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=-5.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 26645C43331 for ; Sat, 28 Mar 2020 01:28:41 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id A77B52071B for ; Sat, 28 Mar 2020 01:28:40 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=nvidia.com header.i=@nvidia.com header.b="RgiI7AVX" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A77B52071B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=nvidia.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 13EE56B0010; Fri, 27 Mar 2020 21:28:40 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0EF666B0032; Fri, 27 Mar 2020 21:28:40 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 003BE6B0036; Fri, 27 Mar 2020 21:28:39 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0176.hostedemail.com [216.40.44.176]) by kanga.kvack.org (Postfix) with ESMTP id DD5806B0010 for ; Fri, 27 Mar 2020 21:28:39 -0400 (EDT) Received: from smtpin02.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id C4474180AD811 for ; Sat, 28 Mar 2020 01:28:39 +0000 (UTC) X-FDA: 76643036358.02.stage70_85c6b5c179322 X-HE-Tag: stage70_85c6b5c179322 X-Filterd-Recvd-Size: 6221 Received: from hqnvemgate26.nvidia.com (hqnvemgate26.nvidia.com [216.228.121.65]) by imf40.hostedemail.com (Postfix) with ESMTP for ; Sat, 28 Mar 2020 01:28:39 +0000 (UTC) Received: from hqpgpgate102.nvidia.com (Not Verified[216.228.121.13]) by hqnvemgate26.nvidia.com (using TLS: TLSv1.2, DES-CBC3-SHA) id ; Fri, 27 Mar 2020 18:28:24 -0700 Received: from hqmail.nvidia.com ([172.20.161.6]) by hqpgpgate102.nvidia.com (PGP Universal service); Fri, 27 Mar 2020 18:28:37 -0700 X-PGP-Universal: processed; by hqpgpgate102.nvidia.com on Fri, 27 Mar 2020 18:28:37 -0700 Received: from [10.2.58.50] (10.124.1.5) by HQMAIL107.nvidia.com (172.20.187.13) with Microsoft SMTP Server (TLS) id 15.0.1473.3; Sat, 28 Mar 2020 01:28:37 +0000 Subject: Re: [Patch v2 2/2] mm/page_alloc.c: define node_order with all zero To: Wei Yang CC: , , , , References: <20200327220121.27823-1-richard.weiyang@gmail.com> <20200327220121.27823-2-richard.weiyang@gmail.com> <4c9d8138-d379-810f-64e7-0d018ed019df@nvidia.com> <20200328002616.kjtf7dpkqbugyzi2@master> <97a6bf40-792b-6216-d35b-691027c85aad@nvidia.com> <20200328011031.olsaehwdev2gqdsn@master> From: John Hubbard X-Nvconfidentiality: public Message-ID: <40facd34-40b2-0925-90ca-a4c53fc520e8@nvidia.com> Date: Fri, 27 Mar 2020 18:28:36 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200328011031.olsaehwdev2gqdsn@master> X-Originating-IP: [10.124.1.5] X-ClientProxiedBy: HQMAIL111.nvidia.com (172.20.187.18) To HQMAIL107.nvidia.com (172.20.187.13) Content-Type: text/plain; charset="utf-8"; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nvidia.com; s=n1; t=1585358904; bh=4ZdoLhpeCl5eOQ49GFbds6K+x5pnaKcmo16+au32dnM=; h=X-PGP-Universal:Subject:To:CC:References:From:X-Nvconfidentiality: Message-ID:Date:User-Agent:MIME-Version:In-Reply-To: X-Originating-IP:X-ClientProxiedBy:Content-Type:Content-Language: Content-Transfer-Encoding; b=RgiI7AVXQXAkRF6pVEN0tDCSFfXnakhk6m/mU7qN1FgbjQSK40jywNeTFGzlwtkHg TrFoDlVMQZyLrYO1dZnp2ErM4roJuIW/2j/nqorFAb1WlG8wRpDmAIVuUKlFxWPHgM 8jmDG7iKqoJlHJvfGWhaLqdwUp1Nl2lz+bB5Y6ExaCWpYFDLZy0Di6P/CGsl3SYU1l 1q6VxwfQ3bC8I0070TXqmkdp5hBCXruocFzOwPmY3hEbwHd+K6fkNkvKi1GPrtamf4 X7uy9SVZ9UOePaPbkWHLOBRvSqKfVIIpwOnUmbFfEa+caMF7eouj6x4PjNgv/LC16W pioJgsOMjajrA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 3/27/20 6:10 PM, Wei Yang wrote: ... >> It's not just about preserving the value. Sometimes it's about stack space. >> Here's the trade-offs for static variables within a function: >> >> Advantages of static variables within a function (compared to non-static >> variables, also within a function): >> ----------------------------------- >> >> * Doesn't use any of the scarce kernel stack space >> * Preserves values (not always necessarily and advantage) >> >> Disadvantages: >> ----------------------------------- >> >> * Removes basic thread safety: multiple threads can no longer independently >> call the function without getting interaction, and generally that means >> data corruption. >> >> So here, I suspect that the original motivation was probably to conserve stack >> space, and the author likely observed that there was no concurrency to worry >> about: the function was only being called by one thread at a time. Given those >> constraints (which I haven't confirmed just yet, btw), a static function variable >> fits well. >> >>> >>> My suggestion is to remove the static and define it {0} instead of memset >>> every time. Is my understanding correct here? >> >> >> Not completely: >> >> a) First of all, "instead of memset every time" is a misconception, because >> there is still a memset happening every time with {0}. It's just that the >> compiler silently writes that code for you, and you don't see it on the >> screen. But it's still there. >> >> b) Switching away from a static to an on-stack variable requires that you first >> verify that stack space is not an issue. Or, if you determine that this >> function needs the per-thread isolation that a non-static variable provides, >> then you can switch to either an on-stack variable, or a *alloc() function. >> > > I think you get some point. While one more question about stack and static. If > one function is thread safe, which factor determines whether we choose on > stack value or static? Any reference size? It looks currently we don't have a > guide line for this. > There's not really any general guideline, but applying the points above (plus keeping in mind that kernel stack space is quite small) to each case, you'll come to a good answer. In this case, if we really are only ever calling this function in one thread at a time, then it's probably best to let the "conserve stack space" point win. Which leads to just leaving the code nearly as-is. The only thing left to do would be to (optionally, because this is an exceedingly minor point) delete the arguably misleading "= {0}" part. And as Jason points out, doing so also moves node_order into .bss : diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 4bd35eb83d34..cb4b07458249 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -5607,7 +5607,7 @@ static void build_thisnode_zonelists(pg_data_t *pgdat) static void build_zonelists(pg_data_t *pgdat) { - static int node_order[MAX_NUMNODES] = {0}; + static int node_order[MAX_NUMNODES]; int node, load, nr_nodes = 0; nodemask_t used_mask = NODE_MASK_NONE; int local_node, prev_node; Further note: On my current testing .config, I've got MAX_NUMNODES set to 64, which makes 256 bytes required for node_order array. 256 bytes on a 16KB stack is a little bit above my mental watermark for "that's too much in today's kernels". thanks, -- John Hubbard NVIDIA