On Wed, 5 Oct 2005, Ingo Molnar wrote:
>
> * Thomas Gleixner <[email protected]> wrote:
>
> > On Wed, 2005-10-05 at 10:29 -0400, Steven Rostedt wrote:
> > > Hmm, Ingo,
> > >
> > > Do you know why time goes backwards when I run hackbench as a realtime
> > > process? I added the output of start and stop and it does seem to go
> > > backwards.
> > >
> > > Thomas?
> >
> > Yes. Thats happening. I moved the priority of softirq-timer above
> > hackbench priority and the problem goes away. I look into this
> > further.
>
> wouldnt hackbench then permanently starve run_timer_softirq(), and
> update_times() in particular?
>
It seems that the problem comes down to the call to getnstimeofday in
do_gettimeofday.
void getnstimeofday(struct timespec *ts)
{
nsec_t delta;
unsigned long seq;
/* atomically read __monotonic_clock() */
do {
seq = read_seqbegin(&system_time_lock);
delta = __monotonic_clock() - xtime_last_update;
*ts = xtime_last_update_ts;
} while (read_seqretry(&system_time_lock, seq));
set_normalized_timespec(ts,
ts->tv_sec,
ts->tv_nsec + (long) delta);
}
I found that xtime_last_update is not updated while hackbench is running,
and the delta ends up being 16 billion and some change. So the call to
set_normalized_timespec overflows with the (long) delta.
-- Steve
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