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From:   Stephane Eranian <eranian@google.com>
Date:   Sat, 31 Aug 2019 02:13:05 -0700
Message-ID: <CABPqkBSPfZ6iQ_t1ESZf334q+dNVf=RvoNHdmKZSs5GLokbjFQ@mail.gmail.com>
Subject: Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics
To:     Andi Kleen <ak@linux.intel.com>
Cc:     "Liang, Kan" <kan.liang@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jiri Olsa <jolsa@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>
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Andi,

On Fri, Aug 30, 2019 at 5:31 PM Andi Kleen <ak@linux.intel.com> wrote:
>
> > 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.
>
How is RAPL handled? No deltas there either. It uses the snapshot model.
At each interval, perf stat just reads the current count, and does not compute
a delta since previous read.
With PERF_METRICS, the delta is always since previous read. If you read
frequently enough you do not lose precision.

>
> 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
>