From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S932951AbcLIFWJ (ORCPT <rfc822;w@1wt.eu>);
        Fri, 9 Dec 2016 00:22:09 -0500
Received: from mail-wj0-f196.google.com ([209.85.210.196]:33233 "EHLO
        mail-wj0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1753368AbcLIFWH (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 9 Dec 2016 00:22:07 -0500
Date: Fri, 9 Dec 2016 06:22:03 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        John Stultz <john.stultz@linaro.org>,
        David Gibson <david@gibson.dropbear.id.au>,
        Liav Rehana <liavr@mellanox.com>,
        Chris Metcalf <cmetcalf@mellanox.com>,
        Richard Cochran <richardcochran@gmail.com>,
        Parit Bhargava <prarit@redhat.com>,
        Laurent Vivier <lvivier@redhat.com>,
        "Christopher S. Hall" <christopher.s.hall@intel.com>
Subject: Re: [patch 5/6] [RFD] timekeeping: Provide optional 128bit math
Message-ID: <20161209052203.GA32246@gmail.com>
References: <20161208202623.883855034@linutronix.de>
 <20161208204229.005418487@linutronix.de>
 <20161209040826.GA2595@gmail.com>
 <20161209044849.GY3045@worktop.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161209044849.GY3045@worktop.programming.kicks-ass.net>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


* Peter Zijlstra <peterz@infradead.org> wrote:

> On Fri, Dec 09, 2016 at 05:08:26AM +0100, Ingo Molnar wrote:
> > > +#if defined(CONFIG_ARCH_SUPPORTS_INT128) && defined(__SIZEOF_INT128__)
> > > +static inline u64 timekeeping_delta_to_ns(struct tk_read_base *tkr, u64 delta)
> > > +{
> > > +	unsigned __int128 nsec;
> > > +
> > > +	nsec = ((unsigned __int128)delta * tkr->mult) + tkr->xtime_nsec;
> > > +	return (u64) (nsec >> tkr->shift);
> > > +}
> > 
> > Actually, 128-bit multiplication shouldn't be too horrible - at least on 64-bit 
> > architectures. (128-bit division is another matter, but there's no division here.)
> 
> IIRC there are 64bit architectures that do not have a 64x64->128 mult,
> only a 64x64->64 mult instruction. Its not immediately apparent using
> __int128 will generate optimal code for those, nor is it a given GCC
> will not require libgcc functions for those.

Well, if the overflow case is rare (which it is in this case) then it should still 
be relatively straightforward, something like:

        X and Y are 64-bit:

	X = Xh*2^32 + Xl
	Y = Yh*2^32 + Yl

	X*Y = (Xh*2^32 + Xl)*(Yh*2^32 + Yl)

	    =   Xh*2^32*(Yh*2^32 + Yl)
	      +      Xl*(Yh*2^32 + Yl)

	    =   Xh*Yh*2^64
	      + Xh*Yl*2^32
	      + Xl*Yh*2^32
	      + XL*Yl

Which is four 32x32->64 multiplications in the worst case.

Where a valid overflow threshold is relatively easy to determine in a hot path 
compatible fashion:

	if (Xh != 0 || Yh != 0)
		slow_path();

And this simple and fast overflow check should still cover the overwhelming 
majority of 'sane' systems. (A more involved 'could it overflow' check of counting 
the high bits with 8 bit granularity by looking at the high bytes not at the words 
could be done in the slow path - to still avoid the 4 multiplications in most 
cases.)

Am I missing something?

Thanks,

	Ingo