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); > 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, URIBL_RED,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 A5547C4338F for ; Thu, 5 Aug 2021 11:24:46 +0000 (UTC) Received: from smtp1.osuosl.org (smtp1.osuosl.org [140.211.166.138]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 4A1D76112D for ; Thu, 5 Aug 2021 11:24:46 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 4A1D76112D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=lists.linux-foundation.org Received: from localhost (localhost [127.0.0.1]) by smtp1.osuosl.org (Postfix) with ESMTP id 18D048350A; Thu, 5 Aug 2021 11:24:46 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org Received: from smtp1.osuosl.org ([127.0.0.1]) by localhost (smtp1.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id Iycn3iGZ3vrb; Thu, 5 Aug 2021 11:24:44 +0000 (UTC) Received: from lists.linuxfoundation.org (lf-lists.osuosl.org [140.211.9.56]) by smtp1.osuosl.org (Postfix) with ESMTPS id 2CE4382B81; Thu, 5 Aug 2021 11:24:44 +0000 (UTC) Received: from lf-lists.osuosl.org (localhost [127.0.0.1]) by lists.linuxfoundation.org (Postfix) with ESMTP id 0EC32C001A; Thu, 5 Aug 2021 11:24:44 +0000 (UTC) Received: from smtp2.osuosl.org (smtp2.osuosl.org [140.211.166.133]) by lists.linuxfoundation.org (Postfix) with ESMTP id 3E076C000E for ; Thu, 5 Aug 2021 11:24:42 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by smtp2.osuosl.org (Postfix) with ESMTP id 267A34028F for ; Thu, 5 Aug 2021 11:24:42 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org Received: from smtp2.osuosl.org ([127.0.0.1]) by localhost (smtp2.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id NVKxArySKa-X for ; Thu, 5 Aug 2021 11:24:40 +0000 (UTC) X-Greylist: domain auto-whitelisted by SQLgrey-1.8.0 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp2.osuosl.org (Postfix) with ESMTP id D160040138 for ; Thu, 5 Aug 2021 11:24:40 +0000 (UTC) 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 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-Language: en-GB Cc: linuxarm@huawei.com, iommu@lists.linux-foundation.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org X-BeenThere: iommu@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Development issues for Linux IOMMU support List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Errors-To: iommu-bounces@lists.linux-foundation.org Sender: "iommu" 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); > _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu 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=-16.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,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=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 8CFD3C4338F for ; Thu, 5 Aug 2021 11:26:17 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 561FC60243 for ; Thu, 5 Aug 2021 11:26:17 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 561FC60243 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Content-Type: Content-Transfer-Encoding:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date:Message-ID:From: References:Cc:To:Subject:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=XtSaBZxS4DtXjvZ1ITzIkyAgA65RZSMYz8EWnwBwb78=; b=rrVDKOXGCyvVPEoaS9DS4Ubxdd nVbmcD1E3EpoJAiO32Lv+snB/hzdsuWIpWj0PofK+NqQVmNT3OqsXbNw880j04mGdX++EPrDfMkRq MBYi8FghnTRExMc89WDlGtLcf9P4Y0535IvA9lpA0bTK3EXjCjhrxchZfYZChHjJr92e3zOVVZSjA WVX7cCOZDb9ie9WqjObgSiXDb4JYxZ9DwZFasAFQaax/guoPYaCcI2gtIvFozB0xBHC/6rCIlBfA2 d1WpQp5bG4f20Swsocvrlb1cUfWSl9E4AwqJdKVAK2H3sO7igEFDCGbm4CEgqf86uTA2wfJWJUBc0 mUQAwiJQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1mBbUT-009GHC-Gf; Thu, 05 Aug 2021 11:24:45 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1mBbUP-009GGZ-H5 for linux-arm-kernel@lists.infradead.org; Thu, 05 Aug 2021 11:24:43 +0000 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-Language: en-GB X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210805_042441_671202_C3BAB04A X-CRM114-Status: GOOD ( 20.59 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.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); > _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel