prefer tsc timecounter when it's good

[Andriy Gapon]

on 26/04/2011 20:49 Jung-uk Kim said the following:
> Can you please test attached patch?  You can get it from here, too:
> 
> http://people.freebsd.org/~jkim/tsc_smp_test.diff

I am planning on testing the patch, but I am a little bit busy with other things
at the moment.

The idea looks good to me.  The code is a little bit hard to follow :-)
I would use three separate array instead of a single array with triple size (for
clarity).  The arrays would have to be placed inside a structure for passing to
smp_rendezvous.

Also, perhaps a single rendezvous per iteration would be sufficient, so that you
get four values to compare per a pair of CPUs, instead of current six.  Again to
make the code simpler / more readable.  That would allow to expand TSC_READ
macro as well (two copies of the function would take less lines than the macro).

BTW, not sure if you actually need 'volatile' inside tsc_read_X.

> Currently this patch samples 3,000 times to determine if any CPU has 
> out-of-order TSC but it may be too much, especially for large SMP 
> machines.  If it takes too long, I'll lower the number.  Please 
> report if that's the case.
> 
> Please note this patch also changes HPET, ACPI-fast and TSC qualities.  
> However, TSC on SMP does not change the default quality, i.e., HPET 
> or ACPI timer will be chosen by default, because we cannot be sure if 
> they'll drift later.  If the user is sure that they don't drift AND 
> it is absolutely constant, kern.timecounter.smp_tsc tunable can be 
> used to set better quality.

- You changing the relative priorities of HPET and ACPI-fast.  I support this
change (some others may not), but please make it as a separate commit.

- Not sure if the quality test code is of much use if a user has to set some
tunable to actually use it over HPET or ACPI-fast.  I thought that the whole
point was in automatically choosing the best timecounter.  I would go the
opposite way - if automatic selection of TSC causes any trouble then provide a
way to disable it.

-- 
Andriy Gapon