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

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Message-ID: <8f68278c4a454fd79b39cf21d4c0974f@baidu.com>
Date: Tue, 8 Jul 2025 01:58:00 +0000
From: "Li,Rongqing" <lirongqing@...du.com>
To: Steven Rostedt <rostedt@...dmis.org>
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: 答复: [????] Re: [????] Re: divide error in x86 and cputime

> "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?
> 

Yes, Consider more and more CPUs in machine, I think it is common case


> 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"?


Do not clear how to reset

But mul_u64_u64_div_u64() for x86 should not trigger a division error panic, maybe should return a ULLONG_MAX on #DE (like non-x86 mul_u64_u64_div_u64(),)

> 
> -- Steve