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=-15.4 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 autolearn=ham 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 564F5C4338F for ; Thu, 5 Aug 2021 11:24:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 238C861108 for ; Thu, 5 Aug 2021 11:24:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240947AbhHELYy (ORCPT ); Thu, 5 Aug 2021 07:24:54 -0400 Received: from foss.arm.com ([217.140.110.172]:43080 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240623AbhHELYy (ORCPT ); Thu, 5 Aug 2021 07:24:54 -0400 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 C67151FB; Thu, 5 Aug 2021 04:24:39 -0700 (PDT) Received: from [10.57.36.146] (unknown [10.57.36.146]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A99A63F719; Thu, 5 Aug 2021 04:24:38 -0700 (PDT) Subject: Re: [PATCH] iommu/arm-smmu-v3: Remove some unneeded init in arm_smmu_cmdq_issue_cmdlist() To: John Garry , will@kernel.org Cc: joro@8bytes.org, linux-arm-kernel@lists.infradead.org, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linuxarm@huawei.com References: <1624293394-202509-1-git-send-email-john.garry@huawei.com> From: Robin Murphy Message-ID: Date: Thu, 5 Aug 2021 12:24:33 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.12.0 MIME-Version: 1.0 In-Reply-To: <1624293394-202509-1-git-send-email-john.garry@huawei.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021-06-21 17:36, John Garry wrote: > Members of struct "llq" will be zero-inited, apart from member max_n_shift. > But we write llq.val straight after the init, so it was pointless to zero > init those other members. As such, separately init member max_n_shift > only. > > In addition, struct "head" is initialised to "llq" only so that member > max_n_shift is set. But that member is never referenced for "head", so > remove any init there. > > Removing these initializations is seen as a small performance optimisation, > as this code is (very) hot path. I looked at this and immediately thought "surely the compiler can see that all the prod/cons/val fields are written anyway and elide the initialisation?", so I dumped the before and after disassembly, and... oh. You should probably clarify that it's zero-initialising all the cacheline padding which is both pointless and painful. With that, Reviewed-by: Robin Murphy However, having looked this closely I'm now tangentially wondering why max_n_shift isn't inside the padded union? It's read at the same time as both prod and cons by queue_has_space(), and never updated, so there doesn't appear to be any benefit to it being in a separate cacheline all by itself, and llq is already twice as big as it needs to be. Sorting that would also be a good opportunity to store the value of interest in its appropriate form so we're not needlessly recalculating 1 << shift every flippin' time... Robin. > Signed-off-by: John Garry > > diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c > index 54b2f27b81d4..8a8ad49bb7fd 100644 > --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c > +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c > @@ -727,11 +727,11 @@ static int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu, > unsigned long flags; > bool owner; > struct arm_smmu_cmdq *cmdq = &smmu->cmdq; > - struct arm_smmu_ll_queue llq = { > - .max_n_shift = cmdq->q.llq.max_n_shift, > - }, head = llq; > + struct arm_smmu_ll_queue llq, head; > int ret = 0; > > + llq.max_n_shift = cmdq->q.llq.max_n_shift; > + > /* 1. Allocate some space in the queue */ > local_irq_save(flags); > llq.val = READ_ONCE(cmdq->q.llq.val); >