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

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Message-ID: <20250708120016.4071595d@pumpkin>
Date: Tue, 8 Jul 2025 12:00:16 +0100
From: David Laight <david.laight.linux@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Steven Rostedt <rostedt@...dmis.org>, "Li,Rongqing"
 <lirongqing@...du.com>, Peter Zijlstra <peterz@...radead.org>,
 "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 01:00:57 +0200
Oleg Nesterov <oleg@...hat.com> wrote:

> On 07/07, Steven Rostedt wrote:
> >
> > On Mon, 7 Jul 2025 18:20:56 -0400
> > Steven Rostedt <rostedt@...dmis.org> wrote:
> >  
> > > I would say this should never happen and if it does, let the kernel crash.  
> >  
> > >> [78250815.703852] CPU: 127 PID: 83435 Comm: killall Kdump: loaded Tainted: P           OE K   5.10.0 #1  
> >
> > This happened on a 5.10 kernel with a proprietary module loaded, so
> > honestly, if it can't be reproduced on a newer kernel without any
> > proprietary modules loaded, I say we don't worry about it.  
> 
> Yes, agreed, see my reply to myself.

Except that just isn't relevant.
The problem is that the process running time (across all threads) can
easily exceed 2^64 nanoseconds.

With cpu having more and more 'cores' and software spinning to reduce
latency it will get more and more common.

Perhaps standardising on ns for timers (etc) wasn't such a bright idea.
Maybe 100ns would have been better.

But the process 'rtime' does need dividing down somewhat.
Thread 'rtime' is fine - 564 years isn't going to be out problem!

	David