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=-17.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 9288FC433ED for ; Wed, 7 Apr 2021 09:06:53 +0000 (UTC) Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) (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 2C4E061262 for ; Wed, 7 Apr 2021 09:06:53 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 2C4E061262 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=xen.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Received: from list by lists.xenproject.org with outflank-mailman.106529.203716 (Exim 4.92) (envelope-from ) id 1lU492-0002Ew-1D; Wed, 07 Apr 2021 09:06:40 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 106529.203716; Wed, 07 Apr 2021 09:06:39 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lU491-0002Ep-U0; Wed, 07 Apr 2021 09:06:39 +0000 Received: by outflank-mailman (input) for mailman id 106529; Wed, 07 Apr 2021 09:06:39 +0000 Received: from mail.xenproject.org ([104.130.215.37]) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lU491-0002Ek-8G for xen-devel@lists.xenproject.org; Wed, 07 Apr 2021 09:06:39 +0000 Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1lU491-0001aF-1K; Wed, 07 Apr 2021 09:06:39 +0000 Received: from 54-240-197-225.amazon.com ([54.240.197.225] helo=a483e7b01a66.ant.amazon.com) by xenbits.xenproject.org with esmtpsa (TLS1.3:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1lU490-0000Tu-Mb; Wed, 07 Apr 2021 09:06:38 +0000 X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=xen.org; s=20200302mail; h=Content-Transfer-Encoding:Content-Type:In-Reply-To: MIME-Version:Date:Message-ID:From:References:Cc:To:Subject; bh=S54PR/9ZKM3q1z6Eb9HVnUnTexMcE2bH2i2s7F0TC00=; b=PrUyFe5pf4DzhlGWpsnBIAtTQl 2HIqR4kbHdLGkHnZPFYomxn8Tawh+gdU/UUxdwaR7ZjtO4Fngho5aQqOKY96S32oE5yt5lMdRoyXe NMrUDu4Agt6qiwhfnj4wRrJ7NG83zyMJpqRnjH9MSZ9IZiQglWyCS/CtMZWgv75jI70I=; Subject: Re: [PATCH 02/14] xen/sched: Constify name and opt_name in struct scheduler To: Jan Beulich Cc: Julien Grall , George Dunlap , Dario Faggioli , xen-devel@lists.xenproject.org References: <20210405155713.29754-1-julien@xen.org> <20210405155713.29754-3-julien@xen.org> <5618bed6-a213-d2c9-4cbf-d95cb4dc02b4@suse.com> <00a75c04-fad1-7061-ac39-a811f442db05@xen.org> From: Julien Grall Message-ID: <4088e12a-f641-278d-dac7-d25f52e0dfe2@xen.org> Date: Wed, 7 Apr 2021 10:06:37 +0100 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.9.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 8bit Hi Jan, On 07/04/2021 09:22, Jan Beulich wrote: > On 06.04.2021 20:24, Julien Grall wrote: >> Hi Jan, >> >> On 06/04/2021 09:07, Jan Beulich wrote: >>> On 05.04.2021 17:57, Julien Grall wrote: >>>> From: Julien Grall >>>> >>>> Both name and opt_name are pointing to literal string. So mark both of >>>> the fields as const. >>>> >>>> Signed-off-by: Julien Grall >>> >>> Reviewed-by: Jan Beulich >>> albeit ... >>> >>>> --- a/xen/common/sched/private.h >>>> +++ b/xen/common/sched/private.h >>>> @@ -272,8 +272,8 @@ static inline spinlock_t *pcpu_schedule_trylock(unsigned int cpu) >>>> } >>>> >>>> struct scheduler { >>>> - char *name; /* full name for this scheduler */ >>>> - char *opt_name; /* option name for this scheduler */ >>>> + const char *name; /* full name for this scheduler */ >>>> + const char *opt_name; /* option name for this scheduler */ >>> >>> ... I'd like to suggest considering at least the latter to become >>> an array instead of a pointer - there's little point wasting 8 >>> bytes of storage for the pointer when the strings pointed to are >>> all at most 9 bytes long (right now; I don't expect much longer >>> ones to appear). >> >> I have tried this simple/dumb change on top of my patch: >> >> diff --git a/xen/common/sched/private.h b/xen/common/sched/private.h >> index a870320146ef..ab2236874217 100644 >> --- a/xen/common/sched/private.h >> +++ b/xen/common/sched/private.h >> @@ -273,7 +273,7 @@ static inline spinlock_t >> *pcpu_schedule_trylock(unsigned int cpu) >> >> struct scheduler { >> const char *name; /* full name for this scheduler */ >> - const char *opt_name; /* option name for this scheduler */ >> + const char opt_name[9]; /* option name for this scheduler */ >> unsigned int sched_id; /* ID for this scheduler */ >> void *sched_data; /* global data pointer */ >> struct cpupool *cpupool;/* points to this scheduler's pool */ >> >> GCC will throw an error: >> >> core.c: In function ‘scheduler_init’: >> core.c:2987:17: error: assignment of read-only variable ‘ops’ >> ops = *schedulers[i]; >> ^ >> core.c:2997:21: error: assignment of read-only variable ‘ops’ >> ops = *schedulers[i]; >> ^ >> >> I don't particularly want to drop the const. So the code would probably >> need some rework. > > What's wrong with not having the const when the field is an array? > The more that all original (build-time, i.e. contain in the binary) > instances of the struct are already const as a whole? The scheduler will do a shallow copy of the structure that will be non-const. So opt_name can be modified afterwards (one can argue this is unlikely). If I have to chose between saving overall 20 bytes in the binary and const. I would chose the latter. Cheers, -- Julien Grall