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

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Message-ID: <20250707203057.1b2af73d@gandalf.local.home>
Date: Mon, 7 Jul 2025 20:30:57 -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: [????] Re: divide error in x86 and cputime

On Tue, 8 Jul 2025 00:10:54 +0000
"Li,Rongqing" <lirongqing@...du.com> wrote:

> >         stime = mul_u64_u64_div_u64(stime, rtime, stime + utime);
> > +       /*
> > +        * Because mul_u64_u64_div_u64() can approximate on some
> > +        * achitectures; enforce the constraint that: a*b/(b+c) <= a.
> > +        */
> > +       if (unlikely(stime > rtime))
> > +               stime = rtime;  
> 
> 
> My 5.10 has not this patch " sched/cputime: Fix mul_u64_u64_div_u64() precision for cputime ",
> but I am sure this patch can not fix this overflow issue, Since division error happened in mul_u64_u64_div_u64()

Have you tried it? Or are you just making an assumption?

How can you be so sure? Did you even *look* at the commit?

    sched/cputime: Fix mul_u64_u64_div_u64() precision for cputime
    
    In extreme test scenarios:
    the 14th field utime in /proc/xx/stat is greater than sum_exec_runtime,
    utime = 18446744073709518790 ns, rtime = 135989749728000 ns
    
    In cputime_adjust() process, stime is greater than rtime due to
    mul_u64_u64_div_u64() precision problem.
    before call mul_u64_u64_div_u64(),
    stime = 175136586720000, rtime = 135989749728000, utime = 1416780000.
    after call mul_u64_u64_div_u64(),
    stime = 135989949653530
    
    unsigned reversion occurs because rtime is less than stime.
    utime = rtime - stime = 135989749728000 - 135989949653530
                          = -199925530
                          = (u64)18446744073709518790
    
    Trigger condition:
      1). User task run in kernel mode most of time
      2). ARM64 architecture
      3). TICK_CPU_ACCOUNTING=y
          CONFIG_VIRT_CPU_ACCOUNTING_NATIVE is not set
    
    Fix mul_u64_u64_div_u64() conversion precision by reset stime to rtime


When stime ends up greater than rtime, it causes utime to go NEGATIVE!

That means *YES* it can overflow a u64 number. That's your bug.

Next time, look to see if there's fixes in the code that is triggering
issues for you and test them out, before bothering upstream.

Goodbye.

-- Steve