prefer tsc timecounter when it's good
jkim at FreeBSD.org
Thu Apr 7 20:00:32 UTC 2011
On Thursday 07 April 2011 01:12 pm, Andriy Gapon wrote:
> what do you think about the following change?
> The idea is mark TSC as the best timecounter when it's invariant
> and synchronized between cores. Unfortunately I don't have code to
> auto-detect the synchronization and keep relying on the
> corresponding tunable.
I know Intel is claiming that TSCs for all cores/packages reset to
zero when they receive a "synchronous" INIT/RESET IPI. I haven't
really verified their claim but I think it may be good enough for our
AP startup code. However, AMD processors never had such guarantee,
> I thought about auto-setting it for single-package configurations,
> but even that information is currently not trivial to get out of our
> mp (i386/amd64) machdep code.
It isn't easy ATM. :-/
> --- a/sys/x86/x86/tsc.c
> +++ b/sys/x86/x86/tsc.c
> @@ -169,6 +169,9 @@ init_TSC_tc(void)
> printf("TSC timecounter disabled: APM enabled.\n");
> + if (tsc_is_invariant)
> + tsc_timecounter.tc_quality = 1200;
> #ifdef SMP
> * We can not use the TSC in SMP mode unless the TSCs on all CPUs
Although it looks okay, please don't commit it just yet. I am working
in this area actively. Also, if the Intel's claim is true, i.e.,
TSCs reset to zero when APs start, we cannot use TSC as a timecounter
hardware until all APs are started properly.
More information about the freebsd-current