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=-5.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no 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 BEB42C433E0 for ; Tue, 11 Aug 2020 23:33:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 89041206DA for ; Tue, 11 Aug 2020 23:33:00 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="OqZRqgq6" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726420AbgHKXc7 (ORCPT ); Tue, 11 Aug 2020 19:32:59 -0400 Received: from us-smtp-2.mimecast.com ([205.139.110.61]:32437 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726143AbgHKXc6 (ORCPT ); Tue, 11 Aug 2020 19:32:58 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1597188777; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=X0PZbt2gu9cb6IQ7UVghICiVpHs7kwKAG/AqXZropSQ=; b=OqZRqgq68WJnltknCL/pWWYZ2qoCqSqhbS4VLn4QissQg5r8rQYYKrsuqWWVf0L5rh2x/N awnmEKB7PiPmTduJf9UYuSKTsifZcbiwFSRerihQ0FdS1lrTVFXnX/yFjxLAiPPLW2yGTb t160HrC7qw0V/44cfoMYzYzMKgNtfuA= 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-550-T2xA4c9cMBuy9e3qmW0d2w-1; Tue, 11 Aug 2020 19:32:55 -0400 X-MC-Unique: T2xA4c9cMBuy9e3qmW0d2w-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 E8E691902EA1; Tue, 11 Aug 2020 23:32:53 +0000 (UTC) Received: from llong.remote.csb (ovpn-118-35.rdu2.redhat.com [10.10.118.35]) by smtp.corp.redhat.com (Postfix) with ESMTP id 26FE81001B07; Tue, 11 Aug 2020 23:32:53 +0000 (UTC) Subject: Re: [PATCH v2] sched, mm: Optimize current_gfp_context() To: Andrew Morton Cc: Peter Zijlstra , Ingo Molnar , Mathieu Desnoyers , Michel Lespinasse , linux-kernel@vger.kernel.org References: <20200618212936.9776-1-longman@redhat.com> <20200811152921.1d43e34ad7aff928c6c1267f@linux-foundation.org> From: Waiman Long Organization: Red Hat Message-ID: <3b353d6a-8430-b847-6ddd-21189c87ef6b@redhat.com> Date: Tue, 11 Aug 2020 19:32:52 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200811152921.1d43e34ad7aff928c6c1267f@linux-foundation.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 8/11/20 6:29 PM, Andrew Morton wrote: > On Thu, 18 Jun 2020 17:29:36 -0400 Waiman Long wrote: > >> The current_gfp_context() converts a number of PF_MEMALLOC_* per-process >> flags into the corresponding GFP_* flags for memory allocation. In >> that function, current->flags is accessed 3 times. That may lead to >> duplicated access of the same memory location. >> >> This is not usually a problem with minimal debug config options on as the >> compiler can optimize away the duplicated memory accesses. With most >> of the debug config options on, however, that may not be the case. >> For example, the x86-64 object size of the __need_fs_reclaim() in a >> debug kernel that calls current_gfp_context() was 309 bytes. With this >> patch applied, the object size is reduced to 202 bytes. This is a saving >> of 107 bytes and will probably be slightly faster too. >> >> ... >> >> --- a/include/linux/sched/mm.h >> +++ b/include/linux/sched/mm.h >> @@ -181,18 +181,20 @@ static inline bool in_vfork(struct task_struct *tsk) >> */ >> static inline gfp_t current_gfp_context(gfp_t flags) >> { >> - if (unlikely(current->flags & >> + unsigned int pflags = READ_ONCE(current->flags); > Why use READ_ONCE() here? It was a change suggested by PeterZ. He said without the READ_ONCE(), the compiler may still choose to access current->flags multiple times instead of just reading it once. I think he is right and so I made the change. Cheers, Longman