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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 50EF5C43334 for ; Wed, 15 Jun 2022 12:42:43 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id C5E1C6B0071; Wed, 15 Jun 2022 08:42:42 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C0E016B0072; Wed, 15 Jun 2022 08:42:42 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id AD4E56B0073; Wed, 15 Jun 2022 08:42:42 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id A1FFE6B0071 for ; Wed, 15 Jun 2022 08:42:42 -0400 (EDT) Received: from smtpin29.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay13.hostedemail.com (Postfix) with ESMTP id 72C2B614BD for ; Wed, 15 Jun 2022 12:42:42 +0000 (UTC) X-FDA: 79580434164.29.EE51C76 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf01.hostedemail.com (Postfix) with ESMTP id 0F83D4009F for ; Wed, 15 Jun 2022 12:42:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=3exYkbkGVA6ONvFcqLC/UiBhYiMhudeXJrj19fyXfHU=; b=Rp+lH575wjGmNq83/mfaaH/jyj Qv0XxFkQB1d8mzui7iHsYYRAD1KhfvjfHt2+gjqMXmAyL+qVHrFvMKCZxF5uoYk/g/vG+PPgk/FET 2wmBLCY224PlgsrQAVzIoMic/rLKHys7LgzlAQa+uK+Tmb3b02J71QpePNMyFsalQ8faX7UaI6jJG SM1Xc+ogSUkG2Sz5mouAYWQsY4I+hSJHOByMwz2glWJoIpT+dqlQqVWFmOrMPMPySrpW0LE7Q5gqa +z+38S2rg+wKsi+s0d11+NDgRd35QrJMILGkCZAVP0qHmUDyLNk23UI7w46JrW73UAIetnik/SK5n LwXZn0KA==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1o1SLs-0013Qh-1s; Wed, 15 Jun 2022 12:42:28 +0000 Date: Wed, 15 Jun 2022 13:42:28 +0100 From: Matthew Wilcox To: Miaohe Lin Cc: Joao Martins , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Dan Williams Subject: Re: [PATCH] mm/page_alloc: make calling prep_compound_head more reliable Message-ID: References: <20220607144157.36411-1-linmiaohe@huawei.com> <20220607113257.84b1bdd993f19be26b8c4944@linux-foundation.org> <65e5da9c-32d1-17d7-d8c6-96cbfac23fec@oracle.com> <4a30f026-789a-9235-2fbd-f553e4d7b45d@huawei.com> <40a07ce5-414a-a3b8-53ee-6c348635f03a@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <40a07ce5-414a-a3b8-53ee-6c348635f03a@huawei.com> ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1655296962; a=rsa-sha256; cv=none; b=o8rhjitPFAHcyAgAaKTF6S2ANwGnFV76y8hlxKz5Oayy/cUbLvdPj4EhcVSoN/nmvdFQRq 5gSxnNXeoOkNcKYkCHlGd0ZeNfQUuerIZkbrEQt5lbilROC6tXND5RbM/kFe8GfB2lM4nv AosNxHBFfh1DcU76vfSbOMk75hA/DHM= ARC-Authentication-Results: i=1; imf01.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=Rp+lH575; dmarc=none; spf=none (imf01.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1655296962; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=3exYkbkGVA6ONvFcqLC/UiBhYiMhudeXJrj19fyXfHU=; b=uyEchVqj0h3VFq/UpCU56YNIeWtbSzlf8JApUnae7H41BDqfYv6XZD1txAYpRJ36iZznTy fSjucwwJFwD1eANgvlNRqJFGGpjC9lkNdH6GLWw5kY2/JxYRjnemPHQIhnHxgzpgVOAxnO qRteWK+S1NSQqs+SDTzvHM2Xeorv83I= X-Rspamd-Queue-Id: 0F83D4009F X-Rspam-User: Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=Rp+lH575; dmarc=none; spf=none (imf01.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org X-Rspamd-Server: rspam06 X-Stat-Signature: wiygsd71g49urnjmwyfi7rkq3c7xrw1r X-HE-Tag: 1655296961-821377 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 Wed, Jun 15, 2022 at 03:44:06PM +0800, Miaohe Lin wrote: > > We definitely don't need the unlikely here. > > Could you please give me a more detailed explanation? IIUC, the above if condition > will only meet at a probability of 1/512. So unlikely tells the compiler to do some > optimization around it. Or am I miss something? Only add unlikely() when the compiler can't figure out for itself that it's unlikely. You should also check the generated code and/or benchmark the results to be sure that it's actually an improvement. Using unlikely() needs to be backed up with more than just a feeling.