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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1C7D8C433EF for ; Thu, 20 Jan 2022 09:46:02 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1359643AbiATJp7 (ORCPT ); Thu, 20 Jan 2022 04:45:59 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:30257 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1359635AbiATJp6 (ORCPT ); Thu, 20 Jan 2022 04:45:58 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1642671957; 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=AnozGUjZxfRbxL/uXbhZzAjPZpfXbAj0xuaYn46Ym1Q=; b=NJjkkARJZezA/0RNYzYtAyUaELpC13zuVbpPYd71OXyj70UzLoMWexlc36tM7JQoslamGY AHYCJ65dGQihq2v+EWjLcM3uqwIoCBcdVlmfJDbqECw1fgs/57OTgiXgigB0/5C3H8cyuq AunP6l6IPuhHjPSRScLhZ+h1MXbcyZI= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-556-Q1acZg4LMjWZJjJMpLE5jw-1; Thu, 20 Jan 2022 04:45:52 -0500 X-MC-Unique: Q1acZg4LMjWZJjJMpLE5jw-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7497D108087A; Thu, 20 Jan 2022 09:45:49 +0000 (UTC) Received: from localhost (ovpn-13-177.pek2.redhat.com [10.72.13.177]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0F2C510013C1; Thu, 20 Jan 2022 09:45:42 +0000 (UTC) Date: Thu, 20 Jan 2022 17:45:39 +0800 From: Baoquan He To: Jisheng Zhang Cc: Alexandre Ghiti , Russell King , Catalin Marinas , Will Deacon , Paul Walmsley , Palmer Dabbelt , Albert Ou , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , x86@kernel.org, hpa@zytor.com, Eric Biederman , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org, kexec@lists.infradead.org, Alexandre ghiti Subject: Re: [PATCH v2 0/5] kexec: use IS_ENABLED(CONFIG_KEXEC_CORE) instead of #ifdef Message-ID: <20220120094539.GC18398@MiWiFi-R3L-srv> References: <20211206160514.2000-1-jszhang@kernel.org> <20220116133847.GE2388@MiWiFi-R3L-srv> <20220119080859.GB4977@MiWiFi-R3L-srv> <20220119093322.GC4977@MiWiFi-R3L-srv> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/19/22 at 07:44pm, Jisheng Zhang wrote: > On Wed, Jan 19, 2022 at 05:33:22PM +0800, Baoquan He wrote: > > On 01/19/22 at 09:52am, Alexandre Ghiti wrote: > > > Hi Baoquan, > > > > > > On Wed, Jan 19, 2022 at 9:11 AM Baoquan He wrote: > > > > > > > > On 01/18/22 at 10:13pm, Jisheng Zhang wrote: > > > > > On Sun, Jan 16, 2022 at 09:38:47PM +0800, Baoquan He wrote: > > > > > > Hi Jisheng, > > > > > > > > > > Hi Baoquan, > > > > > > > > > > > > > > > > > On 12/07/21 at 12:05am, Jisheng Zhang wrote: > > > > > > > Replace the conditional compilation using "#ifdef CONFIG_KEXEC_CORE" > > > > > > > by a check for "IS_ENABLED(CONFIG_KEXEC_CORE)", to simplify the code > > > > > > > and increase compile coverage. > > > > > > > > > > > > I go through this patchset, You mention the benefits it brings are > > > > > > 1) simplity the code; > > > > > > 2) increase compile coverage; > > > > > > > > > > > > For benefit 1), it mainly removes the dummy function in x86, arm and > > > > > > arm64, right? > > > > > > > > > > Another benefit: remove those #ifdef #else #endif usage. Recently, I > > > > > fixed a bug due to lots of "#ifdefs": > > > > > http://lists.infradead.org/pipermail/linux-riscv/2021-December/010607.html > > > > > > > > Glad to know the fix. While, sometime the ifdeffery is necessary. I am > > > > sorry about the one in riscv and you have fixed, it's truly a bug . But, > > > > the increasing compile coverage at below you tried to make, it may cause > > > > issue. Please see below my comment. > > > > > > > > > > > > > > > > > > > > > For benefit 2), increasing compile coverage, could you tell more how it > > > > > > achieves and why it matters? What if people disables CONFIG_KEXEC_CORE in > > > > > > purpose? Please forgive my poor compiling knowledge. > > > > > > > > > > Just my humble opinion, let's compare the code:: > > > > > > > > > > #ifdef CONFIG_KEXEC_CORE > > > > > > > > > > code block A; > > > > > > > > > > #endif > > > > > > > > > > If KEXEC_CORE is disabled, code block A won't be compiled at all, the > > > > > preprocessor will remove code block A; > > > > > > > > > > If we convert the code to: > > > > > > > > > > if (IS_ENABLED(CONFIG_KEXEC_CORE)) { > > > > > code block A; > > > > > } > > > > > > > > > > Even if KEXEC_CORE is disabled, code block A is still compiled. > > > > > > > > This is what I am worried about. Before, if CONFIG_KEXEC_CORE is > > > > unset, those relevant codes are not compiled in. I can't see what > > > > benefit is brought in if compiled in the unneeded code block. Do I miss > > > > anything? > > > > > > > > > > This is explained in Documentation/process/coding-style.rst "21) > > > Conditional Compilation". > > > > Thanks for the pointer, Alex. > > > > I read that part, while my confusion isn't gone. With the current code, > > CONFIG_KEXEC_CORE is set, > > - reserve_crashkernel_low() and reserve_crashkernel() compiled in. > > Although the code block will be compiled, but the code block will be > optimized out. > > > CONFIG_KEXEC_CORE is unset, > > - reserve_crashkernel_low() and reserve_crashkernel() compiled out. > > > > After this patch applied, does it have the same effect as the old code? > > I compared the .o, and can confirm they acchieve the same effect. Checked the .o, it's truly as you said. I didn't know this before, thank you and Alex, learned this now. Seems only static function has this effect. I tested your x86 patch, those two functions are all optimized out. If I remove the static, the entire reserve_crashkernel_low() exists, while reserve_crashkernel() will be optimized as a empty function.