Re: do_posix_clock_monotonic_gettime() returns negative nsec

[Herbert Poetzl <herbert@13thfloor.at>]

On Thu, Dec 02, 2004 at 07:08:23PM -0800, Andrew Morton wrote:
> Herbert Poetzl <herbert@13thfloor.at> wrote:
> >
> > 
> > Hi Folks!
> > 
> > recent kernels (tested 2.6.10-rc2 and 2.6.10-rc2-bk15)
> > produce funny output in /proc/uptime like this:
> > 
> > 	# cat /proc/uptime
> > 	  12.4294967218 9.05
> > 	# cat /proc/uptime
> > 	  13.4294967251 10.33
> > 	# cat /proc/uptime
> > 	  14.4294967295 11.73
> > 
> > a short investigation of the issue, ended at
> > do_posix_clock_monotonic_gettime() which can (and 
> > often does) return negative nsec values (within
> > one second), so while the actual 'time' returned
> > is correct, some parts of the kernel assume that
> > those part is within the range (0 - NSEC_PER_SEC)
> > 
> >         len = sprintf(page,"%lu.%02lu %lu.%02lu\n",
> >                         (unsigned long) uptime.tv_sec,
> >                         (uptime.tv_nsec / (NSEC_PER_SEC / 100)),
> > 
> > as the function itself corrects overflows, it would
> > make sense to me to correct underflows too, for 
> > example with the following patch:
> > 
> > --- ./kernel/posix-timers.c.orig	2004-11-19 21:11:05.000000000 +0100
> > +++ ./kernel/posix-timers.c	2004-12-03 02:23:56.000000000 +0100
> > @@ -1208,7 +1208,10 @@ int do_posix_clock_monotonic_gettime(str
> >  	tp->tv_sec += wall_to_mono.tv_sec;
> >  	tp->tv_nsec += wall_to_mono.tv_nsec;
> >  
> > -	if ((tp->tv_nsec - NSEC_PER_SEC) > 0) {
> > +	if (tp->tv_nsec < 0) {
> > +		tp->tv_nsec += NSEC_PER_SEC;
> > +		tp->tv_sec--;
> > +	} else if ((tp->tv_nsec - NSEC_PER_SEC) > 0) {
> >  		tp->tv_nsec -= NSEC_PER_SEC;
> >  		tp->tv_sec++;
> >  	}
> 
> Doesn't this imply that do_posix_clock_monotonic_gettime_parts() is
> returning a negative tv_nsec?

nope, not necessarily, because after that ...

        tp->tv_sec += wall_to_mono.tv_sec;
        tp->tv_nsec += wall_to_mono.tv_nsec;

might add a negative value, which explains the
underflow ...

and if you look closer:

	xtime.tv_sec = get_cmos_time();
        wall_to_monotonic.tv_sec = -xtime.tv_sec;
        xtime.tv_nsec = (INITIAL_JIFFIES % HZ) * (NSEC_PER_SEC / HZ);
        wall_to_monotonic.tv_nsec = -xtime.tv_nsec;

#define INITIAL_JIFFIES ((unsigned long)(unsigned int) (-300*HZ))

which might need a fix too ... but that's a 
different story ...

> If so, that would point back at getnstimeofday().  What is your setting of
> CONFIG_TIME_INTERPOLATION?

# grep TIME .config
# CONFIG_HPET_TIMER is not set
# CONFIG_HANGCHECK_TIMER is not set

best,
Herbert