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=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 BC0E5C28CF6 for ; Fri, 27 Jul 2018 02:09:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 7038C20857 for ; Fri, 27 Jul 2018 02:09:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7038C20857 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732079AbeG0D3A (ORCPT ); Thu, 26 Jul 2018 23:29:00 -0400 Received: from mga02.intel.com ([134.134.136.20]:56556 "EHLO mga02.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1731974AbeG0D3A (ORCPT ); Thu, 26 Jul 2018 23:29:00 -0400 X-Amp-Result: SKIPPED(no attachment in message) X-Amp-File-Uploaded: False Received: from orsmga003.jf.intel.com ([10.7.209.27]) by orsmga101.jf.intel.com with ESMTP/TLS/DHE-RSA-AES256-GCM-SHA384; 26 Jul 2018 19:09:28 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.51,407,1526367600"; d="scan'208";a="70311575" Received: from unknown (HELO [10.239.13.97]) ([10.239.13.97]) by orsmga003.jf.intel.com with ESMTP; 26 Jul 2018 19:09:26 -0700 Message-ID: <5B5A7FD9.2010105@intel.com> Date: Fri, 27 Jul 2018 10:13:45 +0800 From: Wei Wang User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Yury Norov CC: linux-kernel@vger.kernel.org, akpm@linux-foundation.org, corbet@lwn.net, linux@rasmusvillemoes.dk, dgilbert@redhat.com, Andy Shevchenko Subject: Re: [PATCH] linux/bitmap.h: fix BITMAP_LAST_WORD_MASK References: <1532592471-21177-1-git-send-email-wei.w.wang@intel.com> <20180726093728.GA9069@yury-thinkpad> <5B599F5F.2070705@intel.com> <20180726121042.GA11481@yury-thinkpad> In-Reply-To: <20180726121042.GA11481@yury-thinkpad> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/26/2018 08:10 PM, Yury Norov wrote: > On Thu, Jul 26, 2018 at 06:15:59PM +0800, Wei Wang wrote: >> External Email >> >> On 07/26/2018 05:37 PM, Yury Norov wrote: >>> On Thu, Jul 26, 2018 at 04:07:51PM +0800, Wei Wang wrote: >>>> The existing BITMAP_LAST_WORD_MASK macro returns 0xffffffff if nbits is >>>> 0. This patch changes the macro to return 0 when there is no bit needs to >>>> be masked. >>> I think this is intentional behavour. Previous version did return ~0UL >>> explicitly in this case. See patch 89c1e79eb3023 (linux/bitmap.h: improve >>> BITMAP_{LAST,FIRST}_WORD_MASK) from Rasmus. >> Yes, I saw that. But it seems confusing for the corner case that nbits=0 >> (no bits to mask), the macro returns with all the bits set. >> >> >>> Introducing conditional branch would affect performance. All existing >>> code checks nbits for 0 before handling last word where needed >>> explicitly. So I think we'd better change nothing here. >> I think that didn't save the conditional branch essentially, because >> it's just moved from inside this macro to the caller as you mentioned. >> If callers missed the check for some reason and passed 0 to the macro, >> they will get something unexpected. >> >> Current callers like __bitmap_weight, __bitmap_equal, and others, they have >> >> if (bits % BITS_PER_LONG) >> w += hweight_long(bitmap[k] & BITMAP_LAST_WORD_MASK(bits)); >> >> we could remove the "if" check by "w += hweight_long(bitmap[k] & >> BITMAP_LAST_WORD_MASK(bits % BITS_PER_LONG));" the branch is the same. > But your patch doesn't remove external conditional, and it fact > introduces overhead, right? Also, in some cases it's not so trivial to > remove it. Consider __bitmap_intersects() for example. > > Anyway, this patch changes the very basic API. In that case you should > check every user of the macro to be safe against the change, including > possible performance downsides. > > If you find this corner case behavior of macro confusing, I think that > the better option would be introducing detailed comment to the > BITMAP_LAST_WORD_MASK(), or writing wrapper around it that handles > nbits == 0 as you expect. > OK. Thanks Yury and Andy for the discussion. It seems the more preferred way is just to add comments as a note. Agree with that. Best, Wei