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

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Date:   Mon, 9 Sep 2019 09:40:26 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>
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
Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown
 metrics

On 8/31/2019 5:19 AM, Peter Zijlstra wrote:
>>> Then there is no mucking about with that odd counter/metrics msr pair
>>> reset nonsense. Becuase that really stinks.
>> You have to write them to reset the internal counters.
> But not for ever read, only on METRIC_OVF.

The precision are lost if the counters were running much longer than the 
measured delta. We have to reset the counters after every read.
For example, the worst case is:
The previous reading was rounded up and the current reading is rounded 
down. The error of METRICS is 1/256 - 1/SLOTS, which is related to 
SLOTS. The error for each Topdown event is  (1/256 - 1/SLOTS) * SLOTS.
With the SLOTS increasing, the precision will get worse and worse.
Therefor, we have to reset the counters periodically.

Thanks,
Kan