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 smtp3.osuosl.org (smtp3.osuosl.org [140.211.166.136]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 94557C433F5 for ; Tue, 31 May 2022 21:56:10 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by smtp3.osuosl.org (Postfix) with ESMTP id 4E26660B6C; Tue, 31 May 2022 21:56:10 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org Received: from smtp3.osuosl.org ([127.0.0.1]) by localhost (smtp3.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id qeBT0Wv9mh26; Tue, 31 May 2022 21:56:09 +0000 (UTC) Received: from lists.linuxfoundation.org (lf-lists.osuosl.org [140.211.9.56]) by smtp3.osuosl.org (Postfix) with ESMTPS id 3FD5C60B74; Tue, 31 May 2022 21:56:09 +0000 (UTC) Received: from lf-lists.osuosl.org (localhost [127.0.0.1]) by lists.linuxfoundation.org (Postfix) with ESMTP id 171CEC0032; Tue, 31 May 2022 21:56:09 +0000 (UTC) Received: from smtp4.osuosl.org (smtp4.osuosl.org [IPv6:2605:bc80:3010::137]) by lists.linuxfoundation.org (Postfix) with ESMTP id C32E1C002D for ; Tue, 31 May 2022 21:56:08 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by smtp4.osuosl.org (Postfix) with ESMTP id A2CAD4184A for ; Tue, 31 May 2022 21:56:08 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org Received: from smtp4.osuosl.org ([127.0.0.1]) by localhost (smtp4.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 52UUqMT2aBid for ; Tue, 31 May 2022 21:56:03 +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 smtp4.osuosl.org (Postfix) with ESMTP id CA7B441847 for ; Tue, 31 May 2022 21:56:03 +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 0032C23A; Tue, 31 May 2022 14:56:03 -0700 (PDT) Received: from [10.57.81.38] (unknown [10.57.81.38]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 8C7883F66F; Tue, 31 May 2022 14:56:01 -0700 (PDT) Message-ID: <417bd608-0eeb-b3a0-31e3-8e241ab75e59@arm.com> Date: Tue, 31 May 2022 22:55:57 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:91.0) Gecko/20100101 Thunderbird/91.9.1 Subject: Re: [PATCH 04/10] dmapool: improve accuracy of debug statistics Content-Language: en-GB To: Tony Battersby , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <9b08ab7c-b80b-527d-9adf-7716b0868fbc@cybernetics.com> From: Robin Murphy In-Reply-To: Cc: Tony Lindgren , iommu@lists.linux-foundation.org, Matthew Wilcox , Andy Shevchenko , Keith Busch , kernel-team@fb.com 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 2022-05-31 20:52, Tony Battersby wrote: > On 5/31/22 15:48, Robin Murphy wrote: >> On 2022-05-31 19:17, Tony Battersby wrote: >> >>> pool->name, blocks, >>> - (size_t) pages * >>> - (pool->allocation / pool->size), >>> + (size_t) pages * pool->blks_per_alloc, >>> pool->size, pages); >>> size -= temp; >>> next += temp; >>> @@ -168,6 +168,9 @@ struct dma_pool *dma_pool_create(const char *name, struct device *dev, >>> retval->size = size; >>> retval->boundary = boundary; >>> retval->allocation = allocation; >>> + retval->blks_per_alloc = >>> + (allocation / boundary) * (boundary / size) + >>> + (allocation % boundary) / size; >> Do we really need to store this? Sure, 4 divisions (which could possibly >> be fewer given the constraints on boundary) isn't the absolute cheapest >> calculation, but I still can't imagine anyone would be polling sysfs >> stats hard enough to even notice. >> > The stored value is also used in patch #5, in more performance-critical > code, although only when debug is enabled. Ah, fair enough. On second look I think 64-bit systems could effectively store this for free anyway, if patch #2 moved "size" down past "dev" in struct dma_pool, such that blks_per_alloc then ends up padding out the hole again. FWIW the mathematician in me also now can't help seeing the algebraic reduction to at least "(allocation + (allocation % boundary)) / size", but is now too tired to reason about the power-of-two constraints and whether the intermediate integer truncations matter... Cheers, Robin. _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu