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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,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 98DC8C2D0CE for ; Tue, 21 Jan 2020 11:11:26 +0000 (UTC) Received: from hemlock.osuosl.org (smtp2.osuosl.org [140.211.166.133]) (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 5C1D32253D for ; Tue, 21 Jan 2020 11:11:26 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 5C1D32253D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=iommu-bounces@lists.linux-foundation.org Received: from localhost (localhost [127.0.0.1]) by hemlock.osuosl.org (Postfix) with ESMTP id 2B34086370; Tue, 21 Jan 2020 11:11:26 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org Received: from hemlock.osuosl.org ([127.0.0.1]) by localhost (.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 1oQuOtLWq7gE; Tue, 21 Jan 2020 11:11:21 +0000 (UTC) Received: from lists.linuxfoundation.org (lf-lists.osuosl.org [140.211.9.56]) by hemlock.osuosl.org (Postfix) with ESMTP id 2276887860; Tue, 21 Jan 2020 11:11:21 +0000 (UTC) Received: from lf-lists.osuosl.org (localhost [127.0.0.1]) by lists.linuxfoundation.org (Postfix) with ESMTP id 0621FC1D80; Tue, 21 Jan 2020 11:11:21 +0000 (UTC) Received: from whitealder.osuosl.org (smtp1.osuosl.org [140.211.166.138]) by lists.linuxfoundation.org (Postfix) with ESMTP id 6EE39C0174 for ; Tue, 21 Jan 2020 11:11:19 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by whitealder.osuosl.org (Postfix) with ESMTP id 660B086378 for ; Tue, 21 Jan 2020 11:11:19 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org Received: from whitealder.osuosl.org ([127.0.0.1]) by localhost (.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id uNciGoQoSSn1 for ; Tue, 21 Jan 2020 11:11:14 +0000 (UTC) X-Greylist: domain auto-whitelisted by SQLgrey-1.7.6 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by whitealder.osuosl.org (Postfix) with ESMTP id DB61F84540 for ; Tue, 21 Jan 2020 11:11:13 +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 33E6A1FB; Tue, 21 Jan 2020 03:11:13 -0800 (PST) Received: from [10.1.196.37] (e121345-lin.cambridge.arm.com [10.1.196.37]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 808883F6C4; Tue, 21 Jan 2020 03:11:12 -0800 (PST) Subject: Re: [Patch v3 1/3] iommu: avoid unnecessary magazine allocations To: Cong Wang , iommu@lists.linux-foundation.org References: <20191218043951.10534-1-xiyou.wangcong@gmail.com> <20191218043951.10534-2-xiyou.wangcong@gmail.com> From: Robin Murphy Message-ID: Date: Tue, 21 Jan 2020 11:11:08 +0000 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20191218043951.10534-2-xiyou.wangcong@gmail.com> Content-Language: en-GB Cc: 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 18/12/2019 4:39 am, Cong Wang wrote: > The IOVA cache algorithm implemented in IOMMU code does not > exactly match the original algorithm described in the paper > "Magazines and Vmem: Extending the Slab Allocator to Many > CPUs and Arbitrary Resources". > > Particularly, it doesn't need to free the loaded empty magazine > when trying to put it back to global depot. To make it work, we > have to pre-allocate magazines in the depot and only recycle them > when all of them are full. > > Before this patch, rcache->depot[] contains either full or > freed entries, after this patch, it contains either full or > empty (but allocated) entries. How much additional memory overhead does this impose (particularly on systems that may have many domains mostly used for large, long-term mappings)? I'm wary that trying to micro-optimise for the "churn network packets as fast as possible" case may penalise every other case, potentially quite badly. Lower-end embedded systems are using IOMMUs in front of their GPUs, video codecs, etc. precisely because they *don't* have much memory to spare (and thus need to scrape together large buffers out of whatever pages they can find). But on the other hand, if we were to go down this route, then why is there any dynamic