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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id E3B3DEE2084 for ; Fri, 6 Feb 2026 10:24:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Content-Type: Content-Transfer-Encoding:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:From:References:Cc:To:Subject: MIME-Version:Date:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=/vaLMLX8xTo8NwibDz3MsXSv8tgAdewHV5Ngzn+uBo4=; b=mpbhflxlVzAUI6 PjTjBfQm37flrbrsPd6T0vdE0Mp3tjtmtJrK4RyoEVc+ARN0Ei/5hhVDdToKtUzUJQm4RR5NdwHdu leEqsf8uJJMiBk7fSZdVRIMrMKRNOkE4pVm08GgdfMZNv+i28USotf8RFNWMnaVxWNWZMGtjv74KA 3/3UnXqB27i31DdnW/h7eo+x7e8XmBp7yWEOjpzqIINn0c1IkuP7KmwAE0TxZV+4bNF3CvbhyR89G 9HEOqHI1cBAQ8sylLazop5+XHSVHqTSr1edMJpW0FCXkbu4exjgdjEj+E0WK287r16DT6nQPhANOn 1/4eVLqO9tZ3G65Vl91Q==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1voJ0K-0000000BAup-0mpt; Fri, 06 Feb 2026 10:24:02 +0000 Received: from sea.source.kernel.org ([172.234.252.31]) by bombadil.infradead.org with esmtps (Exim 4.98.2 #2 (Red Hat Linux)) id 1voJ0G-0000000BAuQ-2v6P for linux-riscv@lists.infradead.org; Fri, 06 Feb 2026 10:23:59 +0000 Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 01C7444214; Fri, 6 Feb 2026 10:23:56 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id EA86DC19422; Fri, 6 Feb 2026 10:23:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770373435; bh=CVRfjenhPNZ53C5ptXLpHu6G+l5qThn6T1w+S3R43J4=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=lkgsOVjM8345QQjYJiRfQfJ0oSMWqIL/dF3b44KB/OYvtFa5wtksT4epKW7dc7zQK n13iIxkGzKpBzMaIzGDTNhk7k+VRqM3ICVJzDUOySg0Dee4z13wjFn7Cl2EN1ekcLW XWt4/jPFLIpPhf3JY1BeyzKTbyedA1v9JDaYVUAjI9tYMJtEuJsfrVQR+lDly5qhC1 zCwWghcKG4Qbwjg/A1hG6Xsrw28MryrTIl0yqvYqtw7HzK9njc9BhbXFrF7jt0cFdj aloqve3suzVNAbni2tS3Lc0IjWEudBVqDr37kT5BZXF3tUmnGa4ntBhXN5Von7/OEX EKyVfiMLATw8A== Message-ID: Date: Fri, 6 Feb 2026 11:23:48 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCHv6 15/17] mm: Remove the branch from compound_head() To: Kiryl Shutsemau , Andrew Morton , Muchun Song , Matthew Wilcox , Usama Arif , Frank van der Linden Cc: Oscar Salvador , Mike Rapoport , Vlastimil Babka , Lorenzo Stoakes , Zi Yan , Baoquan He , Michal Hocko , Johannes Weiner , Jonathan Corbet , Huacai Chen , WANG Xuerui , Palmer Dabbelt , Paul Walmsley , Albert Ou , Alexandre Ghiti , kernel-team@meta.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, loongarch@lists.linux.dev, linux-riscv@lists.infradead.org References: <20260202155634.650837-1-kas@kernel.org> <20260202155634.650837-16-kas@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset="us-ascii"; Format="flowed" Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On 2/2/26 16:56, Kiryl Shutsemau wrote: > The compound_head() function is a hot path. For example, the zap path > calls it for every leaf page table entry. > > Rewrite the helper function in a branchless manner to eliminate the risk > of CPU branch misprediction. > > Signed-off-by: Kiryl Shutsemau > Reviewed-by: Muchun Song > Reviewed-by: Zi Yan > --- > include/linux/page-flags.h | 27 +++++++++++++++++---------- > 1 file changed, 17 insertions(+), 10 deletions(-) > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h > index 1aaa604f4b9b..16384cb6f962 100644 > --- a/include/linux/page-flags.h > +++ b/include/linux/page-flags.h > @@ -224,25 +224,32 @@ static __always_inline bool compound_info_has_mask(void) > static __always_inline unsigned long _compound_head(const struct page *page) > { > unsigned long info = READ_ONCE(page->compound_info); > + unsigned long mask; > + > + if (!compound_info_has_mask()) { > + /* Bit 0 encodes PageTail() */ > + if (info & 1) > + return info - 1; > > - /* Bit 0 encodes PageTail() */ > - if (!(info & 1)) > return (unsigned long)page; > - > - /* > - * If compound_info_has_mask() is false, the rest of compound_info is > - * the pointer to the head page. > - */ > - if (!compound_info_has_mask()) > - return info - 1; > + } > > /* > * If compoun_info_has_mask() is true the rest of the info encodes > * the mask that converts the address of the tail page to the head page. > * > * No need to clear bit 0 in the mask as 'page' always has it clear. > + * > + * Let's do it in a branchless manner. > */ > - return (unsigned long)page & info; > + > + /* Non-tail: -1UL, Tail: 0 */ > + mask = (info & 1) - 1; > + > + /* Non-tail: -1UL, Tail: info */ > + mask |= info; > + > + return (unsigned long)page & mask; > } > > #define compound_head(page) ((typeof(page))_compound_head(page)) Nice! Acked-by: David Hildenbrand (Arm) -- Cheers, David _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv