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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id CE5A5C433EF for ; Wed, 8 Dec 2021 18:40:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S239156AbhLHSoJ (ORCPT ); Wed, 8 Dec 2021 13:44:09 -0500 Received: from foss.arm.com ([217.140.110.172]:38846 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230040AbhLHSoI (ORCPT ); Wed, 8 Dec 2021 13:44:08 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 86E1A11FB; Wed, 8 Dec 2021 10:40:36 -0800 (PST) Received: from [10.57.34.58] (unknown [10.57.34.58]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6A5483F73D; Wed, 8 Dec 2021 10:40:35 -0800 (PST) Message-ID: Date: Wed, 8 Dec 2021 18:40:31 +0000 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:91.0) Gecko/20100101 Thunderbird/91.3.2 Subject: Re: [PATCH v3 1/1] iommu/arm-smmu-v3: Simplify useless instructions in arm_smmu_cmdq_build_cmd() Content-Language: en-GB To: John Garry , "Leizhen (ThunderTown)" , Will Deacon , Joerg Roedel , linux-arm-kernel@lists.infradead.org, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org References: <20211207094109.1962-1-thunder.leizhen@huawei.com> <20211207094109.1962-2-thunder.leizhen@huawei.com> <9da73d96-c61d-4ed2-607d-4861e6bfd696@huawei.com> <58e8e7c6-a47b-7816-ac74-aefa5b55dd17@huawei.com> <4f296b55-a59a-49b4-ad4d-902bb83a203c@huawei.com> From: Robin Murphy In-Reply-To: <4f296b55-a59a-49b4-ad4d-902bb83a203c@huawei.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021-12-08 18:17, John Garry wrote: >>> Did you notice any performance change with this change? >> >> Hi John: >>    Thanks for the tip. I wrote a test case today, and I found that the >> performance did not go up but down. > > I very quickly tested on a DMA mapping benchmark very similar to the > kernel DMA benchmark module - I got mixed results. For fewer CPUs (<8), > a small improvement, like 0.7%. For more CPUs, a dis-improvement - > that's surprising, I did expect just no change as any improvement would > get dwarfed from the slower unmap rates for more CPUs. I can check this > more tomorrow. > >> It's so weird. So I decided not to >> change it, because it's also poorly readable. So I plan to make only >> the following modifications: >> @@ -237,7 +237,7 @@ static int queue_remove_raw(struct arm_smmu_queue >> *q, u64 *ent) >>   static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct >> arm_smmu_cmdq_ent *ent) >>   { >>          memset(cmd, 0, 1 << CMDQ_ENT_SZ_SHIFT); >> -       cmd[0] |= FIELD_PREP(CMDQ_0_OP, ent->opcode); >> +       cmd[0] = FIELD_PREP(CMDQ_0_OP, ent->opcode); >> >>          switch (ent->opcode) { >>          case CMDQ_OP_TLBI_EL2_ALL: >> >> This prevents the compiler from generating the following two inefficient >> instructions: >>       394:       f9400002        ldr     x2, [x0]    //x2 = cmd[0] >>       398:       aa020062        orr     x2, x3, x2    //x3 = >> FIELD_PREP(CMDQ_0_OP, ent->opcode) >> >> Maybe it's not worth changing because I've only seen a 0.x nanosecond >> reduction >> in performance. But one thing is, it only comes with benefits, no side >> effects. >> > > I just think that with the original code that cmd[] is on the stack and > cached, so if we have write-back attribute (which I think we do) then > there would not necessarily a write to external memory per write to cmd[]. > > So, apart from this approach, I think that if we can just reduce the > instructions through other efficiencies in the function then that would > be good. Not sure if it's still true, but FWIW last time the best result actually came from doing the ridiculously counter-intuitive: https://lore.kernel.org/linux-iommu/141de3c3278e280712d16d9ac9ab305c3b80a810.1534344167.git.robin.murphy@arm.com/ Robin.