linux-kernel - Re: perf_counter Atom patch

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Message-ID: <7c86c4470906230219o23e1b85u7111cc0f527ab906@mail.gmail.com>
Date:	Tue, 23 Jun 2009 11:19:59 +0200
From:	stephane eranian <eranian@...glemail.com>
To:	Yong Wang <yong.y.wang@...ux.intel.com>
Cc:	"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 10:53 AM, Yong Wang<yong.y.wang@...ux.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@...glemail.com> 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.
>> >
>> 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. BTW, why should ONLY generic counters be on by default?
>
Glad you asked.
I think, it's for backward compatibility with processors such as Core Duo with
architected perfmon v1 which did not have the GLOBAL_* controls.
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