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Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown
 metrics
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     acme@kernel.org, mingo@redhat.com, linux-kernel@vger.kernel.org,
        tglx@linutronix.de, jolsa@kernel.org, eranian@google.com,
        alexander.shishkin@linux.intel.com, ak@linux.intel.com
References: <20190826144740.10163-1-kan.liang@linux.intel.com>
 <20190826144740.10163-4-kan.liang@linux.intel.com>
 <20190828150238.GC17205@worktop.programming.kicks-ass.net>
From:   "Liang, Kan" <kan.liang@linux.intel.com>
Message-ID: <9e5f9292-9767-d17a-2223-37dff9e759c7@linux.intel.com>
Date:   Wed, 28 Aug 2019 15:35:02 -0400
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On 8/28/2019 11:02 AM, Peter Zijlstra wrote:
>> Reset
>> ======
>>
>> The PERF_METRICS and Fixed counter 3 have to be reset for each read,
>> because:
>> - The 8bit metrics ratio values lose precision when the measurement
>>    period gets longer.
> So it musn't be too hot,
> 
>> - The PERF_METRICS may report wrong value if its delta was less than
>>    1/255 of SLOTS (Fixed counter 3).

The "delta" is actually for the internal counters. Sorry for the
confusion.

> it also musn't be too cold. But that leaves it unspecified what exactly
> is the right range.
> 
> IOW, you want a Goldilocks number of SLOTS.
> 
>> Also, for counting, the -max_period is the initial value of the SLOTS.
>> The huge initial value will definitely trigger the issue mentioned
>> above. Force initial value as 0 for topdown and slots event counting.
> But you just told us that 0 is wrong too (too cold).

We set -max_period (0x8000 0000 0001) as initial value of FIXCTR3 for 
counting. But the internal counters probably starts counting from 0.
PERF_METRICS is fraction of internal counters / FIXCTR3.
So PERF_METRICS will never works.
We have to make FIXCTR3 count from 0 as well.

Thanks,
Kan