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=-8.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,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 5570EC433DF for ; Thu, 18 Jun 2020 20:59:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 33E9520890 for ; Thu, 18 Jun 2020 20:59:28 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="ijjGELKA" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728291AbgFRU70 (ORCPT ); Thu, 18 Jun 2020 16:59:26 -0400 Received: from us-smtp-2.mimecast.com ([207.211.31.81]:27003 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726478AbgFRU7Z (ORCPT ); Thu, 18 Jun 2020 16:59:25 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1592513964; 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=2iDTwr+/iWvsdg/YE4W4q4L8qhRtiVj32UpGEhBg6Zs=; b=ijjGELKAgzCuAxQiEFaeMtxy0HDr59QFyDgsOphcvYUVjO5Ftkecj1gPJ8vNE20vFFZ77H vnY2On8n/JVR/W7TVguU8Vi0jGt6Q/C/V6QaOP7ecD2fMAHsYyF2tsCsYVaa23yUw3z/9f K3C3RXF13C2eI1gMcojfLi2FDMXt1T8= 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-375-CkW3-5CMNT632YrXDGbmzw-1; Thu, 18 Jun 2020 16:59:20 -0400 X-MC-Unique: CkW3-5CMNT632YrXDGbmzw-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 442C4107ACF4; Thu, 18 Jun 2020 20:59:19 +0000 (UTC) Received: from llong.remote.csb (ovpn-118-66.rdu2.redhat.com [10.10.118.66]) by smtp.corp.redhat.com (Postfix) with ESMTP id 70B4A5C1D0; Thu, 18 Jun 2020 20:59:18 +0000 (UTC) Subject: Re: [PATCH] sched, mm: Optimize current_gfp_context() To: Peter Zijlstra Cc: Ingo Molnar , Andrew Morton , Mathieu Desnoyers , Michel Lespinasse , linux-kernel@vger.kernel.org References: <20200618155847.30975-1-longman@redhat.com> <20200618160726.GL576905@hirez.programming.kicks-ass.net> From: Waiman Long Organization: Red Hat Message-ID: <6333186c-224a-fb84-7304-ca7c6c903c13@redhat.com> Date: Thu, 18 Jun 2020 16:59:18 -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: <20200618160726.GL576905@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 6/18/20 12:07 PM, Peter Zijlstra wrote: > On Thu, Jun 18, 2020 at 11:58:47AM -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. >> >> Signed-off-by: Waiman Long >> --- >> include/linux/sched/mm.h | 10 ++++++---- >> 1 file changed, 6 insertions(+), 4 deletions(-) >> >> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h >> index 480a4d1b7dd8..09e01909d192 100644 >> --- 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 = current->flags; > Per the above, I would've expected a READ_ONCE() there, otherwise the > compiler can still re-load. > Right. Will do that. Thanks, Longman