Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles

Mathieu Desnoyers
2008-01-17T02:40:17Z

* john stultz (johnstul@us.ibm.com) wrote:
>
> On Wed, 2008-01-16 at 18:33 -0500, Steven Rostedt wrote:
> > Thanks John for doing this!
> >
> > (comments imbedded)
> >
> > On Wed, 16 Jan 2008, john stultz wrote:
> > > + int num = !cs->base_num;
> > > + cycle_t offset = (now - cs->base[!num].cycle_base_last);
> > > + offset &= cs->mask;
> > > + cs->base[num].cycle_base = cs->base[!num].cycle_base + offset;
> > > + cs->base[num].cycle_base_last = now;
> >
> > I would think that we would need some sort of barrier here. Otherwise,
> > base_num could be updated before all the cycle_base. I'd expect a smp_wmb
> > is needed.
>
> Hopefully addressed in the current version.
>
>
> > > Index: monotonic-cleanup/kernel/time/timekeeping.c
> > > ===================================================================
> > > --- monotonic-cleanup.orig/kernel/time/timekeeping.c 2008-01-16 12:21:46.000000000 -0800
> > > +++ monotonic-cleanup/kernel/time/timekeeping.c 2008-01-16 14:15:31.000000000 -0800
> > > @@ -71,10 +71,12 @@
> > > */
> > > static inline s64 __get_nsec_offset(void)
> > > {
> > > - cycle_t cycle_delta;
> > > + cycle_t now, cycle_delta;
> > > s64 ns_offset;
> > >
> > > - cycle_delta = clocksource_get_cycles(clock, clocksource_read(clock));
> > > + now = clocksource_read(clock);
> > > + cycle_delta = (now - clock->cycle_last) & clock->mask;
> > > + cycle_delta += clock->cycle_accumulated;
> >
> > Is the above just to decouple the two methods?
>
> Yep. clocksource_get_cycles() ended up not being as useful as an helper
> function (I was hoping the arch vsyscall implementations could use it,
> but they've done too much optimization - although that may reflect a
> need up the chain to the clocksource structure).
>

The problem with vsyscall is that we will have a hard time disabling
preemption :( Therefore, insuring that the read of the data is done in a
timely manner is hard to do.

Mathieu

> thanks
> -john
>
>

--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68