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SubjectRe: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics

On Fri, Aug 30, 2019 at 5:31 PM Andi Kleen <> 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

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