Messages in this thread | | | Date | Thu, 1 Feb 2007 16:23:40 +0100 | From | Vojtech Pavlik <> | Subject | Re: [patch 9/9] Make use of the Master Timer |
| |
On Thu, Feb 01, 2007 at 03:29:31PM +0100, Jiri Bohac wrote: > On Thu, Feb 01, 2007 at 12:36:05PM +0100, Andi Kleen wrote: > > On Thursday 01 February 2007 11:00, jbohac@suse.cz wrote: > > > > > + case VXTIME_TSC: > > > + rdtscll(tsc); > > > > Where is the CPU synchronization? > > > > > + cpu = smp_processor_id(); > > > + rdtscll(t); > > > > Also no synchronization. It's slower, but needed. > > Hmm, I wasn't sure. Why is it needed? How outdated can the > result of RDTSC / RDTSCP be? > > If I do: > rdtscll(a) > ... > rdtscll(b) > is it guaranteed that (b > a) ?
On a single CPU this is always guaranteed. Even on AMD.
> > > unsigned long long sched_clock(void) > > > { > > > - unsigned long a = 0; > > > - > > > - rdtscll(a); > > > - return cycles_2_ns(a); > > > + return monotonic_clock(); > > > } > > > > This is overkill because sched_clock() doesn't need a globally monotonic > > clock, per CPU monotonic is enough. The old version was fine. > > OK, thanks for spotting this. I'll change it to use __guess_mt(). > (more or less equal to cycles_2_ns(), no need to maintain yet another > tsc->ns ratio just for cycles_2_ns().
Will this also work correctly during CPU frequency changes?
> > > - tv->tv_sec = sec + usec / 1000000; > > > - tv->tv_usec = usec % 1000000; > > > + sec += nsec / NSEC_PER_SEC; > > > + nsec %= NSEC_PER_SEC; > > > > Using while() here is probably faster (done in vdso patchkit where > > gtod got mysteriously faster). Modulo and divisions are slow, even > > for constants when they are large. > > OK, will do that
I'd suggest benchmarking the difference.
-- Vojtech Pavlik Director SuSE Labs - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |