Re: divide error in x86 and cputime

[Oleg Nesterov <oleg@redhat.com>]

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?

Oleg.

On 07/08, Oleg Nesterov wrote:
>
> On 07/07, Li,Rongqing wrote:
> >
> > [78250815.703847] divide error: 0000 [#1] PREEMPT SMP NOPTI
>
> ...
>
> > It caused by a process with many threads running very long,
> > and utime+stime overflowed 64bit, then cause the below div
> >
> > mul_u64_u64_div_u64(0x69f98da9ba980c00, 0xfffd213aabd74626, 0x09e00900);
> >
> > I see the comments of mul_u64_u64_div_u64() say:
> >
> > Will generate an #DE when the result doesn't fit u64, could fix with an
> > __ex_table[] entry when it becomes an issu
> >
> > Seem __ex_table[] entry for div does not work ?
>
> Well, the current version doesn't have an __ex_table[] entry for div...
>
> I do not know what can/should we do in this case... Perhaps
>
> 	static inline u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div)
> 	{
> 		int ok = 0;
> 		u64 q;
>
> 		asm ("mulq %3; 1: divq %4; movl $1,%1; 2:\n"
> 			_ASM_EXTABLE(1b, 2b)
> 			: "=a" (q), "+r" (ok)
> 			: "a" (a), "rm" (mul), "rm" (div)
> 			: "rdx");
>
> 		return ok ? q : -1ul;
> 	}
>
> ?
>
> Should return ULLONG_MAX on #DE.
>
> Oleg.