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=-5.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 83FE3C433E3 for ; Thu, 16 Jul 2020 11:32:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 58B732074B for ; Thu, 16 Jul 2020 11:32:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728402AbgGPLcV (ORCPT ); Thu, 16 Jul 2020 07:32:21 -0400 Received: from lhrrgout.huawei.com ([185.176.76.210]:2491 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728032AbgGPLcU (ORCPT ); Thu, 16 Jul 2020 07:32:20 -0400 Received: from lhreml724-chm.china.huawei.com (unknown [172.18.7.106]) by Forcepoint Email with ESMTP id 2518E8DCE03CF4265438; Thu, 16 Jul 2020 12:32:19 +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 12:32:18 +0100 Subject: Re: [PATCH 0/4] iommu/arm-smmu-v3: Improve cmdq lock efficiency To: Robin Murphy , Will Deacon CC: "joro@8bytes.org" , "trivial@kernel.org" , "linux-arm-kernel@lists.infradead.org" , "iommu@lists.linux-foundation.org" , "linux-kernel@vger.kernel.org" , Linuxarm , "maz@kernel.org" References: <1592846920-45338-1-git-send-email-john.garry@huawei.com> <20200716101940.GA7036@willie-the-truck> <20200716102233.GC7036@willie-the-truck> <20200716102814.GD7036@willie-the-truck> From: John Garry Message-ID: Date: Thu, 16 Jul 2020 12:30:30 +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: 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 12:22, Robin Murphy wrote: > On 2020-07-16 11:56, John Garry wrote: >> On 16/07/2020 11:28, Will Deacon wrote: >>> On Thu, Jul 16, 2020 at 11:22:33AM +0100, Will Deacon wrote: >>>> On Thu, Jul 16, 2020 at 11:19:41AM +0100, Will Deacon wrote: >>>>> On Tue, Jun 23, 2020 at 01:28:36AM +0800, John Garry wrote: >>>>>> As mentioned in [0], the CPU may consume many cycles processing >>>>>> arm_smmu_cmdq_issue_cmdlist(). One issue we find is the cmpxchg() >>>>>> loop to >>>>>> get space on the queue takes approx 25% of the cycles for this >>>>>> function. >>>>>> >>>>>> This series removes that cmpxchg(). >>>>> >>>>> How about something much simpler like the diff below? >> >>>> Ah, scratch that, I don't drop the lock if we fail the cas with it held. >>>> Let me hack it some more (I have no hardware so I can only build-test >>>> this). >>> >>> Right, second attempt... >> >> I can try it, but if performance if not as good, then please check mine >> further (patch 4/4 specifically) - performance is really good, IMHO. > > Perhaps a silly question (I'm too engrossed in PMU world ATM to get > properly back up to speed on this), but couldn't this be done without > cmpxchg anyway? Instinctively it feels like instead of maintaining a > literal software copy of the prod value, we could resolve the "claim my > slot in the queue" part with atomic_fetch_add on a free-running 32-bit > "pseudo-prod" index, then whoever updates the hardware deals with the > truncation and wrap bit to convert it to an actual register value. > That's what mine does. But I also need to take care of cmdq locking and how we unconditionally provide space. Cheers, John