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

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Date:   Fri, 30 Aug 2019 17:31:10 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown
 metrics

> the same manner. It would greatly simplify the kernel implementation.

I tried that originally. It was actually more complicated.

You can't really do deltas on raw metrics, and a lot of the perf
infrastructure is built around deltas.

To do the regular reset and not lose precision over time internally
you have to keep expanded counters anyways. And if you do that
you can just expose them to user space too, and have everything
in user space just work without any changes (except for the final
output)

-Andi