linux-kernel - Re: [PATCH next 0/3] lib: Implement mul_u64_u64_div_u64

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Message-ID: <20250518144315.2c2b4cb8@pumpkin>
Date: Sun, 18 May 2025 14:43:15 +0100
From: David Laight <david.laight.linux@...il.com>
To: Nicolas Pitre <npitre@...libre.com>
Cc: Uwe Kleine-König <u.kleine-koenig@...libre.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org,
 Oleg Nesterov <oleg@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Biju Das <biju.das.jz@...renesas.com>
Subject: Re: [PATCH next 0/3] lib: Implement mul_u64_u64_div_u64_roundup()

On Fri, 16 May 2025 11:49:44 -0400 (EDT)
Nicolas Pitre <npitre@...libre.com> wrote:

> On Fri, 16 May 2025, David Laight wrote:
> 
> > On Fri, 16 May 2025 11:47:58 +0200
> > Uwe Kleine-König <u.kleine-koenig@...libre.com> wrote:
> >   
> > > Hello David,
> > > 
> > > On Sat, Apr 05, 2025 at 09:45:27PM +0100, David Laight wrote:  
> > > > The pwm-stm32.c code wants a 'rounding up' version of mul_u64_u64_div_u64().
> > > > This can be done simply by adding 'divisor - 1' to the 128bit product.
> > > > Implement mul_u64_add_u64_div_u64(a, b, c, d) = (a * b + c)/d based on the
> > > > existing code.
> > > > Define mul_u64_u64_div_u64(a, b, d) as mul_u64_add_u64_div_u64(a, b, 0, d) and
> > > > mul_u64_u64_div_u64_roundup(a, b, d) as mul_u64_add_u64_div_u64(a, b, d-1, d).
> > > > 
> > > > Only x86-64 has an optimsed (asm) version of the function.
> > > > That is optimised to avoid the 'add c' when c is known to be zero.
> > > > In all other cases the extra code will be noise compared to the software
> > > > divide code.
> > > > 
> > > > I've updated the test module to test mul_u64_u64_div_u64_roundup() and
> > > > also enhanced it to verify the C division code on x86-64.
> > > > 
> > > > Note that the code generated by gcc (eg for 32bit x86) just for the multiply
> > > > is rather more horrid than one would expect (clang does better).
> > > > I dread to think how long the divide loop takes.
> > > > And I'm not at all sure the call in kernel/sched/cputime.c isn't in a
> > > > relatively common path (rather than just hardware initialisation).
...
> > > I wonder what happend to this series. I'd like to make use of
> > > mul_u64_u64_div_u64_roundup() so I'd be interested to get this into the
> > > mainline.  
> > 
> > I've a WIP rewrite of the divide code, speeds it up considerably for
> > 'not amd-64'.  
> 
> May I suggest you simply submit the new API now (addressing my latest 
> comments if possible) and submit the divide optimization later?

I've just 'lobbed in' a 'v2' that excludes the last patch (extra changes
to the test module) and replaces 'if (!divisor) 1/0;' with BUG_ON(!divisor).

I'll to the speedup changes on top.

	David