[RFC] Enabling invariant TSC timecounter on SMP

[Andriy Gapon]

on 31/05/2011 23:16 Jung-uk Kim said the following:
> On Tuesday 31 May 2011 07:18 am, Andriy Gapon wrote:
>> on 24/05/2011 20:56 Jung-uk Kim said the following:
>>> I think it's about time to enable invariant TSC timecounter on
>>> SMP by default.  Please see the attached patch.  It is also
>>> available from here:
>>>
>>> http://people.freebsd.org/~jkim/tsc_smp_test4.diff
>>>
>>> avg convinced me enough that it should be an opt-out feature
>>> going forward. :-)
>>
>> Not sure if I really did that.
>> My position is this:
>> - if we think that TSC is SMP-safe then it should have the best
>> priority
>> - we should do our best to auto-guess if TSC is SMP-safe 
>> unless a user explicitly overrides that property (either via
>> explicit testing or by making guesses based on CPU model or etc)
> 
> I am sorry if I misunderstood your intention.  However, I added 
> explicit boot-time TSC sanity check (to do our best to auto-guess) 
> and I think TSC is fairly SMP-safe.  Hence, I thought that it is 
> about time for the change.

In this case - yes.  But I remember that you were thinking about either
improving or simplifying that check or both.

>>> Comments?
>>
>> Perhaps I missed it, but I don't remember the "lowres" part of the
>> patch being discussed.
> 
> No, it wasn't discussed with you.  Do you see any problem with that 
> code?

I don't see any obvious problem, but I also don't understand rationale of using
smaller max_freq for the ncpus > 1 case.
Maybe these two logical changes should be done as two separate commits.

-- 
Andriy Gapon