Re: perf_counter Atom patch

[Yong Wang <yong.y.wang@linux.intel.com>]

On Tue, Jun 23, 2009 at 11:47:17AM +0200, Ingo Molnar wrote:
> 
> * Yong Wang <yong.y.wang@linux.intel.com> wrote:
> 
> > On Tue, Jun 23, 2009 at 10:40:45AM +0200, stephane eranian wrote:
> > > Yong,
> > > 
> > > On Tue, Jun 23, 2009 at 10:27 AM, stephane
> > > eranian<eranian@googlemail.com> wrote:
> > > > Hi,
> > > >
> > > > On Tue, Jun 23, 2009 at 9:59 AM, Yong Wang<yong.y.wang@linux.intel.com> wrote:
> > > >> On Tue, Jun 23, 2009 at 09:45:03AM +0200, stephane eranian wrote:
> > > >>>
> > > >>> Unfortunately, I don't have a N270 to compare with your results.
> > > >>> We need to verify whether or not N270 implements the fixed counters.
> > > >>> Does it report architected perfmon v3 or v1?
> > > >>>
> > > >>
> > > >> All Atom processors report perfmon v3 as specified in SDM. N270 is no
> > > >> exception.
> > > >>
> > > > V3 does not set a minimal number of fixed counters, could be zero. But
> > > > that seems
> > > > odd. Let me ask around.
> > > >
> > > Second thought on this:
> > >        x86_pmu.num_counters_fixed      =
> > > max((int)edx.split.num_counters_fixed, 3);
> > > 
> > >         rdmsrl(MSR_CORE_PERF_GLOBAL_CTRL, x86_pmu.intel_ctrl);
> > > 
> > > 
> > > Forcing num_counter_fixed is not enough, you need to make sure 
> > > they are actually activated in GLOBAL_CTRL, i.e., make sure bits 
> > > 32-34 are set in intel_ctrl. Depending on which machine you're 
> > > on, the power on value for GLOBAL_CTRL changes. The correct 
> > > value for it should be that ONLY generic counters are on by 
> > > default.
> > > 
> > 
> > Oh, this might be why fixed counter do not work on my Atom box. I 
> > will look into it. [...]
> 
> Thanks - having a different bootup default for the global ctrl 
> indeed sounds like a good and plausible explanation - please send a 
> patch for that if you've tested it, removing that quirk and adding 
> the global-enable ctrl logic.
> 

The root cause of fixed counters not working on Atom is indeed related
to global counter control MSR. The power-on value on Atom is 0x3 which
means only general purpose counters are enabled by default. The power-on
value on Core2 is 0xffffffffffffffff which I believe is also the case
for Nehalem. That's why Core2 and Nehalem do not have the problem.