From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753338AbbJ2Mwb (ORCPT ); Thu, 29 Oct 2015 08:52:31 -0400 Received: from unicorn.mansr.com ([81.2.72.234]:59906 "EHLO unicorn.mansr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752670AbbJ2MwV convert rfc822-to-8bit (ORCPT ); Thu, 29 Oct 2015 08:52:21 -0400 From: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= To: Alexey Brodkin Cc: linux-kernel@vger.kernel.org, linux-snps-arc@lists.infradead.org, Vineet Gupta , Ingo Molnar , Stephen Hemminger , "David S. Miller" , Nicolas Pitre , Russell King Subject: Re: [PATCH] __div64_32: implement division by multiplication for 32-bit arches References: <1446072455-16074-1-git-send-email-abrodkin@synopsys.com> Date: Thu, 29 Oct 2015 12:52:17 +0000 In-Reply-To: <1446072455-16074-1-git-send-email-abrodkin@synopsys.com> (Alexey Brodkin's message of "Thu, 29 Oct 2015 01:47:35 +0300") Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Alexey Brodkin writes: > Existing default implementation of __div64_32() for 32-bit arches unfolds > into huge routine with tons of arithmetics like +, -, * and all of them > in loops. That leads to obvious performance degradation if do_div() is > frequently used. > > Good example is extensive TCP/IP traffic. > That's what I'm getting with perf out of iperf3: > -------------->8-------------- > 30.05% iperf3 [kernel.kallsyms] [k] copy_from_iter > 11.77% iperf3 [kernel.kallsyms] [k] __div64_32 > 5.44% iperf3 [kernel.kallsyms] [k] memset > 5.32% iperf3 [kernel.kallsyms] [k] stmmac_xmit > 2.70% iperf3 [kernel.kallsyms] [k] skb_segment > 2.56% iperf3 [kernel.kallsyms] [k] tcp_ack > -------------->8-------------- > > do_div() here is mostly used in skb_mstamp_get() to convert nanoseconds > received from local_clock() to microseconds used in timestamp. > BTW conversion itself is as simple as "/=1000". > > Fortunately we already have much better __div64_32() for 32-bit ARM. > There in case of division by constant preprocessor calculates so-called > "magic number" which is later used in multiplications instead of divisions. > It's really nice and very optimal but obviously works only for ARM > because ARM assembly is involved. > > Now why don't we extend the same approach to all other 32-bit arches > with multiplication part implemented in pure C. With good compiler > resulting assembly will be quite close to manually written assembly. > > And that change implements that. > > But there's at least 1 problem which I don't know how to solve. > Preprocessor magic only happens if __div64_32() is inlined (that's > obvious - preprocessor has to know if divider is constant or not). > > But __div64_32() is already marked as weak function (which in its turn > is required to allow some architectures to provide its own optimal > implementations). I.e. addition of "inline" for __div64_32() is not an > option. > > So I do want to hear opinions on how to proceed with that patch. > Indeed there's the simplest solution - use this implementation only in > my architecture of preference (read ARC) but IMHO this change may > benefit other architectures as well. I tried something similar for MIPS a while ago after noticing a similar perf report. Adapting Nico's ARM code gave some nice speedups, but only when I used MIPS assembly for the long multiplies. Apparently gcc is still too stupid to do the sane thing. -- Måns Rullgård mans@mansr.com