476d515c9a94@huawei.com> <5c041ef1-05ec-f49f-7ada-fa58d5aec8ba@redhat.com> From: Miaohe Lin Message-ID: Date: Mon, 6 Jun 2022 11:14:50 +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: <5c041ef1-05ec-f49f-7ada-fa58d5aec8ba@redhat.com> Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.177.76] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected Authentication-Results: imf28.hostedemail.com; dkim=none; dmarc=pass (policy=quarantine) header.from=huawei.com; spf=pass (imf28.hostedemail.com: domain of linmiaohe@huawei.com designates 45.249.212.255 as permitted sender) smtp.mailfrom=linmiaohe@huawei.com X-Stat-Signature: pdffk5xjr373u4yhqf6begmuqwjoopiz X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 94BA3C0037 X-HE-Tag: 1654485280-38899 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 2022/6/2 16:41, David Hildenbrand wrote: > On 02.06.22 09:29, Miaohe Lin wrote: >> 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. > > At least makes sense to me, AFAIKU, these are not statistics one could > easily use to tune system performance because they are not easily > accessile. Maybe simply propose removal? I tend to agree with you. Will try to do it soon. Thanks! > >