linux-kernel - Re: divide error in x86 and cputime

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Message-ID: <20250707214143.59ce744e@gandalf.local.home>
Date: Mon, 7 Jul 2025 21:41:43 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "Li,Rongqing" <lirongqing@...du.com>
Cc: Oleg Nesterov <oleg@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 David Laight <david.laight.linux@...il.com>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "vschneid@...hat.com"
 <vschneid@...hat.com>, "mgorman@...e.de" <mgorman@...e.de>,
 "bsegall@...gle.com" <bsegall@...gle.com>, "dietmar.eggemann@....com"
 <dietmar.eggemann@....com>, "vincent.guittot@...aro.org"
 <vincent.guittot@...aro.org>, "juri.lelli@...hat.com"
 <juri.lelli@...hat.com>, "mingo@...hat.com" <mingo@...hat.com>
Subject: Re: divide error in x86 and cputime

On Tue, 8 Jul 2025 01:17:50 +0000
"Li,Rongqing" <lirongqing@...du.com> wrote:

> Stime is not greater than rtime in my case, (stime= 0x69f98da9ba980c00,
> rtime= 0xfffd213aabd74626, stime+utime= 0x9e00900. So utime should be
> 0x960672564f47fd00 ), and this overflow process with 236 busy poll
> threads running about 904 day, so I think these times are correct
> 

But look at rtime, it is *negative*. So maybe that fix isn't going to fix
this bug, but rtime is most definitely screwed up. That value is:

  0xfffd213aabd74626 = (u64)18445936184654251558 = (s64)-807889055300058

There's no way run time should be 584 years in nanoseconds.

So if it's not fixed by that commit, it's a bug that happened before you even
got to the mul_u64_u64_div_u64() function. Touching that is only putting a
band-aid on the symptom, you haven't touched the real bug.

I bet there's likely another fix between what you are using and 5.10.238.
There's 31,101 commits between those two. You are using a way old kernel
without any fixes to it. It is known to be buggy. You will hit bugs with
it. No need to tell us about it.

-- Steve