Re: perf_counter Atom patch

[Ingo Molnar <mingo@elte.hu>]

* Yong Wang <yong.y.wang@linux.intel.com> wrote:

> 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.

I suspect it can also be firmware/BIOS and microcode version 
dependent - it's better to not rely on bootup state like that 
indeed.

thanks guys,

	Ingo