[Bugme-new] [Bug 11432] New: tick clock too slow by 0.5%

bugme-daemon at bugzilla.kernel.org bugme-daemon at bugzilla.kernel.org
Tue Aug 26 08:32:31 PDT 2008


http://bugzilla.kernel.org/show_bug.cgi?id=11432

           Summary: tick clock too slow by 0.5%
           Product: Timers
           Version: 2.5
     KernelVersion: 2.6.26.3
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: high
          Priority: P1
         Component: Other
        AssignedTo: johnstul at us.ibm.com
        ReportedBy: olha at users.sourceforge.net


The clock whose values are returned by the C library function "times" runs too
slow on my system, by about 0.5%. I think this must be a kernel bug, because
glibc forwards the call directly to sys_times (right?).

I had the bug with SMP kernel 2.6.25.10 and 2.6.26.3 (unpatched from
kernel.org) on an Intel Core2Duo machine, under OpenSuse 11.0 64-bit. The same
system copied to an Athlon machine and with kernel 2.6.25.10 reconfigured
non-SMP does not have the bug. I also have an elder system on the Intel
machine, Suse 9.3 with SMP kernel 2.6.23.9, but this one even does not have the
bug. So the bug started somewhere between 2.6.23.10 and 2.6.25.10, and it still
nests in 2.6.26.3, and it seems to concern SMP kernels only, or Intel machines.

The bug can be seen in the stopwatch of Eagle Mode 0.71.0
(http://eaglemode.sf.net). That stopwatch uses both, the "time" and "times"
functions, in order to be persistent across reboots and to show centiseconds as
long as the system is not rebooted. Through the bug, the stopwatch stops
showing centiseconds after about three to five minutes, because it realizes a
too large difference between the two clocks. More precise: First, the stopwatch
shows results from the times function with centiseconds, then it toggles a
while (quite ugly), and then it shows results from the time functions without
centiseconds. By comparison with a real-world stopwatch it can be seen that the
first phase loses nearly one second in three minutes (~0.5%).

Eagle Mode 0.72.0 will be resistant against the bug, but I think fixing it is
quite important, because bad clocks can bring satellites down. Therefore I
apply a high severity.


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