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

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 7 Jul 2025 23:41:14 +0000
"Li,Rongqing" <lirongqing@baidu.com> wrote:

> > On a second thought, this
> > 
> >     mul_u64_u64_div_u64(0x69f98da9ba980c00, 0xfffd213aabd74626,
> > 0x09e00900);
> >                         stime               rtime
> > stime + utime
> > 
> > looks suspicious:
> > 
> > 	- stime > stime + utime
> > 
> > 	- rtime = 0xfffd213aabd74626 is absurdly huge
> > 
> > so perhaps there is another problem?
> >   
> 
> it happened when a process with 236 busy polling threads , run about 904 days, the total time will overflow the 64bit
> 
> non-x86 system maybe has same issue, once (stime + utime) overflows 64bit, mul_u64_u64_div_u64 from lib/math/div64.c maybe cause division by 0
> 
> so to cputime, could cputime_adjust() return stime if stime if stime + utime is overflow
> 
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 6dab4854..db0c273 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -579,6 +579,10 @@ void cputime_adjust(struct task_cputime *curr, struct prev_cputime *prev,
>                 goto update;
>         }
> 
> +       if (stime > (stime + utime)) {
> +               goto update;
> +       }
> +
>         stime = mul_u64_u64_div_u64(stime, rtime, stime + utime);
>         /*
>          * Because mul_u64_u64_div_u64() can approximate on some
> 

Are you running 5.10.0? Because a diff of 5.10.238 from 5.10.0 gives:

@@ -579,6 +579,12 @@ void cputime_adjust(struct task_cputime *curr, struct prev_cputime *prev,
        }
 
        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;
 
 update:


Thus the result is what's getting screwed up.

-- Steve