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

["Li,Rongqing" <lirongqing@baidu.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?
> 

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


Thanks

-Li


> 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.