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

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Message-ID: <20240726130401.GB21542@redhat.com>
Date: Fri, 26 Jul 2024 15:04:01 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Zheng Zucheng <zhengzucheng@...wei.com>
Cc: mingo@...hat.com, peterz@...radead.org, 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, 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...

> 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.

Oleg.