From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752315AbaAPJau (ORCPT ); Thu, 16 Jan 2014 04:30:50 -0500 Received: from mx0.aculab.com ([213.249.233.131]:48266 "HELO mx0.aculab.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1752246AbaAPJap (ORCPT ); Thu, 16 Jan 2014 04:30:45 -0500 From: David Laight To: "'Eric Dumazet'" , Daniel Borkmann CC: "davem@davemloft.net" , "netdev@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Jakub Zawadzki , Hannes Frederic Sowa Subject: RE: [PATCH net-next 1/2] random32: add prandom_u32_lt_N and convert "misuses" of reciprocal_divide Thread-Topic: [PATCH net-next 1/2] random32: add prandom_u32_lt_N and convert "misuses" of reciprocal_divide Thread-Index: AQHPEnTDLEe58ej+DkOnUyMNeJ8BfJqHE+Ew Date: Thu, 16 Jan 2014 09:28:33 +0000 Message-ID: <063D6719AE5E284EB5DD2968C1650D6D45D899@AcuExch.aculab.com> References: <1389828228-30312-1-git-send-email-dborkman@redhat.com> <1389828228-30312-2-git-send-email-dborkman@redhat.com> <1389842050.31367.396.camel@edumazet-glaptop2.roam.corp.google.com> In-Reply-To: <1389842050.31367.396.camel@edumazet-glaptop2.roam.corp.google.com> Accept-Language: en-GB, en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-originating-ip: [10.202.99.200] Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Transfer-Encoding: 8bit X-MIME-Autoconverted: from base64 to 8bit by mail.home.local id s0G9Ut33016548 From: Eric Dumazet > On Thu, 2014-01-16 at 00:23 +0100, Daniel Borkmann wrote: > > > @@ -1220,7 +1219,7 @@ static unsigned int fanout_demux_hash(struct packet_fanout *f, > > struct sk_buff *skb, > > unsigned int num) > > { > > - return reciprocal_divide(skb->rxhash, num); > > + return (u32)(((u64) skb->rxhash * num) >> 32); > > } > > > > This is unfortunate. > > (This reverts one of your patch : f55d112e529386 ) > > Please add a helper to explain what's going on here, and on many other > spots we do this computation (as in get_rps_cpu()). > Few people really understand this. > > Or keep reciprocal_divide() as is, and introduce a new set of functions > for people really wanting the precise divides. Maybe rename the current reciprocal_divide() as well? In the above code it isn't immediately obvious that the result of a division is as useful as that of a modulus! OTOH the high 32 bits of a 32x32 multiply are (slightly) more obviously in the required range. David {.n++%ݶw{.n+{G{ayʇڙ,jfhz_(階ݢj"mG?&~iOzv^m ?I