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=-5.3 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 B854CC43466 for ; Mon, 21 Sep 2020 14:01:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8BCDE20756 for ; Mon, 21 Sep 2020 14:01:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727423AbgIUOBE (ORCPT ); Mon, 21 Sep 2020 10:01:04 -0400 Received: from lhrrgout.huawei.com ([185.176.76.210]:2906 "EHLO huawei.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726688AbgIUOBC (ORCPT ); Mon, 21 Sep 2020 10:01:02 -0400 Received: from lhreml724-chm.china.huawei.com (unknown [172.18.7.106]) by Forcepoint Email with ESMTP id C85C03125A0D1D39BC16; Mon, 21 Sep 2020 15:01:00 +0100 (IST) Received: from [127.0.0.1] (10.210.166.25) by lhreml724-chm.china.huawei.com (10.201.108.75) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256) id 15.1.1913.5; Mon, 21 Sep 2020 15:01:00 +0100 Subject: Re: [PATCH v2 0/2] iommu/arm-smmu-v3: Improve cmdq lock efficiency To: Will Deacon CC: , , , , , , References: <1598018062-175608-1-git-send-email-john.garry@huawei.com> <20200921134324.GK2139@willie-the-truck> From: John Garry Message-ID: Date: Mon, 21 Sep 2020 14:58:10 +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: <20200921134324.GK2139@willie-the-truck> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.210.166.25] X-ClientProxiedBy: lhreml701-chm.china.huawei.com (10.201.108.50) To lhreml724-chm.china.huawei.com (10.201.108.75) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 21/09/2020 14:43, Will Deacon wrote: > On Fri, Aug 21, 2020 at 09:54:20PM +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 a lot of time once we start getting many >> CPUs contending - from experiment, for 64 CPUs contending the cmdq, >> success rate is ~ 1 in 12, which is poor, but not totally awful. >> >> This series removes that cmpxchg() and replaces with an atomic_add, >> same as how the actual cmdq deals with maintaining the prod pointer. > > I'm still not a fan of this. :( > Could you try to adapt the hacks I sent before, > please? I know they weren't quite right (I have no hardware to test on), but > the basic idea is to fall back to a spinlock if the cmpxchg() fails. The > queueing in the spinlock implementation should avoid the contention. OK, so if you're asking me to try this again, then I can do that, and see what it gives us. John