Re: [????] Re: [????] Re: divide error in x86 and cputime

[David Laight <david.laight.linux@gmail.com>]

On Mon, 7 Jul 2025 20:30:57 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

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

It can't be relevant.
That change is after the mul_u64_u64_div_u64() call that trapped.
It is also not relevant for x86-64 because it uses the asm version.

At some point mul_u64_u64_div_u64() got changed to be accurate (and slow)
so that check isn't needed any more.

	David