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Herbert Poetzl wrote:
> 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;
Yep, that IS the problem. It should be normalized here, I.e.
set_normalized_timespec(wall_to_monotonic,
wall_to_monotonic.tv_sec - xtime.tv_sec,
wall_to_monotonic.tv_nsec - xtime.tv_nsec);
with the obvious delets :)
Still, this should be corrected by the first settimeofday, which most systems do
on the way up, or is that just those who use NTP?
--
George Anzinger george@mvista.com
High-res-timers: http://sourceforge.net/projects/high-res-timers/
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