Messages in this thread | | | Date | Fri, 3 Dec 2004 04:20:25 +0100 | From | Herbert Poetzl <> | Subject | Re: do_posix_clock_monotonic_gettime() returns negative nsec |
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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
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