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 07797C54E49 for ; Mon, 26 Feb 2024 02:55:53 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 95C9094000E; Sun, 25 Feb 2024 21:55:52 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 8BB79940008; Sun, 25 Feb 2024 21:55:52 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 6E7F694000E; Sun, 25 Feb 2024 21:55:52 -0500 (EST) 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 37947940008 for ; Sun, 25 Feb 2024 21:55:52 -0500 (EST) Received: from smtpin19.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay09.hostedemail.com (Postfix) with ESMTP id DB25480668 for ; Mon, 26 Feb 2024 02:55:51 +0000 (UTC) X-FDA: 81832440102.19.5EAA8D7 Received: from szxga06-in.huawei.com (szxga06-in.huawei.com [45.249.212.32]) by imf19.hostedemail.com (Postfix) with ESMTP id 9AA591A0004 for ; Mon, 26 Feb 2024 02:55:48 +0000 (UTC) Authentication-Results: imf19.hostedemail.com; dkim=none; dmarc=pass (policy=quarantine) header.from=huawei.com; spf=pass (imf19.hostedemail.com: domain of zhangpeng362@huawei.com designates 45.249.212.32 as permitted sender) smtp.mailfrom=zhangpeng362@huawei.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1708916149; h=from:from:sender: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=Ybg6QR3yO78pXiEUHGELXL9hrpDeY4FGjFAKPrdLeZ0=; b=In9H7jqSiCx1HY7ZolUXIoT9+oLC7cZdYkxxkWZTsSEzckWIQY+HMvHnCmvSayQSrTHvvG Q4mf84om26X8qT1A2hwI1Ad59AZVHidM3hGfDwUkbqfIfbnPFt4elJoiL+JjTDl+edxpd+ wYMpLeQGMkLaRMoXldjK689g5N8+WcI= ARC-Authentication-Results: i=1; imf19.hostedemail.com; dkim=none; dmarc=pass (policy=quarantine) header.from=huawei.com; spf=pass (imf19.hostedemail.com: domain of zhangpeng362@huawei.com designates 45.249.212.32 as permitted sender) smtp.mailfrom=zhangpeng362@huawei.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1708916149; a=rsa-sha256; cv=none; b=WDwCv5KJ+x1eaSizVydmg9yLpzzPJoGdCPaL62ptFadSHh5ezC9FEDEX/7+U6yq56b8opu Jyn4sa2nB6iE60njXFlhCsqYrmth5+r+tjx3HM9rd1C8GWArCvZ7/yusJ65mKELYijBVdb 57B7T+tFDDEoL/jCGkOgfv29kTUftv8= Received: from mail.maildlp.com (unknown [172.19.162.112]) by szxga06-in.huawei.com (SkyGuard) with ESMTP id 4TjlbJ4zD0zqhdk; Mon, 26 Feb 2024 10:55:08 +0800 (CST) Received: from kwepemm600020.china.huawei.com (unknown [7.193.23.147]) by mail.maildlp.com (Postfix) with ESMTPS id DA1D01400D3; Mon, 26 Feb 2024 10:55:44 +0800 (CST) Received: from [10.174.179.160] (10.174.179.160) by kwepemm600020.china.huawei.com (7.193.23.147) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.35; Mon, 26 Feb 2024 10:55:43 +0800 Message-ID: Date: Mon, 26 Feb 2024 10:55:43 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.9.0 Subject: Re: [PATCH v2] filemap: avoid unnecessary major faults in filemap_fault() To: "Huang, Ying" CC: , , , , , , , , , , Nanyong Sun References: <20240206092627.1421712-1-zhangpeng362@huawei.com> <87jznhypxy.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Language: en-US From: "zhangpeng (AS)" In-Reply-To: <87jznhypxy.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.179.160] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To kwepemm600020.china.huawei.com (7.193.23.147) X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 9AA591A0004 X-Stat-Signature: n6xtbc44kbsktgx8yfdcg3c5iarpabxn X-Rspam-User: X-HE-Tag: 1708916148-24644 X-HE-Meta: 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 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: List-Subscribe: List-Unsubscribe: On 2024/2/7 10:21, Huang, Ying wrote: > Peng Zhang writes: >> From: ZhangPeng >> >> The major fault occurred when using mlockall(MCL_CURRENT | MCL_FUTURE) >> in application, which leading to an unexpected performance issue[1]. >> >> This caused by temporarily cleared PTE during a read+clear/modify/write >> update of the PTE, eg, do_numa_page()/change_pte_range(). >> >> For the data segment of the user-mode program, the global variable area >> is a private mapping. After the pagecache is loaded, the private anonymous >> page is generated after the COW is triggered. Mlockall can lock COW pages >> (anonymous pages), but the original file pages cannot be locked and may >> be reclaimed. If the global variable (private anon page) is accessed when >> vmf->pte is zeroed in numa fault, a file page fault will be triggered. >> >> At this time, the original private file page may have been reclaimed. >> If the page cache is not available at this time, a major fault will be >> triggered and the file will be read, causing additional overhead. >> >> Fix this by rechecking the PTE without acquiring PTL in filemap_fault() >> before triggering a major fault. >> >> Testing file anonymous page read and write page fault performance in ext4 >> and ramdisk using will-it-scale[2] on a x86 physical machine. The data >> is the average change compared with the mainline after the patch is >> applied. The test results are within the range of fluctuation, and there >> is no obvious difference. The test results are as follows: > You still claim that there's no difference in the test results. If so, > why do you implement the patch? IMHO, you need to prove your patch can > improve the performance in some cases. I'm sorry that maybe I didn't express myself clearly. The purpose of this patch is to fix the issue that major fault may still be triggered with mlockall(), thereby improving a little performance. This patch is more of a bugfix than a performance improvement patch. This issue affects our traffic analysis service. The inbound traffic is heavy. If a major fault occurs, the I/O schedule is triggered and the original I/O is suspended. Generally, the I/O schedule is 0.7 ms. If other applications are operating disks, the system needs to wait for more than 10 ms. However, the inbound traffic is heavy and the NIC buffer is small. As a result, packet loss occurs. The traffic analysis service can't tolerate packet loss. To prevent packet loss, we use the mlockall() function to prevent I/O. It is unreasonable that major faults will still be triggered after mlockall() is used. In our service test environment, the baseline is 7 major faults/12 hours. After applied the unlock patch, the probability of triggering the major fault is 1 major faults/12 hours. After applied the lock patch, no major fault will be triggered. So only the locked patch can actually solve our problem. The test data provided is intended to prove that the patch does not have a major impact on the performance of the page fault itself. >> processes processes_idle threads threads_idle >> ext4 private file write: -1.14% -0.08% -1.87% 0.13% >> ext4 shared file write: 0.14% -0.53% 2.88% -0.77% >> ext4 private file read: 0.03% -0.65% -0.51% -0.08% >> tmpfs private file write: -0.34% -0.11% 0.20% 0.15% >> tmpfs shared file write: 0.96% 0.10% 2.78% -0.34% >> ramdisk private file write: -1.21% -0.21% -1.12% 0.11% >> ramdisk private file read: 0.00% -0.68% -0.33% -0.02% >> >> [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@huawei.com/ >> [2] https://github.com/antonblanchard/will-it-scale/ >> >> Suggested-by: "Huang, Ying" >> Suggested-by: Yin Fengwei >> Signed-off-by: ZhangPeng >> Signed-off-by: Kefeng Wang >> --- >> v1->v2: >> - Add more test results per Huang, Ying >> - Add more comments before check PTE per Huang, Ying, David Hildenbrand >> and Yin Fengwei >> - Change pte_offset_map_nolock to pte_offset_map as the ptl lock won't >> be used >> >> RFC->v1: >> - Add error handling when ptep == NULL per Huang, Ying and Matthew >> Wilcox >> - Check the PTE without acquiring PTL in filemap_fault(), suggested by >> Huang, Ying and Yin Fengwei >> - Add pmd_none() check before PTE map >> - Update commit message and add performance test information >> >> mm/filemap.c | 34 ++++++++++++++++++++++++++++++++++ >> 1 file changed, 34 insertions(+) >> >> diff --git a/mm/filemap.c b/mm/filemap.c >> index 142864338ca4..a2c1a98bc771 100644 >> --- a/mm/filemap.c >> +++ b/mm/filemap.c >> @@ -3238,6 +3238,40 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) >> mapping_locked = true; >> } >> } else { >> + if (!pmd_none(*vmf->pmd)) { >> + pte_t *ptep; >> + >> + ptep = pte_offset_map(vmf->pmd, vmf->address); >> + if (unlikely(!ptep)) >> + return VM_FAULT_NOPAGE; >> + /* >> + * Recheck PTE as the PTE can be cleared temporarily >> + * during a read+clear/modify/write update of the PTE, >> + * eg, do_numa_page()/change_pte_range(). This will >> + * trigger a major fault, even if we use mlockall, >> + * which may affect performance. >> + * We don't hold PTL here as acquiring PTL hurts >> + * performance. So the check is still racy, but >> + * the race window seems small enough. >> + * >> + * If we lose the race during the check, the page_fault >> + * will be triggered. Butthe page table entry lock >> + * still make sure the correctness: >> + * - If the page cache is not reclaimed, the page_fault >> + * will work like the page fault was served already >> + * and bail out. >> + * - If the page cache is reclaimed, the major fault >> + * will be triggered, page cache is filled, >> + * page_fault also work like the page fault was >> + * served already and bail out. >> + */ > IMHO, this is too long. It can be shorten to like, > > If we lose the race, major fault may be triggered unnecessary. This > hurts performance but not functionality. OK, I'll fix it in the next version. >> + if (unlikely(!pte_none(ptep_get_lockless(ptep)))) >> + ret = VM_FAULT_NOPAGE; >> + pte_unmap(ptep); >> + if (unlikely(ret)) >> + return ret; >> + } >> + >> /* No page in the page cache at all */ >> count_vm_event(PGMAJFAULT); >> count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT); > -- > Best Regards, > Huang, Ying -- Best Regards, Peng