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 C8DE2C43331 for ; Sat, 28 Mar 2020 11:25:31 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 62F16206E6 for ; Sat, 28 Mar 2020 11:25:31 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="cUr2PJjp" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 62F16206E6 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id BDA8F6B0010; Sat, 28 Mar 2020 07:25:30 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id B8BC26B0032; Sat, 28 Mar 2020 07:25:30 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id AA22A6B0036; Sat, 28 Mar 2020 07:25:30 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0201.hostedemail.com [216.40.44.201]) by kanga.kvack.org (Postfix) with ESMTP id 9608E6B0010 for ; Sat, 28 Mar 2020 07:25:30 -0400 (EDT) Received: from smtpin19.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay01.hostedemail.com (Postfix) with ESMTP id 14772180AD80F for ; Sat, 28 Mar 2020 11:25:30 +0000 (UTC) X-FDA: 76644540420.19.rest86_6a30559da6d37 X-HE-Tag: rest86_6a30559da6d37 X-Filterd-Recvd-Size: 6352 Received: from us-smtp-delivery-74.mimecast.com (us-smtp-delivery-74.mimecast.com [63.128.21.74]) by imf18.hostedemail.com (Postfix) with ESMTP for ; Sat, 28 Mar 2020 11:25:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1585394729; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=MkhqG+9pJjWDz5YSa/ZfjozmXA6/H2qbLbttt1sv2Dw=; b=cUr2PJjpwc5rlN9rtn2oGPB4HIzmCUu1bNyX9KHcSbowFpkdUyYgrSL4Xo6x2Q0G+/Z+0Y Bh98V5GqVqmQnQF1SQQtxjVVTEf2uUiINFLkNbnDM5DUgqBpTQ+LGWkYlLZtRVK3+gcoz2 SkaUock4e9ullio2xJPXJ+qTSgGKXwI= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-56-T21r-Y38N2GSk0dDrCOBYg-1; Sat, 28 Mar 2020 07:25:25 -0400 X-MC-Unique: T21r-Y38N2GSk0dDrCOBYg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 9A4B2100726A; Sat, 28 Mar 2020 11:25:23 +0000 (UTC) Received: from localhost (ovpn-12-98.pek2.redhat.com [10.72.12.98]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1F36519756; Sat, 28 Mar 2020 11:25:19 +0000 (UTC) Date: Sat, 28 Mar 2020 19:25:17 +0800 From: Baoquan He To: John Hubbard Cc: Wei Yang , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, jgg@ziepe.ca, david@redhat.com Subject: Re: [Patch v2 2/2] mm/page_alloc.c: define node_order with all zero Message-ID: <20200328112517.GR3039@MiWiFi-R3L-srv> 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> <40facd34-40b2-0925-90ca-a4c53fc520e8@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <40facd34-40b2-0925-90ca-a4c53fc520e8@nvidia.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 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 03/27/20 at 06:28pm, John Hubbard wrote: > 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". Oh, so Michal was deliberate to do so. I have CONFIG_NODES_SHIFT as 10 in my laptop config. That truly will cost much kernel stack. Thanks for telling this.