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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 3B3CBC433FE for ; Tue, 15 Nov 2022 03:35:54 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4NBBfJ4Bhsz3cKM for ; Tue, 15 Nov 2022 14:35:52 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=huawei.com (client-ip=45.249.212.187; helo=szxga01-in.huawei.com; envelope-from=yangyicong@huawei.com; receiver=) Received: from szxga01-in.huawei.com (szxga01-in.huawei.com [45.249.212.187]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4NBBdj6LZxz3bhR for ; Tue, 15 Nov 2022 14:35:19 +1100 (AEDT) Received: from canpemm500009.china.huawei.com (unknown [172.30.72.54]) by szxga01-in.huawei.com (SkyGuard) with ESMTP id 4NBBXf463pzqSRP; Tue, 15 Nov 2022 11:30:58 +0800 (CST) Received: from [10.67.102.169] (10.67.102.169) by canpemm500009.china.huawei.com (7.192.105.203) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Tue, 15 Nov 2022 11:34:44 +0800 Subject: Re: [PATCH v5 2/2] arm64: support batched/deferred tlb shootdown during page reclamation To: Anshuman Khandual , , , , , , , References: <20221028081255.19157-1-yangyicong@huawei.com> <20221028081255.19157-3-yangyicong@huawei.com> <86fbdc8c-0dcb-9b8f-d843-63460d8b1d6a@arm.com> <9982dac0-9f2e-112a-d440-467c8e8f8aa4@huawei.com> <40f1b5ad-2165-bb81-1ff5-89786373fa14@arm.com> From: Yicong Yang Message-ID: <5bbd5364-fcec-5fc2-4780-e9b44f2ab135@huawei.com> Date: Tue, 15 Nov 2022 11:34:44 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.5.1 MIME-Version: 1.0 In-Reply-To: <40f1b5ad-2165-bb81-1ff5-89786373fa14@arm.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.67.102.169] X-ClientProxiedBy: dggems702-chm.china.huawei.com (10.3.19.179) To canpemm500009.china.huawei.com (7.192.105.203) X-CFilter-Loop: Reflected X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: wangkefeng.wang@huawei.com, darren@os.amperecomputing.com, peterz@infradead.org, yangyicong@hisilicon.com, punit.agrawal@bytedance.com, Nadav Amit , guojian@oppo.com, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, zhangshiming@oppo.com, lipeifeng@oppo.com, corbet@lwn.net, Barry Song <21cnbao@gmail.com>, Mel Gorman , linux-mips@vger.kernel.org, arnd@arndb.de, realmz6@gmail.com, Barry Song , openrisc@lists.librecores.org, prime.zeng@hisilicon.com, xhao@linux.alibaba.com, linux-kernel@vger.kernel.org, huzhanyuan@oppo.com, linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 2022/11/14 22:19, Anshuman Khandual wrote: > > > On 11/14/22 14:16, Yicong Yang wrote: >> On 2022/11/14 11:29, Anshuman Khandual wrote: >>> >>> On 10/28/22 13:42, Yicong Yang wrote: >>>> +static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm) >>>> +{ >>>> + /* >>>> + * TLB batched flush is proved to be beneficial for systems with large >>>> + * number of CPUs, especially system with more than 8 CPUs. TLB shutdown >>>> + * is cheap on small systems which may not need this feature. So use >>>> + * a threshold for enabling this to avoid potential side effects on >>>> + * these platforms. >>>> + */ >>>> + if (num_online_cpus() <= CONFIG_ARM64_NR_CPUS_FOR_BATCHED_TLB) >>>> + return false; >>>> + >>>> +#ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI >>>> + if (unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI))) >>>> + return false; >>>> +#endif >>> should_defer_flush() is immediately followed by set_tlb_ubc_flush_pending() which calls >>> arch_tlbbatch_add_mm(), triggering the actual TLBI flush via __flush_tlb_page_nosync(). >>> It should be okay to check capability with this_cpu_has_cap() as the entire call chain >>> here is executed on the same cpu. But just wondering if cpus_have_const_cap() would be >>> simpler, consistent, and also cost effective ? >>> >> ok. Checked cpus_have_const_cap() I think it matches your words. >> >>> Regardless, a comment is needed before the #ifdef block explaining why it does not make >>> sense to defer/batch when __tlbi()/__tlbi_user() implementation will execute 'dsb(ish)' >>> between two TLBI instructions to workaround the errata. >>> >> The workaround for the errata mentioned the affected platforms need the tlbi+dsb to be done >> twice, so I'm not sure if we defer the final dsb will cause any problem so I think the judgement >> here is used for safety. I have no such platform to test if it's ok to defer the last dsb. > > We should not defer TLB flush on such systems, as ensured by the above test and 'false' > return afterwards. The only question is whether this decision should be taken at a CPU > level (which is affected by the errata) or the whole system level. > > What is required now > > - Replace this_cpu_has_cap() with cpus_have_const_cap ? > - Add the following comment before the #ifdef check > Have respin the series according to the above comments: https://lore.kernel.org/lkml/20221115031425.44640-3-yangyicong@huawei.com/ Thanks. > /* > * TLB flush deferral is not required on systems, which are affected with > * ARM64_WORKAROUND_REPEAT_TLBI, as __tlbi()/__tlbi_user() implementation > * will have two consecutive TLBI instructions with a dsb(ish) in between > * defeating the purpose (i.e save overall 'dsb ish' cost). > */ > > . >