linux-kernel - Re: perf_counter Atom patch

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Message-ID: <20090623083420.GB23534@ywang-moblin2.bj.intel.com>
Date:	Tue, 23 Jun 2009 16:34:20 +0800
From:	Yong Wang <yong.y.wang@...ux.intel.com>
To:	eranian@...il.com
Cc:	Yong Wang <yong.y.wang@...ux.intel.com>,
	"Wang, Yong Y" <yong.y.wang@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: perf_counter Atom patch

On Tue, Jun 23, 2009 at 10:27:47AM +0200, stephane eranian wrote:
> Hi,
> 
> On Tue, Jun 23, 2009 at 9:59 AM, Yong Wang<yong.y.wang@...ux.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.
> 

The Atom spec update says CPUID.0AH.EDX should be 0x0503 in errata
section. I assume future offerings will more likely to add more new
counters in stead of remove existing ones. Correct me if I'm wrong;-)

> >> > The return value of CPUID(0xa) is indeed bogus, too and there is another quirk for that in
> >> > intel_pmu_init() in arch/x86/kernel/cpu/perf_counter.c
> >> >
> >> > x86_pmu.num_counters_fixed	= max((int)edx.split.num_counters_fixed, 3);
> >> >
> >> > Is this what you were talking about?
> >>
> >> Not quite, because with the max() you'd have a problem on Intel Core
> >> Duo/Solo processors
> >> as they do implement the first generation of architected perfmon and
> >> that one did not have
> >> fixed counters. So you'd have to special case family=6 model=14.
> >
> > That has been taken into account actually. Only perfmon v2 and above are
> > supported as you see in intel_pmu_init().
> >
> > 	if (version < 2)
> > 		return -ENODEV;
> >
> I assume this is a current limitation of the implementation. If you
> see version <  2
> you could simply consider having 0 fixed counters and everything else would work
> as expected. But there is a catch, unfortunately, in that there is erratum AE49
> which says that there is only one enable bit to control the two generic counters
> on Core Duo/Solo.
> 

Ideally we could consider having 0 fixed counters for Core Duo/Solo. But
like you said, the erratum you pointed out just complicates things much.

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