linux-kernel - Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics

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Message-ID: <fd95a255-4499-2907-8af9-d340f157da68@linux.intel.com>
Date:   Thu, 29 Aug 2019 09:31:37 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     acme@...nel.org, mingo@...hat.com, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, jolsa@...nel.org, eranian@...gle.com,
        alexander.shishkin@...ux.intel.com, ak@...ux.intel.com
Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown
 metrics

On 8/28/2019 11:19 AM, Peter Zijlstra wrote:
>> +static int icl_set_topdown_event_period(struct perf_event *event)
>> +{
>> +	struct hw_perf_event *hwc = &event->hw;
>> +	s64 left = local64_read(&hwc->period_left);
>> +
>> +	/*
>> +	 * Clear PERF_METRICS and Fixed counter 3 in initialization.
>> +	 * After that, both MSRs will be cleared for each read.
>> +	 * Don't need to clear them again.
>> +	 */
>> +	if (left == x86_pmu.max_period) {
>> +		wrmsrl(MSR_CORE_PERF_FIXED_CTR3, 0);
>> +		wrmsrl(MSR_PERF_METRICS, 0);
>> +		local64_set(&hwc->period_left, 0);
>> +	}
> This really doesn't make sense to me; if you set FIXED_CTR3 := 0, you'll
> never trigger the overflow there; this then seems to suggest the actual
> counter value is irrelevant. Therefore you don't actually need this.
> 

Could you please elaborate on why initialization to 0 never triggers an 
overflow?
As of my understanding, initialization to 0 only means that it will take 
more time than initialization to -max_period (0x8000 0000 0001) to 
trigger an overflow.

Maybe 0 is too tricky. We can set the initial value to 1.
I think the bottom line is that we need a small initial value for 
FIXED_CTR3 here.
PERF_METRICS reports an 8bit integer fraction which is something like 
0xff * internal counters / FIXCTR3.
The internal counters only start counting from 0. (SW cannot set an 
arbitrary initial value for internal counters.)
If the initial value of FIXED_CTR3 is too big, PERF_METRICS could always 
remain constant, e.g. 0.

Thanks,
Kan