allocation/freeing left at all? Once both the depot and the rcaches are preallocated, then AFAICS it would make more sense to rework the overflow case in __iova_rcache_insert() to just free the IOVAs and swap the empty mag around rather than destroying and recreating it entirely. Perhaps there's a reasonable compromise wherein we don't preallocate, but still 'free' empty magazines back to the depot, such that busy domains will quickly reach a steady-state. In fact, having now dug up the paper at this point of writing this reply, that appears to be what fig. 3.1b describes anyway - I don't see any mention of preallocating the depot. Robin. > > Together with a few other changes to make it exactly match > the pseudo code in the paper. > > Cc: Joerg Roedel > Cc: John Garry > Signed-off-by: Cong Wang > --- > drivers/iommu/iova.c | 45 +++++++++++++++++++++++++++----------------- > 1 file changed, 28 insertions(+), 17 deletions(-) > > diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c > index 41c605b0058f..cb473ddce4cf 100644 > --- a/drivers/iommu/iova.c > +++ b/drivers/iommu/iova.c > @@ -862,12 +862,16 @@ static void init_iova_rcaches(struct iova_domain *iovad) > struct iova_cpu_rcache *cpu_rcache; > struct iova_rcache *rcache; > unsigned int cpu; > - int i; > + int i, j; > > for (i = 0; i < IOVA_RANGE_CACHE_MAX_SIZE; ++i) { > rcache = &iovad->rcaches[i]; > spin_lock_init(&rcache->lock); > rcache->depot_size = 0; > + for (j = 0; j < MAX_GLOBAL_MAGS; ++j) { > + rcache->depot[j] = iova_magazine_alloc(GFP_KERNEL); > + WARN_ON(!rcache->depot[j]); > + } > rcache->cpu_rcaches = __alloc_percpu(sizeof(*cpu_rcache), cache_line_size()); > if (WARN_ON(!rcache->cpu_rcaches)) > continue; > @@ -900,24 +904,30 @@ static bool __iova_rcache_insert(struct iova_domain *iovad, > > if (!iova_magazine_full(cpu_rcache->loaded)) { > can_insert = true; > - } else if (!iova_magazine_full(cpu_rcache->prev)) { > + } else if (iova_magazine_empty(cpu_rcache->prev)) { > swap(cpu_rcache->prev, cpu_rcache->loaded); > can_insert = true; > } else { > - struct iova_magazine *new_mag = iova_magazine_alloc(GFP_ATOMIC); > + spin_lock(&rcache->lock); > + if (rcache->depot_size < MAX_GLOBAL_MAGS) { > + swap(rcache->depot[rcache->depot_size], cpu_rcache->prev); > + swap(cpu_rcache->prev, cpu_rcache->loaded); > + rcache->depot_size++; > + can_insert = true; > + } else { > + mag_to_free = cpu_rcache->loaded; > + } > + spin_unlock(&rcache->lock); > + > + if (mag_to_free) { > + struct iova_magazine *new_mag = iova_magazine_alloc(GFP_ATOMIC); > > - if (new_mag) { > - spin_lock(&rcache->lock); > - if (rcache->depot_size < MAX_GLOBAL_MAGS) { > - rcache->depot[rcache->depot_size++] = > - cpu_rcache->loaded; > + if (new_mag) { > + cpu_rcache->loaded = new_mag; > + can_insert = true; > } else { > - mag_to_free = cpu_rcache->loaded; > + mag_to_free = NULL; > } > - spin_unlock(&rcache->lock); > - > - cpu_rcache->loaded = new_mag; > - can_insert = true; > } > } > > @@ -963,14 +973,15 @@ static unsigned long __iova_rcache_get(struct iova_rcache *rcache, > > if (!iova_magazine_empty(cpu_rcache->loaded)) { > has_pfn = true; > - } else if (!iova_magazine_empty(cpu_rcache->prev)) { > + } else if (iova_magazine_full(cpu_rcache->prev)) { > swap(cpu_rcache->prev, cpu_rcache->loaded); > has_pfn = true; > } else { > spin_lock(&rcache->lock); > if (rcache->depot_size > 0) { > - iova_magazine_free(cpu_rcache->loaded); > - cpu_rcache->loaded = rcache->depot[--rcache->depot_size]; > + swap(rcache->depot[rcache->depot_size - 1], cpu_rcache->prev); > + swap(cpu_rcache->prev, cpu_rcache->loaded); > + rcache->depot_size--; > has_pfn = true; > } > spin_unlock(&rcache->lock); > @@ -1019,7 +1030,7 @@ static void free_iova_rcaches(struct iova_domain *iovad) > iova_magazine_free(cpu_rcache->prev); > } > free_percpu(rcache->cpu_rcaches); > - for (j = 0; j < rcache->depot_size; ++j) > + for (j = 0; j < MAX_GLOBAL_MAGS; ++j) > iova_magazine_free(rcache->depot[j]); > } > } > _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu