linux-kernel - Re: [PATCH v2 -next] sched/cputime: Fix mul_u64_u64_div

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Message-ID: <20240726133029.GC21542@redhat.com>
Date: Fri, 26 Jul 2024 15:30:30 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Zheng Zucheng <zhengzucheng@...wei.com>, mingo@...hat.com,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 -next] sched/cputime: Fix mul_u64_u64_div_u64()
 precision for cputime

On 07/26, Peter Zijlstra wrote:
>
> On Fri, Jul 26, 2024 at 03:04:01PM +0200, Oleg Nesterov wrote:
> > On 07/26, Oleg Nesterov wrote:
> > >
> > > On 07/26, Zheng Zucheng wrote:
> > > >
> > > > before call mul_u64_u64_div_u64(),
> > > > stime = 175136586720000, rtime = 135989749728000, utime = 1416780000.
> > >
> > > So stime + utime == 175138003500000
> > >
> > > > after call mul_u64_u64_div_u64(),
> > > > stime = 135989949653530
> > >
> > > Hmm. On x86 mul_u64_u64_div_u64(175136586720000, 135989749728000, 175138003500000)
> > > returns 135989749728000 == rtime, see below.
> >
> > Seriously, can you re-check your numbers? it would be nice to understand why
> > x86_64 differs...
>
> x86_64 has a custom mul_u64_u64_div_u64() implementation.

Yes sure, but my user-space test-case uses the "generic" version,

> > > But perhaps it makes sense to improve the accuracy of mul_u64_u64_div_u64() ?
> > > See the new() function in the code below.
> >
> > Just in case, the usage of ilog2 can be improved, but this is minor.
>
> I meant to go look at this, it seems to loop less than your improved
> version, but I'm chasing crashes atm. Perhaps it provides inspiration.
>
> https://codebrowser.dev/llvm/compiler-rt/lib/builtins/udivmodti4.c.html#__udivmodti4

Thanks... I'll try to take a look tommorrow, but at first glance I will
never understand this (clever) code ;)

Oleg.