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=-5.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,HK_RANDOM_FROM,MAILING_LIST_MULTI,NICE_REPLY_A, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no 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 A1C95C4363D for ; Fri, 25 Sep 2020 12:29:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5B64321D7A for ; Fri, 25 Sep 2020 12:29:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727248AbgIYM36 (ORCPT ); Fri, 25 Sep 2020 08:29:58 -0400 Received: from mga12.intel.com ([192.55.52.136]:29934 "EHLO mga12.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727044AbgIYM36 (ORCPT ); Fri, 25 Sep 2020 08:29:58 -0400 IronPort-SDR: S13Rs3MywnsojtvA43xI5ULhDA5vmklcFPsojsBXEEbTvBvUnTI+WJQqhLohqSqbaozjCQefrl Lq1qWdRzDQyw== X-IronPort-AV: E=McAfee;i="6000,8403,9754"; a="140926049" X-IronPort-AV: E=Sophos;i="5.77,302,1596524400"; d="scan'208";a="140926049" X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga004.jf.intel.com ([10.7.209.38]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 25 Sep 2020 05:29:58 -0700 IronPort-SDR: OatOg+mI2cktpFK85pdpyPjv0iQQ+N/4tqSMOhX/do3enQSWt2VmuP8C841vmZkqHsUfbL0sie MthwJq7tkFmA== X-IronPort-AV: E=Sophos;i="5.77,302,1596524400"; d="scan'208";a="455808533" Received: from mlevy2-mobl.ger.corp.intel.com (HELO [10.251.176.131]) ([10.251.176.131]) by orsmga004-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 25 Sep 2020 05:29:53 -0700 Subject: Re: [Intel-gfx] [PATCH rdma-next v3 1/2] lib/scatterlist: Add support in dynamic allocation of SG table from pages To: Jason Gunthorpe Cc: Leon Romanovsky , Christoph Hellwig , Doug Ledford , linux-rdma@vger.kernel.org, intel-gfx@lists.freedesktop.org, Roland Scheidegger , dri-devel@lists.freedesktop.org, David Airlie , VMware Graphics , Maor Gottlieb , Maor Gottlieb References: <20200922083958.2150803-1-leon@kernel.org> <20200922083958.2150803-2-leon@kernel.org> <118a03ef-d160-e202-81cc-16c9c39359fc@linux.intel.com> <20200925071330.GA2280698@unreal> <20200925115833.GZ9475@nvidia.com> From: Tvrtko Ursulin Organization: Intel Corporation UK Plc Message-ID: Date: Fri, 25 Sep 2020 13:29:49 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20200925115833.GZ9475@nvidia.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-rdma@vger.kernel.org On 25/09/2020 12:58, Jason Gunthorpe wrote: > On Fri, Sep 25, 2020 at 12:41:29PM +0100, Tvrtko Ursulin wrote: >> >> On 25/09/2020 08:13, Leon Romanovsky wrote: >>> On Thu, Sep 24, 2020 at 09:21:20AM +0100, Tvrtko Ursulin wrote: >>>> >>>> On 22/09/2020 09:39, Leon Romanovsky wrote: >>>>> From: Maor Gottlieb >>>>> >>>>> Extend __sg_alloc_table_from_pages to support dynamic allocation of >>>>> SG table from pages. It should be used by drivers that can't supply >>>>> all the pages at one time. >>>>> >>>>> This function returns the last populated SGE in the table. Users should >>>>> pass it as an argument to the function from the second call and forward. >>>>> As before, nents will be equal to the number of populated SGEs (chunks). >>>> >>>> So it's appending and growing the "list", did I get that right? Sounds handy >>>> indeed. Some comments/questions below. >>> >>> Yes, we (RDMA) use this function to chain contiguous pages. >> >> I will eveluate if i915 could start using it. We have some loops which build >> page by page and coalesce. > > Christoph H doesn't like it, but if there are enough cases we should > really have a pin_user_pages_to_sg() rather than open code this all > over the place. > > With THP the chance of getting a coalescing SG is much higher, and > everything is more efficient with larger SGEs. Right, I was actually referring to i915 sites where we build sg tables out of shmem and plain kernel pages. In those areas we have some open coded coalescing loops (see for instance our shmem_get_pages). Plus a local "trim" to discard the unused entries, since we allocate pessimistically not knowing how coalescing will pan out. This kind of core function which appends pages could replace some of that. Maybe it would be slightly less efficient but I will pencil in to at least evaluate it. Otherwise I do agree that coalescing is a win and in the past I have measured savings in a few MiB range just for struct scatterlist storage. Regards, Tvrtko