Messages in this thread | | | Date | Sat, 31 Aug 2019 11:18:02 +0200 | From | Peter Zijlstra <> | Subject | Re: [RESEND PATCH V3 3/8] perf/x86/intel: Support hardware TopDown metrics |
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On Thu, Aug 29, 2019 at 12:56:02PM -0400, Liang, Kan wrote: > On 8/29/2019 9:52 AM, Peter Zijlstra wrote:
> > What what? The PERF_METRICS contents depends on the FIXCTR3 value ?! > > Yes. > > For current implementation, PERF_METRIC MSR is composed by four fields, > backend bound, frontend bound, bad speculation and retiring. > Each of the fields are populated using the below formula for eg: > PERF_METRIC[RETIRING] = (0xFF * > PERF_METRICS_RETIRING_INTERNAL_48bit_COUNTER) > / FIXCTR3
So it really depends on the actual exposed FIXCTR3 _value_ to compute the PERF_METRIC field? *mind boggles*, that's really unspeakable crap. And this isn't documented anywhere afaict.
I was thinking they've have an internal counter for the SLOTS value too, so the PERF_METRIC fields are indenpendent; which would be like 'sane'.
Exposing the internal counters would've been _soooo_ much better, just add 4 more fixed counters and call it a day.
> The METRICS_OVF indicates the overflow of any internal counters.
OK, but I'm thinking that by that time the fraction in PERF_METRIC will be too coarse and we're loosing precision. Reconstruction will be inaccurate.
> The internal counters only start counting from 0, which cannot be programmed > by SW. But resetting the PERF_METRIC would implicitly resetting the internal > counters.
The only possible option given the choices.
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