linux-kernel - Re: perf_counter Atom patch

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Date:	Tue, 23 Jun 2009 09:45:03 +0200
From:	stephane eranian <eranian@...glemail.com>
To:	"Wang, Yong Y" <yong.y.wang@...el.com>
Cc:	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

Hi,

On Tue, Jun 23, 2009 at 5:38 AM, Wang, Yong Y<yong.y.wang@...el.com> wrote:
>> From: stephane eranian [mailto:eranian@...glemail.com]
>>
>> I would like to better understand what makes you think
>> this is the case.
>>
>
> Because I observed that the output of 'perf stat -e 0:0 -e 0:1 -e 0:6 <cmd>'
> is always like below without the quirk.
>
>  Performance counter stats for '<cmd>':
>
>              0  cycles
>              0  instructions
>              0  bus-cycles
>
>> Perfmon is working on Atom and there, fixed counters work perfectly:
>> $ head -6 /proc/cpuinfo
>> processor     : 0
>> vendor_id     : GenuineIntel
>> cpu family    : 6
>> model         : 28
>> model name    : Intel(R) Atom(TM) CPU  230   @ 1.60GHz
>> stepping      : 2
>> ...
>
> My cpuinfo is below and the only difference I can see is 270 vs 230.
>
> processor       : 0
> vendor_id       : GenuineIntel
> cpu family      : 6
> model           : 28
> model name      : Intel(R) Atom(TM) CPU N270   @ 1.60GHz
> stepping        : 2
>
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?

> 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.
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