Re: [RFC PATCH 13/22 -v2] handle accurate time keeping over long delays

Steven Rostedt
2008-01-10T00:25:17Z

On Wed, 9 Jan 2008, john stultz wrote:
> > Index: linux-compile-i386.git/kernel/time/timekeeping.c
> > ===================================================================
> > --- linux-compile-i386.git.orig/kernel/time/timekeeping.c 2008-01-09 14:07:34.000000000 -0500
> > +++ linux-compile-i386.git/kernel/time/timekeeping.c 2008-01-09 15:17:31.000000000 -0500
> > @@ -448,27 +449,29 @@ static void clocksource_adjust(s64 offse
> > */
> > void update_wall_time(void)
> > {
> > - cycle_t offset;
> > + cycle_t cycle_now, offset;
> >
> > /* Make sure we're fully resumed: */
> > if (unlikely(timekeeping_suspended))
> > return;
> >
> > #ifdef CONFIG_GENERIC_TIME
> > - offset = (clocksource_read(clock) - clock->cycle_last) & clock->mask;
> > + cycle_now = clocksource_read(clock);
> > #else
> > - offset = clock->cycle_interval;
> > + cycle_now = clock->cycle_last + clock->cycle_interval;
> > #endif
> > + offset = (cycle_now - clock->cycle_last) & clock->mask;
>
> It seems this offset addition was to merge against the colliding
> xtime_cache changes in mainline. However, I don't think its quite right,
> and might be causing incorrect time() or vtime() results if NO_HZ is
> enabled.

Yeah, this had a bit of clashes in its life in the RT kernel.

>
> > + clocksource_accumulate(clock, cycle_now);
> > +
> > clock->xtime_nsec += (s64)xtime.tv_nsec << clock->shift;
> >
> > /* normally this loop will run just once, however in the
> > * case of lost or late ticks, it will accumulate correctly.
> > */
> > - while (offset >= clock->cycle_interval) {
> > + while (clock->cycle_accumulated >= clock->cycle_interval) {
> > /* accumulate one interval */
> > clock->xtime_nsec += clock->xtime_interval;
> > - clock->cycle_last += clock->cycle_interval;
> > - offset -= clock->cycle_interval;
> > + clock->cycle_accumulated -= clock->cycle_interval;
> >
> > if (clock->xtime_nsec >= (u64)NSEC_PER_SEC << clock->shift) {
> > clock->xtime_nsec -= (u64)NSEC_PER_SEC << clock->shift;
> > @@ -482,7 +485,7 @@ void update_wall_time(void)
> > }
> >
> > /* correct the clock when NTP error is too big */
> > - clocksource_adjust(offset);
> > + clocksource_adjust(clock->cycle_accumulated);
>
>
> I suspect the following is needed, but haven't been able to test it yet.

Thanks, I'll pull it in and start testing it.

-- Steve