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=-8.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 6E5F2C433E3 for ; Thu, 16 Jul 2020 10:28:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 540232078C for ; Thu, 16 Jul 2020 10:28:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726812AbgGPK2T (ORCPT ); Thu, 16 Jul 2020 06:28:19 -0400 Received: from lhrrgout.huawei.com ([185.176.76.210]:2487 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726332AbgGPK2T (ORCPT ); Thu, 16 Jul 2020 06:28:19 -0400 Received: from lhreml724-chm.china.huawei.com (unknown [172.18.7.107]) by Forcepoint Email with ESMTP id 3B92680604F5E70695B5; Thu, 16 Jul 2020 11:28:18 +0100 (IST) Received: from [127.0.0.1] (10.210.168.254) by lhreml724-chm.china.huawei.com (10.201.108.75) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.1913.5; Thu, 16 Jul 2020 11:28:17 +0100 Subject: Re: [PATCH 4/4] iommu/arm-smmu-v3: Remove cmpxchg() in arm_smmu_cmdq_issue_cmdlist() To: Will Deacon CC: , , , , , , , References: <1592846920-45338-1-git-send-email-john.garry@huawei.com> <1592846920-45338-5-git-send-email-john.garry@huawei.com> <20200716102037.GB7036@willie-the-truck> From: John Garry Message-ID: <36fe9596-745b-b01c-181c-b87a544a413b@huawei.com> Date: Thu, 16 Jul 2020 11:26:29 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.1.2 MIME-Version: 1.0 In-Reply-To: <20200716102037.GB7036@willie-the-truck> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.210.168.254] X-ClientProxiedBy: lhreml714-chm.china.huawei.com (10.201.108.65) To lhreml724-chm.china.huawei.com (10.201.108.75) X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 16/07/2020 11:20, Will Deacon wrote: > On Tue, Jun 23, 2020 at 01:28:40AM +0800, John Garry wrote: >> It has been shown that the cmpxchg() for finding space in the cmdq can >> be a bottleneck: >> - for more CPUs contending the cmdq, the cmpxchg() will fail more often >> - since the software-maintained cons pointer is updated on the same 64b >> memory region, the chance of cmpxchg() failure increases again >> >> The cmpxchg() is removed as part of 2 related changes: >> >> - Update prod and cmdq owner in a single atomic add operation. For this, we >> count the prod and owner in separate regions in prod memory. >> >> As with simple binary counting, once the prod+wrap fields overflow, they >> will zero. They should never overflow into "owner" region, and we zero >> the non-owner, prod region for each owner. This maintains the prod >> pointer. >> >> As for the "owner", we now count this value, instead of setting a flag. >> Similar to before, once the owner has finished gathering, it will clear >> a mask. As such, a CPU declares itself as the "owner" when it reads zero >> for this region. This zeroing will also clear possible overflow in >> wrap+prod region, above. >> >> The owner is now responsible for all cmdq locking to avoid possible >> deadlock. The owner will lock the cmdq for all non-owers it has gathered >> when they have space in the queue and have written their entries. >> >> - Check for space in the cmdq after the prod pointer has been assigned. >> >> We don't bother checking for space in the cmdq before assigning the prod >> pointer, as this would be racy. >> >> So since the prod pointer is updated unconditionally, it would be common >> for no space to be available in the cmdq when prod is assigned - that >> is, according the software-maintained prod and cons pointer. So now >> it must be ensured that the entries are not yet written and not until >> there is space. >> >> How the prod pointer is maintained also leads to a strange condition >> where the prod pointer can wrap past the cons pointer. We can detect this >> condition, and report no space here. However, a prod pointer progressed >> twice past the cons pointer cannot be detected. But it can be ensured that >> this that this scenario does not occur, as we limit the amount of >> commands any CPU can issue at any given time, such that we cannot >> progress prod pointer further. >> >> Signed-off-by: John Garry >> --- >> drivers/iommu/arm-smmu-v3.c | 101 ++++++++++++++++++++++-------------- >> 1 file changed, 61 insertions(+), 40 deletions(-) > > I must admit, you made me smile putting trivial@kernel.org on cc for this ;) > Yes, quite ironic :)