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=-8.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,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 A6E36C433E2 for ; Thu, 16 Jul 2020 10:19:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 80EF420853 for ; Thu, 16 Jul 2020 10:19:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594894788; bh=3Kfj2U0gvDfDloWASjQO57LR6xuFE6ghFAVQobdH6ks=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=EcIyWjL5NsjC76zbWlBUY/crjb7jYPgyDmhbslh4nks8QOb+G1JBoIggQVaoqwnqC KEgJjy1AY8ufC0yr1Cdbbbr/0upa9HfJlX9IsQayKuhAw5QR4/WWho4JOm2AgPuZVZ NnVsY77TPNHg4yrU+kVu1+7KVFotltDEectTZ6kI= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727828AbgGPKTr (ORCPT ); Thu, 16 Jul 2020 06:19:47 -0400 Received: from mail.kernel.org ([198.145.29.99]:39526 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726027AbgGPKTq (ORCPT ); Thu, 16 Jul 2020 06:19:46 -0400 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 791782074B; Thu, 16 Jul 2020 10:19:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1594894786; bh=3Kfj2U0gvDfDloWASjQO57LR6xuFE6ghFAVQobdH6ks=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=B4iWlTi9n5/jNVzGiiyq/aD+XME5+rlz4gpt0p4upnXV/Im/YL7X+2qNX8fIrM5P0 djtcRTQhlMZz05N6beqSbpgNlaFKKjDiqdb98e9mWioHJg2tt7hK8KPKx6t0kdlVSL ll7K9O4OpFj4wprqEokl/98nt+0WnI17uXg0TZHA= Date: Thu, 16 Jul 2020 11:19:41 +0100 From: Will Deacon To: John Garry Cc: robin.murphy@arm.com, joro@8bytes.org, trivial@kernel.org, linux-arm-kernel@lists.infradead.org, iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, linuxarm@huawei.com, maz@kernel.org Subject: Re: [PATCH 0/4] iommu/arm-smmu-v3: Improve cmdq lock efficiency Message-ID: <20200716101940.GA7036@willie-the-truck> References: <1592846920-45338-1-git-send-email-john.garry@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1592846920-45338-1-git-send-email-john.garry@huawei.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? Will --->8 diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c index f578677a5c41..705d99ec8219 100644 --- a/drivers/iommu/arm-smmu-v3.c +++ b/drivers/iommu/arm-smmu-v3.c @@ -560,6 +560,7 @@ struct arm_smmu_cmdq { atomic_long_t *valid_map; atomic_t owner_prod; atomic_t lock; + spinlock_t slock; }; struct arm_smmu_cmdq_batch { @@ -1378,7 +1379,7 @@ static int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu, u64 cmd_sync[CMDQ_ENT_DWORDS]; u32 prod; unsigned long flags; - bool owner; + bool owner, cas_failed = false; struct arm_smmu_cmdq *cmdq = &smmu->cmdq; struct arm_smmu_ll_queue llq = { .max_n_shift = cmdq->q.llq.max_n_shift, @@ -1387,27 +1388,35 @@ static int arm_smmu_cmdq_issue_cmdlist(struct arm_smmu_device *smmu, /* 1. Allocate some space in the queue */ local_irq_save(flags); - llq.val = READ_ONCE(cmdq->q.llq.val); do { u64 old; + llq.val = READ_ONCE(cmdq->q.llq.val); - while (!queue_has_space(&llq, n + sync)) { + if (queue_has_space(&llq, n + sync)) + goto try_cas; + + if (cas_failed) + spin_unlock(&cmdq->slock); + + do { local_irq_restore(flags); if (arm_smmu_cmdq_poll_until_not_full(smmu, &llq)) dev_err_ratelimited(smmu->dev, "CMDQ timeout\n"); local_irq_save(flags); - } + } while (!queue_has_space(&llq, n + sync)); +try_cas: head.cons = llq.cons; head.prod = queue_inc_prod_n(&llq, n + sync) | CMDQ_PROD_OWNED_FLAG; old = cmpxchg_relaxed(&cmdq->q.llq.val, llq.val, head.val); - if (old == llq.val) - break; + cas_failed = old != llq.val; + + if (cas_failed) + spin_lock(&cmdq->slock); + } while (cas_failed); - llq.val = old; - } while (1); owner = !(llq.prod & CMDQ_PROD_OWNED_FLAG); head.prod &= ~CMDQ_PROD_OWNED_FLAG; llq.prod &= ~CMDQ_PROD_OWNED_FLAG; @@ -3192,6 +3201,7 @@ static int arm_smmu_cmdq_init(struct arm_smmu_device *smmu) atomic_set(&cmdq->owner_prod, 0); atomic_set(&cmdq->lock, 0); + spin_lock_init(&cmdq->slock); bitmap = (atomic_long_t *)bitmap_zalloc(nents, GFP_KERNEL); if (!bitmap) {