linux-kernel - Re: [????] Re: divide error in x86 and cputime

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Message-ID: <20250707215322.0e4ec431@gandalf.local.home>
Date: Mon, 7 Jul 2025 21:53:22 -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: [????] Re: divide error in x86 and cputime

On Tue, 8 Jul 2025 01:40:27 +0000
"Li,Rongqing" <lirongqing@...du.com> wrote:

> > That would be:
> > 
> >                                    minutes    days
> >                                       v        v
> >   9223372036854775808 / 1000000000 / 60 / 60 / 24 / 365.25 = 292.27
> >                            ^               ^         ^
> >                         ns -> sec       hours       years
> > 
> > So the report says they have threads running for a very long time, it would still
> > be 292 years of run time!  
> 
> Utime/rtime is u64, it means overflow needs 292.27*2=584 year,
> 
> But with multiple thread, like 292 threads, it only need two years, it is a thread group total running time
> 
> 
> void thread_group_cputime_adjusted(struct task_struct *p, u64 *ut, u64 *st)
> {
>     struct task_cputime cputime;
> 
>     thread_group_cputime(p, &cputime);
>     cputime_adjust(&cputime, &p->signal->prev_cputime, ut, st);
> }
> 

So you are saying that you have been running this for over two years
without a reboot?

Then the issue isn't the divider, it's that the thread group cputime can
overflow. Perhaps it needs a cap, or a way to "reset" somehow after "so long"?

-- Steve